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KISS, exterior wall insulating plan?

Problem: My recently painted 1920's Craftsman in zone 5 has cedar lap siding over disintegrated asphalt paper over 3/4" lap sheathing, 2x4 studs, no insulation, LOTS of ventilation.

Plan: From the inside, fur out the walls 2", staple Tyvek against the inside of the sheathing and on studs, R21 Batting in bays then 1" XPS and 1/2" drywall onto studs. Simple right? Any foreseeable problems or alternatives with this?

Asked by James Anderson
Posted Tue, 12/17/2013 - 18:49


1 Answer

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If you fur out the studs to 5.5" to accomodate the R21s you've missed the opportunity to do some effective thermal breaking using a "Mooney wall" approach, where the furring runs lateral to the studs, with fiber in both the 2x4 cavities and the furring cavities.

But there may be an issue with any batt solution, if (like MY 1920s bungalow) the framing uses full-dimension lumber. In those installations even if 16" o.c. the width of the batts is too wide for a perfect fit, creating a propensity to buckle creating a gap. Worse still, the full-loft of an R15 isn't quite 4". This can be overcome to some degree by compressing an R19 into that space, but it's still less than ideal.

In a Mooney wall you get a decent amount of thermal breaking over the studs with the fiber at the depth of the furring. If you used 2x2 furring you'd have 1.5" of fiber, which is worth about R6 if using rock wool or HD fiberglass, R5.5 if blown cellulose, making it comparable in performance to your interior foam concept.

If you DO add interior foam, use EPS or polyiso. Iso has higher thermal performance than XPS at the same thickness in this application, and uses a much lower global warming impact blowing agent (about 200x lower GWP!). EPS is the same polymer as XPS, but uses the same blowing agents as polyiso.

With lap sheathing it's worth air sealing it from the interior prior to adding the fiber. One method is to use #15 felt stapled from the inside, sealed at the felt edges with caulk or foam, which puts a cheap but "smart" vapor retarder between the sheathing and the planking. Carl Seville did the cavity fill in open cell foam, which air-seals as it expands- you'd have to use caulk or can foam:

With a #15 felt air barrier in place the air leakiness of the siding & sheathing are a plus- it allows the siding and sheathing to dry fairly rapidly to the exteirior without contributing much to air infiltration losses- it's definitely "worth it". While not quite the protection of a "rainscreen", the effects are similar.

Answered by Dana Dorsett
Posted Tue, 12/17/2013 - 19:10

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