The not-too-rocket-science of a heated house (part two)

Posted Wednesday, September 28, 2011 in Sustainable Maine

The not-too-rocket-science of a heated house (part two)

Air Moisture Leak, courtesy Passipedia.

by Paul Kando

Moisture can enter a home from a number of sources in a number of ways. A leak in the roof is the most obvious one, perhaps the result of an ice dam. In most homes, the main source is under the house, in the cellar or crawl space. If ground water can flow in, it will. It can also wick up through a dirt or concrete floor through capillary action. Another large moisture source is the occupants taking showers, cooking, doing laundry, etc.

Whatever its source, once the furnace or boiler is fired up, a lot of the moisture will be absorbed and carried upward by the warmed air. It will travel through any leak it can find right into the attic. Here the warm air will come in contact with the cool underside of the roof and its moisture will condense. This can cause rot or mold damage.  The condensing moisture also releases its latent heat of vaporization, the energy it took for it to vaporize. That heat is now transferred to the roof, warming it.

If enough heat escapes into the attic, the roof will become warm enough to melt any accumulated snow. The melt-water will cascade down until it reaches the roof area over the outer wall and roof overhang. Here, not being warmed any longer from below, the water will re-freeze into an ice dam, which will cause the cascading melt to back up between rows of roof shingles, and drip down into the building. A leaky ceiling is an expensive nuisance to fix, but at least the damage is visible. However, the water may, instead, drip down inside the wall cavity where it may not be noticed before a lot of damage occurs. Soaked fibrous insulation is no insulation at all, but it will help hold the water so it causes rot, mold and mildew.

Airborne moisture can also enter through leaks in the walls, such as leaky electrical outlets. Inside the wall cavity it will follow a path toward the outside, until it cools enough to condense. This usually happens on the inside surface of the home’s outer sheeting, or on a stud, wooden components vulnerable to unseen water damage. Even small leaks in walls and ceilings can convey large amounts of condensate. German researchers have found that through a leak a mere millimeter in diameter as much as 360 grams (13 oz.) of water per day can enter and condense inside a wall cavity.

What to do?  First, prevent moisture from entering the building, including cellar and craw space. The origin of most ground water is rain and snow off the roof. This runoff should be channeled away from the building. To prevent moisture entering from below ground by capillary action, permeable (e.g. dirt, broken concrete) cellar or crawl space floors should be covered with a continuous, non-permeable membrane, such as a polyethylene sheet sealed to the walls, with all seams taped. A cellar window left open during the summer “to dry things out” is another source: it only invites moisture to condense out of warm summer air. 

Second, water that cannot be kept out must be removed. If you can, drain liquid water to daylight. If not, a sump pump is in order. Water vapor generated inside (showers, cooking, laundry) must be vented outdoors. Third, all air leaks in the perimeter of the heated space – lowest floor, walls, ceiling/roof – must be sealed. Since moisture travels with the air that absorbed it, stopping air movement also stops the movement of moisture.

Seal the basement/ crawl space ceiling (the underside of the bottom floor). Insulate it as well – unless you want your living room radiating heat into the cooler basement. Likewise seal air leaks in all walls and ceilings. Caulk and seal individual leaks as best you can. However the best way to seal a house is by wrapping it in a sealed layer of rigid foam insulation, boosting the R value of the exterior at the same time. Because this last move also involves re-siding the house and framing out windows and doors, it is best done as part of re-siding or remodeling.

Maine houses lose over a third of their heating energy through air leaks. The only sure way to locate those leaks is to have a blower door test. You may as well have a complete energy audit which will include such a test. The energy audit report will also tell you how much you may seal the building before you must add (heat recovery) ventilation. It should provide a moisture analysis and present a prioritized list of steps you can take to increase the energy efficiency of your home along with a corresponding estimate of energy/ cost savings.

Next week: Principles of a thoughtful energy rehab.

blog comments powered by Disqus