House As A System
The component systems of a home are interrelated; changing
one part without consideration of the others can create a
domino effect with negative outcomes. This became apparent
when the building industry increased envelope tightness only
to find decreased home performance in air quality and moisture
control. With knowledgeable consideration of the house as
a system, changes in one part can augment the performance
of other component system and the house as a whole. Understanding
the interrelations and accommodating them requires effort
from the design phase through all stages of construction.
The net result is a home that performs better on many fronts:
energy efficiency, comfort, durability, safety, and affordability.
Training for builders
and code officials on the systems approach to home building
is available through E-Star™.
Historical Development
of House-As-A-System Approach
Natural gas prices
increased five fold over a decade beginning in 1973 (Monthly
Energy Review, Energy Information Administration, January
2001, p. 131), paralleling other fuel price increases such
as in heating oil, and contributing to higher electricity
costs. These energy issues plus increasing consumer demand
for home comfort drove a trend in the residential building
industry towards increased building envelope tightness. Increased
tightness means a less drafty house, with fewer air changes
per hour between the indoor and outdoor air. The advantage
is less loss of conditioned air to the outside, whether warmed
or cooled, which means greater energy efficiency as well as
more consistent temperatures inside the home. However, the
isolation of indoor air as a solution creates another problem.
With reduced air exchange, indoor air quality suffers as moisture
and pollutants become trapped inside the tightly sealed home.
In a worst case scenario, exhaust gases from a combustion
appliance such as a furnace or water heater are unable to
escape through passive venting due to pressure differentials.
The hazardous gasses can backdraft, infiltrating the indoor
air. In 1994, the EPA stated that “indoor air pollution
is one of the five most urgent environmental problems facing
the United States.” In 2000, the American Lung Association
concurred with a Pew
Commission report stating that asthma rates in the U.S.
are increasing rapidly and are expected to double by the year
2020.
The
United States Environmental Protection Agency ranks poor
indoor air quality among the top five environmental risks
to public health.
The home structure itself can suffer damage or mold problems
from trapped moisture. Going back to building drafty houses
was not a viable option. However, the question remained as
how to retain energy efficiency and comfort gains while improving
home durability, safety, and health for inhabitants, while
keeping the home competitively priced in the marketplace.
Independent energy consultants identified ways in which home
systems work together, and developed a philosophy of design
and construction that must be implemented at the design phase.
The house as a system approach was born and later adopted
by the U.S. Department of Energy’s Building
America program. At the same time, progressive builders
of energy-efficient and quality homes initiated the house
as a system approach for their companies. E-Star supports
these efforts through public education and technical training
for builders, code officials, and homeowners.
Interrelation of Systems
When homes began to be built tighter indoor air quality decreased
and moisture issues cropped up. To counter such problems,
moisture and water must be kept out of the structure, combustion
appliances such as furnaces and water heaters properly ventilated,
and fresh air provided through controlled, mechanical ventilation.
It is important to note, however, that the interrelation of
home systems can work in favor of the homeowner. For example,
when low-E windows are installed in a tight, well-insulated
house with sealed ductwork, the same cooling and heating comfort
can be attained with a smaller HVAC system as was previously
done with a much larger furnace and air conditioner. A homeowner
owning a house that works as a system spends less to condition
the living space and maintain the home. In sum, the house-as-a-system
approach accepts the reality that the various components and
systems of the home do not work independently, and gives appropriate
consideration to the effects of each on the others to maximize
energy efficiency, comfort, durability, air quality, and affordability.
From the Design Phase
On
The house-as-a-system approach begins at the
design phase. Deciding on ductwork location, the sealing of
air barrier boundaries, appropriate levels of insulation and
window selection, and ‘right-sizing’ of HVAC starts
here. The house-as-a-system approach requires a high level
of organization by builders, training and skill on the part
of trades people, and orchestrated timing during the construction
phase. The area behind a bathtub cannot be insulated after
installation has taken place. Trades persons creating holes
for home wiring must understand the importance of sealing
the holes post-installation be required to do so. “The
‘systems approach’ requires that a builder go
beyond good workmanship and good materials … builders
must focus on the interactive impacts that new products and
practices have on home performance.” (“Partnering
for Systems Performance,” Steve Andrews, Professional
Builder, August 2002).
