| Yet at the same time, most builders are continuing to
rely on uncontrolled air leakage through holes and cracks in the
building envelope to provide adequate ventilation for the occupants.
Newspaper articles and trade journals dramatizing indoor air quality
problems in new airtight houses have begun to alarm both builders and
homeowners. It is not uncommon these days for builders to hear
complaints of excess moisture or stale odors in their brand new houses. The question on many builders minds these days is;
how tight should I be building my houses? After more than two decades of
sealing up houses to make them more energy efficient, many people are
wondering whether we've gone too far. The answer may be that we haven't
gone far enough.
New Houses Are Getting Tighter
It used to be that builders rarely worried about the
air tightness level of the houses they built. Standard construction
practices would typically produce a house that generated few if any
callback complaints related to air tightness such as moisture on windows
or stale odors. By leaving houses with a significant level of air
leakage, builders were actually incorporating a passive ventilation
system into every house they built. In addition, the lower efficiency
natural draft combustion appliances routinely installed in new houses
acted like exhaust fans drawing large quantities of outside air into the
house. This passive ventilation system was crude, uncontrollable, and
created some comfort complaints, but it did satisfy the ventilation
needs of most of the houses built more than two decades ago.
But over the past 20 years, building practices have
changed. The advent of the 1973 oil crisis created the first large
demand for more energy efficient houses. Increasing insulation levels
was the first response to this new energy conscious market. As the cost
of energy has continued to increase, other industry responses developed
as well. Most notably, building products and construction practices have
been developed and adopted which reduce the overall size of air leaks in
the building envelope. In fact, many builders are now building much
tighter houses without even realizing it.
Walk down any new development under construction and
it is common to see the use of house wraps, tight fitting exterior
sheathings, vapor barriers, and untold caulks, foams and sealants. While
10 years ago, many of these air tightening products where used by only a
few custom builders, they are now an integral part of standard new
construction practice. Much detail and time are now spent installing
continuous vapor barriers, sealing penetrations in the exterior envelope
with specially designed foam, and adding gaskets at outlets and plumbing
chases.
In cold climates a better understanding that attic
moisture and ice dams problems are caused by warm air leaking into attic
spaces has led builders to do a better job sealing attic penetrations.
New higher efficiency combustion appliances being installed use much
less air for combustion and as a result significantly reduce the amount
of outside air being drawn into the house. Add to this the new tighter
window technologies and it is no wonder why the average house being
built today is significantly tighter than its counterpart of 20 or 30
years ago.
And as houses are made tighter, it also becomes
easier for exhaust devices such as range hoods and clothes dryers to
create negative pressures in houses large enough to back draft furnaces,
water heaters or fireplaces. This can introduce carbon monoxide and
other combustion related pollutants directly into the house. Gas ranges
in kitchens can also become a concern if they are producing high levels
of carbon monoxide. While not all of the thousands of carbon monoxide
poisonings each year can be attributed to tight buildings, it is clear
that tight houses are increasingly susceptible to this problem.
Measuring House
Air tightness
Despite the growing importance of house
air tightness,
few builders actually know how tight they are building their houses.
Until recently, the building community has tended to rely on subjective
estimates of air tightness. Unfortunately, it is virtually impossible to
accurately estimate the tightness level of houses by visual inspection
alone. And without knowing house air tightness, it is difficult to assess
the need for, or to design an effective approach to ventilation or
indoor air quality.
When discussing these issues, it is important to
distinguish between two terms; air tightness and natural ventilation (or
natural infiltration). The air tightness of a house is related directly
to the cumulative size of all the holes and penetrations in the exterior
building envelope. The natural ventilation rate is determined by the
forces driving air in or out through the leaks in the building envelope.
The easiest way to measure house
air tightness is with
a diagnostic tool called a blower door. The blower door consists of a
powerful, calibrated fan that is temporarily sealed into an exterior
doorway. The fan blows air out of the house to create a slight pressure
difference between inside and outside. This pressure difference forces
air through all holes and penetrations in the exterior envelope. Blower
door tests are typically performed at a pressure difference of 50 Pa
(0.2 inches of water column).
