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Indoor Air Quality and Tight Houses


Robert Jordan
July 16, 2009
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Good IAQ starts with a tight building. It is often said a house has tobreathe. This brings to mind a Prohibition-era movie where one gang hascaptured members of their rival. Lined up in a bleak warehouse, theyare sweating bullets and running fingers around their collars. The Caposays to his machine-gun toting lieutenant, "He says he can't breathe.Fill him full of holes." Rat-a-tat-tat. This is not breathing. Peoplebreathe with their respiratory system, and in a building it is aventilation system.

When a builder assertively says a househas to breathe, I ask where he puts the holes and how he knows how manyto put. Like you want to make it leaky, but not too leaky right? Thatusually gets an annoyed look, but come on, he is just blowing smoke.

Thereis an effect known as the stack effect. Warm air rises and createsslight pressure at the ceiling where it tries to escape. This draws airin at the foundation to replace the escaping air. Since this is drivenby the temperature difference between outdoors and indoors wouldn'tthere be more ventilation on a cold, blustery winter day than on a mildApril day?

A tight building keeps out allergens, dust andhumid air that can lead to mold if it is trapped in the buildingassembly. Internal causes of bad IAQ are best controlled by excludingthe source. Even so, outside air needs to be introduced to ridpollutants from occupants, cooking and off gassing household items, aswell as controlling humidity. This can be done with a Heat RecoveryVentilator or exhaust only ventilation using a continuously runningbath fan and strategically placed air inlets. The HRV is more effectivein terms of thoroughness and extracts heat from the outgoing air, butcosts more to operate than the bath fan option.

In addition to better IAQ, Tight homes:

Keepmoisture in a building in the winter, maintaining desirable humiditylevels. Proper humidity makes us more comfortable at lower thermostatsettings and also keeps our skin and nasal passages from drying out. Ofcourse, there may be cases where humidity gets too high. Multiple teenstaking hour long showers or a rain forest of plants, for example.

Keep humid air out in the summer, reducing the AC load.

Make a house more energy efficient because the heat (or cooling) isn't leaking out.

Have fewer drafts, thus more comfort.

Allowdesigners to accurately design heating and AC systems. Air infiltrationcan be measured, instead of being guessed at, in performing a heat lossor cooling load calculation.

Are more forgiving relative to distribution since the heat stays in the house. Shouldn't be any hard to heat rooms

Wellinsulated walls are warmer and consequently you feel warmer at lowerthermostat settings. Think of how cold you feel next to a cold windowin winter. You are radiating body heat to the window. Same effect witha cold wall.

When I tell builders they need a completethermal envelope, they look at me with a "Duh, of course" kind of lookand then proceed to put forty holes in the ceiling. You know, recessedlights.

Knowing how to build a tight house requires somestudy. Unfortunately, too many builders think there is nothing to itand just go do it. It involves defining the thermal envelope and theair barrier prior to construction and coordinating with the trades sothe continuous envelope can actually be built. We need far morecooperation than is usually the case with our bottom-line process,which usually leaves the homeowner with a poorly performing house thatis often cost prohibitive to fix.

In case you think all ofthis is just one person's opinion, or if you want more information, goto the American Lung Association's website for their Health Houseprogram and check out their builder guidelines.

(Robert Jordan, Dr. Warm; This Cold House Doctor. e-mail: [email protected])