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Rigid foam insulation on ceiling

Tony2slow | Posted in General Questions on

I’m finishing off a new construction room above a garage. Currently into the insulation phase, and discussing this with a contractor friend, he told me to be cautious of my plan as it might not be best.

Room has scissor trusses with energy heals at 24” on center, strapped every 16”. Plan is this…
1. Two lifts of R23 Roxul batt insulation between the trusses laying on the strapping.
2. 1” (or 3/4”) of foil-faced polyiso rigid board insulation over the entire ceiling, taped and sealed
3. Re strap under the insulation, perpendicular to the original strapping
4. Ceiling finish will be T&G pine to the strapping

I believe that this will give me an R value of >51. (R46 batts + R5 foam + air space) This is in zone 6 (Maine), min required is R49. The taped up foam will be my vapor barrier, and it’s all good, right?

Walls are 2×6 and will have Roxul batts, and there are a couple of skylights that will be sealed with 5 to 7 inches of foam insulation all around.

My contractor friend told me to be careful of the foil face board due to possible moisture problems later on. Not sure why this is, I thought that the foam was an added benefit for thermal bridging, plus I need a vapor barrier anyway, right?

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  1. GBA Editor
    Martin Holladay | | #1

    The approach you suggest is used all the time in New England, and it won't cause any moisture problems.

    There is a small risk that over a long time-frame -- say, ten years -- the rigid foam will shrink, popping the tape used at the seams. That's why taped rigid foam may not be the best choice as your interior air barrier. For more peace of mind, you might want to include (adjacent to the rigid foam layer) one of the European air barrier membranes sold by 475 High Performance or by Small Planet Workshop.

    For more information on rigid foam shrinkage, see "Using Rigid Foam As a Water-Resistive Barrier." (Scroll down to the section that begins with the subhead that reads, "Do rigid foam panels shrink?")

  2. Dana1 | | #2

    As long as the attic space is properly vented above the insulation it's a pretty safe stack up. But fitting batts to work well in trusses with NO voids or compressions is pretty difficult. Damp sprayed cellulose works fine on sloped ceiling, and would have a more perfect fit.

  3. STEPHEN SHEEHY | | #3

    Tony: Can you access the truss space from above or from the gable ends? Dumping 15" of cellulose on top of the ceiling will be much easier and cheaper than the roxul/ foam board option. Just put an air barrier membrane under the bottom chord and strap it, then drywall then install the cellulose on top.

  4. Tony2slow | | #4

    Thanks guys, this is a great site with really good information.

    Stephen: Unfortunately there is no easy access from above or gable ends. I agree that blown in cellulose would be easier, and maybe cheaper for this one part of my insulation project. I got quotes for the entire project, and did not look at doing just a small portion. Contractors were going to do like you said, insulweb the ceiling and then send one of their smaller monkey guys up to fill it in. I have about 4 feet from bottom to top of truss at the peak, so not a lot of room up there, for a normal sized adult anyway.

    Dana: None of contractors mentioned damp cellulose; they were going to do loose fill. One of three mentioned that they would use baffles to reduce/eliminate sliding. I have 25% complete, fitting the batts has not been too bad so far. I suspect the trickiest part will be when I try and place the last few sections of batts with no access to above from the adjoining truss bay.

    Martin: Rigid foam panels shrink – just something else to think about. I’ll have to do some extra research now. I will definitely look into an air barrier membrane. Would this go on the room side or truss side of the foam boards?

    By the way, I have done a lot of research for this project, and have quite a few estimates for my insulation job. Unfortunately, I was unimpressed with the large insulation contractors in my area. Enough so, that I have decided to do the work myself. It would have been nice to get this work completed in 2 or 3 days by a work crew. But I know that I will feel better knowing that I did the best that I could, and that things are done the way I think they should be.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "I will definitely look into an air barrier membrane. Would this go on the room side or truss side of the foam boards?"

    A. Either way would work. If I were you, I would tape the seams of both layers -- the membrane seams as well as the rigid foam seams.

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