Elements of an
Energy Efficient House
Designing and building an energy-efficient
home that conforms to the many considerations faced by home builders can be a
challenge. However, any house style can be made to require relatively minimal
amounts of energy to heat and cool, and be comfortable and healthy. It's easier
now to get your architect and builder to use improved designs and construction
methods. Even though there are many different design options available, they all
have several things in common: a high R-value, tightly sealed thermal envelope;
controlled ventilation; and lower than usual heating and cooling bills.
Some designs are more expensive to build
than others, but none of them need to be extremely expensive to construct.
Recent technological improvements in building elements and construction
techniques, and heating, ventilation, and cooling systems, allow most modern
energy saving ideas to be seamlessly integrated into any type of house design
without sacrificing comfort, health, or aesthetics. The following is a
discussion of the major elements of energy-efficient home design and
construction systems. The EREC has other, more specific information on these
topics available.
The Thermal Envelope
A "thermal envelope" is
everything about the house that serves to shield the living space from the
outdoors. It includes the wall and roof assemblies, insulation, windows,
doors, finishes, weather-stripping, and air/vapor retarders. Specific items to
consider in these areas are described below.
Wall and Roof Assemblies
There are several alternatives to the
conventional "stick" (wood stud) framed wall and roof construction
now available and growing in popularity. They include:
Optimum Value Engineering (OVE.) This is
a method of using wood only where it does the most work, thus reducing costly
wood use and saving space for insulation. However, workmanship must be of the
highest order since there is very little room for construction errors.
Structural Insulated Panels (SIP.) These
are generally plywood or oriented strand board (OSB) sheets laminated to a
core of foamboard. The foam may be 4 to 8 inches thick. Since the SIP acts as
both the framing and the insulation, construction is much faster than OVE or
it's older counterpart "stick-framing." The quality of construction
is often superior too since there are fewer places for workers to make
mistakes.
Insulating Concrete Forms (ICF.) These
often consist of two layers of extruded foamboard (one inside the house and
one outside the house) that act as the form for a steel reinforced concrete
center. This is the fastest and least likely technique to have construction
mistakes. Such buildings are also very strong and easily exceed code
requirements for tornado or hurricane prone areas.
Insulation
An energy-efficient house has much
higher insulation R-values than required by most local building codes. For
example, a typical house in New York State might have haphazardly installed
R-11 fiberglass insulation in the exterior walls and R-19 in the ceiling, and
the floors and foundation walls may not be insulated. A similar, but
well-designed and constructed house's insulation levels would be in the range
of R-20 to R-30 in the walls (including the foundation) and R-50 and R-70 in
the ceilings. Carefully applied fiberglass batt or roll, wet-spray cellulose,
or foam insulations will fill wall cavities completely.
Air/Vapor Retarders
These are two things that sometimes can
do the same job. How to design and install them depends a great deal on the
climate and what method of construction is chosen. No matter where you are
building, water vapor condensation is a major threat to the structure of a
house. In cold climates, pressure differences can drive warm, moist indoor air
into exterior walls and attics. It condenses as it cools. The same can be said
for very Southern climates, just in reverse. As the humid outdoor air enters
the walls to find cooler wall cavities it condenses into liquid water. This is
the main reason why some of the old buildings in the South that have been
retrofitted with air conditioners now have mold and rotten wood problems.
Regardless of your climate, it is
important to minimize water vapor migration by using a carefully designed
thermal envelope and sound construction practices. Any water vapor that does
manage to get into the walls or attics must be allowed to get out again. Some
construction methods and climates lend themselves to allowing the vapor to
flow towards the outdoors. Others are better suited to letting it flow towards
the interior so that the house ventilation system (see below) can deal with
it.
The Airtight Drywall Approach and the
Simple CS system are other methods to control air and water vapor movement in
a residential building. These systems rely on the nearly airtight installation
of sheet materials such as drywall or gypsum board on the interior as the main
barrier, and carefully sealed foamboard and/or plywood on the exterior.
Foundations and Slabs
Foundation walls and slabs should be at
least as well insulated as the living space walls. Uninsulated foundations
have a negative impact on home energy use and comfort, especially if the
family uses the lower parts of the house as a living space. Also, appliances
that supply heat as a by-product, such as domestic hot water heaters, washers,
dryers, and freezers, are often located in basements. By carefully insulating
the foundation walls and floor of the basement, these appliances can assist in
the heating of the house.
Windows
The typical home loses over 25% of its
heat through windows. Since even modern windows insulate less than a wall, in
general an energy-efficient home in heating dominated climates should have few
windows on the north, east, and west exposures. A rule-of-thumb is that window
area should not exceed 8-9% of the floor area, unless your designer is
experienced in passive solar techniques. If this is the case, then increasing
window area on the southern side of the house to about 12% of the floor area
is recommended. In cooling dominated climates, its important to select east,
west, and south facing windows with low solar heat gain coefficients (these
block solar heat gain). A properly designed roof overhang for south-facing
windows is important to avoid overheating in the summer in most areas of the
continental United States. At the very least, Energy Star rated windows or
their equivalents, should be specified according to the Energy Star regional
climatic guidelines.
In general, the best sealing windows are
awning and casement styles since these often close tighter than sliding types.
