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The Building Envelope

Written by Bill Younger

The building envelope is made up of the windows, doors, walls, foundation, floor, ceiling and roof and is the barrier between the conditioned indoor environment and the outdoors. Under most circumstances, you will use less energy in your HVAC system to control heating, cooling, outside air, and humidity levels in your building if the envelope works well as a barrier.

Even with a very good envelope, your building will lose heat in cold weather and gain it when it's hot outside. Your basic objective is to minimize unwanted heat gain in the summer and heat loss during the winter.

Envelope Heat Loss

The heat required to maintain a desired indoor temperature at a specified mechanisms by which heat flows into and out of a building also called conduction or radiation.

Conduction
Conduction is the term applied to heat flow within a solid from a high-temperature lower-temperature region through the molecules in the material. Conduction requires that surfaces touch in heat to transfer. Because the different materials in an insulated assembly touch each other, conduction heat loss through solid components of the building envelope. For example, heat flows by conduction from areas to the cooler areas of concrete slabs, window glass, walls, ceilings, and other solid materials.

The unit used for thermal transmittance (heat transfer) or conductance of a single building material or building often called the U-value. U-values are expressed in Btus per hour per square foot of area per degree temperature difference. Windows are commonly described by their U-values. Descriptions of building walls, floors, or ceilings, often use R-values instead of U-values. The U-value or conductance flows through a material and the R-value measures the resistance, or how slowly heat flows. The two terms are reciprocal.
(R=1/U, U=1/R)

Convection
Convection is the process of transferring heat from one place to another by molecular movement through fluids such as water or air. Heat loss by convection commonly results from exfiltration or air leakage. Connective heat loss occurs when warm air is forced out, usually from the building (exfiltration), by cold incoming air, usually in the lower part (infiltration). The rate of transfer is increased when the wind blows against the building or when the temperature difference between the inside and outside increases.

Radiation
Radiation is the heat transfer by electromagnetic waves from a warmer to a cooler surface. The transfer of the sun's heat to the earth or the warmth of a campfire are examples of radiant heat transfer.

Heating and Cooling Loads

To determine the degree in which the thermal quality of the building envelope affects the energy consumption, it is important to evaluate the driving forces behind the heating and cooling loads in the building.

Thermally Light Buildings
A building whose heating and cooling requirements are proportional to the weather is considered a thermally light building. That is, when the outdoor temperature drops below the desired room temperature, heating is required and when the outdoor temperature goes above the desired room temperature, cooling is needed. In a thermally light building, the thermal performance of the envelope becomes a dominate factor in energy use and can usually be seen as seasonal fluctuations in utility consumption.

Thermally Heavy Buildings
When factors other than weather determine the heating and cooling requirements, the building can be considered thermally heavy. The difference between thermally light and thermally heavy buildings is the amount of heat generated by people, lighting, and equipment within the building. Thermally heavy buildings typically have high internal heat gains and, to a certain extent, are considered to be self heating and more cooling dominated. This need to reject heat makes them less dependent on the thermal performance of the building envelope.

Thermal Weight
A simple "rule of thumb" for determining the thermal weight of a building is to look at heating and cooling needs at an outdoor temperature of 60 degrees Fahrenheit. If the building requires heat at this temperature, it can, too, considered thermally light, if cooling is needed, it is thermally heavy.

Some buildings or areas within a building can be both thermally light and thermally heavy depending on their use. A meeting room, for example, can have significant heat gains from people, equipment, and lights when the room is occupied and not require any heating from the HVAC system on a cold day. The same meeting room, however, may require heat at the same outdoor temperature when the room is vacant.

Thermal Mass
Thermal mass saves energy by storing and releasing heat. For a building to take advantage of thermal mass, there must be a source of free or less expensive energy to charge the mass. The existence of thermal mass, such as concrete walls and floors, can have a substantial impact on the operation of HVAC system's and can be difficult to analyze. It can affect the HVAC systems ability to quickly respond to rapid changes in load caused by increased occupancy, equipment, or solar gains through windows.

The effect of thermal mass on the building systems will vary by climate and type of building as well as the location of the mass within the structure. Thermal mass in exterior walls, for example, will slow down heat flow through the wall allowing a reduction in insulation requirements while maintaining performance levels similar to standard frame construction. High levels of mass located within the building tend to reduce the effectiveness of mass in the outside walls.

Buildings that most benefit from thermal mass are typically those with substantial cooling loads. In this case, the thermal mass can be precooled at night using outside air for free cooling or less expensive offpeak electricity for mechanical cooling. This allows the mass to absorb heat the following day, reducing the need for operation of cooling systems during peak utility demand hours.

Generally, thermal mass is part of the integral construction of the building and is not added for conservation reasons. Unfortunately, there are no easy rules to determine how thermal mass will affect different buildings. It is important to note its existence because it may help you understand behavior of the mechanical systems or reasons for some comfort complaints.

