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Old 09-07-2016, 05:55 PM   #1
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Subflooring Insulation: XPS vs Polyiso

Hi all,

So for the past two weeks I have had a nightmare trying to figure out my insulation options for my flooring. I insulated my driving area with 1/2" polyiso insulation without a second thought. However after digging down the insulation rabbithole further, I've read that polyiso insulation performs poorly if it gets wet and that extruded (xps aka 'blue board', 'pink board') polystyrene insulation does very well under moist conditions. Obviously I am not trying to wet my insulation, however I prefer to plan around me F'ing up as I do routinely.

The real challenge I've found is that I can't for the life of me find xps insulation under an Inch within 500 miles of me (in Washington). I could give up that 1/4-1/2 inch of head space, but I'm 6'3" - need I say more?

After 3 weeks on this procurement hunt I feel very restless to make a move, my options below.

1. I found a vendor which can ship over 5 sheets for 3/4 inch Dow xps around $30 a sheet after shipping before tax.

2. Buying units of 80 sheets from home depot at around $16 a sheet before tax.

3. Buying 5 sheets of 1/2 or 3/4 inch polyiso from home depot at around $11-$15 a sheet

Way later on, I plan on attempting to fasten some 2" xps from below the bus, but that's a different pipe dream on some distant back burners.

As always I appreciate and treasure your opinions regarding, thanks in advance!
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Old 09-07-2016, 06:16 PM   #2
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Scratch that, I just found a vendor who carries 1/2" xps insulation at $20ish with no minimum! Life is good! Ooooooh my goodness, these little victories sure feel good.
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Old 09-08-2016, 12:15 AM   #3
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I tested the poly-iso insulation I used in my bus for water resistance. I cut a small piece and weighed it on my reloading scale. dunked it in water overnight then weighed it again after letting it dry in the sun for a half hour. Both weighings were the same so it looks like the insulation is pretty moisture resistant.
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Old 09-08-2016, 12:43 PM   #4
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Thats interesting/strange that its so hard to for XPS there. Every Lowes and Home Depot I have been to in MA carries XPS in 1/2", 3/4", and up.
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Old 09-08-2016, 04:36 PM   #5
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I'm going with XPS on the floor and silver backed poly-iso on my side walls and ceiling. Reason for the difference is the compressability. Put two pieces of each side by side and try squeezing them with your fingers. The poly-iso compress's quite easily and the XPS is rock hard. I don't want poly-iso as my subfloor because I don't want it to compress and possibly mess up my final tongue and groove flooring on top of it.
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Old 09-08-2016, 07:20 PM   #6
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My company uses poly ISO insulation daily in buildings that are not dried in because of its moisture resistance.
We have been doing it for years and the only problem I have had in extreme circumstance is the outer jacket (ASJ-all service white on piping and FSK-foil scrim Kraft on flat boards and even the aluminum foil jacket ) will start to collect mold but it is only on the surface that you can clean with a rag.
They make a insulation paint just for using polyiso exposed outdoors to seal any cuts/taped joint's to weather proof.
True polyiso comes in ridgid sheets or 3' sections for piping whatever thickness and you have to cut the bevels for anything other than flat but it is very water resistant and I have seen some polyiso pipe insulation that I helped install 20-years ago on a underground steam line tunnel that floods every time it rains last 19-years cause I got to go tear out and upgrade the same steam line and put back in polyiso insulation.
Don't know the price ranges your finding so go with what suits your budget?
I have been able to salvage 85%of my 95% complete build inside but I do commercial construction and a lot of companies will throw stuff away just cause 1' of 8' is damaged? It can't be used because it structurability is in question **** like that.
The insulation scraps are first pick for me cause the company I work for is the ones figuring out what is trash and scraps and save able back to the warehouse scraps?
Good luck with your build.
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Old 09-09-2016, 03:07 PM   #7
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Thank you everyone, your input has given me some great food for thought!

Roach: I really appreciate you sharing your tests with the polyiso, I don't think I would've ever known or even thought of performing such a test. This makes me feel very good about using polyiso on the walls.

flyboy: I agree and I think I have come down to the same conclusion.

Whitewhale: It was a helluva struggle but I finally got my 1/2" xps sheets, I guess it's a really slow moving item in the northwest and everyone has replaced it with 1/2" polyiso.

Jolly Roger: It's great having you share your experience, do you know what kind of paint they use to seal the insulation gaps? Maybe it could be useful if I ever try to do exterior insulation(in the unforeseeable future). I love dumpster diving but am prone to taking way more than I need. Currently though time is so short, and I don't have many good opportunities to go through construction scraps so I guess I'm spending money on these "structurally sound" sheets. I should probably ask some of my vendors when I willcall if they have any damaged products they'd let me sort through and give me a discount on or something. I'm not rich by any means but have a nice balance between free time and money to keep up stable work. Thanks for all your help!

Also question: How do we feel on this board about reflectix? I went ahead and got myself a roll because it is cheap enough and seemed cool. I have some crappy windows in my house I'd love to cover with this stuff for the winter. The bus I figure I could use as a curtain over my single pane windows, but is it practical to use this with my foam board to insulate the walls and ceiling?

Originally I was planning to use reflectix, then foam board (silver facing out), then I've read some mixed reviews on the forum. So now I don't know if it's worth doing. Maybe I'll just use it for making insulating curtains or something.
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Old 09-09-2016, 03:51 PM   #8
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I found 4'x8' 2.75" thick used polyiso sheets for $7-8/sheet depending on quantity ordered. I got a couple pallets. I used it in my subfloor, I'm putting a full layer on my wall, and I'm doubling up the ceiling. I'll be more insulated than most new construction houses when I'm done, and it cost me much less than buying 3/4" sheets of new stuff. Craigslist can be a wonderful tool.
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Old 09-09-2016, 06:47 PM   #9
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Taolik the sealant that I am used to seeing used is Childers cp-35 chill perm..
I have never seen the reflectix board you are talking about.
Have a good weekend
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Old 09-10-2016, 07:46 PM   #10
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Holy crap ashandrix, that's a great deal! It's a shame you can't sell me a handful of sheets.

Jolly thanks for the heads up, I will definitely save that for if I ever do exterior insulation. In regards to the reflectix it's not board, it's just shiny bubble wrap.
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