Low Slope Commerical Roof Insulation Options
mattmark
| Posted in Energy Efficiency and Durability on
Hello. Have a commercial low slope unvented roof with 2×10/2×12 roof joists near Barrie, ON. Insulation contractors I’ve spoken to all recommend closed cell under the roof sheathing. If replacing the roof with external insulation isn’t possible at this time (IE not touching the sheathing or roofing material), will ~R35 of CC still be the best choice in this scenario?
I know this has been answered in the past (Apr 2013 / Insulating Low-Slope Residential Roofs), but there seems to be a sentiment that this is riskier than once thought versus something like dense pack + variable membrane, etc.
Thanks,
Matt Mark.
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Replies
Matt,
If you can't vent the roof, your options all involve some combination of foam - sprayed or board.
The two risks associated with spray foam are:
- Using open-cell, where the permanence can cause rot.
- Errors in the installation causing incomplete curing, which can need remediation.
I would suggest closed-cell spray foam in the rafter cavities, and 1 1/2" layer of foam board underneath to reduce thermal bridging , and increase the overall R-value of the assembly.
To alleviate concerns over poor installation, make sure your contract includes provisions for remediation by the contractor should the mix go wrong.
Went to Barrie for the first time this summer to visit relatives.
Thanks Malcolm for your response!
If we don't fill the cavity completely, is an air gap an issue between the rigid board if we end up going in that direction? I assume that doing perpendicular furring strips underneath would give us a good working space for drywall/electrical pot lights, etc.
Matt,
I suspect it is fine as long as the ratio of the spray foam to the rigid below was high enough to avoid condensation. However I'm not familiar enough with foam roofs to rely on my advice. Hopefully others will chime in on that aspect.
if you spray foam between the joists and then get a leak, you won't know it until there is a lot of damage to structure. I'd do a pretty exhaustive close examination of the roof underside for evidence of infiltration. then make sure that the membrane and flashings are in good shape. The last thing you want to do is trap a bunch of water infiltrating from above.
i don't know enough to opine on the assembly within the joist cavities. However you should figure what you are doing now, based on the long term plan of when you plan to replace the roof and what assembly you will have in the future. whatever road you decide, you'll have to do a lot of figuring how to deal with the roof/ceiling to wall transition.