Evaluating the Building Envelope

When evaluating the building envelope, it is important to keep in mind the thermal weight of the building as well as the various types of heat loss and gain to determine the impact of the existing envelope on energy consumption. For example, If you have determined that the building has high internal heat gains and must reject heat a majority of the time, then perhaps the level of effort spent on envelope evaluation should be limited. Time spent measuring glazing areas and determining detailed wall R values may not be justified because of the limited potential for reducing energy consumption by making changes to these components. A small office building with minimal internal heat gains and substantial amounts of west facing glass on the other hand may greatly benefit from a more detailed analysis of the envelope and effects of solar heat gains on the summer cooling load.

It is up to the individual energy auditor to determine the level of effort given to evaluation of envelope components and potential improvements.

Windows

Heat Loss Through Windows
Windows can be one of the single largest sources of unwanted heat gain and loss in the thermal envelope. It is not unusual to find a glass area that comprises only 15 to 25 percent of the surface area of a building while contributing up to 75 percent of the heat loss. Windows typically lose heat through conduction and air movement around the frames, and gain heat through solar radiation.

Window U values
Window U-values are determined by testing the frame and glass as a whole unit. New developments in glass and frame technologies have substantially improved the thermal performance of window units. Technologies such as "low E" glazing, thermally broken frames, and gas fillers between panes are becoming common in commercial building construction.

Solar Heat Gain
Windows are subject to solar heat gains which can have significant impacts on HVAC operation and occupant comfort. The amount of heat gain is dependent on orientation, season, time of day, glazing type, and shading by window coverings, overhangs, other buildings and vegetation. Solar gains through south facing glass will add heat to the building in the winter. East and west surfaces will gain the greatest amount of heat in the early morning and late afternoon hours during summer months. Winter heat gains may be desirable in thermally light buildings while any solar heat gains in a thermally heavy building will only contribute to the cooling load. East and west facing glass are primarily a problem during the summer. Low sun angles in the morning and late afternoon can result in substantial solar heat gains as well as unwanted glare. The problem of excess solar heat gains during the summer can be compounded by the build up of internal heat most buildings experience late in the day. The combination of solar and internal heat gains can greatly increase the energy required for cooling.

Window Evaluation Checklist
When evaluating windows in an existing structure, make note of:

• Single/Double glazing

• Frametype

• Operable windows

• Estimated % of gross wall area

• Daylighting

• Glazing orientation and cooling zones

• Glazing coatings

• Cracked or broken panes

• Alignment of operable windows

• Weatherstripping condition

• Skylights

Skylights
Skylights will have an effect on the energy balance of the building in several ways. Illumination from skylights can reduce the need for additional illumination from the lighting systems. Heating loads may be decreased in winter due to solar heat gains while the summer cooling load will be increased. The amount of solar heat gain through skylights is largely dependent on the angle of the glazing. A typical skylight set at a low angle will have minimal heat gains in the winter but will have significant gains in the summer due to the high angle of the summer sun.

Windows can serve a variety of purposes including light, view, heat, and ventilation. If glazing modifications are considered, intended use and interactions with other systems should be assessed. Because window replacements and retrofits are typically expensive on a cost per square foot basis, they are often difficult to justify on the basis of energy savings alone.

Doors

Heat Loss Through Doors
Exterior doors generally comprise a small area of the building envelope. Even though most door types may not be very well insulated, they usually do not contribute substantially to the overall heat transfer of the envelope. The primary source of heat loss related to doors is through air leakage due to poor fitting doors and weatherstripping, and through the door being physically opened for building access.

Vestibules
A vestibule is an intermediate space between the inside and outside environment. During the winter, this helps prevent the dumping of cold, unconditioned air directly into the building every time someone enters or exits the building.

If an exterior door is used very intensely for several short periods of the day, then installation of a vestibule door may not offer much savings. In areas of high use, revolving doors are typically more effective in controlling infiltration.

Vestibules can be expensive to install depending on the situation. If you have to construct exterior sidewalls and a roof, the cost can be fairly expensive as opposed to simply installing a set of doors and surrounding wall in the end of a corridor.

The energy auditor should identify doors most frequently used and evaluate potential for installation and effectiveness of various entry systems. Vestibules, usually unconditioned, help reduce both air leakage and conductive heat losses.

Overhead Doors
Overhead doors used for loading and unloading material or vehicle access are often left open for convenience. If used frequently, overhead doors can cause excessive air leakage and result in substantial heat loss or gain. This can lead to unnecessary cycling of heating and cooling systems as well as reduce comfort in surrounding areas.

