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Old 07-25-2012, 01:09 AM   #1
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retrofit cathedral ceiling


I have an older house with post and beam construction and cathedral ceilings. The roof is framed out of 4x6" timbers, 4' OC. There is wood paneling on the inside, plywood decking on the outside, and an approx 6" cavity in the middle for insulation, which has 3" fiberglass in it. There must be some other roof structure going laterally between the 4' OC roof members, but I haven't been able to find a place to see it without tearing part of the roof or ceiling off.

This is zone 6, so the insulation is way sub-code by modern standards, especially for an unvented roof.

The roof is cedar shake and I'm replacing it with a new cedar shake roof next month. I'm trying to finalize my plan of attack on the ventilation and insulation since now is the time to fix it if I'm going to do anything. Anything I do with the insulation would have to be done from the outside.

It seems like some combination of closed cell polyurethane spray foam and SIP panels may be the best solution, but I don't know if I can do that at sane expense given that it's zone 6. A SIP panel would need to be quite thick, no? That would affect dormers and such. 6" x 3500 sqft of closed cell spray foam is expensive, too.

I could also try to retrofit a vented cold roof but I don't think I'll be able to get anywhere near code requirements with only a few inches to play with for insulation. Still, it seems that would be much cheaper to implement and still a big upgrade.

Any advice?

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Old 07-25-2012, 07:59 AM   #2
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retrofit cathedral ceiling


Post up a picture and that will get you more feedback.

Feel free to PM me contact information as there are a bunch of ways to skin this cat and you could type out a book on it.

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Old 07-25-2012, 07:49 PM   #3
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retrofit cathedral ceiling


Here's a few pictures.
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retrofit cathedral ceiling-interior.jpg   retrofit cathedral ceiling-wide.jpg   retrofit cathedral ceiling-gambrel.jpg  
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Old 07-26-2012, 06:45 AM   #4
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retrofit cathedral ceiling


I wouldn't bother with trying to vent that assembly.

I would go with a combination of spray foam and rigid foam to fill the rafter cavity followed by a layer of rigid foam to the exterior, follow by an overdeck vent, followed by your new roof.

Not sure I would use wood either while there are great synthetic and metal options for shake.
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Old 08-03-2012, 10:46 AM   #5
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retrofit cathedral ceiling


How old is your current roof? Is there a vapour barrier currently? Venting with wood shingles is very important, otherwise they tend to fail prematurely.

You could remove the plywood and insulate the 6" space (either with rigid foam or fiberglass, or both) and possibly sister additional lumber onto the existing to give you more depth. Then you could add vertical strapping to give you an airspace and reapply your plywood. Similar to Figure 3 here http://www.cedarbureau.org/cms-asset...1.mrm-11p3.pdf. I recommend reading over that site, as it is very helpful when it comes to wood shingles/shakes.
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Old 08-03-2012, 06:30 PM   #6
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retrofit cathedral ceiling


The roof is about 35 years old. I'm not sure if there is a vapor barrier or not, but I suspect not. It will be interesting to see what is really going on in there once the existing roof comes off.

I understand the need for ventilation, some sort of furring strips or cedar breather will be required. If I end with a vented solution, it would probably be something like you describe.

If I end up with an unvented solution, then it would be something like Windows on Wash suggested with cedar breather on top of the deck.

I need to figure out how I would deal with the dormers given an increase in the deck height. Perhaps just let them recess into the roof and leave them alone...

Thanks for the input!
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Old 08-04-2012, 09:46 AM   #7
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retrofit cathedral ceiling


Strap the roof and over deck vent it to allow the wood to breathe if you are going back with wood.

I would also look at metal since they have some very nice metal shakes.

On the dormers, you are going to have to cut the siding back/remove it, and get your flashing details re-done. Don't bury it behind the deck, it will be a leak point in the future.

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