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Old 12-28-2020, 09:08 PM   #1
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Stacking iso foam board insulation w/ foil backing?

We're considering using iso rigid foam board in the floor and/or walls.
In both cases, we've got ~1.5" depth to fill. The floor will be framed (2x2s).
Local hardware stores only stock .5" & 1" sheets, with foil backing on one side.


Our questions:


1) Is it kosher to stack a .5" sheet & 1" sheet to make up the thickness?
2) If so, how should the foil backing of each be oriented?
3) Is there any possibility 1" & .5" stacked will add up to more than 1.5", to the point it exceeds the height of the floor framing? I know this is probably a stupid question, but not sure how exact the advertised dimensions are.
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Old 12-28-2020, 09:40 PM   #2
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The foil backing won't have any effect unless it's facing a source of high heat and has an air gap of at least 1" adjacent to it (with no air gap, the foil layer reflects radiant energy into the adjacent solid material instead, with no net benefit). So it would be useful in hot weather in direct sunlight (about +2 to the R value) on the walls if it was placed against the ribs (as opposed to in between them) with the foil facing outwards (a second foil layer would have no effect, with or without its own air gap). Otherwise it wouldn't make any difference what side the foil was on.

I don't know about the tolerances with polyiso, but I've had 2" XPS vary between 2" and 1 7/8" - never over 2". I don't think you'd have any problem with layering the 1" and the 0.5".
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Old 12-28-2020, 09:44 PM   #3
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Thanks so much, musigenesis. Think I'm squared away!
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Old 12-28-2020, 10:40 PM   #4
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Quote:
Originally Posted by TheHubbardBus View Post
We're considering using iso rigid foam board in the floor and/or walls.
In both cases, we've got ~1.5" depth to fill. The floor will be framed (2x2s).
Local hardware stores only stock .5" & 1" sheets, with foil backing on one side.


Our questions:


1) Is it kosher to stack a .5" sheet & 1" sheet to make up the thickness?
2) If so, how should the foil backing of each be oriented?
3) Is there any possibility 1" & .5" stacked will add up to more than 1.5", to the point it exceeds the height of the floor framing? I know this is probably a stupid question, but not sure how exact the advertised dimensions are.
I used two layers of 3/4” XPS on my ceiling and it was exactly 1 1/2” thick. No variation at all. I thought the foil side was supposed to be facing the living quarters. Not positive though.
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Old 12-29-2020, 12:12 AM   #5
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I'd go with the 2in, it looks to cost the same as 1" + .5", use some thicker wood instead of less foam. As was said, the foil side makes no difference in a wall.
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Old 12-29-2020, 03:47 PM   #6
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Don't think 2" would work, unfortunately. Not only do I doubt we'd be able to find it, since we can't even find 1.5", but what wood would we use? The 2x2s I'm planning are nominal 1.5". I'm not familiar with any common cuts of lumber that are 2.0" nominal.


It's a moot point, regardless. In an effort to preserve headroom, we've pretty much settled on 1.5" of insulation. We're just trying to get the highest R value we can w/in that limit at a reasonable cost.
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Old 12-29-2020, 07:26 PM   #7
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I don’t think iso board would be the best to use for a floor. I’m pretty sure xps is stronger from a psi rating standpoint than iso which is more ideal for a floor. Also iso is not water resistant like xps. I know a lot of people use it but there’s a reason that xps is spec’d for radiant (under slab) insulation when pouring a concrete pad.
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Old 12-29-2020, 08:39 PM   #8
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I appreciate the advice, RamRod. But I'm framing the floor, so I don't imagine I'll have any issues w/ compression. Regarding water absorption, I figured both being closed-cell, performance would be similar, and according to at least one online reference, they are:


https://inspectapedia.com/Energy/Pol...Insulation.php


I'm reasoning that if I get as much water underneath my flooring as would be in contact with an underground slab, I've got bigger problems than r-value. For reference, we plan on marine-grade plywood on top of the joists, all edges sealed w/ polyurethane caulk, with stone polymer composite plank flooring on top of that in a glue-down installation.


If my facts or reasoning are in error, please let me know. I'm far from an expert, on this bucking bronco of a learning curve!
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Old 12-30-2020, 05:59 AM   #9
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Sounds like you have it covered between the sealing and the framing. I'm about this stage on my bus as well. I am back and forth on whether I want to frame out the floor with furring strips in 4x8 sections or just put the foam down and anchor the plwood to the floor through the foam.
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Old 12-30-2020, 07:40 AM   #10
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Quote:
Originally Posted by RamRod4 View Post
Sounds like you have it covered between the sealing and the framing. I'm about this stage on my bus as well. I am back and forth on whether I want to frame out the floor with furring strips in 4x8 sections or just put the foam down and anchor the plwood to the floor through the foam.
I'm planning to just lay down the foam and put the ply right on top of it.
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Old 12-30-2020, 09:16 AM   #11
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Main reason we're framing is some spots of our floor got a little tweaked from me trying to pull old screws w/ a crowbar (bad idea jeans). Otherwise I'd probably float it too. It's not bad, but enough that I'm worried we wouldn't get a truly flat surface, which according to what I've read regarding the vinyl plank, is a factor in long-term performance.

Going to use the 3M 550FC I've been using for everything else to secure the joists, so no new holes in the floor. Thinking that by using a construction adhesive instead of fasteners, I'll be able to level the joists instead of forcing them to confirm to the floor surface. Might also have some usefulness as a thermal break (?). You can lay it on thick or thin, so it should kind of work like liquid shims (I'm trade-marking that lol!).

I've really fallen in love with this stuff. Among the things we've used it for, one was joining the wood frame we constructed for the AC to the inside of the metal roof. Since the roof isn't flat, the steel is forced to deform along the leading & trailing edges to mate with the steel. That's ALOT of force trying to pull the two apart, especially w/ me doing all the work I've been doing lately hoofing around on the roof. In the interest of full disclosure, it did fail once after the wood got soaked from an unexpected rain storm we didn't cover up for (after which I added a few countersunk fasteners around the periphery), but that's not a fair test. The fan hole was constructed similar - which is metal on metal - and it held up fine.

Best thing is it's a popular roofing product, so a nearby supply house stocks it by the case for very reasonable prices. They sell online if anyone's interested:

https://www.bestmaterials.com/detail.aspx?ID=20460
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Old 12-30-2020, 11:06 AM   #12
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Looks like some good stuff. May try a tube myself. thanks!
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