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We have insulated a cathedral ceiling with soffit airflow panels going all the way up to the ridge vent, faced and unfaced batt insulation to get to R-49, and blown insulation to top off. We are in zone 7, so we need a lot of insulation. We want to put up a tongue and groove ceiling as the final finish, but because of the high ceiling, we prefer not to do drywall. I have been researching possible other methods, and seem to keep coming back to drywall as the preferred method to achieve the needed air barrier and fire retardant. I have read that foam board insulation with taped seams and edges have been used by some. Talking with a contractor friend, he doesn't use either one, just nails the T&G up with no air barrier, but I keep reading that the air barrier is critical.

If using foam board is an acceptable air barrier:
1 - what thickness does it need to be,
2 - does it need to be foil faced,
3 - any consideration to make it more fire retardant.

Thanks for any help.
 

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We have insulated a cathedral ceiling with soffit airflow panels going all the way up to the ridge vent, faced and unfaced batt insulation to get to R-49, and blown insulation to top off. We are in zone 7, so we need a lot of insulation. We want to put up a tongue and groove ceiling as the final finish, but because of the high ceiling, we prefer not to do drywall. I have been researching possible other methods, and seem to keep coming back to drywall as the preferred method to achieve the needed air barrier and fire retardant. I have read that foam board insulation with taped seams and edges have been used by some. Talking with a contractor friend, he doesn't use either one, just nails the T&G up with no air barrier, but I keep reading that the air barrier is critical.

If using foam board is an acceptable air barrier:
1 - what thickness does it need to be,
2 - does it need to be foil faced,
3 - any consideration to make it more fire retardant.

Thanks for any help.



First of all, regarding a fire barrier, while I have heard people talk about this, I have never actually seen a code for such a thing. Correct me if I am wrong but the only fire barriers in a typical single family home that I know of are between a garage and the rest of the house (and also fire blocking in the walls but that's sort of a different thing).


Drywall is an air barrier, but not a vapor barrier. You would need to seal it with a vapor impermeable paint. And seal it to any penetrations.



Foam is great (yes foil faced) but you do need a plan to seal it to any penetrations, electric boxes, etc. Also, it will not work so well to install t and g paneling over it.


I think the best thing would be a 6 mil poly vapor/air barrier. Tape all the seams, use a bead of acoustical sealant on the wall plates, use gasketed boxes. To really make it tight, add a 1x2 firring strip to each rafter on top of the poly, and then panelling over that. The 1x2 will limit the damage from all the nail holes you are going to put in it from nailing the t and g.
 

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The ceiling is only insulated so far. Faced fiberglass batts (R-19) are stapled to the rafters, with unfaced batts (R-30) placed on top of those, and blown in insulation on top of that. My understanding is that the faced paper acts as the vapor barrier, but not an air barrier, so need to plan how we will proceed to finish the ceiling to accomplish the air barrier and T&G.
 

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Hi fivecubs and welcome to the forum.
Don't try to save a nickle by re-inventing the process. Facts:
1. An air barrier needs to be a rigid layer.
2. You want the ceiling to be a fire barrier.
3. R-49 in zone 7 is a minimal approach so be neat and use the drywall (well sealed).
If you have penetrations, like recessed lights, select ones that are also well sealed and use low heat lamps. like LED.
4. Be sure you have sufficient ventilation. When running the vent panels all the way up to the ridge, each rafter bay has to deal with its own issues, like a recessed light. In your case the air flow from other rafter bays is not shared.

I'll watch for your reply.

Bud
 

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Discussion Starter #6
Each rafter bay has its own panels to the ridge vent, so hopefully no issues there. We had ceiling lights installed that are air sealed and IC rated, but from what I have been reading, even these have issues with air penetrations. At this point I don't think we would change them out, but will seal well around them.

Putting up the drywall will be challenging, to say the least, as it is a lot heavier and harder to handle than insulation. Which is why we were looking at other options like foam board that may be more manageable for us.
 

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I would use the same roll sheet products (underlay) that are used over subflooring before nailing on a wood floor. It could be stapled along the seams to hold it in place while the T&G is being added.

I would be cautious considering where you live in terms of when it is best to install the T&G. The wood will expand as it absorbs moisture and contract as it dries over the course of the year. A home humidifier can offset this if it is running during the drier winter months. Something to discuss with the installer and worth getting a second opinion.
 
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