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Old 06-07-2017, 07:58 PM   #1
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OSB vs Attic Foil?


Hi,

A crossover post from my upcoming roofing adventure.

I want to increase the R level efficiency while I am replacing my roof. Now, from what I understand, I can lay OSB sheathing with the foam radiant board over my existing sheathing OR staple attic foil against my attic ceiling.

Are there any other options that I am overlooking? How effective is the OSB vs the attic foil? Obviously, there is a substantial cost difference between the two but it seem the OSB will give me a slight bump in R level (R6 I believe) so should be a little better during the winter months.

I currently have blown insulation in my attic but that might have to be revisited as well (either blow in some more or upgrade to a roll insulation).

Thanks!
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Old 06-07-2017, 08:03 PM   #2
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Re: OSB vs Attic Foil?


Not even close to enough info.
What's the location?
Attic a living space?
Has it been air sealed.
How much insulation in the attic?
https://www.energystar.gov/index.cfm...sulation_table
Adding foiled back OSB is going to do nothing to add to the R value.
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Old 06-07-2017, 09:05 PM   #3
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Re: OSB vs Attic Foil?


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Originally Posted by joecaption View Post
Not even close to enough info.
What's the location?
Attic a living space?
Has it been air sealed.
How much insulation in the attic?
https://www.energystar.gov/index.cfm...sulation_table
Adding foiled back OSB is going to do nothing to add to the R value.
1) Southern California
2) No
3) Not really, it needs to be reviewed.
4) This will have to be measured.

Fundamentally, I would like to learn the best practices with respect to making a home more comfortable in extreme heat conditions and the winter season.
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Old 06-07-2017, 10:11 PM   #4
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Re: OSB vs Attic Foil?


What style roof?
How is it vented?
What type and size soffit vents.
How old are the windows, single pane?
Any insulation in the walls or under the floors?
Crawl space or basement every been air sealed.
I'd be far more concerned with these issues then what's on the roof.
All foil is going to do is super heat the shingles.
Need to be concerned with the house envelope first.
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Old 06-07-2017, 10:32 PM   #5
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Re: OSB vs Attic Foil?


It's a single story home, where the attic crawl space is only available 75% of the floor plan. Windows are dual pane, Milgard Tuscany series (these made a huge difference). The home was built in 1969 and I don't believe there is much, if any in the walls, but during a kitchen remodel I did find some vintage insulation bats in the walls. I am sure there are soffit vents but none that I can see from the exterior.

I am under the impression that the attic is currently under ventilated and this will be an area that will be addressed during the re-roof. The foil (http://www.atticfoil.com/) is breathable on both sides so I am hoping it won't cook the shingles. I am going to be using a title 24 shingle as well.
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Old 06-08-2017, 04:47 AM   #6
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Re: OSB vs Attic Foil?


You have enough posts at this point, how about some pictures.
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Old 06-13-2017, 05:52 PM   #7
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Re: OSB vs Attic Foil?


What kind of pictures would you like to see?
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Old 06-16-2017, 08:37 AM   #8
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Re: OSB vs Attic Foil?


Your best choice is big enough inlet vent and big enough outlet fan. You don't need passive venting through the ridge as that doesn't work to move enough air to keep the attic cool enough.
Foil under or over existing roof deck needs to be researched more. What I read about it is that foil can reflect enough radiant heat where it can damage the roofing. Foil sheathing also must be touching least framing as possible. If you add foil osb to existing deck, heat will transfer and makes the foil useless.
If you can afford it, better bet is adding second sheathing with a space in between. This is passive venting but much better than venting inside. Add a passive metal chimney vent at the ridge. Metal gets hot and better to aid with hot-air-rises principle.
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Old 06-16-2017, 02:40 PM   #9
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Re: OSB vs Attic Foil?


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What I read about it is that foil can reflect enough radiant heat where it can damage the roofing. Foil sheathing also must be touching least framing as possible. If you add foil osb to existing deck, heat will transfer and makes the foil useless..
So my existing roof has a "closed sheathing" deck where 1x6 beams are laid much closer to each other than compared to pictures I've seen of decking used for shake wood roofs. Would you think the OSB (with radiant board) will be an issue when directly laid over this type of deck?
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Old 06-30-2017, 12:09 PM   #10
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Re: OSB vs Attic Foil?


