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Sheetrock or Foam insulation board?

brucepirger | Posted in Energy, Heating & Insulation on June 5, 2002 09:34am

Getting close to hanging the sheetrock…and a question has surfaced. I plan to install tongue and groove pine on the interior of all my exterior walls. The plan has been to hang the sheetrock, then install the pine on top.

It seems like it would be better to install foam insulation instead of sheetrock…but I believe the foam board (polyiso 0.5″) is not fire rated for this. Is this correct? Yet I also believe I recall reading/seeing a fire rated verision of this foam board…

I assume it will be somewhat more expensive, insulation vs. sheetrock is some $10 vs. $5 a sheet (4x8x0.5″). But I won’t have to mud and tape the seems…just apply tape to the insulations joints I’d guess?

Same plan for the cathedral ceiling. R-19 and R-38 fiberglass in the studs or trusses. One thing I figure…if anyone every wanted to pull down the T&G, it might be nice to see sheetrock behind it as opposed to foil faced insulation…only drawback I can think of.

All comments would be most appreciated! Thanks much.

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  1. Mooney | Jun 06, 2002 02:18am | #1

    Your post has several points to discuss.

    First of all dw and foam board are two different reasons. If I was after the hole bucket of bolts , I would use sprayed wall insulation [greater r value and fire proof]and I would nail the siding up. What are you hoping to gain with dw?
    Is there an insurance break for this ?A true firewall is 5/8s dw.  However I do see the point in the foam board. But where do you live to justify extreme heat or cold ? Well, you never mentioned stud size or what you have on the exterior?

    For example ; If you lived in the south , a six inch wall,3/4 blue board,  and brick  would be a fortress here .

    I never want to spend more than 70  percent of what the house and lot will appraise .

    Tim Mooney

  2. ClevelandEd | Jun 06, 2002 02:37am | #2

    I'm very happy that this was posted, because I have the same issue - hanging t&g pine on my kitchen ceiling and wondering weather or not I can use the polyiso board under it.  I think the polyiso board I looked at has a warning that a fire barrier, such as drywall, needs to be installed if using the polyiso.  Is that really standard practice?  If it is, I will just skip the polyiso board and install the t&g lumber directly to the ceiling joists.  Also am more than a little concerned about physical deterioration of the polyiso over the decades.  Is is stable?  If not, there are going to be a lot of loose  t&g boards on the ceiling. 

    1. brucepirger | Jun 06, 2002 08:11pm | #3

      Sorry for the lack of info...though I did mention R-19 in the walls (2x6) and R-38 in the ceiling (trusses). Building is in Berkshire NY, about an hour south of Syracuse, NY.

      Exterior siding will be log siding. From the outside in, log siding, layer of TYVEK, .5" OSB, R-19 fiberglass, then the drywall or polyiso, covered with T&G pine on the inside surface. If I hang drywall, I will put up plastic before the drywall. If I hang polyiso, I believe it serves as a good vapor barrier, when I properly tape the seams, correct?

      My understanding is that standard 0.5" polyiso sheets are not allowed to be exposed or directly covered due to fire concerns. However, I have heard of a "reinforced" polyiso sheet material, reinforced I think with fiberglass in the polyiso, that has both sides covered with the aluminium foil...hence providing a good vapor barrier.

      But my information comes from an un-remembered source, no doubt something from the web, and I am trying to ask the experts.

      My thoughts are that polyiso would be a better insulation and vapor barrier than the drywall (although maybe not drywall + plastic) and I think easier to hang (or at least not more difficult) than the drywall. If for the extra few dollars per sheet (or some $300-$400 for the whole house), I'd just hang the polyiso. But, the next question of course is, how much more expensive is the reinforced polyiso?

      You certainly are correct about the loose T&G is the polyiso were to fall apart. I have no idea about the long term life of polyiso sheeting. And I do agree, I'd love to spray polyiso in all my studs...the cat's meow I think...but I guess since I can't do it myself, the cost is too prohibitive.

      Sorry for the rambling...

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