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XPS below rafters in cathedral ceiling zone 4

We are finishing off some attic space (most over a garage, some over main part of house) for very occasional use as a guest bedroom, kid's playroom. Total sq ft is about 350, with cathedral ceiling (8 ft at the collar ties) with ridge vent and soffits. I'm fairly certain that the roof structure is just shingles, paper and plywood. Air vent baffles were installed between all 2 x6 rafters and OSB was used as backing behind the 5 ft knee walls (and was thoroughly air sealed). JM Spider high density fiberglass was blown in to fill all cavities in the slopes and knee walls and R38 batts were placed between the collar ties (only about a 3 ft span). We ended up with R30 in the slopes (which is all that was required by code) and since the knee walls were actually almost 10" deep (had to install 2 x 6 5 ft kneewalls in front of the existing 2x4 knee walls (because the original builder only made them 4' 3" high), the R value in the knee walls is much higher than code.

I realize that this is not the most energy efficient way that could have been utilized. We didn't want to use spray foam because of cost and health concerns and did not want to furr out the rafters and lose the limited amount of head space we have. As I said, this room will be used infrequently, and there is a ductless 26 seer minisplit in place to take care of heating and cooling as needed.

Given all that, I have a question about adding a layer of XPS below the rafters before the drywall goes up. I understand the concept of thermal bridging, but other than providing greater R value, is there any other advantage to putting up the XPS? If I don't put it in, what are the chances of "ghosting" where the rafters meet the drywall? Mold on the inside of the drywall? If I do put a layer up, is 1/2" sufficient to prevent any issues, or do I have to use 3/4" or 1"?

Thanks for your help!

Asked by Rob Silbajoris
Posted Oct 17, 2012 10:16 AM ET
Edited Oct 17, 2012 10:17 AM ET


6 Answers

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If you have 2x6 rafters with vent baffles, how can Spider fiberglass give you "R-30 in the slopes"?

Assuming that you have the minimum ventilation gap required by code (1 inch), and some type of baffle material that is extremely thin, then you still have a maximum insulating thickness of 4.5 inches. If we assume that high-density Spider has an R-value of R-4.1 per inch, that gives you about R-18.5 in the slopes, not R-30.

Answered by Martin Holladay
Posted Oct 17, 2012 1:31 PM ET


My bad- I meant 2 x 8 rafters

Answered by Rob Silbajoris
Posted Oct 17, 2012 1:38 PM ET


We're getting closer. But a 2x8 has room for 6.25 inches of insulation (after providing a 1-inch ventilation gap), so the maximum R-value with Spider is R-25.6.

If this were my house, I would install at least 1 inch of rigid foam under the rafters to address thermal bridging and to improve the R-value of the assembly. Of course, 2 inches would be better.

Answered by Martin Holladay
Posted Oct 17, 2012 2:15 PM ET


I really appreciate all the knowledge I gained and all the help I've received from this website, and I don't want to come across as obnoxious (not sure if that's the right word), but I understand all that you are saying. At this point, all I'm trying to figure out definitively, if possible, is what would be detrimental about not adding the rigid foam at all, other than the effect on thermal bridging and R-value. What are the chances of "ghosting" where the rafters meet the drywall? If there is a chance of that happening, what is the minimum amount of rigid foam that would prevent that from happening?

Answered by Rob Silbajoris
Posted Oct 17, 2012 2:38 PM ET


I have drywalled right over the foam on a wall but strapping the ceiling over the foam will help mitigate the problem AND give you the additional air space recommended on foil faced foam. You'd want the foil face pointing toward the floor. Strap it if you can spare the space, it will make for a better install. You can "make it work" with out strapping also.

Answered by David Burke
Posted Oct 23, 2012 6:07 PM ET


You may have misunderstood my basic question. So far I only have dense pack Spider fiberglass blown between the rafters. My question is: if I do not put a layer of foam board (XPS) below the rafters, and just add drywall up against the rafters, what are the chances that I will have "ghosting" where the rafters are in contact with the drywall? If I do add XPS, I was not planning on using any with foil backing.

Answered by Rob Silbajoris
Posted Oct 23, 2012 7:20 PM ET

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