concrete wall insulation
Bill37
| Posted in General Questions on
I’m renovating a home made of old concrete blocks. I’m in the insulation stage and need assistance regarding the first-floor walls; they are made of concrete blocks with 2×6 studs 1/2 inch from the concrete wall. What is the best way to insulate the walls? Do I put a vapor barrier between the concrete blocks and the 2×6 studs and add faced fiberglass batts in between the studs? Or should I not use a vapor barrier between the concrete wall and the studs and just put faced fiberglass insulation between the studs with the paper facing the drywall? The 2×6 studs are not pressure-treated.
I’m in Pennsylvania, so it’s a cold climate.
please see image attached
Thank you!
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Replies
Check out these articles.
https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
does it make a defense if its basement or first floor? also, i already have the framing installed and don't want to remove it to install foam board.
Yes, basement vs. 1st floor makes a different because detailing is different below vs. above grade. Best practice would be to remove the framing and put up the foam board, but maybe you could also slide 1/2" polyiso boards behind the framing and tape the seams and tape all to studs? Seems like taking down the framing is the best way to go for a long - lasting detail. Maybe another way is to cut the sheets of foam insul. to fit the studbays and then use canned foam to seal them into the stud bays and seal the bottom plate and top plate.
Or just closed-cell sprayfoam in the cavity and use batts behind it. And you definitely want at least your bottom plate to be pressure treated if you have no separation between the framing and the concrete. But again - if you take the framing down you could add a gasket or piece of rigid foam below the bottom plate and ensure a well done and safe detail.
Either way I think best practice calls for following one of the articles or many retrofit details in the detail library. Putting in the work to do it right will be worth the time it takes to redo the framing, and you should be able to reuse all of it and "fix" it pretty easily.
I think this might be a good scenario for a "flash & batt" system ...
2-3" of closed-cell, spray foam (R12 to 18) - followed by a batt of your choice to get the Rvalue you are looking for.
This is Option #1 in the "Three ways to Insulate a basement wall" link above ...
i understand this is the best way to go, but is it acceptable to not use a foam board or spray foam, leave the concrete block 1/2 from the studs and just put faced fiber glass batts in the studs cavity and drywall? will it guarantee issues? cause that's what my architect is telling me to do and what the plans call for. thank you.
Will putting a foam board and rain screen behind the exterior siding be a solution? Or it doesn't solve the interior issue?
Yes and no. If all the basement concrete walls you’re insulating are above grade, you should be ablue insulate all to outside provided they’re detailed properly. See this Q&A thread here that answers your question.
https://www.greenbuildingadvisor.com/question/on-concrete-walls-that-are-fully-exposed-above-grade-for-a-new-residence-should-there-be-an-air-space
I referring only to first floor concrete walls, not basement.
If using 1/2" polyiso boards, should I have the silver side facing the concrete block or the interior? also, Should I use faced or unfaced fiberglass batts in the studs cavity?
Thank you!
Above grade masonry is not the same a foundation. It gets much colder and you also have to deal with solar moisture drive. You can read through the various options here:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-105-avoiding-mass-failures
I've had double brick wall turn into a moldy mess when I tried to insulated with interior rigid.
The one guaranteed to work are exterior insulation or interior spray foam. With either option, once you have enough insulation for condensation control, the rest of the insulation can be fluffy.
In your case, if you are planning on siding, the simplest would be the exterior rigid. With 2x6 walls, in guessing zone 5, you need R7.5 of exterior rigid.
If you go with the spray foam route, I would aim for about 2" of SPF, this is more than you need for condensation control but leaves you just enough space for a 2x4 batt in the wall.
With either assembly, you don't want any poly vapor barrier anywhere. Your stack-up would be:
rigid with WRB either over or under it, depends on where your windows are
masonry
2x6 stud wall with fluffy insulation
drywall
or
masonry
2" SPF
R14 batt
drywall
does it make a difference which foam board? eps, xps or poly ? do i need a rainscreen if the siding is vinyl? the fiberglass batts insulation doesn't need to be faced? and is it ok for fiberglass insulation and 2x6 studs to be 1/2 from the masonry wall ?
also, my contractor wrapped the house with tyvek, but he put wood strips under the tyvek directly on the masonry wall, is that ok?
The type of exterior rigid doesn't really matter, whatever the cheapest you can easily find.
You don't need a rainscreen for vinyl but you will need something to nail the vinyl to.
It doesn't matter if the insulation is faced or not as long as the facing is not poly or foil. Kraft facer is fine. The insulation should be tight against the masonry. A gap directly behind the drywall is not an issue.
The housewrap over the wood strips doesn't work, if you are installing rigid it needs to go. If you don't want to redo the strapping, you can cut the Tyvek, install 3/4" thick rigid between the strapping followed by another layer of rigid over the strapping to bring it up to R7.5. You can then nail the vinyl siding through the outside layer into the strapping.
You can re-install the Tyvek over the rigid or if using faced rigid insulation can be taped to work as a WRB. Your windows should be at the same plane as your WRB.
The issue I have is that the masonry wall is not 100% leveled, so he used the wood strips to get it straight. The windows are flashed with the masonry wall, so the Tyvek needs to be on the masonry wall. In that case, I need to have the rigid board over the Tyvek. What should I do? Thank you for being so helpful!
Okay, thank you. Just to confirm, I'm removing the current Tyvek and shims from the masonry wall.
The correct order from outside should be
Vinyl siding / furring strips/ tyvek / unfaced foam board minimum r6.3/ tyvek / masonry wall/ r 21 batts/ drywall?
Understood, thank you. I'm in climate zone 6, is r 7.5 sufficient? Which foam board do you recommend for the exterior? eps, xps or polyiso?
I currently have a polyiso foam board over a vapor barrier in the enclosed crawl space. Is it okay to have an external rigid foam board on the first floor and an interior foam board in the crawl space?
In zone 6 you want a bit more, R11.25 in case of a 2x6 wall.
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
With craft faced batts the insulation ratio can be reduced though so up to you if you want to install that much. Around me (zone 5 and 6) and walls with R5 exterior rigid are code and work fine.
Interior rigid in a crawl is perfectly fine. It works there just as well as in a basement. It doesn't matter what it on the first floor.
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I'm using R21 Karft batts on the interior, so can I use the R5 foam board on the exterior instead of R11.25?
With R21 you need a bit more in colder climate, so R6.3. If you are on the warmer end of zone 6, you can go with R5.
In either case, I would go for one of the more permeable foam options such as the perforated GPS or unfaced EPS/XPS. This would allow for some additional drying to the exterior which is always a good thing.