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OK to put foam board over top of fiberglass batts?

Post in 'DIY and General non-hearth advice' started by rmcfall, Nov 5, 2007.

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  1. rmcfall

    rmcfall Feeling the Heat

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    I've got two types of walls in my house--stud walls and block walls. With the block walls, I am going to place foam board up against the masonry, then build 2x4 walls that contain fiberglass bats, and then follow up with drywall. With my already existing stud walls, I am going to fill the cavities with fiberglass insulation, and I was wondering if it would then be OK to place a layer of foam board over the entire insulated wall? That is, insulation between wood studs, then foam board over the studs. I would then cover the foam board with drywall, attaching the drywall with screws through the foam and into the studs.

    I'd like to include this layer of foam board to increase the R-value since my stud walls are only 2x4. Since I'd like to keep as much floor space as possible, I thought foam board would be the way to go instead of extra fiberglass bats and framing. I also figure the foam board would provide an extra air tight seal. Does this sound good?

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  2. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    As long a you don't have any "Leaks" coming through those block walls, I guess it sounds ok. The more insulation- the better off you will be. You might have a time hanging drywall to the studs through the foam board insulation- could be a stretch ????
    Most times I have seen installing the foamboard and waterproofing (Membrane) on the outside of the foundation walls- but you would need asscess to the outside wall and no- one really wants to dig the perimiter of their house up- unless that 'Have-to'.
    So, again-
    any leaks through the block walls ?- stop here.
    no leaks !- go ahead.
  3. jklingel

    jklingel Feeling the Heat

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    I think you will be OK w/ the foam boards between the sheetrock and the studs, but it will be a bit more of a challenge to leave studs available in the corners for screwing the 'rock to. No biggie, just don't forget them. It is common up here to put 2" R-tech on the outside of houses, over the studs and under the siding. However, siding can shift a little and not show cracks like sheetrock will. An obvious option that you know will work is to use 2x6s and no foam boards; faster, too, but not quite the R. Another idea: how spendy would it be to frame w/ 2x4s, then sheetrock them and have the hollows filled w/ spray foam. Worth it????? Good luck, and let us know what you try.
  4. Sandor

    Sandor Minister of Fire

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  5. velvetfoot

    velvetfoot Minister of Fire

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    I've also seen un-papered sheetrock.
  6. Sandor

    Sandor Minister of Fire

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    Lowes is selling a new type of mold-proof sheetrock. (Not green-board)
  7. jklingel

    jklingel Feeling the Heat

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    Sandor: That building science link is fantastic. Thanks. j
  8. rmcfall

    rmcfall Feeling the Heat

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    I've actually printed out several of those building sciences articles...they are a great resource. My concern was regarding my existing stud walls--can I put fiberglass in between the studs and then cover this with the foam board? That is, the foam board would be closest to the interior (or drywall), and the fiberglass would be closest to the exterior. This seems like a reverse sequence of things, as it appears that foamboard is normally installed first, then the fiberglass batts. I'd like to do it in the reverse sequence so I can simply put up a full sheet of foam board at a time, which would give me the benefit of a completely sealed wall (no gaps in insulation b/c of wood studs). It seems like it would be fine as long as the fiberglass batts were unfaced?
  9. Nofossil

    Nofossil Moderator Emeritus

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    I did exactly that in our house, except we have 2x6 wall studs. I used 1" foil-faced foam and taped all the joints to make the foam board my vapor barrier. It's important to get your vapor barrier towards the inside - the temperature at the vapor barrier must be higher than the dew point of the air inside the house. If not, moisture from inside the house will condense on your vapor barrier or inside your insulation. Bad Thing.

    If I had it to do over, I would have taped a plastic film over the studs and fiberglass. then nailed horizontal furring strips over that, then attached the foam boards to the furring strips. Gives the sheetrock screws a bigger target, and creates a 3/4" air gap to help the foil face of the foam board to do its thing. Would also provide a convenient wire chase (local codes permitting).
  10. rmcfall

    rmcfall Feeling the Heat

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    So did you use unfaced insulation since the foam board was placed over top of it?

    I was just at Lowes and they have several foam boards. The one with the highest R value (3/4" at R=4) was just a blue styrofoam board. Some of the others were labeled as polystyrene, but had a lower R value. Is the blue styrofoam the same thing as the polystyrene? If not, what exactly is polystyrene?

