I have a cathedral ceiling with no insulation. Construction is 2x6 pine tongue and groove sitting on 4x10 beams. These beams are spaced 4 feet on center. The roofing material is Cedar Shake. What I would like to do is install 4" styrofoam sm (R20) between the beams and finish with pine again to retain the original look. Will I encounter condensation problems? If so what needs to be done to rectify the problem? Thanks.
You need to:
1) Put in a ridge vent, soffit vents, and chutes to connect them. 2) Put in a plastic vapor barrier before you put the tongue & groove ceiling back up.Polyisocyanurate will get you better than the R-5 per inch that you are planning on for the styrofoam.
JK
I never had one, and I'm sure there is more than one design, but my friend has a cathedral ceiling that has a foot or two between the inside surface and the outside surface. If you have one like that, shouldn't the insulation be put in the inside, no need for more pine.
He went in their and did something once, but he says he's not going in again. He's thin and healthy, and could do it just as well as before, but I don't think he likes this kind of stuff as mmuch as many of the people here do. And he's 63 and I think you gotta really love attics to do this at 63.
I assume you are in a cold climate since you mention condensation in a cathedral ceiling. I would go with polystyrene over polyiso for two reasons: polyiso can absorb water, so if you do get any condensation, your foam could get soaked, and second, poly iso isn't much fun to cut with a circular saw. It is true that it gives you more R per inch. A difference between the 2 types is that polystyrene will act as a vapor barrier as it is (in thicknesses over 1 inch),,,polyiso has to have a foil face or some other coating to act as a vapor barrier.
Your trick will be to get an airtight fit with the foam to the 4x10 (if I am picturing this right). Definitely buy a pro grade foam gun. Might even be best to leave a gap between the foam and the 4x10 and fill this with expandable gun foam. Alternately, you could go for a tight fit, and caulk the foam to the 4x10. You MUST use a flexible caulk. Acoustical sealant is the preferred product. Also tape the seams. If you do a good job, a poly vapor barrier is redundant. By the way, a poly vapor barrier would have to be caulked to the 4x10's as well, with acoustical sealant. .
Are you absolutely sure there is insulation? Anything built in 1960s on up, the common practice was to lay down some sort of fiber panels over the T&G, then paper and shingle over that. Are there slats or something under the shakes to keep the back of them dry? How old are the shakes?
I'd get a good roofing contractor in, and price what it would take to insulate from the outside- pull off the shakes and any low-R fiber panels, reskin the roof with modern high-R foam panels designed for exterior use, and lay a new roof surface over that, with suitable flashing and trim around the edges to make it look right. (My preference would be some sort of standing seam metal- I hate the upkeep of shakes. If you want the shake look, they make plastic ones now that outlast wood, and don't rot.)Note that this would take a roofing company experienced in commercial-grade work, not the typical residential re-roofer that uses low-buck crews hired from anywhere.
Unless you give up a foot of headroom, and install a whole 'nother ceiling system and faux beams below, I don't think there is any good way to do what you are proposing, and have it come out looking right. Lots and lots of cracks between warm and cold material, and if condensation happens, lots of crevices for stuff to grow.
aem sends....
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