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Designing and Building a Solar Greenhouse or Sunspace

Written by Mike Nuess

Solar greenhouses and sunspaces remain a popular design for both new and existing homes. They can produce heat, provide an environment for intensive food production, and offer a very pleasant living space. However, a greenhouse or sunspace can seldom do all three things well. Consequently one of the most Important first steps in designing and budding a solar greenhouse or sunspace is to develop a very clear idea of the purpose of the space.

Be Clear About Your Expectations

You may choose to design the space as one whose primary purpose is aesthetic pleasant living space. It would be a space designed for light, beauty, and visual contact with the outdoor environment. It may contain house plants or garden starts and it may gain considerable warmth from the sun at times. But it would not be primarily designed either to intensively produce food or to maximize solar energy gains. Though it would pay an increased energy penalty for facing directions other than south it could do so and still fulfill its purpose.

Or, the purpose could be to maximize the benefits of solar energy gain in order to reduce space heating costs. It would have more restrictive design requirements than a purely aesthetic space. For example it would have to face south, or nearly so, in order to gain the most solar energy. While this might often times provide a very aesthetic space, it might also be too hot at times for comfortable living and so bright that one might have to squint uncomfortably. At night it should be closed off from the house to avoid excessive heat loss. This would make the space uncomfortably cold. It might have plants, but fewer types of plants would tolerate the temperature extremes, and it certainly could not produce tomatoes in the winter.

If the purpose is to maximize the production of plants for food, then you will have little space for easy chairs and other pleasantries of a living space. You may have to sacrifice some solar gains in order to moderate high temperatures or ventilate excess humidity. At night you may have to provide some supplemental heat in order to keep plants healthy.

These three purposes are not entirely exclusive of each other. Through compromise it is possible to blend them in many ways to suit your individual purposes. For example, you can design an exquisitely refreshing living space-one with flowers, fragrance and light-that provides net energy benefits to the home. In addition, the right design choices might enable you to use the space to extend the growing season for some herbs and food plants.

But it is important to understand the technical requirements of each purpose in order to succeed in obtaining the kind of space you desire. Without good design and operation, some sunspace or greenhouse designs could actually make your home use more energy than it would have without the space. For example, if thermally poor windows replace a well insulated wall, and the sunspace is not isolated from the house at night, then the addition would increase your energy costs. If the attached space has glass on several sides, the increased energy use could be large.

How you operate the space may be critically important too. Certain designs will require you to manually close the space off from the house at nights and on cold cloudy days in order to prevent increased energy costs.

Designing solar greenhouses and solar sunspaces can be quite complex. Maximizing their performance potential requires sophisticated analysis, thorough grasp of a number of basic principles, and an in-depth knowledge of a wide range of available materials. However, the ability to design, construct and manage an adequately performing, net energy gaining, and personally satisfying solar sunspace or solar greenhouse is within the grasp of those of us who haven't the time or inclination to first become experts. Fortunately there exists a well developed body of literature addressing the subject. By using the resources referenced in this factsheet you can learn to design and build your own solar sunspace or solar greenhouse or follow one of the more specific "cookbook" paths offered.

In this factsheet we shall use the term solar sunspace to refer to spaces designed to emphasize living and heating purposes. Solar sunspaces include spaces meant to have many house plants or ornamentals. Solar sunspaces may even have a zone set aside for starting food plants for the summer garden, but they stop short of becoming a full production greenhouse. The term solar greenhouse shall refer specifically to those spaces designed to maximize plant production.

Siting

Attaching the solar sunspace or solar greenhouse to the new or existing home can either reduce or increase the cost of heating the home. It is critically important that the attached space is located where it will have adequate solar availability. A solar site survey can assist in that determination and is advised in all but the most obviously complete solar exposures.

A Caution About Rules of Thumb

They can be valuable guidelines, helping you develop a solar sun space or greenhouse design, and a number are offered herein. They can also be dangerous, because they are often meant to apply to several different climates. If you develop a design based upon rules of thumb and don't learn to perform more detailed calculations, it may be wise to have a design professional review your design. This caution may be less important for those building temporary low cost spaces, and more important for larger, more expensive, more permanent spaces.

Glazing

Orientation
The optimum orientation for solar sunspace and solar greenhouse glazing is due south, though solar sunspace and solar greenhouse perform well when the glazing faces within about 25º of true south (not magnetic south). Within this range 90 percent or more of the solar energy that would be obtained with an exact south orientation will be available Beyond 25º the amount of available solar energy falls off rapidly and thermal performance will be impaired.

