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).
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.
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