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Old 12-15-11, 01:40 PM   #11
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Here is the spec sheet for the "150" XPS. They also have a 250 XPS which I'm not sure what the difference is (perhaps tongue and groove vs flat sides).

http://insulation.owenscorning.com/a...a29e2fd5bc.pdf

With a 40F temperature differential this stuff has an r-value of 5.5. At 75F differential it is R5.

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Old 12-15-11, 03:05 PM   #12
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The spray foam I used is rated at R-7 per inch x 3 inches = 21, plus 5 more inches of cellulose at R-3 per inch = 15 + 21 = R-36. I added a second offset 2x4 stud wall for the extra thickness, so thermal bridging is minimal.
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Old 12-15-11, 03:55 PM   #13
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The spray foam I used is rated at R-7 per inch x 3 inches = 21, plus 5 more inches of cellulose at R-3 per inch = 15 + 21 = R-36. I added a second offset 2x4 stud wall for the extra thickness, so thermal bridging is minimal.
hamsterpower,

Without a doubt, you are doing this the right way... possibly more right than you realize...

Your 3" of foam on the outside walls not only gives excellent R-value, and a complete seal against infiltration, but it also sets up a thermal gradient, such that moist air will never reach the dew point inside your cellulose fill.

Just couldn't be better.

Had I started with the knowledge I have now, and the technologies that are available now, I would do exactly what you are doing.

As it is I will probably do a variant of your method on the next room I tackle. My little house is only 700 square feet, and I'm trying to minimize intrusion into my living space... so some balance is called for here.

The offset studs idea is great... I think I can do a version of that, next room.

Great advice, great work!

-AC_Hacker
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Old 12-15-11, 05:08 PM   #14
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EPS has a lower R than XPS which is R 5. 4.something is about right for EPS.
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Old 12-15-11, 08:18 PM   #15
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Thanks AC, for the kind words.

I think my wife took some pictures along the way. I hope to put up a build thread of the process some time soon.

My house is only 800 sf too. We feel that if just right for the three of us.
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Old 12-15-11, 09:02 PM   #16
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My house is only 800 sf too. We feel that if just right for the three of us.
Well, I don't know if God looks any more kindly on those of us who live in small houses, but I do know that the heating bills are less...

-AC_Hacker

P.S.: [Everybody tells me I should add onto my house, but for 35 years I have successfully resisted!]
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Old 12-16-11, 02:02 AM   #17
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Interesting conversation here regarding putting EPS & XPS in wall cavities, etc.

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Old 12-16-11, 07:40 AM   #18
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Dunno if you posted the right link. That guy is using polyiso in his wall, not EPS or XPS... Only one other guy mentions using XPS. I think polyiso would be the way to go since its indoors and has a significantly higher r-value.
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Old 12-16-11, 11:31 AM   #19
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Dunno if you posted the right link. That guy is using polyiso in his wall, not EPS or XPS... Only one other guy mentions using XPS. I think polyiso would be the way to go since its indoors and has a significantly higher r-value.
I think you're right.

I was mostly focused on the technique.

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Old 12-16-11, 12:11 PM   #20
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Daox,

Gotta thank you on this one... Turns out that I can get the XPS at the same price or slightly cheaper locally, because there is a manufacturing plant close to my town.

Quote:
Originally Posted by Daox View Post
Here is the spec sheet for the "150" XPS. They also have a 250 XPS which I'm not sure what the difference is (perhaps tongue and groove vs flat sides).
The number refers to the ability of the foam to support a compressive load, 150 psi & 250 psi... The 250 is more dense and has better R-value, but I'm not sure what the value is.

Quote:
Originally Posted by Daox View Post
http://insulation.owenscorning.com/a...a29e2fd5bc.pdf

With a 40F temperature differential this stuff has an r-value of 5.5. At 75F differential it is R5.
Looks like just about all insulation has some kind of declining curve as the delta T becomes greater.

I went through the literature, and the companies do their best to make it difficult to do an apples-to-apples comparison.

Polyiso is higher R-value, but is subject to "R creep." R-6.5/in. seems to be the spec. There is also a foil face that can be used with a dead air space to increase R-value.

EPS is or should be cheaper, but for me, locally, it is not. They're coating one side with a metalized layer, and if you use it correctly, with the dead air space and all, it has higher R-value than XPS, but the EPS + dead air system takes up more space. No significant R creep. Disregarding the metalized layer and dead air space, R-3.8 to R-4+ per in. seems to be the spec. Otherwise, R-5.5 with the dead air space (3/4" to 3").

XPS has no foil, no R-creep and R-5 value. That's what I'll be using. Most people seem to be using saws with this stuff and the dust & debris from the sawing seems to be a real problem. I'm going to see if I can hack up some kind of a hot wire tool.

Thanks for the XPS tip...

-AC_Hacker

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