Comparing Two Wall Assemblies Based on Costs
I’m planning a renovation for a 110 year old house in Southern VT (zone 6). The house has cedar clapboards with board sheathing and no WBR. Exterior walls are 2X4 7/8″ studs and have never seen insulation.
I’m looking for advice on the relative costs (material & labor) for two potential wall assemblies for general comparison, plus any thoughts and feedback on these options in general.
In either scenario interior surfaces and windows will be replaced. We’d inspect for major water problems and fix them. Old window weight pockets will be foamed in. New windows to be <=U0.17 and don’t represent an outsized % of the wall area. We’d add flashing, etc. at the windows as feasible. 3 stories – 3rd floor under the rafters. Nice big overhangs at the eaves so we have some protection of the siding if we keep the exterior as is or room to add exterior continuous insulation if we go that route.
Option 1 – use spray foam and avoid major exterior work
Outside to inside:
Keep the existing clapboards and sheathing.
Install dimpled house wrap on the back of sheathing, stapled to studs, for some moisture management/insurance… maybe help the paint stay on.
2x2s furred to interior of studs horizontally to extend existing cavity depth to 6 3/8.
HDSF to fill cavity and 1″ in front of the old studs for a thermal break. Foam depth of 5 7/8″ with the last 1/2″ of the cavity as air assumed behind the drywall since it’s HDSF.
No additional vapor barrier.
I believe this total wall assembly (excluding windows) ends up around R29 (U 0.34) and would meet alternative path code requirement of U0.45 or less. (~R14 framing path, ~R39 insulation path, 20% framing factor).
Pros: One solution for air and vapor control. Avoids complexity with exterior insulation. Retain historic character.
Cons: Higher risk of water issues. Paint retention may be a concern. Lead paint still there to deal with.
Option 2 – No-spray foam route with exterior insulation
Replace the clapboards and trim, freeze boards, etc. and add exterior CI and proper WRB/AIR barrier, flashing, etc.
Outside to inside:
New fiber cement or cedar clapboards
3/4″ rain-screen gap / furring
3″ of Rockwool comfortboard 80 OR recycled EPS
WBR (Henry blueskin)
Densepack / spray cellulose in the existing 4 7/8″ cavity.
Vapor retarder? – maybe smart membrane, maybe just paint the drywall and call it good? (Seems like I can use a class III VR since the mix of interior and exterior CI should provide the right hygrothermal management and this assemble can dry both ways)
Perhaps… we also use aerobarrier before we fill the cavities to improve the air barrier vs. relying only on the blueskin with an old structure and tricky details at transition points.
I think total wall assembly (excluding windows) ends up around R24 or 25 (~U 0.4) and would meet alternative path code requirement of U0.45 or less. (R17~18 framing path, R27~28 insulation path).
What do you think of these options?
Can you suggest sources where I can find information to estimate per sq ft install costs?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part