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Thread: Dow Super Tuff-R

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  1. #1
    DIY Member jdon88's Avatar
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    Default Dow Super Tuff-R

    Anyone use these Dow insulation boards for interior walls? I'm thinking about using them in my place. 60 year old Pennsylvania house, exterior brick face over blocks. Currently no interior insulation between block wall and drywall, just 3/4" wood strips between drywall and block. I'm thinking about gluing these foam boards between the wood strips then attaching drywall to the strips. Any issues with moisture being trapped causing mold? The rooms are above ground. I've read some forums where people don't like attaching these boards directly to masonry walls, causes mold, but they are usually referring to basement walls. I've contacted DOW but they say it is OK but they recommend air space between the foam board and drywall.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    You would want a vapor barrier in the wall. The only way to get an air space with 3/4" furring strips would be to use 1/2" panels and they would probably have such a low R value that they would be almost worthless.

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    In the trades Dana's Avatar
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    Foil-faced iso like Tuff-R IS a vapor barrier, which can be both good & bad, but in this case maybe good, but it depends. Is there's a cavity/gap between the brick face and the block wall? How are the floor/ceiling joists mounted to the block?

    Mold & moisture problems are only created when moisture can get to the wood, but can't leave. Before insulating, air leaks and vapor diffusion through the gypsum on the interior reaches the cold block wall and condenses when the block is below the dew point of the interior air (typically 35-40F, depending on how warm & humid you keep it indoors), but the block can wick it in without damage, and releases it quickly when the block warms up. After insulating between the gypsum & block the block stays colder (much much colder), and as it's humidity rises, puts the furring and joist connections at risk. With a good air seal on the foam the volume of diffusion & air transported moisture is small, and if there's a vented cavity between the block and brick moisture will be released to the outdoors over time, even in winter. With a cavity-wall the overall performance would be enhanced with an interior vapor-barrier (like foil-facers on Tuff-R), but with a solid masonry wall moisture drives from the exterior during the spring/summer/fall could be much worse than the winter drives from the interior through 3/4" of XPS. While XPS allows more moisture into the block via vapor diffusion in winter, it allows several orders of magnitude better drying toward the interior. Since the block is mostly impervious to the wintertime moisture drives (unlike wood-sheathed buildings).

    A typical brick-veneer CMU block wall comes in at a whopping ~R2-2.5 if uninsulated, and they leak air. If the block wall is hollow CMU you can typically double that by filling the hollows with a vapor permeable non-expanding injection foam such a Core Fill or Tripolymer. That would be step-1, bringing the wall's R value up to R4-5, which is only about half the R value of a typical 2x4 studwall with R13 batts, but WAY better than R2.

    Then...

    With a vent cavity between the walls you can safely add 3/4" foil-faced goods between the furring. Without it, use 3/4" XPS (pink or blue board) insulating sheathing, which is semi-permeable to water vapor to allow the block wall to dry toward the interior, lowering the risk to any wood in contact with the block. With either type of foam board you may still have to leave 6" clearances around joist ends to allow sufficient drying depending on how it's attached, but the overall risk is much lower in a cavity-wall than a solid type.

    Cut the rigid foam for a loose fit and glue it to the masonry using blobs of foam board construction adhesive, and use 1-part can-foam (Dow Great Stuff, or similar) to seal the edges of the foam-board to the furring. Trim the can-foam flush with the furring with a knife or saw after the foam has cured.

    Alternatively you could inject the same vapor-permable non-expanding foam into that mini-cavity as well as to the CMU cores, if the installer thinks they can do it all without stripping the gypsum off the studs. The vapor-diffusion would be about 5-10x as high through the injection foam as through XPS, so this would be best done on cavity-walls only, since cavity walls dry toward the exterior so much better.

    With 3/4" of iso (Tuff-R) you'd be adding R4-5 to the center cavity R value, but only ~R3.5 to the whole-wall R value (whole-wall is with the thermal bridging of the furring & framing factored in.) With 3/4" of XPS (typically cheaper), you'd be only adding R3.3-3.8 center cavity, but the it would add ~R3 to the whole-wall R (as would a non-expanding foam injection into that cavity.)