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So my existing roof has a "closed sheathing" deck where 1x6 beams are laid much closer to each other than compared to pictures I've seen of decking used for shake wood roofs. Would you think the OSB (with radiant board) will be an issue when directly laid over this type of deck?
You will get no benefit putting radiant barrier decking directly over the sheathing as shown in the picture. Radiant barrier foil MUST have an airspace on one side to either work off the emissivity quality or reflectivity quality. When you "sandwich" the foil it won't help reduce heat transfer. I always tell people this would be like putting a piece of foil on a hot skillet, then cracking an egg on top and expecting it NOT to cook. It's going to cook right through the foil. NOW, if you lifted the foil off the hot skillet about 1/2" it would take a long time for the egg to cook since the airspace is provided and the bottom of the foil is reflecting the heat. Here is an article an video on Why An Airspace Is Required For Radiant Barrier To Work. And roofs getting too hot and cause damage is a myth. The fact is that a roof over a radiant barrier will increase less than 10 degrees. Finally, in a ventilated attic, putting R-Value on the roof will basically do nothing. This is like holding a jacket over your head. You want r-value on the attic floor and install a radiant barrier between the hot roof and the insulation to keep it cooler. How To Install A Staple Up Radiant Barrier In Your Attic. Hope this helps. Ed Any questions contact me.
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Old 06-30-2017, 01:57 PM   #11
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Re: OSB vs Attic Foil?


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You will get no benefit putting radiant barrier decking directly over the sheathing as shown in the picture. Radiant barrier foil MUST have an airspace on one side to either work off the emissivity quality or reflectivity quality. When you "sandwich" the foil it won't help reduce heat transfer. I always tell people this would be like putting a piece of foil on a hot skillet, then cracking an egg on top and expecting it NOT to cook. It's going to cook right through the foil. NOW, if you lifted the foil off the hot skillet about 1/2" it would take a long time for the egg to cook since the airspace is provided and the bottom of the foil is reflecting the heat.
Thanks for the information!

I would like to really hear your opinion on thermasheet (http://www.low-e.com/thermasheet.html). They claim this product can be installed directly over the decking and no air space is required. Which goes against everything I have heard and your own comments on the air space requirement. A local roofing supplier said this is the only item that is worth the effort and $$ to install (vs OSB).
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Old 07-01-2017, 10:22 AM   #12
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Re: OSB vs Attic Foil?


Thanks for the question. Let me quote their website: "Without an airspace beneath composite shingles, Therma-Sheet provides an excellent thermal break that, without reflectivity, still lowers the temperature below your roof deck 8-12 degrees." So, they are being a little vague, but are saying it is NOT a radiant barrier but IS a "thermal break". So, the foil is doing nothing and the 1/4" or 1/8" foam will provide a little r-value. If you want to put a little foam under shingles (and there are a LOT of products out there) that's fine, but it's a waste of money to pay extra for the aluminum foil layer when it's doing nothing. Let's assume the bottom of the roof deck DOES drop 8-12 degrees with 1/4" foam under the shingles. On a hot-sunny 90 degree day the bottom of the roof deck can easily be 150 degrees or hotter, so this isn't much of and impact. WITH the airspace, the bottom of the roof deck will be close to ambient (I'm all for this application and I've got videos explaining this at RoofingFoil.com). The bottom line is that it's all about keeping the insulation as cool as possible. The best way for your application is to "break" the path of the radiant heat from the hot roof deck to the relatively cool insulation with a staple up radiant barrier. Be wary of any roofing company that is pushing putting foil under shingles. There was a case a few years back where a roofing company was pushing a "radiant barrier" under shingles by installing a similar product without an airspace and charging crazy money for it. The roofing company settled since their lawyer looked at it and basically said "There's no way I can defend you. You don't have a radiant barrier installed that way." Finally, IF it did work, don't you know ALL the big roofing manufactures (GAF, Certainteed, etc.) would be making products to compete in this space.
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Old 07-03-2017, 01:01 PM   #13
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Re: OSB vs Attic Foil?


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Thanks for the question. Let me quote their website: "Without an airspace beneath composite shingles, Therma-Sheet provides an excellent thermal break that, without reflectivity, still lowers the temperature below your roof deck 8-12 degrees." So, they are being a little vague, but are saying it is NOT a radiant barrier but IS a "thermal break". So, the foil is doing nothing and the 1/4" or 1/8" foam will provide a little r-value.
Seems like I would be better off spending the money towards improving the attic space to combat radiant heat and insulate it further. So much for doing any fancy low-e tricks while replacing the shingles. I guess with such closed spaced sheathing, even OSB is out of the question for me.

Just to add more data to this thread, thermasheet claims their product under an asphalt shingle has an R-Value = 1.03
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Old 07-03-2017, 02:31 PM   #14
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Re: OSB vs Attic Foil?


Just joined the thread and only skimmed the previous posts, but have you selected and installed your shingles as yet? If not, that is where some radiant reflection can help and they do make shingles with improved properties.

Once the sun heats up the roof you then have to deal with the heat transferred to the inside. Before I go into details can you summarize where your project stands.

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Old 07-03-2017, 03:30 PM   #15
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Re: OSB vs Attic Foil?


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Once the sun heats up the roof you then have to deal with the heat transferred to the inside. Before I go into details can you summarize where your project stands.

Bud
Currently, I am about to purchase all my needed material and hope to get started in the next two weeks.

Thanks!
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