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Old 06-02-2013, 08:02 PM   #1
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2x6 Vaulted Ceiling insulation... again


I'm sure this topic has been discussed at length a million times, but I have an idea I would like some opinions on. I am working on a bathroom on my small weird old house. The room itself is a poorly built 40 year old addition that is sort of like a lean-to on the side of the house (clearly done without any permits or inspections). The roof was insulated poorly and was rotted. The outside wall is low so the vaulted ceiling is necessary. Since I can't afford to tear the whole room off and rebuilt it right, I am just fixing it as correctly as I can (under the radar since I can't imagine any inspector would let it stand as is). I'm also on a budget, but want to do a good job. Sorry for the rambling, hope that explains my situation. I am in the northeast, cold winters, hot summers, zone 5.

Here is my roof insulation idea, let me know what you think.

The rafters are 2x6, the ceiling is vaulted. What I would like to do is use Dow Tuff R closed cell rigid foam. The top layer will be installed 1" below the tops of the rafters and sealed at the edges with Great Stuff to allow for air to move from a soffit vent to a ridge vent. From there I will use the same insulation to fill the rest of the rafter (a remaining space of 4 1/2"). Under that I will cross hatch the rafters with 2x4s attached on their flat sides for an additional 1 1/2" of space. I will fill this with Tuff R sealed with great stuff again, for a total insulation depth of 6". Tuff R at a 6" depth will give me an R value of 39. 38 is required in my zone, and no other idea I've come up with even gets me close to this. To the flat 2x4s I will attach the drywall with no other vapor barrier because the closed cell foam shouldn't need it. I want to do this as a poor man's version of spray foam (got the idea from this youtube video) since I can't really afford the real thing right now.

The main summary of my idea is 6 inches of closed cell foam with an air gap at the roof sheathing and I can actually get my required R value. Will this work or am I asking for a moisture problem?


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Old 06-02-2013, 08:58 PM   #2
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2x6 Vaulted Ceiling insulation... again


Why not just use batt insulation? It'll have to be less expensive and considerably less messy than screwing around with what will probably end up being at least a half-dozen cans of that foam.

If you're not getting a permit for the work then who would be 'requiring' that R value? Sure, you want something in the way of insulation but not at the hassle and expense of screwing around with those foam cans.

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Old 06-02-2013, 09:43 PM   #3
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Why not just use batt insulation? It'll have to be less expensive and considerably less messy than screwing around with what will probably end up being at least a half-dozen cans of that foam.

If you're not getting a permit for the work then who would be 'requiring' that R value? Sure, you want something in the way of insulation but not at the hassle and expense of screwing around with those foam cans.
I am attempting to get as close to code as I can just to do it right. Also, I want as much insulation as possible for my own heating and air conditioning costs and with batts I can only get to like R15 at most. A little more hassle and expense now will pay off every time I have to call the oil man.
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Old 06-02-2013, 09:52 PM   #4
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2x6 Vaulted Ceiling insulation... again


It ain't just a little more hassle dealing with foam cans, especially not when you're trying to dispense them overhead. You do realize the cans only work when upside-down, right? How's that going to work when it's up in the rafters?

God help you if you get it on yourself. So plan on wearing a lot of cover. Get it in your hair and you'll be going for the buzz cut look real quick. And be sure nothing below gets hit.

I only focus on this from having to deal with it buttoning up the few stray places the spray foam guys missed on our new house. The stuff is great and has it's place, I'm just not sure it'll work out as well as you're speculating.
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Old 06-02-2013, 10:00 PM   #5
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I've used great stuff before, im familiar with it and how it works. I'm only using it to seal around the edges of the rigid foam. On the top side ill be working from the roof with no sheathing in place so I have total easy access. The cans of foam are the least of my worries. My real question is just wether or not closed cell insulation with an air gap at the sheathinh is a good idea.
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Old 06-02-2013, 11:08 PM   #6
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2x6 Vaulted Ceiling insulation... again


http://www.buildingscience.com/docum...n-roof-venting

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Old 06-03-2013, 06:17 AM   #7
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Based on that article it sounds like my idea would work, ideally with a 2 inch air gap instead of 1" since with my closed cell rigid foam and sealed at the edges the space will be air tight, and it will have painted drywall on the inside. I can furr up the roof deck with 1x3s to get the bigger gap.
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Old 06-03-2013, 10:43 AM   #8
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2x6 Vaulted Ceiling insulation... again


That will work quite well.

The only thing that rigid foam doesn't account for in this situation is the thermal bridging of the rafters.
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Old 06-03-2013, 01:12 PM   #9
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That will work quite well.

The only thing that rigid foam doesn't account for in this situation is the thermal bridging of the rafters.
If I added some type of insulation above the rafters would I then furr the decking up off of that and leave the air gap just below the decking?

I will most likely have to compromise on the thermal bridging to keep form having to re-engineer the roof because it's not simple geometry. It meets hips and vallies. The space is small, only about 9 feet across, so I will live with whatever heat loss I get throuogh the rafters as it will be 1000% better than the house was. The plan down the road is to tear this room off the house and redo it correctly. What I am doing is a semi-temporary rot-correction solution for the time being. But if I never got around to the major renovation of fixing it correctly I don't want to encourage rot with what I am doing so that is why I am willing to do it as correctly as possible. I'm hoping the thermal bridging through the rafters will not be significant to create a condensation problem, the closed cell foam won't absorb water, and an adequate air gap will hopefully allow any condensation to dry out like a properly built roof should.
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Old 06-03-2013, 09:25 PM   #10
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2x6 Vaulted Ceiling insulation... again


I was thinking about the thermal bridging problem. Since Tuff R is sold as a sheathing could I use it under the rafters without the cross hatched 2x4s and attach the drywall right to it with long screws to penetrate into the rafters?
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Old 06-04-2013, 06:09 AM   #11
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Yes. That would work.

You would effectively be making your own SIP deck.

The idea of above deck ventilation is great if you are okay with the added expense of the project and you are designing a great roof system.

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