There is good reason to avoid east and west facing orientations as most glazing systems will be net energy losers over the winter months. West facing orientations tend to overheat the space in the non-heating season months. However, extreme off south orientations may be useful for some applications where optimal thermal performance is not possible. For example, if solar gain from the south is blocked by an unmovable obstruction, one can orient in an easterly direction and obtain early morning warm-up of the space while reducing afternoon overheating.

Tilt
This refers to the angle between the northern horizon and the plane of the glazing. Any tilt ranging between 60º and 90º will perform well.

Figure 1

The optimum range of glazing tilt is between 60° and 90°.

Vertical glazing has a number of advantages over sloped glazing. Vertical glazing may be the tilt of choice for most solar sunspace applications. Vertical glazing is easier to install than sloped glazing, is much less likely to leak, easier to clean, and significantly reduces the difficulty of avoiding summer overheating. Figure 2 shows the beneficial reduction in summer solar transmission through vertical glazing during the non-heating season months.

Figure 2

Transmittances of Sloped and Vertical Glazing During the Year

The amount of sunlight transmitted into the sunspace decreases dramatically during the summer months with vertical glazing, while it stays relatively steady throughout the year with sloped glazing.

In most solar sunspace applications vertical glazing with an insulated roof may be quite sufficient and is technically the least complex. If some overhead light or view is desired one or two skylights can be included and the difficulties of overhead glass avoided.

Solar greenhouses on the other hand, have more rigid design requirements. They need to maximize both the quality and quantity of light available to plants. Some overhead glazing is required in most cases. Vertical glazing in combination with sloped overhead glazing is a common compromise.

Figure 3

A good solar greenhouse choice is to combine vertical glazing with some sloped glazing.

Glazing Type
Choosing the type of glazing you use is one of the most difficult decisions in the design of a solar sunspace or solar greenhouse. There are many different materials to consider. There is no single complete source of information detailing the costs and benefits of all the glazing options. The field is changing very rapidly as new products continue to surface and the properties of existing products continue to change.

Glazing types can be grouped into three major categories: glass systems, rigid plastics, and roll plastics. Within each category there are a number of choices, each with its own characteristics. Generally, the roll plastics (so called because they are available in roll rather than sheet form) are the least costly, have the shortest lifetimes, and require the most maintenance. The rigid plastics are more costly but more durable. The glass systems are the most expensive, but have the greatest lifetimes and require the least maintenance. The good news is that good solar performance can be obtained from choices in each group.

Probably the most often used glazing system for solar performance has been a double glazed glass system that utilized movable insulation in order to reduce nighttime heat losses. The performance is good but the insulation can be costly. An effective insulation also requires good edge sealing, ease of operation, and consistent operation by the occupant in order to perform well.

Further Considerations For Glass
A major change occurring in glazing systems has been the recent emergence of new glass systems with greatly enhanced solar and thermal performance. Some of these improve the thermal efficiency of the sealed double glass unit by adding a low emissivity (low-e) coating to one of the glass surfaces inside the unit. The low-e coating reflects escaping heat radiation back into the sunspace.

An even more efficient glazing system first incorporates a polyester film inside the sealed glass unit in order to increase air spaces, then adds a low-e coating to the film.

New energy codes have helped to make these high performance windows readily available. Also, new window efficiency advancements continue to emerge, such as the use of vinyl window frames and nonmetallic spacers between the panes of glass.

What to Consider When Selecting Glazing

• Availability
  Some glazings may not have local suppliers and/or may have limited sizes.

• Cost
  The range of costs is great. Standard sizes cost significantly less. For glass units one can sometimes buy factory seconds, such as odd sized units cut to the wrong size, or units that were ordered but not purchased.

• Durability
  Glass is vulnerable to breakage. Acrylics have lower abrasion resistance. Polyesters must be installed tightly. "Slack" polyester installations will fatigue and break at flexing points. Ultraviolet light is a major culprit affecting the durability of plastics. Some plastics are more vulnerable than others. Some are treated with ultraviolet inhibitors in order to lengthen their lifetimes.

• Strength
  Snow and wind loads must be addressed.

• Ease of Installation
  Sealing out moisture is critical. Large glass units have considerable weight. Thermal expansion must be allowed for. For some materials optimal installation should occur when materials are near the mean annual temperature.

• Safety
  Glass units, especially overhead, can be very dangerous. Check local building codes for restrictions governing their use.

• Light Transmitting Qualities
  Incident solar radiation (sunlight) is composed of ultraviolet, visible and near infrared. We only see the visible which is about 50 percent of the total. The purpose of the sunspace may favor certain light transmission qualities over others.

