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Tue, June 18

Column: Too much ventilation can be a good thing

Attic and crawlspace ventilation requirements may be changing soon.

I've had several questions about attic and crawlspace ventilation lately. Most of these came from people who just moved here from cold climates and are wondering if their ventilation is sufficient. Keep in mind while you read this that the main reason for crawlspace and attic ventilation is moisture control. Some of this moisture results from condensation, which can cause major structural damage over the long term, not to mention mold or other nasty things. Moisture control is not the only reason for ventilation. For example if you have a gas furnace in the attic or crawlspace ventilation is necessary to supply combustion air.

I'll talk about crawlspace ventilation first. I inspected a home on Gail Gardner Drive built in the 1970s. The crawlspace ventilation is about 30 years ahead of its time, because there is none. There is a plastic vapor barrier on the ground, and Styrofoam insulation on the crawlspace side of the block stem walls. The furnace, located in an interior closet, blew all the warm air directly into the crawlspace. The supply vents in the interior floor were simply openings into the crawlspace to allow the heated air to enter the home. In effect, the entire crawlspace was the duct.

This may sound strange, but this has become a more common building technique in the past five years. Current research is showing that making the crawlspace part of the conditioned space can reduce humidity in the crawlspace. It will be very important to keep moisture out of the crawlspace as much as possible, primarily by installing a gutter downspout system and maintaining good site drainage adjacent to the home. I believe this building technique will become more common in the years to come, especially after building departments around the country become more familiar with this.

Many homes in our area have crawlspaces, and most of these crawlspaces have had moisture in them on occasion. This can be from broken downspouts or poor site drainage. It is also common to get moisture in crawlspaces after a snowfall. Rainwater will quickly drain away, but snow will slowly melt and completely saturate the soil around a home. Some crawlspaces get moisture from snow when they have never had moisture entry from even the heaviest monsoon rain. Ventilation is needed in these crawlspaces so any moisture that does enter can dry out. The vents should be on opposite or at least different walls to provide cross ventilation. Most homes in our area have adequate ventilation.

Attic ventilation is another story. The main arguments for attic ventilation are to prevent condensation on the bottom of the roof sheathing, cool the attic (and therefore the home) in summer, minimizing ice dams (not a concern here), and extend the life of the roofing (shingles, etc.). In fact, some shingle manufacturers void their warranty if there is no attic ventilation.

For decades, the building industry relied on attic ventilation studies dating to the 1930s and 1940s. These include building guidelines issued by the FHA in 1942, and several studies and technical papers from the Housing and Home Finance Agency in the 1940s. There were several studies done in the 1990s that conflict with the earlier studies, even suggesting that attic ventilation can increase the chances of moisture and condensation.

In cold, humid climates the air entering the attic through vents will have more moisture content.In hot humid climates, the higher moisture content of the outside air increases the chances of condensation.

Many older homes in our area have only gable vents, while many areas of the country require low (soffit or wall vents) and high (gable or roof vents).

When I inspect a home in our area with only gable vents, the comment in my report is "the attic ventilation is typical for our area." I have not documented any moisture in attics as a direct result of insufficient attic ventilation. I frequently find moisture from air conditioners in the attic. This can be from condensation on the air conditioner evaporator, refrigerant lines or even the supply ducts. I have also found moisture from leaking or broken air conditioner condensate lines (the white plastic line that drains away the moisture that an air conditioner will remove from the interior air).

So, as with crawlspace ventilation, in the coming years we may see a major change in the way homes are built. Conditioning the attic space may be the best way to control condensation and other moisture that may enter the attic. And indoor humidity is becoming less of a concern because we are building more "airtight" homes today than in the past.

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