Hybrid Roof Insulation Ratio
steve41
| Posted in General Questions on
I’m juggling some elevation / roof thickness challenges for an unvented low slope roof. Originally the roof was planned for all insulation to be over the roof deck. Now, to reduce the roof thickness I’m considering a hybrid roof. My understanding is that for zone 6A, this would translate to having a minimum of 51% of the R value above the roof deck.
If I install R28, for example, over the deck, are there any code issues by under-filling the rafter cavities in order to maintain the correct ratio.
Thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Since your overall assembly has the correct R value, there is nothing wrong with underfilling the rafter cavity. You do have to make sure there is no gap between the insulation and the roof sheathing, so this might mean needing insulation wires to keep the batts in place. A gap between the batts and drywall is fine.
Unvented roofs with rigid insulation are expensive to build. In most cases there is no financial ROI or climate benefit for going above code min. I would check your local code as you might be able meet code based on whole assembly U factor (could be R38 equivalent) instead of center of cavity R value. There is surprisingly little energy saved going from an R38 to R49 roof even in Zone 6. You can run your roof assemblies through a calculator such as this to see what works:
https://www.ekotrope.com/r-value-calculator
Thanks Akos, really appreciate your feedback.
The calculator link has been a good resource for exploring various roofing options.
Also, I didn't really have a plan for how to keep the batts against the sheathing, so the insulation wires suggestion was helpful.
I do plan to pursue the U factor alternate approach for the assembly, mostly because I feel that a little less insulation in the rafter cavities will be safer for the sheathing. Thankfully, my code officer is receptive to the U factor alternative approach.