• Solar Transmitting Qualities
  The range of transmitting qualities is very great. For examples some "cool" glazings reflect most of the near infrared but pass visible light, resulting in a 40 percent reduction of solar heat gain. While great for reducing air conditioning loads, such a choice would be very inappropriate for a solar greenhouse. Be careful to define the properties you want. Often the vendor's terminology can be subject to misinterpretation. For example, 90 percent "transmission" might refer to either visible light or total solar.

• Appearance
  Glazings seldom look as nice in the field as they do in manufacturers' literature. Fiberglass ultimately yellows with age. Acrylics may get scratched up over the years. Some roll plastics expand and "sag" in warmer weather, then shrink and tighten up in colder weather: if installed in cold weather they will expand and look sloppy in hot weather.

• Insulating Ability
  In an efficiently built solar sunspace or solar greenhouse the glazings will be the major avenue of heat loss. Maximizing the glazings insulating ability is desirable for reducing both heat loss and condensation of moisture. One must be cautious about maximizing insulating ability at the expense of solar transmission, because it is possible to reduce the net energy performance of the sunspace. Also one might impair the quality of light to the plants in a solar greenhouse.

Thermal Mass

Thermal mass is a material or combination of materials that has the ability to absorb or release a relatively large quantity of heat while undergoing a relatively small temperature change. Solar energy is typically delivered to a solar sunspace as in intense "pulse" of energy lasting only a few hours. The energy delivered in those hours is often more than what it takes to heat the space. As the space gets warmer, a properly designed thermal mass will absorb the extra heat. This helps to moderate the temperature increase and prevent overheating. At night the space cools and eventually its temperature drops below that of the mass. The mass then begins to release the heat, this time serving to moderate the temperature drop and preventing "overcooking." The mass performs three very valuable functions:

1. It stores solar energy that would otherwise have been useless,

2. It releases that energy later, when it is most valuable, and

3. It limits the temperature swings within the space.

Three key properties enable the mass to perform: absorptance, thermal conductivity, and high density. A good mass readily absorbs (and releases) radiant energy, conducts the absorbed energy into itself, and holds a large quantity of heat.

Absorptance
This is a measure of the ability of a material to absorb rather than reflect solar radiation and is expressed as a percentage of the light absorbed. Uncolored concrete with an absorptance of .65 absorbs 65 percent of the insulation Thermal mass materials in direct sunlight should generally have absorptances over 70 percent. Darker colors have higher absorptances while lighter colors are more reflective.

Thermal Conductivity
This is a measure of the mass's ability to conduct absorbed heat from its surface into its interior. High conductivity is best.

High Density
In solid materials high density is a good indicator of the material's ability to hold a large quantity of heat. Materials like concrete, tile, and brick are the usual choices.

More on Mass
Other issues to consider are sizing, back-up heat, location of mass material, and non-mass objects in the space.

Sizing
Thermal mass needs to be properly sized to the solar sunspace or greenhouse. Proper sizing is complex but if water is the only mass, rule of thumb guides suggest a minimum of three gallons of water for every ft² of south facing glazing. If concrete or brick is the only mass the suggestion is about 1 ft³ of concrete or brick for every ft² of south facing glazing.

Extensive cloudy periods during Washington winters make long term heat storage less practical, so a good approach is to design the mass to minimize the daily temperature swings. In order to do this we want the mass to be able to quickly absorb and release heat, and this is best accomplished by increasing the surface area of the mass in relation to its volume. For example, five gallon water containers would be more effective than 55 gallon drums. Concrete slabs and walls need be no thicker than 4 to 6 inches. If masonry is the only mass 3 ft² of masonry surface area per ft² of south glazing is a good minimum.

When deciding how much thermal mass should be used it is important to keep in mind the purpose of the space. If the space is primarily a collector of heat for the house and it is intended to supply less than 1/3 of the building's heat, then the amount of mass could be less, roughly about half of the above levels. An insulated concrete floor is usually sufficient mass for a sunspace in Washington. On the other hand the level of mass should remain high for a solar greenhouse that needs to maintain reasonable temperatures for plants.

Back-Up Heat
A solar sunspace or solar greenhouse in Washington will experience extended cold cloudy periods that exhaust all stored thermal mass. If warmer temperatures must be maintained, backup heat will have to be provided from the attached house or from heaters in the space. If not the space can be closed off from the house until solar gains are again available. Closing the space off from the rest of the house at night and on cold cloudy days will save the most energy. In some sunspaces, failure to appropriately close off the space will result in large energy penalties.

Location of the Mass
"Couple" the mass. Direct exposure of the mass to sunlight is best. Indirect exposure is second best and still good. The mass must be in the space receiving the sunlight so it can "see" reflected light and reradiated heat. Remote mass (such as rock bin storage) is worst. You would then need a means, either mechanical or natural, of distributing heated air to the mass. Avoid covering mass with rug materials. Adding masonry tiles to a concrete floor is a good way to make the space attractive yet retain high quality mass, but bed tiles and slate well in mortar to provide a good bond (without air spaces) to the concrete below.

