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What we learned from updating a 16-year-old deep-energy retrofit. Reprinted with permission from Fine Homebuilding, February/March 2012, pages 55-59.
Update: Correction (download "Correction" .pdf here)
Update: Letter to the Editor from Fine Homebuilding, April/May 2012, page 14
Previous articles in FHB detailing rain-screen walls have called for strips of felt between the furring strips and siding, but not this article. Are they necessary? Plus, the author used 1x2 furring strips. Can thinner stock be used? Also, how important is that trim be furred off the walls?
Executive editor Daniel Morrison replies: I've not seen felt strips specified between the furring strips and the siding except at joints in siding to direct water that may get behind the siding back to the surface. This is a good practice regardless of whether you build a rain-screen wall. Second, Joe Lstiburek's minimum thickness for furring strips, which is based on Canadian building codes, is about 3/8 in. When it comes to furring the trim off the wall, the corner boards on Joe's original barn retrofit may offer some clues. They were not furred out in such a way that there was air ciculation behind them, and Joe paid a price for it: The paint didn't hold up. The guys from Synergy Construction who do the work for Joe prefer to use solid backing behind corner boards because with such thick foam, it's hard to angle the screws or nails into solid framing. They use wide strips of 3/4-in. AdvanTech subfloor as backing. To make up for the lack of back ventilation, they used plastic trim boards from Azek this time around, which will hold up better than wood.
Joe Lstiburek's response: Dan and I have a minor disagreement here. In my view there is no need for strips of felt between the furring strips and siding. Plus for the record I used 1x4 furring. Thinner furring would not work structurally for this assembly. A minimum 3/4-inch thickness of wood is necessary for withdrawal strength for the siding attachment. In theory you could go to 1/2-inch strips of plywood - but why worry about a 1/4-inch? The 3/8-inch is for minimum drainage and back ventilation and would work for a 1 inch thick layer of foam sheathing where the 3/8-inch strip is just a spacer and the siding is attached through the foam, through the furring and directly into the studs.