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Thread: Dow Super Tuff-R

  1. #16
    In the trades Dana's Avatar
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    Jan 2009


    Quote Originally Posted by dimmi View Post
    Just finished reading this thread. Great info Dana. I have a couple of questions. I have a cement block wall above grade. This room is pretty cold in the winter. The wall was insulated with 2 inch unfaced fiberglass bat between the 2x2 furring that is nailed into the block, covered with plastic and 5/8" drywall. Ideally I would like to remove the wood strips and cover the wall with 2" xps foam and then studs and then drywall. However, i'm conserned on how to attach the wood to the block thru the foam since I can't see where the screws are going into the block (hollow part or web or mortar joint) and the damage to the block removal will cause. Other option is to leave the 2x2's in place and glue 1.5" xps in between (2x2's are really 1.5") and then drywall.

    If the drywall is touching the xps will is absorb the moisture coming thru and get moldy?
    Should I use 1" xps leaving 1/2" gap between the xps and drywall?
    Any real benefit to doing any of this verses just leaving the 2'" unfaced fiberglass and plastic sheeting?
    Any other ideas?

    By the way the drywall is down.

    Strip the existing furring and clean up any high spots in the mortar etc, and GLUE the XPS (use t & g or ship-lap variants, or 2 layers of 1" sheathing with the seams of the layers offset by a foot or so.) Seal the seams of the XPS with housewrap tape or duct mastic for a super-air seal. If you're trying to minimize the loss of room area, use 1x4 or 1x3 furring rather than 2x, and through-screw it to the block wall with 3-3/4 or 4" concrete anchor screws (Tapcon, or similar), 16" on center or so, then mount the drywall to the furring with 1-1/4" screws. Use a masonry-bit and a standard drill motor (not a hammer-drill/roto-hammer) for drilling the CMU. If you use the bit included in the boxes of Tapcons you won't damage the CMU or masonry.

    Since it's not bearing a lot of load, the mounting of the furring to the CMU is not critical, but you can't rely on adhesives alone, gluing the drywall to the foam, since the drywall is a code-required ignition barrier for the foam, and must attach to something structural with fasteners. By screwing the furring to the walls, it becomes the code-required mechanical connection to the structural CMU.

    Since it's above grade you could also use 2" foil faced polyiso instead of XPS without creating any problems, and get about R13 out of it (vs. R10 with XPS), maybe even a bit more due to the low-E foil facer and the 3/4" air gap of the 1x furring. Seal the seams with 2" aluminum duct tape (aka "FSK tape".) This would perform comparably to a 2x6 studwall 16" o.c. with blown cellulose or R21 batts.

    At the moment you have poly vapor barrier with the moisture-susceptible wood furring on the exterior. CMU wicks up rain & dew, then releases it when the sun hits it, causing VERY high moisture levels, even condensation on the poly in summer, and I'd imagine at least some of the old furring shows signs of mold or rot. With XPS or foil faced iso between then CMU and furring there's a vapor retarder between the CMU and wood, and the wood will stay at the temperature & humidity of the interior as long as you don't use any vapor retarders on the interior such as vinyl/foil wallpaper, or heavy oil paints. The 1.5" of compressed batt has an R value of only ~R6 (best case), but with the thermal bridging of the furring and the minimal R value of the CMU itself factored in the whole-wall performance is only about R4.5.

    Going from R4.5 to R10 or R13 you BET you'll be able to feel the difference, winter and summer!

  2. #17


    Thanks for the quick response. I took your advise and stripped down the existing furring and put up 2 layers of 1" XPS. I used Tyvek tape to seal the seams and Great Stuff foam for the surrounding gaps. The 2 layers worked better than 1. I was able to slip the 1st under the metal electrical conduit and electrical boxes and the 2nd layer was cut around it. Great Stuff filled the cuts around the conduit. I will put 1x3 furring on top of it. The 3/4" gap created between the xps and drywall is just enough space for the conduit.
    I am planning on placing the furring 24"OC. ( 4 screws to cmu per 8' strip) Since I am using 5/8" drywall I read that this is no problem. Agree?
    Also, 3/16" tapcon or 1/4" tapcon for attaching the furring?
    The tapcon instructions/website state that a hammer drill must be used on the cmu in conflict with your recommendation above. I tried a regular drill in the past and it has

    By the way. Know an easy way to cut a channel in XPS?
    Last edited by dimmi; 10-31-2011 at 03:39 AM.

