EPS / XPS on interior ceiling
Problem: I have a very inefficient home in Southern California that was built in the late ’70’s with an exposed, pitched ceiling that slopes from 7′ all the way up to about 16′. Even with a brand new air conditioning unit, the home is stifling hot with the 90+ degree weather we’ve had lately; and my AC unit never shuts off.
The construction of the roof/ceiling assembly consists entirely of: composite roof shingles, plywood underlayment, 2″ of rigid polyiso insulation, and exposed 2×4 t&g hemlock wood. Because the rigid insulation was installed 35 years ago, I’m guessing its R-value is down to about 13.
Proposed solution: To help mitigate my problem, I was thinking of attaching to the underside of the exposed wood ceiling 4″ of either expanded polystyrene (EPS) or extruded polystyrene (XPS), and then putting a finish of either acrylic plaster or more t&g wood (although probably only about 1/2″ thick). This solution should provide an additional R-value of from 15 (EPS) to 20 (XPS).
Questions:
1. What do you think of this proposal?
2. Is there a code issue with installing EPS or XPS on the interior of a home (due to potential toxic gases that might be emitted in a fire)?
3. Besides cost, can you think of any other negatives to this proposal?
4. Can you suggest any better alternatives?
Thanks in advance for everyone’s feedback.
Michael
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Michael,
Q. "What do you think of this proposal?"
A. It will work, but -- if I understand correctly -- the new rigid foam insulation will be interrupted by the rafters. The rafters are thermal bridges, and they will reduce the thermal performance of the insulation.
Q. "Is there a code issue with installing EPS or XPS on the interior of a home (due to potential toxic gases that might be emitted in a fire)?"
A. The code issue concerns flammability, not toxic gases. In most cases, rigid foam has to be covered on the interior with 1/2-inch gypsum drywall or an approved equivalent. If you have any doubts, contact your local building inspector to make sure that you are in compliance with local regulations.
Q. "Besides cost, can you think of any other negatives to this proposal?"
A. The thermal bridging issues.
Q. "Can you suggest any better alternatives?"
A. It would be better to add more rigid foam above your existing roof sheathing. Of course, that work would require the installation of new roof sheathing and new roofing.
Thanks, Martin. I didn't even think about the thermal break but of course you're absolutely correct. I did think initially about adding a layer of rigid on top but discounted the idea believing it would be cost prohibitive; but really, it's probably no more expensive than what I was proposing - plus a lot less disruptive.
So new questions (and maybe these each need their own post):
1. Can anyone recommend a good source for learning more about adding rigid on the topside of a roof assembly?
2. Are there certain roof types that would deflect heat better than others, and would a standing seam roof be one such type?
3. Would photovoltaic panels help in deflecting heat away from the interior?
Michael,
Q. "Can anyone recommend a good source for learning more about adding rigid on the topside of a roof assembly?"
A. GBA is a good source of this information. If you enter relevant search words into our GBA search box, you'll get lots of information. You might start with How to Build an Insulated Cathedral Ceiling.
Q. "Are there certain roof types that would deflect heat better than others, and would a standing seam roof be one such type?"
A. For more information on so-called cool roofs, see Staying Cool with a Metal Roof.
Q. "Would photovoltaic panels help in deflecting heat away from the interior?"
A. Yes. Make sure that you include a ventilated air space between the roofing and the underside of your PV modules.