Adding rigid foam to interior?


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Old 12-15-23, 10:21 AM
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Adding rigid foam to interior?

I have a 1920ís home built with 2x4 exterior walls. We are doing some renovating and have the drywall down in one room and was considering putting up 1/2 or 3/4 inch of rigid foam on the inside of the exterior walls before drywalling. Is this worth the cost and hassle? Also if I do that would that mean I donít need vapor barrier? Also I am in a cold climate. All opinions are welcomed - thanks!
 
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Old 12-15-23, 12:24 PM
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Foam works as a thermal break if it is thick enough, continuous and with all seams taped. Without being able to get to the wall outside of the studs to accomplish this (like installing in an unfinished basement before erecting the stud wall), I don't see any benefit to foam with fiberglass or mineral wool over it - I would just install the fiberglass or mineral wool.
 
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Old 12-15-23, 01:01 PM
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no - The wall cavities are already filled with batts - I am talking about adding the foam before drywalling on top.



 
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Old 12-15-23, 01:34 PM
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When I converted a walk-up attic to a bedroom suite in 1986, the wall spaces between the studs were filled with fiberglass batts (no vapor barrier.) Then taped 1 inch foil-faced foam insulation boards were attached to the studs, and drywall screwed over that. Long drywall screws were used to screw into the studs. The insulation value of the wall is the total of the materials used.

Since we were building the area from scratch, the electrical boxes were positioned to be flush with the finished wall. Any existing boxes you have will have to be relocated or have extension rings installed to bring the front edge of the box flush with the drywall.
 
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Old 12-16-23, 02:18 PM
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"wall cavities are already filled with batts - I am talking about adding the foam before drywalling on top"

Personally, I don't see this as worth the hassle or expense.
 
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Old 12-17-23, 07:33 AM
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Personally, I don't see this as worth the hassle or expense.
I agree unless you are superinsulating to achieve a Green rating or zero-energy house.

In my case since it was an attic conversion we were offsetting the direct heat gain/loss from the roof as well as the walls.
 
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Old 12-22-23, 03:19 AM
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I'm a little late to the party. But around here (Minnesota), by code a vapor barrier has to have a perm rating of 0.1 or less. You have to be very careful what rigid foam you use, not many qualify as a vapor barrier. Frankly, it's easier and cheaper to just buy the 6 mil poly and not worry about what kind of rigid insulation you're using.
 
 

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