    You mentioned that if you had to do it over you would use a plastic film over the studs. What advantage would the plastic have provided? I always thought plastic was a bad idea because it doesn't allow for any transfer of moisture.

  11. rmcfall

    rmcfall Feeling the Heat

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    I saw that as well. About twice as much though...


  12. Nofossil

    Nofossil Moderator Emeritus

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    I believe that the blue foam is 'Expanded PolyStyrene' - EPS. I use the foil-faced polyisocyanurate, which has a higher R value per inch (at least initially) and has the foil surface which cuts down on radiant heat loss.

    The plastic would provide the highest integrity vapor barrier. You don't want moisture migrating from your indoor air into your insulation. The vapor barrier MUST be on the inside of the bulk of the insulation to avoid condensation.

    I didn't look to see if the building science web site talks about vapor barriers - they might have some good stuff.
  13. rmcfall

    rmcfall Feeling the Heat

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    The articles re basements from the Building Sciences website do talk about basement insulation and how one should NOT use plastic. I suppose this is different from the situation you are describing since you are referring to wood stud walls as opposed to block walls.

    I see what you are saying about not wanting moisture from the indoor air to get into the insulation. However, what about the moisture from the outdoor air getting into the insulation?

    You might have already answered this, but in your case where you used foam board over top of fiberglass insulation, were the fiberglass batts faced or unfaced? I am thinking unfaced since your foam board would serve as your vapor barrier.

  14. rmcfall

    rmcfall Feeling the Heat

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  15. rmcfall

    rmcfall Feeling the Heat

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    Before getting responses here, I posted my question to the Owens Corning website. I didn't think I'd get a response, but I was surprised that I did. I thought I'd share it, so it is copied below. They recommend using a plastic barrier with both block and wood stud walls. I was surprised they said to do that with the block walls...

    ________________

    Thank you for contacting Owens Corning. Owens Corning has no recommendations for installing 2x4 framing over plaster.

    Instead, Owens Corning recommends the following method for both types of walls.

    Remove the plaster from all walls. For the block walls, build a 2x4 stud framing system. Once all the walls have 2x4 framing systems, Owens Corning recommends insulating the walls as follows.

    EXTERIOR WALLS

    RECOMMENDED PRODUCTS:


    2x4 Wall Construction:
    R-15 - Faced - 3 1/2" thick
    R-13 - Faced - 3 1/2" thick

    NOTE: R-15 is a high density product and typically must be special ordered.

    There are 2 basic steps to installing this insulation:

    Step 1: Using pre-cut batts rather than continuous rolls is recommended. Most batts are cut to fit 16 or 24 inches on center and about 92 inches high. (typical dimensions) Pre-cut batts make the job go faster and easier.

    Step 2: The insulation should fit snugly against the studs and completely fill the cavity to the top and bottom plates. Be sure to cut the batt insulation to fit snugly around obstructions such as electrical boxes, plumbing and vent lines. It makes no difference where the flanges are stapled (inset or face), they are both acceptable.


    Once all the fiberglass insulation is installed, slit the facing. Then install up to 1" of Fomaular over the studs. For a vapor retarder, install a 4- or 6-mil poly. Finally, install drywall over the entire assembly.

    Should you require further assistance, please feel free to contact our Customer Care Center at 800-GET-PINK.
  16. Nofossil

    Nofossil Moderator Emeritus

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    Whew - sounds like they are now suggesting almost exactly what I wish I'd done. In answer to your earlier question, I used the kraft faced insulation, just because it's easier to handle. I didn't slit the facing as they suggest, though.

