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 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 foam board. 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 foam board (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 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 foam
board 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, wood stoves, 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 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.