  3. #18
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    I guess it depends on how tough your CMU is, but I've drilled hundreds (thousands? maybe not thousands...) of Tapcons into 1950s vintage CMU walls doing EXACTLY this type of retrofit using a low-speed drill motor rather than a hammerdrill. To be sure it would go quicker with a hammerdrill, and the odds of you cracking a CMU block is pretty remote either way.

    Since it's not really a structural loading issue, 3/16" Tapcons are fine, as long as they've penetrated the masonry by 3/4" and you have them at least every 16-24". I'm not sure if there's a real fastener spacing spec for it embedded in the code for the ignition barrier aspect, but you do want to keep your walls flat, right? :-) A couple of inches of moment-arm isn't going to put a huge stress on the CMU at each fastener with only 5/8" gypsum board.

    Setting the furring 24" o.c. with 5/8" wallboard is fine from all points of view (fire-code and wall-flatness).

    I've used a table saw to cut dados/channels in XPS before installing it. Not sure how I'd attack it if it were already up.
    Last edited by Dana; 11-01-2011 at 04:04 PM.

  4. #19


    Thanks again. Appreciate your help. ******* had 100ct boxes of 1/4, 3 3/4" long tapcons fo $14.99! I snapped up the last box.
    Last question. The 2" XPS is straight edge. You think it make a big difference verses tonge and groove?

  5. #20
    DIY Junior Member Brewcrew10's Avatar
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    Nov 2011
    Buffalo, N.Y.


    Dana, what would you reccomend for adding insulation to these walls. Previous wall surface was plaster and lath. Walls are approximately 5" thick of solid concrete with diamond mesh sheeting and another 2" of stucco on top of that on the exterior (although we have just furred out and sided the entire outside of the house). Currently the walls are furred out with 3/4" furring strips (which are attached to studs - appears the concrete was poured between the studs some how). After taking down the existing plaster and lath, which was right against the concrete walls, there appears to have never been a problem with moisture. Any help would be greatly appreciated.

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  6. #21
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    When there's no insulation the furring & lath stayed pretty much at room temp, and was inherently always above the dew point of the interior air. Stripping the old furring and setting it up just as dimmi's wall using rigid foam would be fine. How thick you go depends on your budget for foam & Tapcons. (The fasteners start to add up when you're looking at 7" screw or something. :-) ) If you can find a local source for recycled roofing foam it gets to be pretty reasonable to go with 3"-3.5" of iso (R18-R22), and 5" screws. (I did my basement this way using scavenged 3" fiber-faced iso @ $20/sheet for 4x8s.) With a solid concrete wall there's no getting around using a hammer-drill/roto-hammer.

    You also need to figure out how you're going to extend the window framing and electrical drops, etc in when going thicker. Be sure to make it air tight using low-expansion foam too- you don't want room air getting between the foam & wall and condensing there if you don't have to. Even though the concrete can handle the moisture, the subfloors & joists won't necessarily. The embedded studs in the concrete will be colder and more susceptible too, but it's primarily air-leaks not vapor diffusion that would cause a problem. In fact, using only relatively vapor-open foam is probably the right way to go- no more than 2" if XPS, 4" of EPS or fiber-faced iso, and definitely not foil-faced goods. (As a sanity check, fiber faced iso is usually somewhat more vapor permeable than the kraft facers on batts.) Air tight with sealed seams & edges is key. In the summer there could be fairly high moisture drives from the exterior from solar-heated damp concrete- you HAVE to let that moisture come through the finish wall, at least a little bit, to help keep those concrete-clad studs dry & happy.

    Dimmi- as long as you seal the joints with foam or tape it's not a big deal to use squared edged goods.

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