To vent or not to vent....

Started by OlJarhead, December 06, 2010, 02:42:06 PM

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OlJarhead

OK I'm in a dilemma.

Somehow I thought it would be easy ;)  Just put up the roof, stuff it with insulation and *poof* warmth :)

Not so much.

OK so the story goes like this:  I have 6" rafters on 24" centers on a 12x12 pitch with felt and metal roofing on the outside.

I planned to insulate with 4 1/2" of insulation leaving 1" of vent room and installed a ridge vent to accommodate that plan.

I then found I could not get 4 1/2" insulation so decided that maybe I'd put in 1" of foam and 3 1/2" insulation but then learned R13 is not made in 24" rolls or batts.  Or if it is it is not available locally.

So that brings me back to a post I read a while back suggesting that a roof did not need to be vented.  If this were the case I could install R19 (6 1/4" x 24") and be done with it -- but what would the hazards/trade offs be?

Bear in mind that this is a 14x24 cabin with 10' walls and a 12x12 pitch.

Thoughts?  Suggestions?  Ideas?
Thanks

MountainDon

I believe the key to the idea of a non vented roof with insulation is to totally remove or isolate the cold plane, the roof sheathing in your case. That is usually done most easily by applying foam board over top the sheathing before the final roofing material is installed. I believe that is the method building science recommends.  The foam insulates enough to remove or 'warm' the plane where moisture will condense. No condensation, no problem. At least that is the gist of it.

After the final roofing is installed it gets harder. Spray foam works; $$. Foam boards would work, but there's a lot of careful cutting and sealing that needs to be done. Then you still need something to finish the insulation.

BTW, R19 installed in 2x6 comes out at about R18, after compression.



Add furring strips to the underside of the rafters and use R19 with the air space above. Probably have to cut your own strips about 1  1/2" thick?  ???


Just because something has been done and has not failed, doesn't mean it is good design.


OlJarhead

How about furring strips and R21?

For some reason I keep finding R19 instead of R21 locally so that's what I've been using.

I actually thought about the furring strips idea before but then dismissed it due to the amount of work needed but I'm wondering now if it wouldn't be worth it?  Of course I'd have to screw them on I think, but it would solve another issue for me, which is the top of the knee wall ledge that I'd like to hide.

MountainDon

R21 is made but for some reason not readily available. I found some at a local installer, but he would not sell it. He would only install it. But he wouldn't drive up to the mountains without a surcharge and that put it out of the question, out of the range of what I wanted to spend.

Try to see see it someone would special order it at a reasonable price. That was a wall I ran into as well.
Just because something has been done and has not failed, doesn't mean it is good design.

Redoverfarm

Seems there is always a trade off.  I think that if you used 3/4" firing strips attached to the upper portion of the rafter adjacent to the roof sheeting followed up with 1/2" foam board that any loss you would have in compression of the R19 would be gained by the foam board.  In essence you would be creating your own "rafter-mates" or "Duravents" .  I believe that those 1/2" blue foam has a R3 Value.  You could check and see if 3/4" foam board is available as well as it is about R5 I think.


OlJarhead

Quote from: Redoverfarm on December 06, 2010, 09:44:45 PM
Seems there is always a trade off.  I think that if you used 3/4" firing strips attached to the upper portion of the rafter adjacent to the roof sheeting followed up with 1/2" foam board that any loss you would have in compression of the R19 would be gained by the foam board.  In essence you would be creating your own "rafter-mates" or "Duravents" .  I believe that those 1/2" blue foam has a R3 Value.  You could check and see if 3/4" foam board is available as well as it is about R5 I think.

This was what I was thinking of doing when I first realized the problem I had.  Only I was thinking 1" of foam and 3 1/2" of fiberglass.

So if the foam were R3 and the glass R13 then I'd at least have R16 right?  And the venting.


Redoverfarm

Quote from: OlJarhead on December 07, 2010, 12:24:02 AM
Quote from: Redoverfarm on December 06, 2010, 09:44:45 PM
Seems there is always a trade off.  I think that if you used 3/4" firing strips attached to the upper portion of the rafter adjacent to the roof sheeting followed up with 1/2" foam board that any loss you would have in compression of the R19 would be gained by the foam board.  In essence you would be creating your own "rafter-mates" or "Duravents" .  I believe that those 1/2" blue foam has a R3 Value.  You could check and see if 3/4" foam board is available as well as it is about R5 I think.

This was what I was thinking of doing when I first realized the problem I had.  Only I was thinking 1" of foam and 3 1/2" of fiberglass.

So if the foam were R3 and the glass R13 then I'd at least have R16 right?  And the venting.



Not real sure but even if you used R19 and it was compressed by 1" you would still get R17-18 and the foam would add R3 ended up with R20 there abouts.  Maybe I am wrong but it would give the highest R value without firing out the rafters.  If you could find a crown stapler for strip attachmnets it would make installation easier.  In addition if your bays were 24"OC I would probably run an extra strip in the center so that the foam board would not collapse. 

It is not the ideal insulation for a ceiling but for a temporary get-a-way it might work.