Non-Mass Objects
Non-mass objects have low heat holding capacities. If they are dark in color, they will quickly overheat and release that heat to the space rather than store it. Light colors on nonmass materials will reduce the solar energy they absorb. So use light colors for non-mass materials to "bounce" the light around and distribute it to mass surfaces where it can be absorbed and stored. This is especially important in the solar greenhouse because it will increase the amount of light available to the leaf surfaces of plants. Distribute mass surfaces as evenly and widely as possible throughout the space in order to increase surface area and collect the scattered light. A north/south mass wall that is in the space receiving the sunlight and is directly sunlit on one side in the morning and on the other in the afternoon is a good way to increase the surface area. Insulated floors and walls make good mass surfaces.

Distribution
Distribution refers to the delivery of excess heat from the attached solar sunspace or solar greenhouse to the house. Thermal mass and a distribution system can work well together to maximize the energy and comfort potential of a solar sunspace or solar greenhouse.

Natural Distribution
The simplest and least expensive distribution system is that of natural convection through window and/or door openings. Warm air flows through the upper portions of door openings and through upper window openings while cooler air flows through the lower openings.High and low openings in the common wall can serve as a "vent pair." The rule of thumb recommended combined area for a vent pair with an 8 ft. vertical separation between the openings is 2.5 ft² for each 100 ft² of south glazing. More area will only help. At night reverse heat flow will occur once the solar sunspace or solar greenhouse cools below the temperature of the house. Warm air will flow to the solar sunspace or solar greenhouse unless these vents are closed. The vents will have to be operated on a regular basis. If adequately sized, natural convection vents can work quite well. However, increased energy losses will occur if the vents are not closed during the periods when the sunspace is cooler than the house.

Figure 4

Natural ventilation can work well but must be carefully managed.

Mechanical Distribution
A mechanical fan can increase the convenience of a distribution system as the fan can be controlled automatically by a thermostat. The thermostat turns the fan on when the sunspace is warmer than the house and the house is not overheated (Controls that also enable venting to the outside may be required). The optimal location for the fan is in the upper portion of the space where the highest temperatures will occur. The minimum rule of thumb capacity is 3 to 4 CFM for each single ft of sunspace glazing. The warm air could be ducted to specific areas of the home besides those adjacent to the sunspace.

More on Distribution
One caution with regard to the solar greenhouse is that there will be occasions when the solar greenhouse has excessive humidity levels and it would be unwise to circulate the moisture laden air into the house. Controls that allow venting to the outside may be required.

An uninsulated masonry common wall between the space and the house usually called a trombe wall combines heat storage with distribution. It is not particularly recommended for the Washington climate and an insulated common wall with closable distribution vents is more appropriate for most applications.

Ventilation

Keeping the solar sunspace or solar greenhouse from overheating in the summer months is of fundamental importance if the space is to be used and enjoyed. Ventilation and shading are the primary strategies and can be effective when properly combined. Ventilation removes excess heat to the outside. Shading prevents excess heat from getting in.

Natural Ventilation
Natural ventilation can perform well but requires relatively large vent openings. Vents should be sized to prevent overheating under worst case conditions. Depending on the design of the solar sunspace or solar greenhouse, these conditions will occur in late summer (overhead glazing and little shading) or fall (vertical glazing and overhang shading). The necessary size of the vents is a function of a number of factors, including available shading, the tilt of the glazing the size of the glazing relative to the size of the sunspace, the vertical separation between vents, and the increase over outdoor temperatures you are willing to accept in the space.

It is important to vent at two levels. The inlet vents should be low and the exhaust vents high. The vertical separation or "stack height" between high and low openings helps determine the airflow. More separation is better. Cross ventilation is also important and openings should be placed to maximize it. Here wind can be a very helpful ally. The low vents should face the prevailing winds during the hotter months in order to complement and enhance the stack effect.

Table 1: Exterior Vent Areas

Source: McFarland and Jones, Solar Age 6/84

Table 1 suggests minimum rule of thumb vent areas (remember screens over openings will reduce the "net free vent area" and must be allowed for) as a proportion of glazing area for four different stack heights. You also have to select for a 60º or 90º glazing tilt and an allowable solar sunspace temperature elevation over the temperature outside. For example if you have a solar sunspace with vertical glazing and an 8 foot stack height, and you wish no more than a 5ºF rise over the outside temperature, the total vent area (high and low combined) must be 22 percent of the glazing area. So if you have 100 ft² of glazing you need .22 x 100 = 22 ft² of net free vent area.

