Rigid Foam on Interior Side of Wall for 1960’s Home Renovation
Hello – I am working on a renovation of a 1960’s home in climate zone 6A in northern NY. My current wall assembly is comprised of cedar exterior siding, tar paper, 3/4″ sheathing, and 2×4 walls filled with fiberglass insulation. The exterior siding is in good condition, so we are evaluation options to boost R value in the wall without continuous exterior insulation. One added detail is that we have hydronic baseboard heat on all exterior walls, so adding interior depth to the wall requires moving the baseboards (which we are willing to do).
Our current plan for the wall assembly is below, but would appreciate any criticism or additional idea’s to improve R value and performance of the original wall assembly.
– Keep exterior siding, tar paper and sheathing untouched
– Air seal stud bay cavities with caulk at all intersections of studs and sheathing, between stud and subfloor, and between any double studs for headers, etc.
– Rockwool R15 comfort batts in 2×4 studs
– 2″ Rigid foam on interior side of wall (foil faced on both sides has been recommended)
– 1″x3″ strapping
– Drywall
Thank you,
Matt
Replies
If you add foil-faced rigid foam sheets inside the studs, you can airseal there rather than doing a lot of caulking in your stud wall. The foil-faced foam will be a water and vapor barrier. Just tape all the joints between your foam panels, and caulk at the top and bottom. Air seal any ceiling penetrations also, and add any insulation you can above the ceiling. That's the biggest bang for the buck.
Thanks on the recommendations. Would certainly save time to airseal the foam panel versus each stud bay. I have a cathedral ceiling in 90+% of the house so limited options to add insulation until we re-roof, but will look at adding to the limited space we can reach.
Attic insulation usually is the biggest impact.
Get a pro foam gun and use it to seal around the foam boards. This will be more effective than trying to caulk all the edges inside the wall assembly.
While you are moving baseboard heat, take time to seal and insulate the rim joists. do so for the first floor from the basement, and if you have a second floor, make sure it also is taken care of while you have the edges of the ceiling open. spray foam around foam boards does a pretty good job. If the basement is unfinished, make sure to cover the foam with fire resistant material.
Thanks for the comments. Sealing/Insulating the rim joists is in the plans, and will try and insulate the attic in the limited amount of space we don't have cathedral ceilings.
https://www.finehomebuilding.com/2012/05/17/insulating-cathedral-ceilings
FH has some good content on insulating cathedral ceilings.
I’ve done the same thing with my 1840 house, but for a different reason. I added polyiso insulation on the interior because the roof overhang would have been reduced too much with 2” of exterior insulation. New fiber cement clapboards over 1/2” rain screen, tar paper wrb, new osb sheathing, rock wool batts in full 4” bays, 2” polyiso nailed to studs with roofing nails (foamed in place, floor and ceiling, and piece to piece, and taped), no strapping, drywall screwed through polyiso to studs.
Thanks, eddo324 for the details. How did you handle outlet boxes in your assembly? Just use box extenders?
Yes, extenders, for most. And I was a bit anal so I taped the area where the extender attached to the box, from the inside. Caulk would probably work too. For other areas I had to install new baseboard heat as part of the new boiler installation, so I used surface mount boxes on top of that for a couple of new boxes. I figured the baseboard was already protruding into the room that a couple of surface mount boxes wouldn't stand out.
I did similar to 1900 farmhouse in northwest Massachusetts: removed interior plaster and lathe on exterior walls, cut and cobbled real 2x4 cavities with unfaced polyiso, taped and caulked 3/4" foil faced polyiso across studs as air barrier and thermal break, then built new 2x3 walls and filled with batt insulation and covered with sheetrock. We used recycled polyiso, which cut the cost and eased our conscience. The new interior 2x3 wall acts as service for electric and plumbing and avoids penetrating the air barrier.
Are you going to use fasteners to attach the rigid foam to the studs? I used cap nails with tape over the heads. I was surprised how much they light up on the thermal image in the winter. I'll rethink my plan next time. Curious what others do.
And that's why I don't own a thermal imager. :-) In my case I used roofing nails, but only enough to tack it in place. The drywall screws through the foam into the studs did the hard work. I'm sure you could use adhesive with temporary screws or bracing, but there's only so much time in a day. I imagine that the roofing nails don't show up any more thermally than the necessary drywall screws.
apexjim | Nov 29, 2022 06:31pm | #12
Ha ha, truth. I agree, the fasteners serve a temporary purpose. In my case, I suspect that the nail hole in the insulation wallowed out while cap nail was being driven. Cap nails can be a little tricky to install cleanly, especially if space is limited (the cap limits your view of the nail's trajectory, they bend easily, and hand driving is probably more prone to unnecessary movement). Using a nail gun probably a better way to go.
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https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall
Worth having a look