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Old 12-09-2016, 12:55 PM   #1
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XPS Foam vs EPS Foam?

I read this and thought i would get the local brain trust opinions.

Choosing Between EPS and XPS Rigid Insulation| Concrete Construction Magazine | Foundation, Concrete Slabs and Floors, Building Envelope, Drainage, Insulation, Moisture Barriers, Moisture in Concrete, Properties of Hardened Concrete, Water Quality, W
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Old 12-09-2016, 01:06 PM   #2
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I went with EPS ala the r-tech insulation from home depot. Couldn't be happier with it. It is everywhere in my build, 3.5" in floor, 3" in ceiling and 3" in the walls.

Either type will be light-years ahead of the factory fiberglass stuff. I chose EPS because it was half the price of XPS when I insulated my bus last year.

Fun little story. It was 4 degrees here last night. I've been burning just a single chunk of firewood at a time in my wood stove and it keeps a pretty consistent 60-75 degrees inside. Being that it was 4 degrees I thought to myself "I should probably kick it up a notch and put two chunks of wood in." Seems reasonable right? Cut to me waking up at 2:45 to a 97 degree sauna and sweat bath... Back to one chunk at a time I guess. So yes the cheaper insulation will work great even in the near zero temps I've been experiencing lately...
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Old 12-09-2016, 02:04 PM   #3
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What about polyiso? Supposedly it has a higher R value per inch, but that value starts to decline under 15C and is significantly lower at -25C, but I dont know HOW much lower it is yet.
Good in hot climates maybe?
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Old 12-09-2016, 03:13 PM   #4
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And, wouldnt spray foam be superior for the simple fact it would eliminate the condensation problem?
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Old 12-09-2016, 07:23 PM   #5
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Condensation happens when air containing water vapor touches a surface whose temperature is below the dew point. For a given indoor and outdoor temperature and dew point, there are just two things to be done to prevent condensation. First is to establish a vapor barrier to limit how far toward the exterior the moist indoor air can travel. Second is to make the insulation thick enough that the vapor barrier surface temperature stays above the dew point.

Closed-cell spray foam is nice because it is (can be) both a vapor barrier and an insulation. But a separate vapor barrier can be used with EPS or XPS and also avoid a condensation problem. I've never resolved one question about vapor barriers in a skoolie, though: if there's a vapor barrier on the inside surface, vapor permeable XPS/EPS/whatever insulation in the middle, and a steel skin on the outside (another vapor barrier, no?) -- then what happens with the water vapor that inevitably gets inside the wall? It seems to me the vapor should permeate through the insulation, condense on the inner surface of the steel skin, and hopefully drain out the bottom of the wall. But I just don't know. Maybe there isn't usually enough vapor getting into the wall to matter.

Interesting note about polyiso losing R value at lower temperatures. I wasn't familiar with that. Building Science Corporation has done some research on that which you might find interesting. Their tests found up to 25% decrease in R value as mean temperature drops toward 25 F.

As for XPS vs EPS.... cost, availability, and handling are probably the biggest factors. Off-hand I'm not coming up with any performance characteristic that would make one a better choice than the other.
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Old 12-09-2016, 10:16 PM   #6
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Always nice to know the less expensive material performs better than the pricey stuff, for whst i would use it for Thanks for the info
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Old 12-11-2016, 12:27 PM   #7
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Is this what you are talking about?
R-Matte Rmax Plus-3 1/2 in. x 4 ft. x 8 ft. R-3.2 Polyisocyanurate Rigid Foam Insulation Board-754404 - The Home Depot
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Old 12-11-2016, 12:48 PM   #8
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What would make a good vapor barrier for ceiling?
I'm using 1" foamular Owens Corning FOAMULAR 150 1 in. x 4 ft. x 8 ft. R-5 Tongue and Groove Insulation Sheathing 68WD at The Home Depot - Mobile

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Old 12-11-2016, 10:40 PM   #9
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If you're asking me then no.
R-Tech 1-1/2 in. x 4 ft. x 8 ft. R-5.78 Rigid Foam Insulation-320817 - The Home Depot

They had this in 2" thickness at my local store...
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Old 12-13-2016, 01:52 PM   #10
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The usual go-to for vapor barrier is polyethylene sheeting, sometimes known by the brand name Visqueen.
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