spray foam in a roof with 12″ rafters, foam is r 6.5 per inch only r 30 required is void ok?
JM76
| Posted in Green Building Techniques on
I plan to use spray foam in a roof with 12″ rafters, my area only requires an R-30 which is 4 1/2″ of material, is it ok to leave that space in the joist or should it be filled with something like batt insulation?
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Replies
JM76,
You have a couple of options.
You can use all spray foam, and yes it is fine to leave the rest of the cavity empty.
You can also get that R-30 (or more) by using a combination of foam and batt insulation in each bay. If you go that route make sure to adhere to the ratio of foam to permeable insulation for your climate zone, to keep the interior face of the spray foam warm enough to avoid condensation. The table in at the end of this link:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
Consider R30 is the minimum required by law. The minimum speed limit on the interstate is 45 MPH do you really want to be that guy?
My opinion is spray foam should be avoided whenever possible.
It is the most expensive way to get an R of insulation. It is the most ungreen form of insulation. It is the riskiest insulation in that failures happen regularly.
You might find this article interesting.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
But done correct, the most forgiving…
begreener,
Can you flesh-out what the roof assembly would look like from in to out?
3" foamboard held 2" off roof (R18)
3" closed cell spray foam to seal in place (R18)
4" (R11) fg batt
R value 47
1/2 sheetrock
begreener,
Sorry I posted that question in the wrong thread,. I was curious about how spray foam would work on the gothic arched cabin we are also currently discussing.
Closed-cell foam brands all claim different R-values but they are all high, the value after aging 6 months in carefully controlled conditions. In the real world, over time air will displace the blowing agents inside the foam cells and the value will drop to around R.5.5 per inch, so by using 4.5" you are really getting R-25. You'll meet code requirements with the higher R-value but that's not what you'll have after a few years. (How many years is impossible to say.) R-25 is very little for a roof anywhere in the US.
I would not just add more foam, though, for the same reasons that Malcolm and Walta mention. When possible, I always vent roofs using safe, low-carbon insulation such as cellulose. When an unvented roof is required, I usually recommend a flash-and-batt or flash-and-fill method, which saves carbon emissions and usually costs less than using all foam.
Foam salespeople will likely push back on using a hybrid system, in my experience, but that's because it's easier for them to send a single crew and they will make more money. Insist on a separate proposal for a hybrid system and it will likely be lower than using all spray foam.
I have some practical experience here, albeit with 2x10s instead of 2x12s. I absolutely agree with Michael that R6.5 per inch is overly optimistic. I've seen some claims of up to R7/inch. I would plan for R6 per inch at most. It does NOT hurt to be conservative here and plan on R5/inch. More R value won't hurt you here.
I would plan to do this as a flash and batt. Put in the minimum allowable closed cell spray foam for your climate zone (not less than 2" though), then fill the remainder of the cavities with cheap fiberglass batts. I have found that leaving a large air space in the rafter bays can allow for circulating air currents that seem to manage to find any moisture in a bay and transport it to the one spot cold enough for it to condense. Filling the void with fiberglass stops this from happening.
Bill