Metal window frames should be avoided, especially in cold climates. Always
seal the wall air/vapor diffusion retarder tightly around the edges of the
window frame to prevent air and water vapor from entering the wall cavities.
Air-Sealing
A well-constructed thermal envelope
requires that insulating and sealing be precise and thorough. Sealing air
leaks everywhere in the thermal envelope reduces energy loss significantly.
Good air-sealing alone may reduce utility costs by as much as 50% when
compared to other houses of the same type and age. Homes built in this way are
so energy-efficient that specifying the correct sizing heating/ cooling system
can be tricky. Rules-of-thumb system sizing is often inaccurate, resulting in
oversizing and wasteful operation.
Controlled Ventilation
Since an energy-efficient home is
tightly sealed, it's also important and fairly simple to deliberately
ventilate the building in a controlled way. Controlled, mechanical ventilation
of the building reduces air moisture infiltration and thus the health risks
from indoor air pollutants, promotes a more comfortable atmosphere, and
reduces the likelihood of structural damage from excessive moisture
accumulation.
A carefully engineered ventilation
system is important for other reasons too. Since devices such as furnaces,
water heaters, clothes dryers, and bathroom and kitchen exhaust fans exhaust
air from the house, it's easier to depressurize a tight house if all else is
ignored. Natural draft appliances, such as water heaters, woodstoves, and
furnaces may be "back drafted" by exhaust fans and lead to a lethal
build-up of toxic gases in the house. For this reason it's a good idea to only
use "sealed combustion" heating appliances wherever possible and
provide make-up air for all other appliances that can pull air out of the
building.
Heat recovery ventilators (HRV) or
energy recovery ventilators (ERV) are growing in use for controlled
ventilation in tight homes. These devices salvage about 80% of the energy from
the stale exhaust air and then deliver that energy to the fresh entering air
by way of a heat exchanger inside the device. They are generally attached to
the central forced air system, but they may have their own duct system.
Other ventilation devices such as
through-the-wall and/or "trickle" vents may be used in conjunction
with an exhaust fan. They are, however, more expensive to operate and possibly
more uncomfortable to use since they have no energy recovery features to
pre-condition the incoming air. Uncomfortable incoming air can be a serious
problem if the house is in a northern climate, and they can create moisture
problems in humid climates. This sort of ventilation strategy is recommended
only for very mild to low humidity climates.
Heating and Cooling
Requirements
Houses incorporating the above elements
should require relatively small heating systems (typically less than 50,000
Btu/hour even for very cold climates). Some have nothing more than sunshine as
the primary source of heat energy. Common choices for auxiliary heating
include radiant in-floor heating from a standard gas-fired water heater, a
small boiler, furnace, or electric heat pump. Also, any common appliance that
gives off "waste" heat can contribute significantly to the heating
requirements for such houses. Masonry, pellet, or wood stoves are also
options, but they must be operated carefully to avoid "back
drafting."
If an air conditioner is required, a
small (6,000 Btu/ hour) unit can be sufficient. Some designs, like the ACT2
houses in arid Davis, CA use only a large fan and the cooler evening air to
cool down the house. In the morning the house is closed up and it stays
comfortable until the next evening.
Beginning a Project
Houses incorporating the above features
have many advantages. They feel more comfortable since the additional
insulation keeps the interior wall temperatures more stable. The indoor
humidity is better controlled, and drafts are reduced. A tightly sealed
air/vapor retarder reduces the likelihood of moisture and air seeping through
the walls. They are also very quiet because of the extra insulation and tight
construction.
There are some potential drawbacks. They
may cost more and take longer to build than a conventional home, especially if
your builder and the contractors are not familiar with them. Even though their
structure may differ only slightly from conventional homes, your builder and
the contractors may be unwilling to deviate from what they've always done
before. They may need education or training if they have no experience with
these systems. Because some systems have thicker walls than a
"typical" home, they may require a larger foundation to provide the
same floor space.
Before beginning a home-building
project, carefully evaluate the site and its climate to determine the optimum
design and orientation. You may want to take the time to learn how to use some
of the energy related software programs that are available to assist you.
Prepare a design that accommodates appropriate insulation levels, moisture
dynamics, and aesthetics. Decisions regarding appropriate windows, doors, and
heating, cooling and ventilating appliances are central to an efficient
design. Also evaluate the cost, ease of construction, the builder's
limitations, and building code compliance. Some schemes are simple to
construct, while others can be extremely complex and thus expensive.
An increasing number of builders are
participating in the federal government's Building America and Energy Star
Homes programs, which promote energy-efficient houses. Many builders
participate so that they can differentiate themselves from their competitors.
Construction costs can vary significantly depending on the materials,
construction techniques, contractor profit margin, experience, and the type of
heating, cooling and ventilation system chosen. However, the biggest benefits
from designing and building an energy-efficient home are its superior comfort
level and lower operating costs. This relates directly to an increase in its
real-estate market value.
EREC is operated by NCI Information
Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of
Energy. The statements contained herein are based on information known to EREC
at the time of printing. No recommendations or endorsement of any product or
service is implied if mentioned by EREC.
Energy Efficiency and Renewable Energy
Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com
Path:
Home>Education>Healthy
Home Information>Elements
of an Energy Efficient House
|
