House Doc: Home Retrofit Problem
Q. Have you ever heard of back plaster? The house we bought a couple of years ago has been diagnosed with this malady, and apparently it’s not uncommon for houses built around the turn of the century where I live to have it. It involves a second layer of lath and plaster inside the exterior walls between the studs, spaced about 2 inches from the inside of the regular lath/plaster, and about ¾ inches from the sheathing. I’m told it was thought to provide insulation, and it gave novice plasterers a place to practice, since it wouldn’t ever be seen. The result is that it’s not as easy or cost effective to blow in cellulose. Contractors tell me they can’t easily drill and dense pack the 2-inch cavity from the outside; from the inside, it would be fairly messy (and difficult in tiled areas) , and the end result is only 2 inches of cellulose, with a ¾-inch bypass cavity in parallel.
I’m told Icynene foam is out of the question, because you’d have to use a low-expansion mixture to avoid blowing the interior plaster off, and it’s very difficult to apply properly. You would think someone would have developed a way to make foam work in this situation, but maybe there’s just not enough of a market. If you have any bright ideas, I’m all ears.
—Scott E., Brookline, Massachusetts
A. You see in the photo immediately above that the back plastering runs up and down. Thankfully they put the cross members—every 4 feet, the length of a standard lathe strip—to the exterior, so there is a clear, top to bottom 2-inch space to the interior. In this house we pulled four courses of cedar siding to gain ability to pull one 1-inch of white oak sheathing. The white oak was too hard to drill through! In the photo at the top right, Rachel is running a standard 1 ½” –inch winter grade tube up the second floor wall and will run the tube down the first floor wall, since we pulled the boards at the band joist. Now those band joist cavities can be stuffed and air-sealed. We did not treat the exterior space since it was narrow, just like yours.
Your may consider this option if you have similar access to the exterior.
Interior access is very tricky, but it can be done. We have used 1 ¼ inch blowing tubes, but the lathe can crack if holes are drilled on the same level. We drill low, below sight line, to hide patching behind the furniture. Some have made chair and/or picture rails to hide holes and plugs. If not tubing, 1¼ inch holes are drilled 3 per stud cavity, then filled with a directional nozzle.
So the end result is 2 inches of insulation, a real R7, plus a bit extra for airspace, and wood. But, if you address the band joist, you accomplish some very good air-sealing at a critical area. (The very end band joist cavities get blown front to end, completing a 360o sweep of the perimeter.)
Hope this helps!
Bob Pfeiffer works as a Building Science Technical Lead Trainer and provides in-field mentoring and technical assistance on the Wisconsin Energy Conservation Corporation (WECC)'s Residential Technical Services Team. WECC emphasizes a hands-on, production-based training approach, which means spending a great deal of time in and around houses.
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