Stud a wall over 2" rigid insulation

Discussion in 'Remodel Forum & Blog' started by faffy, Mar 31, 2013.

  1. faffy

    faffy New Member

    Messages:
    1
    Location:
    Pa
    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. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,244
    Location:
    Land of Cheese
    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.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,491
    Location:
    Cave Creek, Arizona
    Slice it with a knife, then dig the insulation out to make a groove for the stud.
  4. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,244
    Location:
    Land of Cheese
    Cutting recesses in the board reduces the insulation value right down to the point that installing it was a wasted effort.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,896
    Location:
    New England
    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.
  6. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

    Messages:
    3,952
    Location:
    North Vancouver, BC
    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
  7. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,244
    Location:
    Land of Cheese
    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. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    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. Windows on Washington

    Windows on Washington New Member

    Messages:
    33
    Location:
    Washington, DC
    +1

    Cheaper than the HD batts as well.

    Dana going shade tree style.
Similar Threads: Stud wall
Forum Title Date
Remodel Forum & Blog Pipe protrudes past studs entire length of wall Apr 13, 2013
Remodel Forum & Blog Safe to cut out stud from this wall? May 26, 2012
Remodel Forum & Blog Best way to build stud wall where joists run parallel? Jan 25, 2010
Remodel Forum & Blog Dumb question about stud walls Sep 18, 2009
Remodel Forum & Blog Wallboarding - 24" on center studs x 33" Jul 8, 2007

Share This Page