Natural ventilation requires frequent opening and closing of vents, especially in the spring and fall. You can purchase automatic temperature-activated vent operating devices from a number of sources listed in the references. Though more expensive, they offer the convenience of automatic operation.

Mechanical Ventilation
Mechanical ventilation also offers the convenience of automatic temperature (and/or humidity)-activated ventilation. Table 1 lists fan capacities that would provide the same amount of ventilation as the recommended natural ventilation openings. With mechanical ventilation the area of openings can be reduced. Low air intake openings and high exhaust fan location is the recommended arrangement. Again work with and not against prevailing winds. Backdraft dampers are important to prevent winter heat loss through the fan duct. It is important to seal ductwork joints and insulate ducts to minimize condensation of high humidity air in the duct. Also ducts should be sloped to drain condensate from the duct, and electrical connections located where condensate won't flow.

The strategy of using natural ventilation as a primary ventilation strategy with a mechanical backup to limit extreme temperatures can greatly reduce fan operating costs and allow use of lower capacity fans.

Some consideration should be given to the nature of the site just outside and adjacent to the glazing area. For example a concrete outdoor patio in direct sunlight and well sheltered from the circulation of air could have a much greater impact on summertime solar sunspace temperatures than a grass lawn.

Shading

Trees
For some applications deciduous trees may be appropriate for shading south facing glazing, though their use will result in a winter energy penalty. Use care in selecting and placing deciduous trees. Some don't lose their leaves until late winter or spring. Some have denser branching and will block a greater percentage of sunlight. Others are quite messy and will make it difficult to keep the glazing clean. Deciduous trees are especially appropriate for shading east and west exposures because there is no winter blockage of sunlight.

Overhangs
Solid roofs with properly sized overhangs readily solve the most difficult problem of summer shading and are excellent when appropriate. Solar greenhouses will need direct light to plants and generally cannot use this option, but many solar sunspaces could accomplish their designed purpose with a solid roof and overhangs.

Overhangs for south facing glazings are a simple and elegant way to shade out direct sunlight in summer while allowing its entrance in winter. Overhangs should be sized to fit the particular design requirements of the space. The rule of thumb is that the length of overhang should be 1/3 to 1/2 of the glazing height, and the height of separation between window and overhang should be 1/4 of the window height. There is a limitation to fixed overhangs: the sun is at the same altitude on September 21 and March 21 and delivers the same amount of sunlight to the space, yet due to the gradual summertime warming of the atmosphere it is much cooler outside in March than in September. Hence the space tends to need the light more in March and less in September, and the fixed overhang doesn't allow this. Also in many Washington areas there is a greater percentage of available sunshine in September/October than there is in March. While it may not be necessary for most solar sunspaces, persons seeking to maximize solar energy performance could make the length of overhang adjustable, "downsize" the fixed overhang and complement it with exterior shading; or use exterior shading only.

Exterior Shades
These have flexibility as they can be operated seasonally or more frequently. They are made of a number of different materials of varying cost and durability including aluminum, fiberglass, and bamboo. They can be mounted as rigid panels or as roll-up mechanisms. They can be difficult to mount and must withstand wind, rain, and ultraviolet light. Plastic glazings definitely should not contact exterior shades as they may be scratched through abrasion; or should they get too hot, they may fuse to the shade. It is best to hold the shading off any glazing in order to allow connective cooling around the shade.

Paint-on Shading
This may be appropriate for some solar greenhouse applications. Some paint-on compounds gradually wear off while others must be washed off. Some may not be compatible with plastic glazings. Information can be obtained from commercial greenhouse suppliers.

Interior Shades
These are generally less expensive but less effective. They too can be roll-up or rigid, and are made of a number of different materials. The limitation of interior shades is that the insulation has already entered the space when it contacts the shade and some portion of the light will be absorbed by the shade, some scattered about and absorbed by other materials, some reflected back to the glazing but absorbed by the glazing itself, and finally a portion of the reflected insulation will actually pass through the glass and outside. Since all the retained energy contributes heat to the space, the interior shade acts somewhat like a poorly designed solar collector. Still they can be effective. Generally the more reflective they are the better they perform, as more light gets returned to the outside. However, using reflective shading with certain low-e glass units can result in an excessive buildup of temperatures between the glass panes. This could cause it to fail prematurely. Prior consultation with the manufacturer is recommended.

For solar greenhouses there are some relatively inexpensive shade cloths that are intended more for shading the plants than the solar greenhouse itself. Information can be obtained from commercial greenhouse suppliers. Also, light colored interior shades can reflect some light while diffusing the light passing through the shade. This diffusion scatters the light and is thought to improve plant growth by increasing the light delivered to leaf surfaces that would otherwise have been shaded in direct light conditions.

