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Old 03-16-2012, 01:47 PM   #1
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Insulating unvented attic w/ 2x4 rafters?


I just purchased a tudor style house built in 1931 with converted attic space. The finished attic space is not well conditioned and I know that if I don't better insulate I will have to dump enormous amounts of AC into the space this summer to make it livable. So right now I am trying to figure out the best route to insulate.

More about the space:
1. Total attic space: ~800 sqft
2. 2x4 on 24" center rafters
3. 2x6 on 24" center floor joists (no insulation between joists)
3. There are four roof mounted vents, no ridge vent, no gable vents
4. There are no soffit vents
5. Two knee walls with no insulation
6. Two east facing walls that have been drywalled but not insulated (slope walls, drywall applied to rafters)
7. Ductwork in attic space above finished area
8. Located in Kansas City, Missouri

I put unvented in the thread title because the attic really isn't vented as there is no air intake. I have researched and researched to try and find the best method but I am still not certain which way to go. These are the options I have come up with.

1. Keep the attic unvented. Seal off the vents, seal any air pockets and then have the underside of the sheathing spray foamed. Open cell or closed cell? I have read quite a few debates and opinions about this. The concern with closed cell is that it will trap moisture against the sheathing and rot out the roof. I like the idea of conditioning the space by having it unvented because I have ductwork up there and it would reduce the load of my HVAC. The fact that I also just have 2x4 would make it easier to achieve the necessary R-values than by using batts (correct me if I am wrong). Are the newer fire-rated spray foams now a one step process? Anyone have any experience with price per sq ft, I've seen comments that range from $1.30 to $6?

2. Vent the attic. Drill intake holes in the blocking between the rafters, add rodent mesh, add air baffles, add furring strips to the bottom of the rafters to create necessary depth for high R-value batts. Use a R38 in the rafters (on parallel space to living space), r13 in the knee walls, loose fill between the joists, weatherstrip the access panels in the kneewalls, add insulation to attic ceiling.

Option 1 sounds like the obvious option but I'm not sure how much it will cost, I have inquiries in to several companies to give me a quote. Option 2 sounds like a lot of extra work and in the end my HVAC is still going to have to work harder because the ductwork is located in unconditioned space. What am I missing? What could I do differently? Rigid foam cannot be used instead of spray foam on the underside of the sheathing can it?

Thanks in advance!

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Old 03-16-2012, 01:59 PM   #2
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Insulating unvented attic w/ 2x4 rafters?


Pictures of what I'm working with

Attic space on the side of the living space


Upper attic:


Where rafter meets the header for the outside wall


Space between bottom segment and upper segment of attic

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Old 03-27-2012, 02:55 AM   #3
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Insulating unvented attic w/ 2x4 rafters?


have you considered Structural Insulated Panels for the underside of the roof?
http://www.doityourself.com/forum/in...eilings.html#b
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Old 03-27-2012, 10:08 AM   #4
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Insulating unvented attic w/ 2x4 rafters?


From those pictures you have a mixture of conditioned and un-conditioned. You need to establish what you want and your insulation plans will be dictated by that.
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Old 03-27-2012, 05:28 PM   #5
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Insulating unvented attic w/ 2x4 rafters?


Since this post I have insulated the knee walls with r13 batts which has helped quite a bit but the space is still not as comfortable as I would like.

Considering the amount of extra work to create a proper ventilation system I would rather create a conditioned attic space. However, from what I've seen this can really only be accomplished with spray foam? I'm not sure if that option is immediately within my budget, maybe next year.

Seattle2k, I've considered using foam board on the underside of the rafters, however that link seems to only discuss using it on top of the sheathing? The problem I see with attaching foam board to the rafters is that I would need to put an ignition barrier on underneath it as well.

The other thing I have considered is putting up a radiant barrier on the underside of the rafters. In the attic space parallel to the living space the radiant barrier would have the added benefit of keeping the batts in place between the rafters.

Thoughts?
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Old 04-26-2014, 11:08 AM   #6
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Insulating unvented attic w/ 2x4 rafters?


Quote:
Originally Posted by rion.martin View Post
Since this post I have insulated the knee walls with r13 batts which has helped quite a bit but the space is still not as comfortable as I would like.

Considering the amount of extra work to create a proper ventilation system I would rather create a conditioned attic space. However, from what I've seen this can really only be accomplished with spray foam? I'm not sure if that option is immediately within my budget, maybe next year.

Seattle2k, I've considered using foam board on the underside of the rafters, however that link seems to only discuss using it on top of the sheathing? The problem I see with attaching foam board to the rafters is that I would need to put an ignition barrier on underneath it as well.

The other thing I have considered is putting up a radiant barrier on the underside of the rafters. In the attic space parallel to the living space the radiant barrier would have the added benefit of keeping the batts in place between the rafters.

Thoughts?
Hey there! Any great strides in the last two years? I'm in exactly the same situation as yourself. Would love to hear what you wound up doing.

My house has an addition built perpendicular to either side of the room we're giving a cathedral ceiling, so now only half of one side of the cathedral ceiling sits on an exterior wall. Should I even bother with a ridge vent if only 25% of the ceiling would benefit from soffit vents? I'd love to go closed cell foam (I think?) but I don't have too much money to spend on it right now.

I am leaning toward 24" Reflectix reflective insulation between each 2x4 rafter to act as a vapor barrier and 2" rigid insulation foam, held in place with Great Stuff Pro.

Thoughts?

Last edited by liquidbeef; 04-26-2014 at 11:13 AM.
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Old 04-27-2014, 07:43 AM   #7
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Insulating unvented attic w/ 2x4 rafters?


Pictures.

Not a fan of the radiant barriers. They are cheesy science and can cause more problems than fix.

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