Proper venting when insulating a shed roof

I'm insulating my shed and I have a cathedral ceiling. Initially I was going to fit foam board in between the rafters and give no kind of attic or ventilation space, but then I was made aware that the roof needs to "breathe", or else it would sweat and the rafters would rot out. I've also heard that if foam board is tight and sealed properly, roof sweating is no issue.

So first of all, I want to know if ventilation space is required for foam board? If so, I'm wondering how? At the top of the roof, I have both a copula and ridge vent. If I was to create some kind of ventilation space, what I was going to assume to do was run the insulation under the rafter ties, about 1' down and 1' wide, and leave that open space. Then I would run the rest flush with the ceiling.

I've never heard of rafters sweating, but you do nee to let air in under the roof sheathing to prolong the life of your shingles. Leave at least a 1" air gap under the roof sheathing and make sure there is an air inlet under the eaves and an air outlet at the peak to the outside. Typically you don't need a Cupola with a ridge vent, but it won't hurt anything to have one. If you insulate directly under the roof sheathing the heat from the sun will bake the life out of your shingles.

Gaps in any type of insulation leak either your heat or cooling depending upon the season. Tighter is better.

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Originally Posted by Lee Schierer

I've never heard of rafters sweating, but you do nee to let air in under the roof sheathing to prolong the life of your shingles. Leave at least a 1" air gap under the roof sheathing and make sure there is an air inlet under the eaves and an air outlet at the peak to the outside. Typically you don't need a Cupola with a ridge vent, but it won't hurt anything to have one. If you insulate directly under the roof sheathing the heat from the sun will bake the life out of your shingles.

Gaps in any type of insulation leak either your heat or cooling depending upon the season. Tighter is better.

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Hmm... Maybe I got some rethinking to do then. The issue is that I only got 2x4 rafters which leaves me with 3.5" to insulate with. I believe the ideal roof r-value is 19, and with 3.5" of foam board, I would achieve 17.5. Leave me with 2.5", I will have 12.5. With the copula in ridge vent btw, they are already on the shed

Actually, come to think about it, with foam board I can just leave the rafters open and secure the foam board underneith...

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Originally Posted by Michael Yadfar

Actually, come to think about it, with foam board I can just leave the rafters open and secure the foam board underneith...

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Or, put in 2" between the rafters, leaving your 1" air gap, held in place by another 3.5" on underneath the rafters and you will have R-25-30

Be sure to read the warning note on the foam board. The manufacturer will most likely recommend that it be covered with a non-flammable material.

Michael,

Look up Hot Roof. It's a method of insullating a cathedral ceiling without needing to leave vent space. It's an accepted practice in building codes. Requires that there is no venting at the top or bottom of the roof. Studies show no decrease in shingle or roof life.

Cliff

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Originally Posted by Cliff Polubinsky

Michael,

Look up Hot Roof. It's a method of insullating a cathedral ceiling without needing to leave vent space. It's an accepted practice in building codes. Requires that there is no venting at the top or bottom of the roof. Studies show no decrease in shingle or roof life.

Cliff

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+1 especially if you are in Western or Southern US.

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Originally Posted by Lee Schierer

Be sure to read the warning note on the foam board. The manufacturer will most likely recommend that it be covered with a non-flammable material.

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Yes I've heard this before, and foam board is truly highly flammable stuff. I was originally going to cover it in OSB, but that may not happen due to budget. My budget is to keep it under $1,000, which for a 16x9 shed that sounds easy, some people said I can do it with half that budget. The real cost though is running into this foam board which isn't cheap, a 4x8 3.5" panel is about $50, and I would likely need 5-6 for the roof. I plan to use 1" foam board for the floor covered with 3/4" plywood, which again isn't cheap. Then I got the walls which I hope to use fiberglass for, and the doors which will have to be rebuilt with foam board.

I figure in a woodshop with no flame used, bad electrical is my enemy.

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Originally Posted by Cliff Polubinsky

Michael,

Look up Hot Roof. It's a method of insullating a cathedral ceiling without needing to leave vent space. It's an accepted practice in building codes. Requires that there is no venting at the top or bottom of the roof. Studies show no decrease in shingle or roof life.

Cliff

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I didn't look too far into it yet, but from what I'm seeing, there needs to be an attic for this system. I probably could put in an attic, but it's not what I look to do. Someone mentioned this is big issue down south or out west, so maybe in the east my shingles baking aren't a huge issue... I actually read online that putting the foam board under the rafters is more efficent because I have less gaps, so it may not be an issue just leaving that 4" of rafter space open.

I used foam board and figured if there was ever a fire up there that it likely wouldn't be the foam board causing the problem. I will be reviewing the "hot roof" option though.

Michael,

No attic needed for a hot roof installation. Cathedral ceilings in many houses are done with hot roof systems.

Cliff

What I ended up doing was putting the foam board over the rafter, leaving the 3.5" of space open. That space will be vented by both the copula and ridge vent

I'm currently building a shop with a shed roof as well. On the tall side of my walls, I'll be drilling 2" holes on the blocking between the rafters.
On the lower end of the roof I'll be installing something like this.
http://www.dciproducts.com/html/midroof.htm

my rafters are 2x10, I'll be insulating with the roxul safe n sound product.
This venting solution was recommended by my county and seems like a pretty good solution.

I would have nailed a 2x2 on each side of the rafter against the roof sheathing, and fastened the foam to the 2x2. A 1 1/2" gap between roof sheathing and insulation will allow air to flow if you have soffit vents and ridge vent.