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Old 10-18-2020, 10:56 PM   #1
Mini-Skoolie
 
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GPS foam

Hi everyone,
Has anyone tried gps foam insulation that is foil backed to insulate the ceiling? Same r value as XPS, I know the permeability is lower on XP’s which is 1.5 vs gps which is 5.0 but with a foil backing it actually goes down to 0.5.
It’s less then half the price then XPs foam. It won’t work for floors because the lack of compression strength but for the ceiling, it should work.

Has anyone used it? Am I missing something why it shouldn’t be used?
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Old 10-18-2020, 11:17 PM   #2
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Originally Posted by Markusbc View Post
Hi everyone,
Has anyone tried gps foam insulation that is foil backed to insulate the ceiling? Same r value as XPS, I know the permeability is lower on XPís which is .5 vs gps which is 5.0 but with a foil backing it actually goes down to 0.5.
Itís less then half the price then XPs foam. It wonít work for floors because the lack of compression strength but for the ceiling, it should work.

Has anyone used it? Am I missing something why it shouldnít be used?

Have never heard of GPS foam, a quick google turns up a few results but not a ton, where have you seen it sold?
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Old 10-18-2020, 11:19 PM   #3
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GPS foam

At Home Depot

Comes in 4x8 sheets. 1 1/6 thick or 2 1/8 thickness.
Foil covered on both sides. No off gassing and no loss of r- value over time apparently.
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Old 10-19-2020, 01:49 AM   #4
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After a bit more research, it looks like GPS is a sub-type of EPS



This discussion may interest you
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Old 10-19-2020, 08:50 AM   #5
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GPS foam

So it looks like eps stores less moisture then XPs due to a closer cell structure. Yes it is more permeable. Wouldnít that help with avoiding trapping air against the metal skin.

The down side of eps seems the lower r rating compared to eps and poly... which gps seems to take care of.

I am thinking I might go for gps. Just not sure if I use Vapour barrier over it but then it would allow to trap moisture again. Defeats the purpose.
Any thoughts?
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Old 10-19-2020, 02:29 PM   #6
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Originally Posted by Markusbc View Post
So it looks like eps stores less moisture then XPs due to a closer cell structure. Yes it is more permeable. Wouldnít that help with avoiding trapping air against the metal skin.

The down side of eps seems the lower r rating compared to eps and poly... which gps seems to take care of.

I am thinking I might go for gps. Just not sure if I use Vapour barrier over it but then it would allow to trap moisture again. Defeats the purpose.
Any thoughts?

I was under the impression that people generally regarded XPS as the better options with regards to moisture. But I may have misunderstood, or may have misunderstood a piece of the context for that preference.
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Old 10-19-2020, 02:47 PM   #7
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GPS foam

Yes I also thought so. But from what I have read XPís will absorb moisture over time and wonít release it. GPS apparently absorbs moisture as well but will dry out again fast. So it might avoid trapping humidity between the foam and the metal skin of the bus.

Also one thing i find appealing g is no blowing agents off gassing compared to XPs and no revalue loss over time. Compared to polyc.. foam The r value of gps apparently Increases the colder it gets outside.
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Old 10-19-2020, 09:07 PM   #8
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The R-value of GPS does increase (slightly) with decreasing temperature, but that's normal not just for most types of insulation but also for non-metallic materials in general. Polyiso is the exception to this, with an R-value that increases (slightly) with increasing temperatures (at least within the temperature range that humans inhabit).

The permeability of GPS appears to be intermediate between XPS and EPS; none of these have a low enough permeability to be considered a proper vapor barrier, which is something spray foam achieves. If you're not completely blocking condensation from inside from making its way through the foam to the steel skin, do you want to block it a little or a lot? I dunno, but the difference between XPS and GPS in this case is probably pretty small.

GPS seems like a perfectly viable option for insulation, but not really superior to XPS (unless it's significantly cheaper). I would use it for sure if I found some on Craigslist - in fact I have seen it there before but I didn't know what it was. It looks like the stuff foam sticks for pools are made out of, not exactly promising.
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Old 10-19-2020, 09:14 PM   #9
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Foam

Not arguing itís superior, I was just surprised that I havenít heard of it before. Up here in bc Canada the prices are, $44 for 2x8 foot for 2 inch xps and $35 for 4x8 foot 2 1/8 gps. So itís significantly cheaper.

My post is to open up a conversation of the viability of this type of insulation.
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Old 10-19-2020, 09:16 PM   #10
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Quote:
Originally Posted by musigenesis View Post
The R-value of GPS does increase (slightly) with decreasing temperature, but that's normal not just for most types of insulation but also for non-metallic materials in general. Polyiso is the exception to this, with an R-value that increases (slightly) with increasing temperatures (at least within the temperature range that humans inhabit).

The permeability of GPS appears to be intermediate between XPS and EPS; none of these have a low enough permeability to be considered a proper vapor barrier, which is something spray foam achieves. If you're not completely blocking condensation from inside from making its way through the foam to the steel skin, do you want to block it a little or a lot? I dunno, but the difference between XPS and GPS in this case is probably pretty small.

GPS seems like a perfectly viable option for insulation, but not really superior to XPS (unless it's significantly cheaper). I would use it for sure if I found some on Craigslist - in fact I have seen it there before but I didn't know what it was. It looks like the stuff foam sticks for pools are made out of, not exactly promising.
It looks more like dark grey eps foam.
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Old 10-19-2020, 09:28 PM   #11
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Not arguing itís superior, I was just surprised that I havenít heard of it before. Up here in bc Canada the prices are, $44 for 2x8 foot for 2 inch xps and $35 for 4x8 foot 2 1/8 gps. So itís significantly cheaper.

My post is to open up a conversation of the viability of this type of insulation.
I hope I didn't seem like I was criticizing. I was just trying to add some information.
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Old 10-20-2020, 12:05 AM   #12
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Not at all.

Any information is good to have.the more info the better.
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Old 10-20-2020, 12:33 AM   #13
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Originally Posted by dzl_ View Post
After a bit more research, it looks like GPS is a sub-type of EPS

This discussion may interest you
This thread and the link above (and others that stem from it) are giving me more to think about in designing the walls, ceiling and floor insulation.
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