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Insulation: Above all do no harm

Part 1

by: Paul Kando

Wall section & psychrometric chart
photo credit:

There are times when the best-intended plans come with unintended negative consequences. This is one of several cautionary tales to underscore this point.

When Angie and Pat (pseudonyms to protect their privacy) planned to remove the old clapboards from their century-old home and replace it with new, low maintenance siding, a prospective contractor recommended that a continuous half-inch layer of polyisocyanurate foam board be installed under the new siding. It would add insulation, he said, and also help air-seal the building.

In reality, based on calculations as part of an energy audit of the house, a half-inch foam board would invite a serious moisture problem. As already discussed in previous columns, warm air can carry as much as 13 ounces of water vapor per day through every hole as tiny as one millimeter (a little over 1/32 of an inch) in diameter into a wall cavity. On a cold day, the vapor inside the wall is likely to condense into liquid water on the first cold surface it encounters. This is usually the inner surface of the sheathing, the layer of wood immediately beneath the siding.

In a very leaky old house like Angie and Pat's, much of the moisture-laden air entering the wall cavity is vented right out, so there is little moisture accumulation. The half-inch foam board suggested by the contractor would change this. It would act as an air-vapor barrier on the cold side of the wall assembly. As a result, moisture-laden air will no longer be vented out. It will, instead, accumulate within the structure and condense on the inner surface of the wall sheathing.

There are multiple lessons here: It pays to understand the basic physics of your house because a lot of well meaning contractors, even with years of experience, do not. In the "good old days" of cheap oil, nobody worried about moisture problems as long as the house was "able to breathe." With fuel costs rising, most of us can no longer afford to live in a breathing house. Nor can we afford to defer to folks whose "years of experience" is building such houses. We must understand the physics well enough to be able to insist that work hired out be done exactly as we specify. If unsure, we can ask our energy auditor for advice. Of course, it helps if that auditor is an independent advisor without a financial interest in his/her recommendations, rather than a contractor in auditor's clothing.

Adding board insulation under the new siding of a house is actually a good idea. However this outside layer of insulation must be thick enough (have a high enough R-value) to prevent the original sheathing (now inside the new layer of insulation) ever to cool down enough for moisture to condense on it. In Angie and Pat's case the minimum thickness of the foam board worked out to a minimum of four inches, rather than the half inch recommended by the contractor. (Ideally, while they are at it, they should insulate to a total of about R-60 all around in our Maine climate.)

Angie and Pat have three choices: One, they can spring for a complete passive house upgrade. Their heating costs will be reduced by up to 90 percent, and their conventional heating system will no longer be needed. However, such an upgrade will involve not just the added insulation, but meticulous planning, new carpentry, new siding, the upgrading of windows, and the addition of a heat-recovery ventilation system as well. Most of us cannot afford the cash layout required, because the expenses of essentially gutting and restoring the house from the outside far outweigh the cost of the efficiency improvements. The payback period from fuel cost savings is also longer. Two, they can chose to blow insulation into the outside walls and seal the spaces between the floors and the tops of inside walls, as well as add the minimum four inches of foam board required to avoid moisture trouble under their new siding. This will involve the extra expense of meticulous sealing and carpentry around windows and doors and probably the addition of heat recovery ventilation. As a result, the thermal efficiency of their house will be improved significantly. The cost will be less, but not much so than option one above. And three, they can settle for no exterior foam board at all, blowing insulation into the existing walls, caulking and sealing leaks from the inside as best they can, and delaying the residing project until they can afford to pay for the proper exterior energy upgrade.

What Angie and Pat do not have an option to do is to add less exterior insulation than necessary to prevent moisture condensing inside the exterior wall cavity.