    Took me a while to wrap my head around the whole vapor barrier concept. As I now think of it, the moisture level inside your house is higher than outside. Moist inside air will percolate through your walls until it either finds an impermeable vapor barrier, or reaches the point in your wall where the temperature is below the dew point of the inside air. If it's the second case, it will condense at that point, and you will have wet insulation and eventually rot. The goal is to have a perfect barrier that's close enough to the inside so that it's warm enough that moisture won't condense on it. Everything outside that point should breathe so that any moisture can work its way out over time.
  17. rmcfall

    rmcfall Feeling the Heat

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    I think I see what you are saying, but with block walls it seems like it would be a problem. In fact, articles from the Building Sciences website say that plastic contributes to mold problems in basements due to moisture getting trapped. In my case, I've got block walls in my basement, as well as in parts of my main house. It seems like the same concerns regarding moisture would apply to the block walls that are part of the main level of my house....
  18. Nofossil

    Nofossil Moderator Emeritus

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    Forgive me if I haven't read all of your application description carefully. If you're talking block walls that are below grade, then you MUST NOT use a vapor barrier on the inside. Moisture will seep in from the outside and be trapped.

    Above grade, vapor barrier on the inside. Building sciences seems to confirm that, and it makes sense. In fact, they show an example of a concrete wall that's partially above and partially below grade. They have vapor barrier of sorts on the outside below grade and on the inside above grade. Moisture must have an escape path, and it will tend to migrate from moister to drier if it can.

    Hope this helps.
  19. rmcfall

    rmcfall Feeling the Heat

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    Thanks for clarifying... Also, I guess I didn't see where the Building Science site suggested using a vapor barrier above grade with masonry walls...I'll have to take a closer look. I was under the impression that a vapor barrier shouldn't be used with masonry walls in general, whether above or below grade.

  20. Nofossil

    Nofossil Moderator Emeritus

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    Page 12 of this pdf has a good picture.
  21. rmcfall

    rmcfall Feeling the Heat

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    Sorry to dig up an old thread, but some of my projects have moved ahead slowly and I am just now to the point of insulating my ABOVE ground walls, which are a combination of block walls and framed walls. The new title of this thread should now be "OK to put fiberglass batts over top of foam board?" I am clear on how to use a combination of foam and fiberglass in the basement, but these walls above grade have me a little confused. Here is what I am thinking, but am a little unsure if it would cause moisture problems....

    for the framed walls....

    insert foam board between the studs to seal everything up tight, and then install fiberglass batts over the foam board. Finish with plastic stapled to the studs, then drywall. In this case I am assuming that the plastic will prevent indoor humid air from getting inside the wall. The question is, does the foam board on the other side of the fiberglass prevent any moisture that does happen to get in there from escaping, thereby causing mold problems?

    for the block walls (again, these block walls are above grade)....
    apply foam board directly to the block walls, then build stud walls with fiberglass, and then finish with plastic stapled to the studs followed by drywall. Same question as above...will the foam board, and perhaps the combination of foam board and block wall together, prevent any moisture that happens to get through the plastic barrier from escaping?

    So in both cases, there is fiberglass insulation in between foam board and plastic. Is this a problem? I initially thought that having a sandwich like this might trap moisture, but Figure 14 from http://www.buildingscience.com/docu...t-insulation-systems/view?searchterm=basement insulation
    shows rigid foam board, then insulation, then a polyethylene vapor diffusion retarder on the above grade section of the wall. In this situation, does the foam board allow any trapped moisture to escape outdoors? What about with block walls above grade, would trapped moisture pass through the foam board, then through the block walls?

    Thanks for your help everyone.
  22. richg

    richg Minister of Fire

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    This is an easy one. For the block walls, I'm going to strenuously disagree with one prior post, and say that a vapor barrier is mandatory. Put a 4-6 mil poly sheet over your block. Get yourself a .22 caliber nail gun and put up studs 16 inches on center. The best foam board to use is called polyiso (polyisocynaurate), which has a foil radiant barrier on one side. Cut the foam board to fit between the studs, and use duct tape to temporarily hold it in place. Over top of the foam board goes your drywall, and I have had good luck with the new mold-proof stuff. It is easier to cut than regular drywall, holds paint like a vise, and is water resistant. A bit more expensive, but peace of mind is worth it.
  23. rmcfall

    rmcfall Feeling the Heat

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    I am curious what your rationale is for saying that a vapor barrier against the block is mandatory, as opposed to placement of the foam board against the block as described in the Building Sciences articles?

  24. richg

    richg Minister of Fire

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    Foamboard in and of itself is a vapor barrier, as it is closed cell foam with a perm rating of less than .05. A poly vapor barrier directly against block or poured walls protects the studs from moisture that would otherwise wick into the wood and sheetrock.
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