Options to reduce construction cost and retain wall performance
“Value engineering” wall insulation options
by Tim Oldfield
We are in the Boston area, Zone 5, I believe. The exterior wall section we designed for the renovation of our one-story house, built in 1950, was for existing and new 2×4 walls filled with dense-packed cellulose insulation in the stud cavities, 1/2″ plywood sheathing, a fully adhered W. R. Grace Vycor enV-S weather restrictive barrier, covered by 2″ of polyisocyanurate board insulation, furring strips, 3/4″ wood siding, and “outie” fiberglass windows. The interior will have 1/2″ GWB with veneer plaster and latex paint, no other vapor retarder or vapor barrier. We recently received final pricing updated from the preliminary pricing of a year ago, and the overall price has gone into the stratosphere. Apparently, this is fairly common these days in the Boston area where the construction industry is very hot. We are working with our contractor to reduce costs in all aspects of the house design, the exterior walls included, and I wanted to see if the GBA community might offer other solutions we might miss.
I would like opinions on making the new exterior walls (approx. 112 linear feet) of 2×6 studs with dense-packed cellulose, 1/2″ plywood sheathing, a fully adhered W. R. Grace Vycor enV-S weather restrictive barrier, covered by 1.5″ of XPS board insulation (R-7.5), furring strips, and 3/4″ wood siding. Some have recommended the new walls of 2×6 studs with dense-packed cellulose, Huber Zip System R-Sheathing 1.5″ panels (R-6.6), furring strips, and 3/4″ wood siding. I am concerned that this Zip system is not suited for our Zone 5 because: the R-value is below 7.5 for a 2×6 wall, the polyisocyanurate insulation layer would be on the interior side of sheathing instead of the exterior side, and that sheathing is just impregnated OSB that would be screwed through 56 times times in every 4×8 panel, compromising its effectiveness as an air and water barrier. To economize on the existing 2×4 walls (approx. 77 linear feet), I am thinking of filling the stud cavities (which have 2-cell Infra radiant barrier aluminum foil insulation from 1950) with dense-packed cellulose or open-cell spray foam insulation, installed from the inside and not replacing the existing windows or adding the recommended R-5 sheathing for 2×4 stud walls and then needing to replace the siding. We also have approximately 62 linear feet of insulated window walls facing south, and approximately 58 linear feet of an 8″ CMU wall facing east. Your thoughts? Can you recommend any other cost effective wall sections to achieve a similar R value (approx. R-26) in the exterior stud walls? Thanks for your input.
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