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Nathansdad

Insulating low piched roof rafters?

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I have a small sloped roof with 8' long rafters. Roof pitch is 2/12 and was famed up many years ago. Ceiling is flat. There was 6" of batt insulation on top of the ceiling. Frost would build up on the roof sheeting during the winter likely due to no roof or soffit vents. If I shot 4" of spray foam to the bottom of the roof sheeting would I be o.k. or will I still have a problem?

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Granted I did almost exclusively commercial/industrial architecture, but I was always taught that there must be a vented air space above the insulation in a vaulted ceiling. The very few houses I designed always had the vented space. I think it was sometime in the mid to late seventies that there were a few people advocating completely filling the joist space with insulation, but that idea died after just a few years....I never really heard why.

If it were mine, I would try to find a way to ventilate that space. Frost on the bottom side of the roof sheathing is a bad thing. Is there a vapor barrier on the warm side of the insulation?

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That's maybe a good part of the problem. When you put the ceiling back, you need to add a vapor barrier. But I would still suggest trying to find a way to get ventilation above the insulation.

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I'd be inclined to mount a whirly bird type roof vent on there. My feelings are; you still need to remove the trapped air and moisture.

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I'd be inclined to mount a whirly bird type roof vent on there. My feelings are; you still need to remove the trapped air and moisture.

With a good vapor barrier without holes in it, and good insulation, there shouldn't be warm air or moisture up there that especially needed venting, right?

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Granted I did almost exclusively commercial/industrial architecture, but I was always taught that there must be a vented air space above the insulation in a vaulted ceiling. The very few houses I designed always had the vented space. I think it was sometime in the mid to late seventies that there were a few people advocating completely filling the joist space with insulation, but that idea died after just a few years....I never really heard why.

If it were mine, I would try to find a way to ventilate that space. Frost on the bottom side of the roof sheathing is a bad thing. Is there a vapor barrier on the warm side of the insulation?

You're right. Moisture will always migrate through ceiling electrical receptacles, vent pipe runs, corner joists, etc.You will also get some moisture in the air from very cold air warming up, and this must be removed or it will condense and remain in your airspace.You always need to have roof /soffit vents to provide sufficient air movement, which is the key to removing moisture. Don't let anyone tell you that you don't need vents, you'll pay for it dearly.

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If you ever look into sips panel manufacturers they offer roof systems that are vaulted and are not vented. If you use spray foam I contend you can do it and not have vents. With fiberglass and no vapor barrier that is a whole other thing.

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You're right. Moisture will always migrate through ceiling electrical receptacles, vent pipe runs, corner joists, etc.You will also get some moisture in the air from very cold air warming up, and this must be removed or it will condense and remain in your airspace.You always need to have roof /soffit vents to provide sufficient air movement, which is the key to removing moisture. Don't let anyone tell you that you don't need vents, you'll pay for it dearly.

Uh, cold air warming up humidity goes down. Warm air holds much more moisture than cold air. Venting lets in cold air, and lets out the warmer air.

Some leakage is inevitable, so there probably needs to be some venting. But it is best to block as much infiltration around fixtures and pipes as possible.

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Trying to retro-insulate a much older home (he states "many years ago.." I take it that means an older house) by foaming from above is one heck of a job, to say the least. You'll never be able to hit all the little gaps and voids. Been there done that, working with two of the best roofing/insulation companies this town has to offer, in many attics, years ago. Sorry, but humidity in an attic does not always go down as the air warms up. One of the biggest mistakes people make is sealing up an attic and providing no venting or proper ventilation. Sheathing delamination, buckling, mildew, the list is long. It'll stay much cooler in summer, too.Best bet all around is proper ventilation up there.

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You absolutely need ventilation! Use those prefabricated soffit vents,easy to install, then spray foam over the installed vents. That should seal up most air leaks at the rafter lower end. Considering you have such a small roof , use those vents all the way bottom to top of rafters, making sure you vent out at the top of the rafters. I've had several experiences with a choice of vent or no vent. You will regret it if you don't.

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http://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-low-slope-residential-roofs

The article above has some good info on low slope roofs. You have to keep in mind that part of the concept of venting a standard pitch roof is the warm air will rise to the ridge where it can exit through the vent, but with shallow pitches like the 2/12 in the op situation you don't get the velocity because the path from rave to ridge is much flatter.

You may notice flat roofs have a much different approach to insulating and venting and that 2/12 can be done in much the same way a flat roof is done unless it was originally built to accommodate an acceptable depth of insulation for cellulose or fiberglass and that rarely happens. Often times you can't even get r20 at the heels of the rafters with cellulose and that can lead to condensation That can't be removed with traditional venting methods.

http://foursevenfive.com/unvented-flat-roofs-a-technical-discussion/

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Google a hot roof system--spray the foam on the bottom of the roof, and you will be fine!!!

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