Evaluate loading schedules for frequency of overhead door use and identify problem areas and retrofit potential. Loading dock curtains made of plastic strips can be installed to reduce mixing of outside and conditioned air while permitting access to the loading dock. Other alternatives include reducing the door size or installing air curtains, radiant heating systems, conveyor belts, and controls to lock out HVAC equipment when the doors are open. Overhead doors in conditioned areas should also be insulated and weatherstripped to prevent heat loss when closed.

Insulation

Conductive heat losses can be reduced by adding insulation to exterior walls, floors, ceilings, and roof areas. It is important to identify existing insulation types and levels in each component to evaluate the envelope's impact on energy consumption and building comfort. These levels must also be known to determine the cost effectiveness of adding insulation to the existing envelope.

Roofs and Ceilings
Because warm air collects at the ceiling and increases the temperature difference between the inside and outside surfaces, the rate of conduction also increases. This higher rate of conduction makes ceiling and roof insulation a high priority in controlling heat loss. Keep in mind that hours of operation and night set-back of the heating system as well as costs for heating fuel and climate zone can influence the pay-back of adding insulation. The color of the roof can also have a substantial impact on the operation of heating and cooling systems. When accounting for conductive and radiant contributions, dark colored roofs reduce heat loss in the winter, however this may be outweighed by unwanted heat gain in the summer. The color of the roof is typically dictated by the cooling load of the building. If a building has greater heating needs the majority of the year, a dark colored roof may be desirable. A well ventilated attic space, if one exists, will minimize the impact of roof color.

Walls
Heat loss in walls is primarily by conduction of energy through the wall components. Adding insulation will greatly reduce conductive losses, however, careful consideration must be given to ease of installation to ensure cost effectiveness. Installing blown insulation to wall cavities can be relatively economical if there are large wall surfaces with a minimal amount of surface detail or windows. Applying insulation to interior or exterior wall surfaces can be costly due to finish materials required over the insulation. Wall insulation measures tend to be more cost effective in colder climates.

Foundations and Floors
Foundations and floors can be sources of heat loss that are often overlooked. In addition to saving energy, installing insulation in floors over crawl spaces can make floors more comfortable to building occupants.

Installing perimeter slab insulation in an existing building may not be cost effective due to the relatively low heat loss and high cost of the insulation. There may be situations, however, where there is high heat loss due to heating system piping located on or near the foundation for example that would make this measure more attractive. Ease of installation is also an important factor in determining cost effectiveness of foundation insulation.

Insulation
A variety of materials can be used for roof, wall, foundation and floor insulation. Common insulating products include:

• Blown-in fiberglass

• Blown-in cellulose

• Fiberglass batts

• Rigid board insulation.

The choice of insulation depends on the type of construction and required R-value. Use the chart below to approximate existing R-values in building components.

Roof and Ceiling Evaluation Checklist
When evaluating roofs and ceilings for adequate insulation, make note of;

• Type, thickness, and location of the existing insulation

• Age and condition of roof

• Damaged or wet insulation

• Insulation voids

• Proper attic ventilation

• Space available for additional insulation

• Color of roof membrane

Infiltration and Exfiltration

Infiltration and exfiltration are uncontrolled leakage of outside air into and out of the building through any openings in the building shell. Air leaks are caused by pressure effects of wind and differences in indoor and outdoor air temperature and density. Typical sources of air leakage include cracks around windows and doors, utility penetrations, poorly sealed air dampers, and any locations where different types of construction meet. The problem of infiltration and exfiltration is worse in tall buildings due to the stack effect and can be compounded by vertical shafts such as open stairwells and elevator shafts.

Infiltration of air into the building is similar to the effect of additional ventilation, however, unlike ventilation, it cannot be filtered, conditioned, controlled, or turned off at night.

In addition to infiltration from doors and windows, cracks in building materials, and around utility penetrations are other common sources of infiltration that should not be overlooked.

Building Pressure

HVAC system balance can also influence the amount of air leakage. Buildings can be slightly pressurized by bringing in more intake air than is exhausted to reduce infiltration. An easy method of determining if a building is under positive or negative pressure is to hold an exterior door open about 1 inch on a calm cool day and observe which way the air is flowing. If air is flowing into the building, that part of the building is under negative pressure and may have problems with infiltration.

Summary

Major modifications to the building envelope can be prohibitive when considered solely in terms of return on investment. Other factors may influence a decision to implement changes to the envelope. Sizing and performance of new HVAC equipment, for example, can be dependent on the integrity and overall condition of the building envelope.

Keep conservation in mind when remodeling or making changes to the building structure. A good example is the addition of rigid insulation to the roof deck when replacing the roofing material. While it may not be cost effective to tear off an existing roof membrane just to add insulation, installing insulation when worn out roofing material is being replaced makes sense.

Modifications to the building envelope are typically the most visible of all energy conservation measures and should be treated with special significance. They not only affect the appearance of the facility but also have an impact on public and employee attitudes toward energy conservation.

Washington State University Cooperative Extension

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