By simultaneously measuring the air flow through the
fan and its effect on the air pressure in the house, the blower door
system measures the air tightness of the entire building envelope. The
tighter the building (e.g. fewer holes), the less air you need from the
blower door fan to create a change in house pressure.
Air
tightness measurements are presented in a number
of different formats including:
- square inches of leakage
- air flow needed to generate 50 Pa of pressure
difference (CFM50)
- air changes per hour at 50 Pa of pressure
difference (ACH50)
It takes about 20 minutes to set-up a blower door and
do a test to document the air tightness of a house. In addition to
assessing the overall air tightness level of the building envelope, the
blower door can be used to estimate the amount of leakage between the
conditioned space of the building and attached structural components
such as garages, attics and crawlspaces. It can also be used to estimate
the amount of outside leakage in forced air duct systems. And because
the blower door forces air through all holes and penetrations, these
problem spots are easier to find using chemical smoke, an infrared
camera or simply feeling with your hand. The air tightness measurement
can also help you assess the potential for back drafting of natural draft
appliances by exhaust fans and other mechanical devices.
Estimating Natural (Passive) Ventilation Rates
Natural ventilation rates are difficult to measure
directly because of the small and fluctuating driving forces (such as
wind and indoor/outdoor temperature differences) which actually create
air infiltration. One technique for directly measuring natural
ventilation involves injecting a known amount of inert gas into a house
and carefully measuring the length of time it takes for the gas to be
diluted by natural air infiltration into the house. This technique, call
tracer gas testing provides accurate measurement of house infiltration
rates under actual operating conditions. Unfortunately, because tracer
gas testing is quite expensive and can require significant time to
conduct a test, it is used almost exclusively by building researchers.
A blower door
air tightness measurement can be used to
provide a quick natural ventilation estimate. While not measuring
natural ventilation directly, the blower door test provides us with a
reliable measure of the total hole size in the exterior envelope, and
that measurement can be used along with a simple mathematical model to
provide a rough estimate of the average annual natural infiltration rate
of the house.
This estimated natural ventilation rate can be
compared with published ventilation guidelines to help determine if
additional mechanical ventilation may be needed. However, the
ventilation estimate from a blower door test is sufficiently imprecise
that it alone should probably not be used to determine if a house has
sufficient passive ventilation.
Relying on Passive Ventilation May Be A Mistake
Despite the advent of new tighter housing
construction, most builders continue to rely on a passive ventilation
system of natural infiltration to provide fresh air for their new
houses. The result, in some cases, may be houses with chronic problems
and increased callbacks for builders.
Probably the most widely published symptom of a tight
under ventilated house is condensation and moisture build-up.
Condensation on windows, mold growth on cold surfaces and dust mite
infestations in carpets are becoming more frequent in new houses as
air tightening reduces natural ventilation rates and indoor humidity
levels increase. Moisture on windows not only creates a nuisance for
homeowners, excess moisture levels in houses can prematurely degrade
building components resulting in costly callbacks and replacements. And
mold growth and dust mite concentrations in houses is now suspected as a
leading cause of respiratory problems.
By relying on a passive ventilation system, we are
now less confident that the house has adequate ventilation. The amount
of air provided by passive ventilation is a function of the cumulative
size of the holes in the exterior of the house, and the driving forces
which drive air into and out of those holes. By sealing up holes in the
building exterior, we effectively reduce the capacity of the passive
system to provide fresh air to the house. Also, because we can't see the
holes once the house is finished, we typically don't know the total hole
size remaining in the outside envelope without actually measuring it
To make it even more complicated, the driving forces
pushing air through these holes varies dramatically from one part of the
country to another, and from one season to another. For example, if you
built two identical houses (with the same exterior hole size and
location) and put one in Florida and one in Minnesota, the Minnesota
house would have almost twice the annual infiltration rate from the
passive ventilation system. This is because the two main driving forces
for the passive system, wind and the temperature difference between
inside and outside, tend to be greater in Minnesota than in Florida.
Seasonal variations can also create large differences in ventilation
rates. During the mildest weather, there is less driving force to
operate the passive ventilation system. As a result, ventilation rates
in the spring and fall can be a fraction of the rate during the harsher
winter or summer months.
