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Insulating walls in an old house

My 1927 house has balloon framing, real plaster walls on rock lath, and no vapor barrier. It has cedar shingles over tar paper.

If I blow insulation into the walls, will I create a problem with vapor from the house condensing inside the walls? Even without insulation (current condition), can I close the top of the walls in the attic or will that also allow water vapor to condense in the walls?

Posted Aug 29, 2014 11:40 AM ET
Edited Aug 29, 2014 12:50 PM ET


6 Answers

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Although you didn't mention it, I assume that your walls have board sheathing under the tar paper (asphalt felt).

Working from the attic, you can certainly install some type of blocking (2-by lumber or rectangles of rigid foam, installed with caulk or canned spray foam to ensure airtightness) as an air barrier at the top of your stud cavities. This won't cause any problems.

Your walls can easily be insulated with dense-packed cellulose insulation. Most cellulose contractors will be familiar with this type of weatherization work and will be happy to explain the details and cost to you if you call them up.

Answered by Martin Holladay
Posted Aug 29, 2014 12:00 PM ET


Your local climate makes a difference, but in most cases you'll be OK. Where are you located?

By insulating may end up with paint issues if the shingles are painted on the exterior face, but not back-primered (especially if there is any low permeability lead paint layers) but that would primarily be in parts of the wall that are less protected by roof overhangs and get period re-wetting from the exterior (either from direct rain, or splash-back from the drips.)

Air leaks from the interior are a much bigger driver of wintertime moisture accumulation in the sheathing than vapor diffusion through a painted plaster wall. If you blow fiber insulation into the wall, dense-packing the insulation (3lbs per cubic foot or higher if cellulose, 1.8lbs or higher if fiberglass), makes a significant difference on air leakage rates. Using cellulose is also more protective of the sheathing than mineral or glass fiber, due to the fact that the hollow fiber structures of cellulose allows it to take on a decent fraction of the moisture load without damage or losing function.

Simply blocking the tops of the open balloon framing won't change the moisture handling characteristics much, but would lower the amount of stack effect infiltration pulling interior air into the wall cavities. How much of a difference that would make on heating energy use depends on how air-tight your plaster and sheathing/tarpaper layers are

Answered by Dana Dorsett
Posted Aug 29, 2014 12:05 PM ET


Thank you both for your advice. I live near Boston Massachusetts on a heavily treed lot. The house gets very little direct sunlight, so any moisture in the walls is not baked out in the summer. The shingles were originally unstained red cedar, so I doubt that they are back primed .We have added semi-transparent stain twice - the last time latex as oil-base house stains are no longer available. And yes, there is probably board sheathing beneath the tar paper. The house is as dry as the Sahara in the winter, so it is probably leaky. It has the original double-hung windows with weights (really they are so much easier to open and close than the modern windows); the windows have metal weather stripping and aluminum storm windows. It's a 2-story house with full attic and basement. The attic is full of beloved junk and has tongue and groove flooring beneath which is a little rock wool. We have gable and ridge vents but still have big ice dams in the winter. I'd like to improve the house's energy efficiency but I want to be sure that I don't trade heat loss for mold problems. I don't really trust contractors to do the right thing or be knowledgeable about water vapor issues. Their natural interest is in getting work so they can earn money. I've been getting conflicting advice on whether to insulate the walls. In the attic, I am unsure whether to use Icynene to insulate under the roof or tear up the floor - which will probably have room for only 4 or 6" of insulation.

Posted Aug 29, 2014 9:19 PM ET


Ruth you just described my old house, circa 1890. My home is on the shoreline in Connecticut. I suggest you follow Dana Dorsett's recommendation. If you want to chat about your old house feel free to call me.... 860-460-5434. Spray foam in an old home is a risky proposition which could quickly prove to be very costly to you if not handled correctly.

Answered by Richard Beyer
Posted Aug 29, 2014 10:40 PM ET


Martin, you say that Ruth can install some blocking at the top of her walls within the attic & it won't cause any problems. Would problems arise if if she did NOT do this (aside from just being less air-tight)?

Answered by Jeff Watson
Posted Aug 30, 2014 1:09 PM ET


Q. "Would problems arise if if she did NOT do this (aside from just being less air-tight)?"

A. No.

Answered by Martin Holladay
Posted Aug 30, 2014 2:46 PM ET

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