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tikitacky 02-02-2013 11:36 PM

Insulating a Tongue and Groove Cathedral Ceiling
After scouring the internet, this thread is the closest I've come to finding someone else trying to do what we are. We just purchased a 1955 Mid-Century Modern house with vaulted cathedral ceilings. We do not want to lose the wood ceilings. Instead we would like to insulate by using 1.5" foam and installing a second T&G ceiling on top of the existing ceiling. We are in Southern California. There is no venting in the ceiling and never has been any and the wood is all in great shape. The roof of the house was put directly on top of the T&G. I am hoping someone can tell us if what we want to do is feasible. Can a second T&G ceiling be installed with no air space, directly on top of Tuff-R foam over the existing T&G ceiling?

Gary in WA 02-03-2013 09:24 PM

CA has its own Energy code;

Depends on location but minimum may be R-30; Check with your local AHJ:

PS. welcome to the forums!

tikitacky 02-03-2013 10:24 PM

Regulations regarding insulating T&G ceiling
Thank you for the welcome! :-)

I wasn't clear on what I meant about "able to". It was the housing inspector who suggested such a method of insulating so it is allowed as far as building regulations go, however I cannot find anything on the internet about anyone else having done this and if there are any issues inherent with installing a second T&G directly on top of the original one separated only by 1.5 of foam insulation. Does anyone have any personal insight or knowledge of doing this? BTW: the house is built on slab.

joecaption 02-03-2013 10:44 PM

How exactly would you ever nail it up there?

tikitacky 02-03-2013 11:05 PM

Personally? I would not. But the building inspector presented this option when he recommended we insulate and we explained to him that we did not wish to cover the T&G cathedral ceilings in order to insulate. I assumed by his familiarity with the practice that there is a method by which it is done. I did not ask him to recommend someone to estimate the job as they're not allowed to make personal recommendations. I am here specifically to find out more about this potential solution to our lack of insulation and discover any issues we may encounter by installing a 2nd T&G on top of the existing one (with foam insulation in between). My hope is that we have a master on here that can tell me all about it.

joecaption 02-03-2013 11:13 PM

Remove the shingles and add SIP roofing panels is one way.
(google it)
I've never seen long enough fastners to make it through T & G plus 1-1/4 foam.

tikitacky 02-03-2013 11:25 PM

The roof is new and we don't want to take it off to insulate. So it sounds like we will have to wait for 20 years until it's time to replace the roof and insulate then. I wonder why the home inspector suggested it as something that was fairly standard. Odd.

joecaption 02-03-2013 11:42 PM

Can you post a picture of what's there now?
Thinking outside the box it may be possible to add SIP's on the inside of the ceiling, then you would have something to attach the new ceiling to.
May want to contact a company like this and get some ideas.

tikitacky 02-04-2013 06:01 AM

Sure, I'd be glad to. I will take some on Wednesday and post them. Thanks.

Gary in WA 02-04-2013 11:17 PM

If he was a building inspector with a county/city, he should have also mentioned the R-value required for your specific area.

"In Zones 1 through 3, open cell (vapor permeable) foam can be used successfully to enclose building spaces with moderate or dry interior humidities. In higher interior humidity spaces, closed cell foam or other roof designs are recommended." Taken from; see/read about Fig. 8:

Without having the added cavity insulation, your situation is slightly different than these test results;

Before I explain further, where are you located? Nearest large city...
What is the humidity reading in the room most of the year?


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