Read the recent FH article on SIP’s construction and saw the details for a ventilated roof. I’m wondering why the need for the ventilation. Isn’t the primary reason for ventilating a conventional attic to keep the insulation dry? I know it might add some longevity to the roofing, but is the addition expense going to add more than a year of two life to the shingles? I must be missing the big picture here. I’m looking at this from a northern climate view.
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The benefit I see is to allow the underlayment (felt paper) to dry if it becomes wet. Depending on the type of roof you put on, this could be a common or a rare situation. I'm building a SIP house right now and I am considering the idea, although I haven't figured out how to vent the opening at the fascia without it being visible - ie., I don't want vent holes in the fascia.
>>>>>>>>I haven't figured out how to vent the opening at the fascia without it being visible - ie., I don't want vent holes in the fascia.Use vented drip edge (probably not the best alternative) or fir the fascia out slightly and vent from the soffit if you have one.http://grantlogan.net/
http://www.dciproducts.com/html/smartvent.htm
i've never used it, we used one of their other products and i saw this in their literature.
carpenter in transition
It also allows some of the heat buildup to vent out which reduces the cooling load.
http://grantlogan.net/
Merty,
In general it's a good idea to vent any roof to reduce moisture buildup. A SIP roof can't be vented on the inside so it has to be vented on the outside by means of a cold roof.
You're right that one reason to vent with a cold roof is to keep moisture from building up under the shingles and thus on the surface of the OSB.
The other reason is to try to prevent what's called ridging (literally little ridges in the shingles). This is caused by expansion/contraction of OSB in relation to asphalt shingles. It is not a SIP specific issue but it does seem to be a hot button concerning SIPs. Apparently a cold roof will significantly reduce the risk of ridging.
jross -- FH Editorial
I had my current house converted to P&B with SIP's from a stick built plan. Long story but we didn't do it due to the P&B builder. It's sticked. Anyway, the shingles were not going to be covered for warranty unless they were installed over a cold roof. That message came straight from the factory. The yard called and I also spoke to the rep myself on the phone. I was not willing to gamble with a 35 year shingle.
I think that you raise a good question about the value of putting a cold roof over SIPs. We have done it both ways but will have to wait another 15 years to make any kind of evaluation. I don't think that I would recommend it to the next customer unless they had particular concerns. Sometimes peace of mind is worth the price.
Thanks for the feed back. I'm just starting my first complete home. With the project growing bit by bit as unforeseen details arise, I'm inclined to trust the manufacurer and not install the cold roof. Any idea how warrents their asphalt shingles for this application?
One or two manufacturers (I think Elk is one) warrant their shingles over a hot roof (straight on SIPS) but I don't know the warr. terms. I used SIPS over my timberframe, and did a cold roof system. On the soffit, I attached cor-a-vent strips to the edge of the SIPS (on the 2x6 infill) behind the fascia board, and then strapped it out so that the whole fascia has an air space behind it. Then the entire SIP roof got Grace Tri-Flex (Grace I&W for the first four feet and along the entire rake edge) and then 1/2" thick straps on 16" centers, 1/2" plywood with clips, 15# felt and 30-year shingles. Basically two entire roof systems. Expensive, but with the warranty issues I wasn't taking any chances. Plus the house stays cool in the summer and warm in winter, and there are no ice dams.
Jon-- sounds like a great way to get the ventilation. (Bet there's not much storage in that attic)
No attic. Timberframe, whole thing is cathedral ceiling.
You should ask around about how many shingle roofs have their warrantees used in your area. We have only had one roof that needed to use its warranty in almost thirty years and this was a case of clearly defective shingles. I don't doubt that your roofs life could be reduced by up to 10% but it is likely that it might still outlive the warranty.
Run the numbers. Figure out what it will cost to build a cold roof. Compare it to the present value of the lost 5 years of roof life 35 years down the road. You may find that upgrading to a shingle with a better warranty makes more sense. Or not.
Isn't the primary reason for ventilating a conventional attic to keep the insulation dry?
In colder climates, snow country, a conventional roof (insulation between ceiling joists, rafters or trusses supporting plywood roof, covered by shingles) requires ventilation primarily to keep the surface temperature of the entire roof at the same temperature when temps are close to freezing.
If the roof area over the living space is slightly warmer than the overhang when temps are near freezing, snow will melt and refreeze as ice on the overhang, forming an "ice dam". As this thawing and refreezing continues, the ice rises until the melted water creeps under shingles over the living area, causing leaks inside the attic space and, eventually, the living space.
In warmer climates, attic space ventilation serves to reduce temperatures caused by heat radiating into that space through the roof. It also helps to control humidity levels in that space.