Poor insulation in old cathedral ceiling
rlangley
| Posted in GBA Pro Help on
Having read the 5 options for insulating a cathedral ceiling (By Martin Holladay | December 11, 2020) I am in a dilemma…#3 seems the closest. Here is my situation – I am in zone 5b. A portion of my home was built in the 70’s and that portion employs a cathedral ceiling. The pitch is 8/12. The rafters are 2×6 and the sheathing is 1″x10″ rough sawn lumber with asphalt shingles. A ~ 1″ air space was left above the batt fiberglass insulation and the insulation may have been R12 vs R22…not sure. The underside of the rafters have a poly vapour barrier followed by drywall. I am in the process of covering other parts of the house with steel roofing but before I do this portion I would like to increase the insulation. I believe the air gap can be sealed top and bottom (vents in the soffit can be sealed and the ridge vent removed and sealed).
My preference, due to interior limitations, is to add insulation above the rafters rather than below. Any recommendation on how I tackle this will be appreciated.
Thanks
Richard
My preference, due to interior limitations, is to add insulation above the rafters rather than below. Any recommendation on how I tackle this will be appreciated.
Thanks
Richard
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Replies
With a 1” air space in 2x6 rafter cavities, about the most fiberglass batt R value you could fit in the remaining space would be around R19 using high density material, but your probably around R14-ish.
If you can open the roof from above, remove the vent channel and the old batts, and replace with new R21 high density fiberglass batts (or R23 mineral wool batts). You can use unfaced batts here. Put all the rest of the insulation above the roof, using polyiso. Ideally you want at least half the total R value on the exterior for a roof, which is easy here since you don’t have much inside. 3” of polyiso is R19, 3.5” is R22, and that’s continuous insulation too — no thermal bridges.
Bill
Thanks for your response Bill (for some reason I did not receive an email letting me know you had responded).
Do I have to cut out the poly vapour barrier that is currently between the drywall and rafters to prevent trapping any moisture that may end up within the roof assembly? ...or is this not a risk when using polyiso?
Since in this scenario I would be removing the sheathing to get at the rafter bay, to remove the old insulation and vent channels, do I need to replace the old sheathing or just fasten the polyiso directly to the rafters, then add strapping and the metal roofing?
Thanks again,
Richard
It depends on the assembly and where the poly is. Can you include some kind of diagram, or a description of the layers? My first thought though is to remove the poly and replace it with the polyiso.
Bill
From inside out - Drywall, poly vapour barrier, 2x6 rafter [fiberglass insulation and 1' air gap], 1 x 10 rough sawn sheathing, asphalt-saturated felt underlayment, asphalt shingles.
If there is any way to avoid removing the drywall - that would be my preference.
Thanks
If you want to reinsulate without removing the drywall, then you'd have to work from the exterior, removing the roof sheathing for access. You could then leave the drywall and poly in place, with your final assembly from the inside out being: drywall, polyethylene sheet, batt, polyiso, air gap, plywood/OSB sheathing, final roofing materials.
Bill
The poly / polyiso sandwich was my concern...I have read in numerous articles about the risks of sealing wood (the rafters in this case) between two non-permeable barriers. Wouldn't this be a risk? Also, based on your comment about adding an air gap between the polyiso and plywood - if I am using Steel roofing over 1x4 strapping is there any reason to add in this air gap (is there any reason to install the plywood - couldn't I just strap the polyiso and fasten the metal to the strapping? What is the purpose of the air gap in this specific roof assembly?
Thanks
Richard
rlangley,
I see your concern. I'm curious about this. I'll start another Q&A and see if we can't clear it up.