Food Production

The solar greenhouse designed for food production must meet a number of specific requirements in order to create an optimal micro-climate for the plants. In order to sustain the growing process, you must design and build the solar greenhouse to adequately provide for many environmental conditions including light, moisture (in both liquid and vapor form), temperature, and carbon dioxide.

Sunlight
This is obviously of utmost importance, especially since vegetables need more light than house plants. A rule of thumb is to get the direct sun from 9 a.m. to 3 p.m. in the winter and about 10 hours of summer sun for year round production. In winter when the sun is low and in the southern sky, south facing glazing is adequate; but east and west walls may have to be glazed in order to obtain adequate direct light on summer mornings and afternoons. The greater the length of the solar greenhouse in the east/west direction and the narrower its width in the north/south, the less will east and west walls need glazing. One energy saving option might be the seasonal installation of rigid insulation on east and west windows during the winter.

Table 2: Light Intensity

Light levels can also be increased via creative exterior landscaping where lawns, fields, and even buildings can (especially when snow covered) reflect additional light into the solar greenhouse. Also, maximizing light colored non-mass surfaces will increase diffused light and light to the plant leaf surfaces. Since all incoming light is from the south it is important to have relatively large light reflecting areas on the north wall. This should be kept in mind when designing mass.

The availability of sufficient light intensity to plants should be planned for during the design stage when decisions about plant layout and glazing type are made. Providing enough overhead glazing to allow direct light to all plants is recommended. Light intensity can also be measured after the solar greenhouse has been built. (See Table 2.)

Humidity
Plants will grow best when the relative humidity is between 45 and 60 percent. Consequently the solar greenhouse must tolerate higher moisture levels than many other indoor environments and needs to be built accordingly.

Temperature Control
In order to control minimum temperatures some form of backup heat will very likely be necessary. If it is acceptable, the house could be the "backup heater." If not, some means of backup heat will have to be provided to the space. By growing those crops most tolerant of cold weather during the critical periods one can minimize the requirements for backup heat. Soil temperatures are more critical than air temperatures, in that if soil temperatures are kept warm the plants can tolerate colder air temperatures. Strategies for keeping soil temperatures above about 45-50ºF merit attention. Information can be obtained from commercial greenhouse suppliers.

The necessity of utilizing overhead glazing in order to provide direct light of sufficient intensity to food plants makes summer cooling a special concern in the solar greenhouse. Sustained temperatures above 90ºF to 110ºF must be avoided. Emphasizing ventilation over shading is an excellent approach given the need to deliver light to the plants, perhaps sizing the vent in order to maintain the space temperature within a few degrees of the outside temperature. A mechanical backup fan, evaporative cooling, and shading are additional tools for maintaining temperature control.

Construction

One option is to purchase a solar sunspace in kit form. A number of prefabricated solar sunspace kits are available that can be site assembled by either contractors or do-it-yourselfers. One advantage is that all parts come from a single supplier and in many cases the assemblies have been first engineered then refined through many field applications. Special difficulties such as sealing, overhead glazing, and shading devices have often evolved through field experience. There is a considerable menu of sizes shapes and costs to select from. Some are fairly limited in flexibility while others have more customizing potential. Disadvantages are that to some they look like "add ons," and they may not meet specific design purposes. Generally they are designed as living space, with hot tubs and breakfast nooks. Some can be expected to greatly increase energy costs if they are not closed off from the house when the sunspace temperature is lower than the house temperature.

Another option is the custom designed and built solar sunspace or solar greenhouse. Whether owner or contractor built, the solar sunspace or solar greenhouse will require careful planning and several building skills. Most solar sunspaces and especially solar greenhouses are subjected to more severe temperature and moisture conditions than is the home to which they are attached. Quality construction is essential to long life, and the key to quality is attention to detail and thorough research before and during construction. One of the most reliable rules of thumb is that it will always take longer than you think. Unanticipated weather and logistical difficulties will occur and should be allowed for in planning the construction sequence. Nonetheless the project can be as rewarding as a process as it will be as a completed space.

All construction should be based upon sound building practices and comply with building and energy codes. Build the space square, level, and plumb; then fit the house to it rather than try to build it to conform to the existing house. The foundation and floor should be built as is that of a house. Footings for piers or walls should be below frostline. The perimeter of foundation walls should be insulated, typically on the exterior with approximately 2" of extruded polystyrene. If the floor is a concrete slab and is to serve as a thermal mass, insulation under the entire floor is suggested. It is best ( but more costly to install) if the thickness of sub-slab insulation is tapered: thickest at the perimeter and thinnest in the center. Crawl space floors can be quite acceptable, though the weight of solar greenhouse soil beds and some thermal mass materials will require special attention. A well insulated wood joisted crawlspace floor with a 2" lightweight concrete deck provides excellent mass for certain applications.

