Insulating Cathedral ceiling-need help!


Old 08-28-06, 11:01 PM
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Join Date: Mar 2004
Posts: 40
Insulating Cathedral ceiling-need help!

I've searched back about 25 pages and have seen this question posted about two or three times, but none had any reponses. So, I thought I'd try myself. Here goes...

We have the typical cathedral ceilings in our living room. There is no space and no insulation between the ceiling and roof. We're thinking of putting some type of rigid insulation on the ceiling and then finishing it off with sheet rock. Of course, we want max R value so we were thinking of putting up a couple of layers of the insulation. The best R value of rigid insulation we could find on the internet is 10. If we put on three layers would that make the R value 30?

Other questions: Any suggestions? Is R value 10 the best we can get? Remember, the roof shingles are sitting right on top of the ceiling.... no space at all. It's a late 70's ranch home in No. Calif at the base of the Sierra Mountains.

Thanks, Mary
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Old 08-29-06, 04:24 AM
Join Date: Jul 2005
Location: Ottawa, Canada
Posts: 857
What is the drywall attached to on the inside of the ceiling?
Old 08-29-06, 05:07 AM
Join Date: Mar 2005
Location: USA
Posts: 6,126
Insulating Cathedral ceiling-need help!

Don't get totally caught up in the pink panther R-value world.

When comparing rigid insulation to fiberglass, the real or effective insulation value for rigid will be far greater than that of fiberglass with the same idealized, ADVERTISED R-value.

The R-value for the insulation refers to the idealized "insulating" value of the material without regard to how it is used and other real world factors.

When you put fiberglass between joists or studs, you are only insulating a portion of the wall, leaving a thermal "short circuit" at the joists and studs in addition to the obvious gaps every 12" to 24". This installation dramatically reduces the real insulation value by 10% to 40% depending on the spacing and materials.

Rigid insulation provides a continuous insulation layer with no thermal short circuits of gaps and minimal air infiltration.

In comparison, rigid will out perform bulk or batt insulation. The ideal insulation would be a a rigid insulation with the same "R-value" of the present rigid, but weighing much more. This would also give you the benefits of thermal interial that is superior in the real world of dynamic hourly or daily changing temperatures.

I have installed rigid insulation over scissor trusses/fiberglass for a cathedral ceiling and was amazed at the performance and comfort.

You can install over existing sheet rock or you can remove the existing sheetrock. If you are concerned about having too good a ceiling vapor barrier system, you can always slash any existing poly of drilling holes periodically to negate the existing VB.

Just don't get lost in the muopic R-value calculations and keep in mind that many of the numbers are there for for decoration and advertising based on idealized laboratory tests.


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