Venting and polyurethane insulation
I will be adding a second story with cathedral ceilings. I will be using a minimum of 4 inches of spray polyurethane. I have talked with several insulation contractor and everyone agrees that with polyurethane you don’t need a vapor barrior but as far as venting is concern there seems to be confusion. I have heard a lot of different opinions about affecting the life of the shingles but why would it affect them more than on roof constructed of insulated panels? I am in Tennessee with moderate climate and not really any snow accumulation. Any strong opinions?
Thanks
Replies
Check out the insulation manufacturers recommendations yourself.
http://www.Corbond.com is a good one for 2# foam.
The recommended way is to apply directly to the roof deck (inside) with no venting. Some people will always have their own logic but I will trust the manufacturers first.
That should also weed out some less desirable contractors for you also.
Stu
Venting is a load of horse dung, at least in this climate.
I'm in central NC and I sprayed 3500 sq ft of roof deck with 1/2 lb foam (open cell). No vents at all, in fact I couldn't vent it if I wanted too. Too many gross gables, not enough soffit area to meet the stupid code. So I went to the mat with my building department, armed with letter from building science engineer letter specifying roof insulation design, and I got it approved. First time it was ever done in my town (Chapel Hill).
So get some docs together, get help from the foam guys (my foam guy has copies of the newly adopted blah, blah code that specified venting might not be necessary, etc.) Also study buildingscience.com. I had asphalt shingles on my roof which apparently drive moisture into the roof assembly when they get wet and then rapidly get hot (summer rain, sun comes out, etc.). So my engineer wanted the roof to be able to dry to the inside, so he spec'd open cell foam and no vapor barrier, not even vapor barrier paint on the drywall.
If you haven't yet done the work, you have some good options (I had already put on my roof shingles when I got into all this). You can either use a metal roof which emits no vapor of any kind *or* and this is really cool, you can put rigid foam over your roof sheathing. It keeps the roof sheathing warm enough that no moisture will condense on it. Do some reading at buildingscience.com and convince yourself.
Spray foam is awesome!
MERC.
I think the key here is *open cell* foam....
That's right. I used to think open cell sounded like junk. But then once I got some more education and started understanding that it allows your roof and walls to dry, I was sold. I'd be very careful spraying high density foam since it is a vapor barries, and a pretty good one at that. It would certainly keep moisture off you sheathing from the inside, but you also have to deal with the outside and I wasn't convinced that is easy to do for the life of a house.
MERC.
I am very interested in the spray foam (open cell) for a home that I am building in 06. I am reading up like a mad man.
All sounds good for my Hot central TX environment except for one thing that worries me. The roof is my real concern. For instance, if you seal the underside of the roof with the spray insulation how would you ever locate a leak without extensive removal of the quite expensive insulation or roof material. Also, would the insulation also not act to effectively trap the moisture under the deck around the leak leading to further decay?
Yes, those are good questions. Everything in building is a trade off and roof leaks are never fun. I put a 50 year roof on my house to minimize the risk of an unnoticed leak. My foam guy claims that leaks come through in basically the same area as they are occuring. He also claims that it won't rot the roof deck anymore than a roof leak with fg insulation. Says the insulation dries out pretty quickly.
However, I'll certainly be keeping an eye on the roof condition and replacing it when it looks worn (if I happen to be alive in 50 years).
MERC.
I have similar concerns. I have a design idea that has a section with a flat or very low-sloped roof with a deck on top. I have seen a couple of articles in FHB that used a membrane roof material on a very slight pitch and tapered pseudo-joists to get a flat deck surface. It seems to me that when the membrane starts to go you could have a lot of rot before you saw any signs from the inside.For the rest of the house I am budgeting for standing-seam, which should outlive me, but my nervousness may drive my design options for the flat section.Am I stupid? OK, better question: Am I wrong?
Jim-
A great solution for you is to have a foam roof installed!This will double as insulation to there will be no need to worry about anything rotting underneath from holding water there. There are also no seams in a foam roof and it is self flashing - sticks to everything. IF there is ever a leak it is usually from a pin hole and the water goes straight thru - easy to find the spot to repair with normal caulk. A small job like yours might be hard to find someone for but it might be worth it. Stu
Do you know of any on-line sources I can investigate?I remember running across a site a few weeks back but can't seem to find it now, but they sold foam sandwich panels that looked to be a little smaller than 4' X 8' that were either flat or had a taper of maybe 2" in 4'. It looked interesting, but was targeted at commercial building and would still need something like EPDM on top. Any experience with something like this?My building timeline is roughly 2 years from the wife returning to work, so the market could be quite different by then, but it gives me plenty of time (perhaps too much) for research.-j
The temperature of the shingles is impacted far more by shingle color than venting. Venting is counterproductive, and can cause moisture problems.
In some cases, the challenge is to convince the code official.