EPS/XPS or ??? on interior side of cathedral roof in zone 7
I have a vented R40 cathedral ceiling (parallel scissor truss). It currently has two layers of R20 fiberglass batt insulation above the drywall. I would like to add some insulation; the only option is to add it to the interior..
Can I do this and if so how much EPS/or XPS or ??? can I add? I have room to add 6 inches of rigid foam or similar.
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Unless you have a complicated roof, or there's no room in the truss space, can't you just cut a hole in the drywall someplace and blow cellulose on top of the batts?
My house isn't fully sided yet, since we seem to get another foot of snow every other day. Monday, the truss space will have cellulose blown in through a hole we just cut into the sheathing on the gable end just under the peak, above the ceiling air barrier. There isn't a lot of space in the trusses, but enough to crawl in with the hose. You could do that. Removing and replacing a little siding isn't a big deal, at least compared with putting six inches of foam on every ceiling.
The correct answer is that you should only consider the action you contemplate if your rafter bays have a ventilation channel between the top of the fiberglass and the underside of the roof sheathing. If your rafter bays are unvented, the rigid foam has to go above the roof sheathing rather than on the interior.
If you decide that you are able to install interior foam, you can install 6 inches if you want.
For more information, see How to Build an Insulated Cathedral Ceiling.
As for the EPS vs. XPS part of the question, the environmental hit of 1" of XPS is about the same as 5" of EPS of equal density, due to the high global warming potential of the HFC134a blowing agent used (~1400x C02) vs. the much lower potential of the pentane used in blowing EPS (~7x CO2.) What's more, in 50 years after most of the HFC has gotten out, the R-value of XPS will have declined to about that of EPS at any given thickness & density.
Put another way, six inches Type-II (1.5lb density) EPS would run R25.2 on day-1, and at day-20,000 (55 years). With 1.5lb density XPS you'd be a bit over R30 on day-1, but only ~R25.5 by day-20,000. It's unlikely that the additional R4.8 would have covered the original difference in environmental hit during that period in reduced energy use, but that depends a bit on the energy sources.