Catherdral Ceiling, Current Best Practice
acornw
| Posted in Energy, Heating & Insulation on
My home shop has a ‘cathedral’ ceiling with 2×10 rafters and 1/2″ OSB, then shingles. The insulation is a 1/2″ foil board ‘trough – spacer’ to give a 1″ air gap directly under the OSB from eave to peak. The there is 8″ fibreglas with kraft paper to the living space. Then a layer of clear Visqueen.
This has been in place for 20 years with no problems except the Visqueen is getting brittle.
I’m contemplating covering the ceiling with T&G boards, or OSB or drywall. What is the current best practice with this?
Should I add 2×2 spacers to the 2×10’s and nail on the interior finish? Or nail straight to the rafter bottoms?
Remove the sheet plastic or leave it?
Dave S
Acorn Woodworks
Replies
Interesting question.
Sounds like one answer is "it ain't broke so don't fix it".
As you now have it, the plastic is serving as your air barrier. I tend to like more permeable materials, so if I was going to rework your lid and install T&G wood, I'd probably replace the plastic with Tyvek and then install the wood. Housewrap is a good air barrier but much more vapor permeable. Bear in mind that I'm in a mild climate where we don't use plastic behind drywall, but your climate may be different.
If you're in the mood to add framing to the bottom of your rafters, you can add insulation thickness. I will always do that if given the option. I might also add rigid insulation against the sides of the beam to reduce the chances of getting cold air down thru the ridge vent that leaks into the room at the ridge.