Including a drain in the floor will probably pay for itself the first time someone leaves the hose on all night.

Framing can be of wood, steel, or aluminum. Each material has its own advantages and disadvantages relating to cost, weatherability and ease of use. Redwood, cedar, and cypress are the most durable woods in moist climates, but structural lumber can be adequately sealed for long life. Do not use creosote or pentachloraphenol (penta) as wood preservatives. Copper napthanate is the most often recommended preservative. It is especially important to seal the cut ends of wood members. Some sealants dry quickly and allow cutting, sealing, and assembly with little inconvenience. In addition, avoid level wood surfaces. Horizontal surfaces such as ledges can be beveled slightly: they'll still hold planters and drinking cups, but water will run off rather than stand on the surface.

Non-corrosive fasteners must be used. The acids in redwood and the prevalent high moisture levels will quickly deteriorate poorly treated steel. Hot dipped galvanized or aluminum nails are acceptable. Screws, nuts, washers and bolts should be brass, aluminum, stainless or nickel plated.

Moisture control is of critical importance in solar greenhouses. Condensation on glazing surfaces is common. It's also likely to occur on other surfaces as well. Warm air can hold significantly more water vapor than cold air. When moisture laden warm air is carried into the wall cavity it may cool below the "dew point" and some of the water vapor will condense into liquid. Reduced thermal performance, mold and mildew, blistering of exterior paint, and rotting of wood can result. Adherence to the Washington State Energy Code requirements for air and vapor barrier systems is critically important.

Installing and sealing glazing onto a wood framed solar sunspace is one of the more difficult construction procedures, but careful attention to detail can lead to success. Many wood framed solar sunspaces have leaked because they attempted to use wood as a sealing component of the glazing system. Wood makes a poor exterior glazing cap. It will require frequent maintenance and will be difficult to keep from leaking. It can be used for cosmetic purposes if the seal is established by some other means.

There are extruded aluminum glazing caps designed especially for applications such as solar sunspaces. Insulated glass units can sit upon wood framing and be sealed on the outside with the aluminum extrusion. The metal and wood make a nice marriage. With a top class sealant a properly installed exterior metal system could provide more than a 20 year service life with little maintenance. If a leak develops, it can be located then patched by cutting back the sealant, cleaning, and resealing.

Condensation may sometimes form on the inside surface of the glass and run off onto the wood, or worse, slip between the wood and glass, where it could eventually damage the wood. The interior wood should be well sealed and the joint between the wood and glass should be caulked with the proper sealant.

Figure 5

This glazing mounting system provides for a durable seal.

Another method of dealing with the problem of interior surface condensation is to use an extruded aluminum base which is fastened to the wood framing. The glass unit sits upon the base and the glazing cap is fastened from above. The base has a condensation runoff channel so water that condenses onto the inside surface of the glazing will be caught by the channel and prevented from running onto the wood. Of course it's also more expensive than the glazing cap alone. Both systems provide a durable, long term and relatively inexpensive method for glazing the wood framed sunspace.

Overhead glass should be installed carefully according to the manufacturer's instruction. Insulated glass units must lie flat (in the same plane) or the edge seals will be stressed. All panes of the unit must be equally and evenly supported at the bottom. There must be adequate bearing area beneath all edges of the unit. There should be a compressible gasket between the unit and the wood below to equally distribute the bearing load and prevent "point" loading. Nails should be set well below the surface of the wood so they won't "expand up" and break the unit. Watch out for sharp bumps in the wood surface due to small knots. Typically, there must be about an inch of spacing between adjacent units and about one-half inch between the unit and wood framing members.

Acrylic glazings expand and contract considerably more than glass and the fastening and sealing system must allow for it. Other plastic glazings have special fastening and sealing procedures. This is one area where extra long distance calls to the manufacturer's representative may be well worth the effort if no local expertise is available.

Energy Conservation/ Efficiency

Increasing the efficiency of the home's energy use will often dramatically lower the cost of home heating and increase comfort. As well the solar sunspace or solar greenhouse should itself have well-insulated floors, walls, ceilings and doors, and should be of tight, well-sealed construction. Levels of insulation should at least match those of the house. In the case of older homes insulation levels will often exceed those of the house.

