Energy Conservatory Articles
Are Your Houses Too Tight?
By Gary Nelson, Robert Nevitt and Gary Anderson
The airtightness of the building envelope is an important key to understanding the performance of any new house. Uncontrolled air leakage can result in high fuel bills, failure of building components, and increased builder callbacks. As a result, more and more time is being spent by the building trades sealing up new houses in an effort to reduce problems associated with air leakage.
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 airtightness level of the houses they built. Standard construction practices would typically produce a house that generated few if any callback complaints related to airtightness 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 housewraps, tight fitting exterior sheathings, vapor barriers, and untold caulks, foams and sealants. While 10 years ago, many of these airtightening 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 backdraft 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 Airtightness
Despite the growing importance of house airtightness, few builders actually know how tight they are building their houses. Until recently, the building community has tended to rely on subjective estimates of airtightness. Unfortunately, it is virtually impossible to accurately estimate the tightness level of houses by visual inspection alone. And without knowing house airtightness, 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; airtightness and natural ventilation (or natural infiltration). The airtightness 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 airtightness 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 airtightness 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.
Airtightness 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 airtightness of a house. In addition to assessing the overall airtightness 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 airtightness measurement can also help you assess the potential for backdrafting 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 airtightness 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 underventilated 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 airtightening 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 airtightness and natural infiltration rates across the country are showing that many new houses relying on passive ventilation systems are underventilated. 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 airtightness 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 airtightness 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 airtightness 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.
