XPS Foam vs EPS Foam?

[Versatile]

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

[slaughridge85]

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...

[dalez]

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?

[dalez]

And, wouldnt spray foam be superior for the simple fact it would eliminate the condensation problem?

[family wagon]

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.

[Carytowncat]

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

[Versatile]

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

[milkmania]

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

Learned yesterday that it's super easy to cut with table saw

[slaughridge85]

Originally Posted by Versatile

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

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...

[family wagon]

The usual go-to for vapor barrier is polyethylene sheeting, sometimes known by the brand name Visqueen.

[milkmania]

Originally Posted by family wagon

The usual go-to for vapor barrier is polyethylene sheeting, sometimes known by the brand name Visqueen.

Very familiar with it...
But, the data sheet says the xps I'm using is moisture resistant.
I can't see how the visqueen would make it any more moisture resistant. Plus it would add a cavity for moisture to accumulate between the layers.

I might be overthinking it, but I'm not seeing the value in the extra step

[DoubleO7]

Key word is "resistant" which does not equal "proof".
As in waterproof visgueen (as long as there are no punctures)

[milkmania]

Originally Posted by DoubleO7

Key word is "resistant" which does not equal "proof".
As in waterproof visgueen (as long as there are no punctures)

I'll agree that water proof and water resistant are two very different terms.... Wrist Watches are water resistant (to a certain depth) but not water proof for advertising reasons & liability.

But, what about the moisture that's bound to develop between the xps and visqueen?
It won't come through the xps without forced pressure, and it can't go through the visqueen and it won't evaporate without air gap to expel the moisture.

I've seen the moisture from fresh concrete develop under the visqueen and have no where to go, until it was pulled and the air allowed to evaporate the moisture.

I'd say "waterproof" won't allow water to penetrate, and I'd wager that if you had a bowl made from xps, that it wouldn't let water out without pressure. But, icbw.... I've just seen how my xps acts in my climate.

http://www.foamular.com/foam/faqs/ addresses all this to some extent

[family wagon]

When Owens Corning says the foam is moisture resistant I think they mean it resists being damaged by moisture. Vapor permeability is something else. The FAQ linked above includes "Does FOAMULAR® used as sheathing on the exterior of a wall create a double vapor retarder?" to which the answer is no, or at least "not necessarily."

I dug up the technical bulletin FOAMULAR ® Extruded Polystrene Insulation Resisting Water Absorption, the Key for High Performance Foam Plastic Rigid Insulation on their web site. It expounds on the FAQ item starting on page 4. It mentions their 1" Foamular 250 XPS unfaced has perm rating of 1.1, a faced version has rating 0.20, and poly sheeting has rating 0.06. I didn't see the Foamular 150 mentioned but IIRC it has lower density than the 250 and so I suppose might have a higher perm rating. Does it matter? I wish I knew!

I wouldn't worry about moisture being trapped between an unfaced XPS and poly sheet. It'll go through the XPS well enough. The question is, what's on the other side of the foam? I've wondered about this several times. Just now it occurs to me that maybe the joints in the metal skin are vapor-leaky enough that we just don't need to worry about this.