Building Codes, Energy Codes, and Solar Access Ordinances

Building Codes
These are meant to ensure structural soundness and prevent health and fire hazards. Code officials have a responsibility to do this. Usually if they sense a cordial builder who respects their role, relations will be much more productive. Since the addition of a solar greenhouse requires a building permit, and lengthy delays in construction can often occur if the space design must be readjusted to meet code, determining the zoning and building code requirements early in the planning process only makes sense.

Key Safety Issues
The use of glass, especially overhead is another major area worthy of attention. Exact codes vary for different jurisdictions but are all very restrictive about the types of glazing that can be used overhead.

The use of electricity in high moisture areas, such as near hot tubs, water storage containers, and in solar greenhouses requires special attention. Solar greenhouses especially, need to design for the necessity of watering plants near lights, fans and heaters. Ground fault interrupting circuits are an excellent idea, as are high quality materials and skilled installation.

Energy Code
Because building an attached greenhouse represents an addition or alteration to your home, it must comply with the provisions of the Washington State Energy Code (WSEC) and the Ventilation and Indoor Air Quality (VIAQ) Code (as well as the Uniform Building Code). To the extent that the greenhouse uses only renewable solar energy for heating, it would be exempt from energy code requirements. If, however, the greenhouse has a positive heat supply and relies on some type of backup heat for as little as 1 watt or 3.4 btu/hr per square foot, it must comply with the Codes. Additions with large amounts of glazing and relatively small floor areas (such as greenhouses) often have trouble meeting WSEC requirements. In some cases, it may be necessary to make conservation improvements to the existing house as a trade-off to qualify the greenhouse.

Solar Access Protection
Increasingly, jurisdictions have solar access laws that protect solar availability to a lot. Your community's planning department can help you determine if such protection is available to you. If not, investigating the need for negotiating a legal solar easement with southerly neighbors may be warranted.

Cost and Performance

Poor design and operation can result in a large increase in the home's energy use. The very best design, construction and operation might provide over 40% of a home's heating energy. Careful design, construction and operation should typically lead to a decrease in the home's heating energy use, perhaps on the order of 15% to 30%. The "comfort" performance of the space can also range from one seldom used to one used on a daily basis.

An index of approximately 30 prefabricated solar sunspaces and solar greenhouses in 1985 reported construction costs ranging from $10/ft² to slightly over $100/ft².

The cost of a custom solar sunspace or solar greenhouse will depend on its size, materials used, and labor costs An extremely basic space with polyethylene or vinyl glazing and recycled lumber for framing might cost about $5/ft2. A more finished solar sunspace or solar greenhouse with quality wood or aluminum framing and higher quality glazing will be more expensive and costs may be similar to the costs of any other type of home addition. In other words they will range considerably.

Suggested Reading

Dodging the Heat in Dixie, Jerry Germer, Solar Age; August 1984.

The Last Word in Sunspace Design, Robert W. Jones and Robert D. McFarland, Solar Age; June 1984.

Made in the Shade, Steve Bliss, Solar Age; July 1984.

More on Glazing Performance, Peter J. Lunde, Solar Age; October 1984.

New Radiation Data Suggest Higher Levels, Dr. Peter Lunde, Solar Age; December 1983.

Off-South Glazing Performance, Peter J. Lunde, Solar Age; November 1984.

Seasonal Performance of Glazing, Peter J. Lunde, Solar Age; September 1984.

Solar Greenhouses and Sunspaces, National Center for Appropriate Technology, Washington State Energy Office, Olympia, WA., 1985.

Solar Radiation Measurement: A National Disgrace, Francis De Winter, Solar Age; January 1982.

The Bountiful Solar Greenhouse, Shane Smith, John Muir Publications, Santa Fe, CA., 1982

The Complete Greenhouse Book, Peter Clegg & Derry Watkins, Garden Way Publishing; Charlotte, Vermont, 1978.

The Future of High-Performance Glazings, Day Chahroudi, Solar Age; February 1986.

The Passive Solar Energy Book, Edward Mazria, Rodale Press, Emmaus, PA., 1978.

The Solar Greenhouse, Bill Vanda and Rick Fisher, John Muir Publication, Santa Fe, New Mexico, 1976.

The Solar Greenhouse Book, James C. Mccullagh, Rodale Press, Emmaus, PA., 1978.

The Solar Greenhouse Guide for the Pacific Northwest, Ecotope Group, Ad Pro Printing, Lynnwood, WA., 1979.

Sunrooms, Progressive Builder; July 1987.

Sunspace Accessories, Progressive Builder; July 1987.

The Sunspace Guide, Jerry Germer, Solar Age; May 1985.

Sunspace Heat Blockers, Solar Age; May 1985.

Winter Greens, Mark A. Craft, Firefly Books Ltd. Ontario, Can., 1983.

Washington State University Cooperative Extension

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