Insulating unvented roof assemblies
Mill_house
| Posted in General Questions on
I live in Canada in climate zone 7. I have a cape cod house and the second floor has been completely gutted. (I understand there are 3 elements that are required to equal a vented roof: soffit vents, ventilation baffles and ridge vents. I have none of those. So I must have what is called an unvented roof.)
I understand closed cell spray foam can be installed against the underside of the roof sheathing with the rest of the rafter cavity filled with air-permeable insulation. I came across this chart:
This chart is based on table R806.5 of the 2012 IRC. If i’m understanding things correctly, I would need to achieve R30 with the spray foam and then add r19 to bring the room up to r49. I am not adding insulation above the roof sheathing. This will all need to be accomplished from the inside. My question is how can this be achieved with 2×4 rafters? Build them out to 2×8? That seems out of the question. So where does this leave me?(One spray foam company I’ve had out to the house recommended 3 inches between the rafters and didn’t even mention foaming the rafters. When I asked he said he could. Then I mentioned the flash and batt method, he said I must be on the internet too much. He said the flash and batt in Manitoba and Saskatchewan does not work. Said something about moisture issues between the two surfaces of insulation.)
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Replies
The "flash" of "flash'n' batt" that works in your climate zone has to be 3/5 (60%) of the total R. If the foam is less than that yes, the fiber insulation next to the foam gets wet.
To hit comparative performance to R49 between rafters takes a U-factor of about U0.026, or a "whole-assembly R" of about R38.5, that includes the thermal bridging of the rafters, but also the R-value of the roof deck, air films etc. To hit that mark with HFO-blown closed cell foam that completely fills the 2x4 rafter and ecapsulates them and no interior side gyprock would be about 6". To do it with HFC blown foam it would be 7". It would run R42 at center cavity, R22 at the encapsulated rafters.
Most HFO blown foam can be installed at 6" safely in a single pass. Most HFC blown foam required lifts of no more than 2" per pass with a curing wait period between lifts- any thicker and there is a fire risk during the curing period, and possible shrinkage/separation issues as it cures. Even though HFO blown foam is more expensive when blown in thin layers it can be a time & money saver when blown at these thicknesses. It's also a heluva lot greener, since HFC245fa is a powerful greenhouse gas (on the order of 1000x CO2 @ 100 years), and HFO1234ze has VERY low greenhouse gas potential, no more than 3x CO2 by most estimates.
Mill House,
Dana gave you good advice. You can either insulate the existing 2x4 rafters -- and end up with insulation that is far below code minimum requirements -- or you can beef up your rafters to provide more thickness to your insulated roof assembly.
If you want to be conscientious, you should consult an engineer to determine whether your 2x4 rafters are adequate. I suspect they aren't -- so you may need deeper rafters for structural reasons, not just for the insulation.
For more information on your intended insulation strategy, see this article: "Flash-and-Batt Insulation."