Numerous studies of house
air tightness and natural
infiltration rates across the country are showing that many new houses
relying on passive ventilation systems are under ventilated. A study of
64 houses built in 1984 in Minnesota showed that 80 percent failed to
meet recognized ventilation standard when relying on passive natural
ventilation. A study of 472 new homes built in the Pacific northwest
found similar results. In that study between 50 to 85% of the houses
failed to meet current ventilation guidelines, even after accounting for
operation of bath and kitchen exhaust fans by the homeowner.
Not only are houses being built tighter, in some
cases we are introducing greater levels of moisture and pollutants into
the home. The installation of hot tubs, whirlpool baths and saunas can
significantly increase the moisture load in a house. Pollutants such as
formaldehyde and volatile organic compounds that are emitted from the
finished products put in the house can also be a serious problem. Many
new interior finishing products including carpeting, carpet backing and
wood products often emit high levels of indoor pollutants. Yet
homeowners demand that builders include these type of products in every
home they build.
Build Tight and Ventilate
Building leaky houses is no longer an option in
today's energy and comfort conscious marketplace. And trying to build
houses that leak just the right amount is also fraught with problems.
Your best bet for controlling comfort, air quality and structural
durability is to build a reasonably tight building envelope, and provide
controlled mechanical ventilation. This is the only way to ensure
adequate fresh air in mild weather without excessively ventilating in
extreme weather conditions.
So what is a reasonable
air tightness level for new
houses? Our experience from working with many builders across the
country is that 2 to 3 Air Changes Per Hour at 50 Pascals (ACH50) is a
reasonable goal that can be achieved for minimal extra cost, once crews
have been properly trained. There are a variety of approaches to
airtight construction such a poly air barriers, air-tight drywall, and
infiltration resistant foam insulation. Do some homework and choose an
approach that fits best with the skill level of your crews and type of
construction in your area.
Mechanical ventilation for your houses does not have
to be complex or expensive. Something as simple as a quiet 75 CFM
bathroom fan running continuously will be adequate in some houses. In
other cases, balanced ventilation systems and systems with heat and
humidity control may be the best bet. With a properly designed
ventilation system and an air tightness level of 2 to 3 ACH50, most of
the ventilation for the house will be coming through the ventilation
system and will therefore be under control. Adjustments to the
ventilation rate can now be easily made, regardless of the season or
outside weather conditions.
Most experts agree that continuously operating,
low-level ventilation that requires little occupant attention is best.
Studies indicate that homeowners often turn off or permanently disable
mechanical ventilation for a variety of reasons, but especially due to
noise. If you choose the right system and have it properly installed,
noise simply shouldn't be a problem. The bottom line is that if
homeowners can't hear the ventilation system running, it has the best
chance of being used.
Other Recommendations for New Construction
- Have a blower door test done on one of your
typical houses
After getting an accurate measure of the
air tightness
of your houses, and assessing potential sources of moisture and other
pollutants, you can decide how much and what kind of additional
ventilation to provide. The blower door technician should be able to
help you make an assessment, or point to someone who can help.
- Avoid natural-draft combustion appliances
These units are prone to back-drafting in tight
houses. Use sealed-combustion units or units with fan-assisted draft. As
long as sealed-combustion appliances are used, an occasional
intermittent high negative pressure, such as might be caused by a
clothes dryer or kitchen exhaust fan, is usually not a problem.
- Warn your customers about the dangers of open
fireplaces.
Fireplaces can produce large amounts of carbon
monoxide whenever the fire smolders. In tight houses, open fireplaces
can be deadly. If your client insists on having one, steer them to
toward a manufactured unit that has been tested for operation in a
negative pressure environment, or a direct-vent gas unit. Some high
quality manufactured units have very tight-fitting doors and draw
combustion air from outside to ensure that they don't spill combustion
gases into the house. In addition, be sure the chimney is located inside
the thermal envelope of the house.
- Try to minimize sources of indoor pollutants.
New homes typically have numerous sources of
pollutants such as formaldehyde from wood products, or new carpets.
Unfortunately, common ventilation standards (such as the 0.35 air
changes per hour set by ASHRAE) do not account for large sources of
indoor pollutants. Likewise, if there is a large moisture source in a
house (such as improper site drainage of rainwater), you will very like
still have a moisture problem despite meeting a minimum ventilation
guideline.
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