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Insulating a cathedral ceiling

| Posted in General Discussion on December 22, 1998 09:33am

*
I’ve built a 21 x 24 family room addition with a 2 in 12 trussed
cathedral ceiling under a 4 in 12 roof and I am wondering how
to best insulate it. It has a continuous ridge vents, & soffit vents. The space above the trusses is about 30″ at the peak down to 3.5″ at the eaves. The space is penetrated by 3 IC rated canister lights and two heating ducts. Through talking with contractors and browsing the net I’ve seen two options:
(1) Fiberglass batts with a 1.5 inch airspace (Proposed by the guy with the pink panther painted on the side of his truck); and (2) Blown in cellulose, blocking the soffit bays and blowing the ceiling tight. I want to get as much r-value as possible. We have mild winters (usually) and hot humid summers. What are the advantages and disadvantages of the above options? I’m a reasonably competent DIYer. Can I blow in the cels? Any tricks to be aware of? What are the cost factors? Thanks.

I like the new format, BTW.

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  1. Rob_McPheeters | Dec 22, 1998 09:21pm | #1

    *
    Either way, make sure you don't block the soffit vents. You didn't mention whether you were using turtlebacks, turbines, or ridge vents. The idea is to always enable air flow from the soffits to the upper vents without any obstacles. Batts should work just fine, as will blown in cellulose, as long as the air moves freely over the insulation and out the upper venting. You encounter severe problems when the warm moist air gets trapped in the space. It will soak the insulation and give you water stains on the interior drywall, etc. You are actually at the most important phase of this job. What you do next will determine how well the project holds up.

    1. Rob_McPheeters | Dec 22, 1998 09:22pm | #2

      *Excuse me, you did in fact say you were using ridge venting. Best of luck...

  2. Guest_ | Dec 22, 1998 09:32pm | #3

    *
    I thought we were all learning in this very discussion group that any venting was a bad idea. The real way to do this is to create a continuous pressure/vapor barrier accross the whole ceiling (including the cans and ducts. Then filling the entire space with -not- fiberglass; i.e. cellulose or foam - which also act as a radiant heat barrier. In this manner there is NEVER hot moist air in this space to get rid of.

    BTW - John, FYI - I believe you have what is called a scissor truss. It is a pretty common way to get a cathedral ceiling without a ridge beam or collar ties, however at a lower slope. I have seen these detailed with a block at the connection to the top plate of the wall so that you can get a full load of insulation in that space.

    Where would you get fg batts that taper down to 2" anyways?

    This no-vent argument is really convincing when you look into it. I always felt that way (intuitive I guess) but the proof is out there, aside from my own 130 year old non vented, non ice damning, non rotted hip roof.

    -Rob

  3. John_Benshoff | Dec 22, 1998 09:33pm | #4

    *
    I've built a 21 x 24 family room addition with a 2 in 12 trussed
    cathedral ceiling under a 4 in 12 roof and I am wondering how
    to best insulate it. It has a continuous ridge vents, & soffit vents. The space above the trusses is about 30" at the peak down to 3.5" at the eaves. The space is penetrated by 3 IC rated canister lights and two heating ducts. Through talking with contractors and browsing the net I've seen two options:
    (1) Fiberglass batts with a 1.5 inch airspace (Proposed by the guy with the pink panther painted on the side of his truck); and (2) Blown in cellulose, blocking the soffit bays and blowing the ceiling tight. I want to get as much r-value as possible. We have mild winters (usually) and hot humid summers. What are the advantages and disadvantages of the above options? I'm a reasonably competent DIYer. Can I blow in the cels? Any tricks to be aware of? What are the cost factors? Thanks.

    I like the new format, BTW.

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