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Old 09-03-2020, 09:49 PM   #1
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attic pitch and floor insulation question


I can't find any info on this, but live in arizona and my roof gets 140-190 degrees in the summer. I have a stick frame 45 degree pitched roof about 10 inches of blown insulation on the floor. Built in 1999, 5 static roof vents, 3 gables 2 on house and 1 on attached garage. I know i need to add a good amount of blown insulation, but i have a lot of remodeling to do. Electrical and low voltage ect. I don't want to blow in all that new insulation and then have to dig through it to find wires and i don't want to smash it all down. So i was looking into getting some 2 inch Thermasheath Rmax closed cell foam panels up against the inside of the attic roof to keep that radiating heat from getting through into my attic. I have heard mixed info on this and can't get a clear answer. As everyone thinks i want to seal off the attic; I don't. But then some one said you can't have both floor insulation and inside roof pitch insulation as it will create " a moisture barrier" even though my attic is vented. I live in almost 0% humidity 95% of the year. I have an AC-Gas-Pack system on the roof and soft tube duct work in the attic. it was 146 degrees in my attic. My roof is south facing.

So would this be a good or bad idea as a temp solution and then eventually once i put more blow in insulation would i need to remove those panels or can i even do this. I would still leave it a vented attic.

Thanks for any info . insight you might have. Cheers!
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Old 09-03-2020, 10:28 PM   #2
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Re: attic pitch and floor insulation question


It is common in the hot south to use a reflective material directly below the roof sheathing. This reduces the radiation from the roof down to the attic floor. They even make plywood sheathing with a foil bottom surface.

Sounds like you want to apply that 2" rigid to bottom of rafters. Only issue I see is ventilating the cavity above the rigid as well as continuing to vent the attic.

You didn't mention soffit venting and not sure if the 5 roof vents are providing sufficient vent area, that is a calculation you should do.

Note, your foil faced rigid would be performing double duty by reflecting the energy back to the roof as well as reduced radiation down to the attic. Not really sure how much the r-value figures in as the foil facing down is doing most of the work. But the r-value doesn't hurt.

Bud

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Old 09-05-2020, 03:30 AM   #3
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Re: attic pitch and floor insulation question


Hey Bud thanks for getting back to me so fast. I realize i didn't clarify what I meant very well. I meant to cut the foam to place in-between the rafters directly against the underside of the roof board. It would make direct contact with the OSB /Plywood that you see when looking up in the attic. Essentially like you do with spray foam or batting insulation, just with the 2 inch reflective foam. Which I know isn't much, but I effectively want to help combat the heat radiation from the roof board into the attic. I put 3 of these panels inset against 3 of my west windows in a room and it reduced the temperature in that room 10 degrees F because the closed cell blinds weren't helping enough and they are single pane windows. I was like "WOW these worked really well, what if i did the same thing up in my attic, laid inset between the rafters against my roof board?"
I just have never heard of "Dual insulation" in a "vented attic". Usually people only do that if they are sealing off the attic to turn it into a living space or temp controlled space. but I can't imagine that it wouldn't help at least some along with my current insulation. I just can't find anything on the internet about it. I would cut the holes in the foam as to not block the 5 static roof vents cut into the roof board. I don't have any soffit venting i have 3 large gable vents as my house doesn't have an enclosed soffit.

I hope this helps clarify what I'm trying to do. Thanks for the welcome and thanks for taking the time to read this.

Thomas-
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Old 09-05-2020, 05:49 AM   #4
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Re: attic pitch and floor insulation question


Hi Thomas, It will help just as you anticipate and even if you upgrade with more insulation on the attic floor those insert panels can stay.

A similar approach is when someone adds rigid foam over the the roof sheathing before installing shingles it just reduces the heat passing through.