    With both a CMU core fill and 3/4" of rigid foam on the interior you'd be bringing the whole-wall R up to something like R8, (nearly that of a batt-filled studwall) but unlike the typical batted studwall it would nearly air-tight, and the conducted heat loss would be 1/4-1/3 of what it is before treatment. It would be enough to FEEL the difference, and would outperform 2x4 studwall built to "typical" standards as opposed to "best practices, air-tight".
    Last edited by Dana; 04-20-2011 at 11:47 AM.

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    DIY Member jdon88's Avatar
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    Default

    Thanks, thats a lot of info to digest, I need to read it a few times. To answer some of your questions:

    I don't think there is a gap between the brick and block, I'll see if I can find out for sure. The wall I'm working on right now has the floor and ceiling joists running parallel to the block wall, there is about a 4" gap between the joist and wall. Looking from below, the ends of the floor joists are sitting on top of the basement block wall which is about 4" wider than the 1st floor brick/block combo.

    I don't think I'll go with filling the hollow CMUs since I'm not exposing all the walls, just a few rooms one at a time, and it sounds like at lot of effort/expense to do. I may just play it safe and leave out the insulation, I'd rather pay more for heat/cool then risk mold and have to tear it all out again in a few years.

  5. #5
    In the trades Dana's Avatar
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    See if you can't access the wall from the attic to determine whether it's a cavity wall vs. solid. Often the cavity is ventied/open to a vented attic as the means of keeping the wall dry. If the CMU cores can be foamed from the top get quotes. It may be subsidized, and even if not it may be pretty cheap if there's no driling & patching to do.

    At the very minimum 3/4" of XPS (extruded polystyrene) between the existing furring is very low risk, and cuts the heat gain/loss by more than half. Better still would be to pull the existing furring and glue up full sheets of 3/4"-2" of XPS (taping the seams with housewrap tape, and foam-sealing the edges), giving up a very few square feet of interior room area. To hang the drywall cut down 3/8" ply or OSB through-screwed to the CMU (horizontally or vertically) for hanging the 1/2" min wallboard. Alternatively use thinner full-sheet OSB and thinner gypsum (1/2" stackup min), which still meet code as an ignition barrier on the foam.

    With all of the furring inside the XPS it's at the same temp & humidity as the room, with zero mold risk. Without furring bridging the insulation you get pretty much the full-R, and by using full sheets rather than cut'n'cobble it goes up quicker, uses less foam (less waste), and produces a superior result overall. The full-sheet approach is also lower risk than not insulating, because it puts a vapor retardent insulating layer between the cold masonry and the mold-susceptible wood & paper materials.

    Passing up the opportunity to insulate at this time would be crime. It's more than just fuel use- the wall temps will be 5-10F warmer when it's wicked cold out, and it'll be more comfortable even at the same interior air temp. If you don't insulate now you'll be kicking yourself as fuel/utility prices rise, or when you sell the place. This is the cheapest/best time to do it.

  6. #6
    In the trades Dana's Avatar
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    It occurs to me that you may have fairly easy access to the upper floor CMU cores from the basement, which would make a retrofit foam job of the cores pretty easy even if the CMU walls have plates at the top for supporting the rafters. If you google "cmu core insulation foam" you'll find a number of manufacturers and installers.

    The mold risk of a CMU core-fill is zero to negative (that is to say, at worst it's zero, but most of the time it lowers the mold risk issues.) Some non-expanding foam manufacturers & installers are pushing the notion that cavity walls can be filled as well, but don't buy into that. Even though it further lowers the risk of mold, it increases the risk of damage to the brick veneer, particularly in cold climates with freeze/thaw cycles, such as PA where spalling of the brick facing and efflorescence/degradation of the mortar would be increased when the bricks can't dry into the cavity.

    During a core fill there will be some seepage through gaps/cracks into the cavity, but an accidental full cavity fill is extremely unlikely, but if that happens it's not going to affect the whole wall, and a masonry sealer would mitigate any downside. A once/decade treatment of the exterior with a silane/silex based masonry sealer is a good idea independent of the status of the cavity (or whether it even exists), since that reduces dew & rain wicking of moisture into the brick, while decreasing the exterior drying rate only somewhat. The drier the masonry, the lower the risk to wood in contact with the masonry.

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