Moisture / vapor barrier
Hi,
I am planning on using tongue and groove boards for the ceiling of our master bathroom. I would like some input on if I should be worried about installing a smart membrane or 15# roofing felt behind the finished ceiling as this is a high humidity area. I will have a Panasonic bathroom fan installed on a humidity sensing switch. CFM of at least 200 due to the high ceilings and bathroom dimensions of 10’x11′.
My house is in Zone 5, Greater Hartford Area in CT. The bathroom ceiling is a cathedral ceiling that starts at 10′ and goes up to 13-14′. the stack up of the roof, starting on the outside, is:
Solar panels, solar rack, new architectural asphalt shingles, roofing paper, 5/8″ osb deck, furring strips, 3″ foil faced polyiso, 1/2″ plywood deck, dense-packed cellulose in 2×8 rafter bays, flattened kraft faced fiberglass (exiting insulation flattened from dense-pack), 5/8″ drywall, taped and painted with latex paint.
I am planning on running 1×2 furring strips perpendicular to the rafters/joists. Should I place 15# felt against the drywall beforehand?
Thank you for your input.
David
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Replies
The furring strips do provide venting from soffit to ridge beneath the built-up deck. The roof is a simple gable, in case anyone was wondering.
Also, I have 1 penetration for an electrical box for a chandelier. This has been sealed with a sealant to prevent moisture migration into the insulated cavity. The bathroom vent will be mounted near the peak, but in the wall, not the ceiling.
David,
Whether or not your roof assembly is safe is a judgment call. In your climate zone, you want the rigid foam layer of your roof assembly to provide 41% of the R-value of the total roof assembly. (For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.)
Whether or not your roof complies with this target depends on the R-value per inch that you assign to the polyiso layer. Assuming you assign an R-value of R-6 per inch to the polyiso, the rigid foam layer in your roof assembly provides a little more than 40% of the total R-value of the roof assembly. So far, so good.
If you de-rate the performance of the polyiso down to R-5.5 per inch to account for the poor performance of the polyiso in cold weather, however, the foam layer only provides 38% of the R-value of the roof assembly -- not quite as high as you're aiming for.
Frankly, you're close, so I wouldn't worry too much. Installing a smart vapor retarder like MemBrain on the interior of your roof assembly would be relatively cheap insurance to help you sleep at night.
With the polyiso at 37-38% and dense packed cellulose preventing convective moisture transfer within the cavities, but also buffering and distributing the wintertime moisture burden it's not worth worrying about it being somewhat below the IRC prescriptive ratio. It's close enough. and Hartford isn't at the cold edge of zone 5, but somewhere in the middle. In zone 4 it would be fine with only 30%, even with fiber insulation with no moisture buffering capacity to speak of.
The IRC prescriptives still presume an interior vapor permeance of less than 10 perms. The kraft facers would meet that (They're less than 1 perm when dry, over 5 perms when damp enough that it needs to dry), and so would the latex on drywall. Additional vapor retardent layers won't hurt, but aren't really necessary.
I appreciate your time and input on this, Martin and Dana. Your responses were very helpful in steering me in the direction I will go.
Out of curiosity, I was thinking about the Certanteed, and it seems that this would be an inadvisable product to use in a bathroom, as it becomes vapor open in high humidity and vapor closed in low humidity. A bathroom would always have fluctuating humidity levels, sometimes allowing vapor to drive the wrong direction possibly.