Building a house that is too tight?  (C) Daniel Friedman Energy-Efficient, Almost Too-Tight Houses

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Tight houses & indoor air quality concerns:

This article discusses the question: is this house constructed too tight with insufficient fresh air intake for good indoor air quality? Tight houses are not a problem if the builder takes reasonable precautions outlined here. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. The sketch above shows a split-level cantilevered raised ranch with chimney and bay windows - a tough house to build tight.

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Good Indoor Air Quality for Tight Houses

"The Almost Too-Tight-House - not a problem if builders take reasonable precautions" - this article appears in original form (the PDF links below) and an updated/expanded web article just below. The text below paraphrases, quotes-from, updates, and comments an original article original article (see links at article end) "The Almost-Too-Tight-House" by Steven Bliss.

How Well Ventilated are Modern Houses?

Most energy-efficient builders don't make their houses as tight as plastic bags. More often, new energy efficient houses have natural air infiltration rates of 0.2 to 0.5 air changes per hour (ACH). These houses already have almost enough average ventilation to meet current indoor air quality guidelines. But because natural air leakage in and out of buildings varies with outdoor temperature and wind speed, these homes do not ventilate reliably, and also, some parts of the home may be left unvented.

Still in some communities these almost-too-tight-houses are mandated by law or promoted with financial incentives. They often arise as a compromise between hard-core superinsulation dogma and what builders think they can build and sell.

How Air-Tight is Your House?

The two most important factors determining the tightness of a house shell are:

  1. The quality of construction, including choice of materials and attention to best building practice details as the components of the home are assembled (for some examples see FRAMING DETAILS for BETTER INSULATION)
  2. The type of housing - its design. All other things being equal, two-story and split-level houses leak more air than one-story homes. (For examples of leak areas on cape-cod style homes see INSULATION LOCATION for CAPES, CRAWLSPACES.)

Other factors affecting house tightness, increasing house air leakage, include:

With these more-air-leak-prone building design and construction details, chances are that the building is no tighter than 0.25 air changes per hour, and on a blower door test for air leakiness, the home is probably in the range of 4-7 ACH at 50 Pascals of pressure. If you don't actually test a home, estimating its leakiness is simply anybody's guess. (See BLOWER DOORS & AIR INFILTRATION).

House Tightness and Indoor Air Quality

Simply knowing that a home is relatively air tight does not by itself permit a clear statement of the home's indoor air quality.

First, the health effects of exposure to low levels of indoor pollutants are not only unknown in many instances, and more, the sensitivity of individuals to airborne irritants or contaminants varies quite widely. Using instrumentation and careful lab analysis we (DJF) have observed clients who respond with severe respiratory distress to airborne levels of certain mold spores at airborne spore counts in the hundreds of spores per liter of air - lower than normally-accepted standards of problem indication indoors.

Second, the indoor air contaminants and their levels vary widely even between houses of the same design and constructed by the same builder. The particular indoor pollutants that may be present in a specific building vary widely depending on the materials of construction, the furnishings, and for homes that have suffered leaks or water entry, the risk of hidden mold, insects, or allergens. Housekeeping and the presence or absence of pets make an enormous difference in the level of allergens in buildings, and the combination of high indoor humidity with pets increases the level of dust mites in the home.

For at least these reasons and probably others as well, there is no direct relationship between the air-tightness of a home and the level of indoor air pollution in that home.

In general, houses that are reported to have indoor air quality problems have a specific (sometimes obvious) pollution source that can be singled out and corrected, removed, or at least controlled.

Effects on Indoor Air Quality When a Home is Weatherized to Be More Air-tight

In several studies of homes whose air tightness was tested before and after weather sealing, researchers found that cutting air infiltration by 20 to 40 percent in a home - typical of what professional weatherization retrofitting contractors achieve - did not degrade indoor air quality significantly.

Many pollution levels in these homes remained about the same, some actually fell. The only contaminant level that increased in these homes was radon gas. Unlike other indoor air contaminants, the radon level in homes does indeed seem to increase more or less proportionally to the air tightness of the home.

List of Reasonable Air Quality Precautions for Builders of Renovated or New Air-Tight Homes

  1. Building retrofits to improve the energy-efficiency of an existing older home are no cause for an air-quality alarm. Unless the house was very tight to start with or you are gutting the building and adding a continuous air/vapor barrier, it is unlikely that a weatherization retrofit can be too tight. At best, a weatherized home will approach the air-tightness of new housing. If there were no air quality problems before the retrofit, there are probably none afterwards.

    Reasonable indoor air quality precautions for weatherization retrofit projects on older homes include adding kitchen and bath vent fans if they were not present before. If radon levels in the area are known to be high, test for radon levels before and after the retrofit; seal basement cracks and gaps, and cover (and possibly vent) basement sump pits.

    If indoor moisture levels are high in the home, identify the moisture source and correct it before other construction improvements. Following these steps are likely to improve rather than harm the indoor air quality of an older home.
  2. Reasonable precautions for new construction of air-tight homes: in new construction the builder has more control over potential sources of indoor air pollutants. Two key strategies for indoor air quality are pollutant source control and spot ventilation (or building ventilation in general).

    Indoor air pollution source control: seal out radon if radon gas is a problem in the area; be careful in choosing building materials. Avoid large amounts of products made using urea-formaldehyde glues. Steer clear of unvented combustion appliances. If you are installing a gas stove, use a pilotless model.

    For spot ventilation, install high quality (high air flow low noise [low sone]) kitchen and bath vent fans, duct the range hood to outdoors. In the bathroom, hard-wire the vent fan to the light switch. Many occupants hate this feature, but if the bath fan is a high quality quiet unit people are more willing to live with it. Add exhaust fans in high-moisture areas such as laundries.

    All vented combustion appliances should have their own combustion air supplies.
    Atmospherically vented gas appliances such as heaters and water heaters pose special problems because if they backdraft there is little warning and the possible production of carbon monoxide can put lives at risk.

    Your main options with furnaces are sealed-combustion appliances, induced-draft heating equipment, or an isolated and fully vented utility room following the Uniform Mechanical Code (and local building code regulations).
  3. Provide mechanical ventilation for the whole house? If you have taken the steps described above and the house is not airtight, you probably do not need a general ventilation system to dilute indoor air pollutants - in the sense that the indoor air will be no worse than in the average home and will be better than many.

    At some level of tightness, bringing in fresh outdoor air has obvious value for indoor air quality. New homes include products that release trace levels of chemicals that were not common in homes 50 years ago, and we simply don't know the effect of living in modern indoor air with these low but detectable pollutant levels.

    See VENTILATION, WHOLE HOUSE STRATEGIES for details of when, why, and how to choose and implement a whole house venting system.

Inadequacies of Optional IAQ Credit Categories Specified in LEED - White Paper from AIHA

This article explains IAQ expert opinion (from the AIHA) concerning the status of the indoor air quality provisions of the current LEED Green Building certification program.

LEED Indoor Environmental Quality IEQ Provisions


This discussion is now found at LEED GREEN BUILDING CERTIFICATION where we also provide an explanation of what the LEED designation means and how it is achieved.

Original article links:

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.


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