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Thread: Stud a wall over 2" rigid insulation

  1. #1
    DIY Junior Member faffy's Avatar
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    Default Stud a wall over 2" rigid insulation

    I have a concrete slab wall that I am putting 2" rigid insulation against and then studding a wall over the insulation. Of course the concrete wall goes in and out and I have my floor plates in place already so there are some areas where the 2" fits nicely in to place behind the floor plates and I can stud with no problem. Other places the insulation sticks out which in turn makes the stud come out further. Has anyone ever cut grooves in rigid insulation and any suggestions how to do it easily? This will allow me to sink the studs further back in the insulation.

    I know I should have come out further from the wall to compensate for this but I didn't so I am now stuck dealing with it. Luckily, I only did one wall so I know what to do on the rest of the walls.

  2. #2
    Test, Don't Guess! cacher_chick's Avatar
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    I would not cut the insulation. You want a thermal break, which means the framing should not touch the foamboard. Move the wall in another inch or two if you need to.

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    Moderator & Master Plumber hj's Avatar
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    Slice it with a knife, then dig the insulation out to make a groove for the stud.
    Licensed residential and commercial plumber

  4. #4
    Test, Don't Guess! cacher_chick's Avatar
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    Cutting recesses in the board reduces the insulation value right down to the point that installing it was a wasted effort.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    As this is not a load bearing wall, you don't have to use 2x4 studs...you could substitute thinner ones where required. Just keep the front edge flush with the outer edge of the plates.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Palm Router.

    Will remove the material so fast.

    Jig Saw would work but a little rough.

    I like Jim's idea the best if the wall is not getting tile.

    HJ's will work but is a lot of work.

    Would total disagree with what Cacher said about it being a wasted effort. Even 3/4" of foam is going to make a big difference in the warmth of the room.

    JW


    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

  7. #7
    Test, Don't Guess! cacher_chick's Avatar
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    Quote Originally Posted by johnfrwhipple View Post
    Palm Router.

    Would total disagree with what Cacher said about it being a wasted effort. Even 3/4" of foam is going to make a big difference in the warmth of the room.

    JW
    My point was that if you cut a trough 1/2 way through the board, the insulation value of the entire board is greatly reduced. I think it better to change the plan to maintain the full value of the insulation.

  8. #8
    In the trades Dana's Avatar
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    johnfrwhipple wins the Kewpie doll!

    Routers make fast work of it- but attach a vaccum hose to it or you'll be sweeping up insulation bits for the next three weeks.

    The primary purpose of the foam at the stud edge is the thermal break, and even if you cut a full inch into the 2" foam it's more than doubling the R-value at the stud, and has NO effect on dew point/condensation issues in the cavity. The secondary (but still important) purpose is as a vapor retarder and capillary break to prevent moisture from the concrete getting into the stud at a faster rate than it can handle.

    The net effect of cutting in 1" channels for even a dozen 8' studs on whole-wall R is negligible- it's one square foot of the total foam reduced from R10 to R5. The other hundreds of square feet are still at full-R. This would be less than a 1% hit in whole wall performance in most cases. I'm betting that the depth of the cut is less than an inch in most places, even for the sloppiest-waviest of poured concrete basement walls.

    If you're insulating the studwall cavites with batts for higher-R (recommended), use (unfaced only) batts thick enough that it compresses all the way from the foam to the gypsum or it will lose performance to convection currents. Standard R13s are nominally only 3.5", and if there are studs with even 1/4" of air between the stud and foam it's better to use unfaced R19s and compress them. (Compressed to 3.5" an R19 batt performs at R13, according to both Certainteed and Owens-Corning compression charts.)

  9. #9

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    Quote Originally Posted by Dana View Post
    If you're insulating the studwall cavites with batts for higher-R (recommended), use (unfaced only) batts thick enough that it compresses all the way from the foam to the gypsum or it will lose performance to convection currents. Standard R13s are nominally only 3.5", and if there are studs with even 1/4" of air between the stud and foam it's better to use unfaced R19s and compress them. (Compressed to 3.5" an R19 batt performs at R13, according to both Certainteed and Owens-Corning compression charts.)
    +1

    Cheaper than the HD batts as well.

    Dana going shade tree style.

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