Steve Andrews, a Senior Technical Officer at E-Star who consults
with builders on energy-efficiency and the house-as-a-system
approach, identifies the following aspects of a home as those
that require special attention because they are so interrelated
(“A Growing Market for High Performance Homes,”
Steve Andrews, Homebuilder, July 2002):
Insulation. Insulation only performs to its R-value
(i.e., insulation level) if it is properly installed. Fiberglass
batts that have been crushed and compressed into the wall
spaces will not provide an optimal level of thermal control.
In such cases, HVAC systems have to work harder and consume
more energy to keep a home comfortable.
Windows. Windows must be selected for an appropriate
U-value (i.e., amount of heat and energy allowed to penetrate
panes). This value varies depending on the solar orientation
of a structure and its geography. Otherwise, undue heating
of the home can take place or a missed opportunity for free
solar heating may occur.
Tight Construction.
Blower
door testing should indicate low air changes per hour
(ACH) total leakage. [Note: The IECC states .35 ACH. High
Performance Homes aim for .22 ACH.] As with insulation, a
leaky house will unnecessarily tax an HVAC system and the
blower door test identifies how large a “hole”
is in any given home.
Bulk Water Management. Water shedding from the structure
includes positive slope from foundation for surface drainage,
capillary break over footing, perimeter footing drains (to
daylight or sump), as well as drainage planes for walls with
appropriate flashings. Bulk water management heads off moisture
problems in tight, energy-efficient structures.
Water Vapor Management. Interior air barriers are
a must and another important part of avoiding moisture problems.
Other features requiring special attention vis-à-vis
an air barrier include crawl spaces, tubs and showers, fireplaces,
dropped soffits, and floor systems interior to rim joists.
Requirements vary by climate in some cases.
Controlled Mechanical Ventilation. Ventilation requirements
vary for kitchens, baths, bedrooms, and general conditioned
space. A ventilation system must have an indicator light (or
some similar device) to let a homeowner know if it is not
working properly. Ventilation is key for providing fresh air
in tightly constructed homes.
Safe and Efficient Combustion Appliances. Combustion
byproducts must be direct or power vented, or the appliance
must have sealed combustion. Other minimum standards apply
by appliance type. Again, this is a necessary complement to
having a tight home.
Properly Sized HVAC, Sealed Ductwork. Sizing and duct
design by room is determined using ASHRAE Manual
J and D calculations (Coming Soon).
Duct requirements cover types of duct sealing and percent
leakage allowed, designed return ducts (rather than the use
of building cavities), and insulating the outside of a duct
when it is located in an attic or wall. Leaky or inadequately
insulated ducts greatly diminish energy efficiency and comfort
and, as with other poorly designed systems, unnecessarily
overwork HVAC equipment.
Advanced Framing and Engineered Lumber. Engineered lumber
tightens the building, and advanced framing allows more insulation.
The result is better water management and energy performance.
Performance Testing
One of the most important aspects of ensuring a house-as-a-system
approach is performance testing. The energy-efficiency performance
of a home is tested by Energy Raters, such as those trained
and certified by E-Star. Home
Energy Ratings are detailed inspections of a home’s
construction and performance.
Affordability
Building
America projects that “energy consumption of new
houses can be reduced by as much as 50% with little or no
impact on the cost of construction through a systems engineering
approach” (“Systems
Engineering Approach,” Building America, 21 May
2003). The example offered is that the added costs of low-e
windows and building tightening are offset by the smaller
HVAC system needed to maintain comfortable home temperatures.
There are both benefits
and challenges to creating energy-efficient homes and
using the house-as-a-system approach. With implementation
of any such change, costs and effort expended decrease as
the process becomes part of a builder’s construction
process. For homeowners, the affordability of an energy-efficient,
systems oriented home extends throughout its lifetime. Energy
Efficient Mortgages available to homebuyers of energy-efficient
homes can also give homebuyers additional purchasing power
and help builders sell energy-efficient homes to a larger
pool of buyers.
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