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In a cathedral ceiling slope can polystyrene ...

Sent to Home Improvement Experts May 15 7:28 p.m.

In a cathedral ceiling slope can polystyrene insulation be placed on top of fiberglass insulation to get the greatest R-value in the small space of a 2x6 without causig condensation or other problems? I am thinking of using 3 1/2 inch fiberglass insulation and 1 inch polystyrene with a 1 inch air gap on top.

Customer (name blocked for privacy)

Status: Closed Value: $9

Answer
May 15 8:06 p.m. (38 minutes and 24 seconds later)

REPLIED

Hello Customer (name blocked for privacy),

Welcome to Just Answer. If interested, Click here to view my credentials.

You certainly can do this, and it is a good idea. I do not see too many 2x6 rafters, but if I had to insulate them I would do exactly what you are proposing. Rigid insulation costs a little more than batt, but you will get R-5 per inch and air cannot pass thru it, like it can with batt insulation. If you can afford it, you can even lose the batt insulation alltogether and just use thicker rigid. If you seal all the edges of the rigid insulation with spray foam or caulk, you can reduce air movement which will add to the R-value as well. Definitely keep the air gap at the top for venting the eaves to the ridge; this will save your roof.

If you need further help or clarification on this answer, please reply to this post.

Best of Luck, Brian

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Reply to Brian
Sent May 15 8:22 p.m. (15 minutes and 41 seconds later)

I found a manville product that is a fiberglass batt style insulation and has an R-15 value.The insulation has a depth of 3 1/2 inches.Do you think it would be worth the extra time and effort to use only solid?This is a remodel of an existing area and the square footage of this ceiling area is approx.88ft.I do not believe I mentioned that the fiberglass insulation has a kraft facing. Sorry to be so technical and such a pain but that is why I call myself Customer (name blocked for privacy).

Edited by Customer (name blocked for privacy) on May 15 2008 at 8:55pm

Customer (name blocked for privacy)

Answer
May 15 9:40 p.m. (1 hour and 18 minutes later)

ACCEPTED

For that small of an area....I believe you said 88 sq.ft., it is probably not worth doing all solid and all the sealing. Stick to your original idea, I think it is perfect. Kraft paper face is fine for this application. Don't even worry about asking questions, that is why I am here.