Best,
Bud
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Old 09-06-2020, 05:42 AM   #5
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Re: attic pitch and floor insulation question


Foil can't touch anything solid. I think there must be some inches of air. Not sure why but that's the rule I read before. Search for how to use foil.
Foiled foam board directly on the osb sheet may not work since everything gets heated. Not sure what foil does either. Reflect but what? Radiation, heat? Sorry but that's all i know about it.
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Old 09-06-2020, 05:56 AM   #6
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Re: attic pitch and floor insulation question


Foil and all reflective surfaces work to stop heat transfer in two ways, reflectivity and emissivity. Where the foil faces the attic it will greatly slow any heat transfer across that air gap. The insulation properties do also reduce the heat flow.

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Old 09-06-2020, 06:48 AM   #7
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Re: attic pitch and floor insulation question


The potential problem you face is that the roof sheathing will get no ventilation if you put the foil faced polyiso up tight against the roof. Probably better to space it out a few few inches and ensure venting carries air from the soffits and out the ridge. But then you might be compromising venting in the attic space between the floor and the foam-board.
Keep in mind that best practice for insulation and ventilation is much affected by climate area, and Arizona is much different than where most of forum members live. Best to use practices that are proven in your area.
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Old 09-06-2020, 08:34 AM   #8
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Re: attic pitch and floor insulation question


http://www.radiantbarrier.com/resour...eat-gain-loss/


Above. Keep looking and finally one article that gives a simple summary of all things. Kudos bud9051 for pointing the way.

Aluminum sounds magical but it looks like that's what aluminum does.

If i had the time and will, i may try this: foil faced foam board against the roof decking with 1 to 2" of cleats as spacers. One thing I'm not sure about is: is it a must to have foil on entire surface? That is, can i leave 1-2' along the eave (where space is narrow and a hell to work under) and some gap along the ridge and expect the foil to do its work? The article seems to say it will work since it's not the air movement but how aluminum reflect the radiation. Not close to 100% but maybe get half to 70%? In this case, it also feels like foil shouldn't touch the joists to reduce as much conduction. As such, maybe home made reflect panel, 1/4" ply and glue on normal kitchen foil, is cheaper? Instead of cutting the foam/ply board to fit between the joists, cut it as big as possible (to access the attic) and fasten to the joists?
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Old 09-06-2020, 08:58 AM   #9
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Re: attic pitch and floor insulation question


Leaving the eaves as is, is fine.

A radiant surface like foil can have a rating in the range of 98% and 2% when it is shinny and clean. That's 98% rejection of incoming heat with only 2% of heat emitted. Radiant surfaces will usually have complimentary ratings adding up to 100%. Here is what you get with both sides covered with foil and that added air gap.

98% of the heat from the roof is reflected back to the roof, it can handle it. So only 2% is left to heat its way through the foam insulation. When it gets to the attic side the 2% emissivity means only 2% of the 2% coming through is radiated towards the attic floor and that number is reduced by the insulation.

So, 2% of 2% leaves only 0.04% of the heat from the top of the roof coming through to the attic space. Nice low number, but a single foil surface yields a 98% reduction and that 2% that gets through is small compared to the heat coming in through the soffit vents. In other words, the benefits of the second layer of foil are not really worth the cost/effort. Plus, over time that space above the rigid will be contaminated by dust and the 98%/2% rating will decline. The attic side foil is facing down so less susceptible to dust settling and it could be cleaned.

Sorry for the long
Bud
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Old 09-06-2020, 09:44 AM   #10
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Re: attic pitch and floor insulation question


Quite sure those panels will say even if you end up upgrading
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Old 09-07-2020, 06:35 AM   #11
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Re: attic pitch and floor insulation question


Step by step number deductions are just what i need. Reason i did fine on algebra and got stopped cold by calculus. All those intervening formulas.


OP: forgot to add. If high attic temps, solar roof fans should help, with enough intake vents. Maybe in the gables? I have gable power fan and it's been working well although paying to run the fan is a cost.


Not sure if i'd add foil to my roof. It's a small house, so smaller roof. The benefit of reflected radiation may be less compared to running the fan to cool the attic as well as the moisture in it. In nj. But your dry conditions, I think, could benefit from using the foil.
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