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Old 02-28-15, 08:17 AM   #1
gtojohn
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Default Polyiso flooring underlayment

I'm building the sub floors for my cabin right now. This is what I'm considering for my flooring. I'm thinking for my finshed flooring putting a layer of 1/2 to 3/4" polyiso board with 1/4" plywood veneer on top and this will be all on top of 3/4" cdx plywood. The polyiso seems strong enough plus the 1/4" to spread out the load. My main concern is I hate the way cheap floating wood flooring sounds and feels bouncy. I think if I can nail it down it will sound better. Is there any reason I can't walk on polyiso? With less than an 800 sqft footprint I don't expect we'll have heavy furniture. There will be a very heavy wood stove I won't insulate or put veneer under.

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Old 02-28-15, 11:03 AM   #2
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Two standards that apply:

ASTM D 1621 Standard Test Method for Compressive Properties of Rigid Cellular Plastics.

ASTM C 165 Standard Test Method for Measuring Compressive Properties of Thermal Insulations.


16 to 25 psi (110 to 172 kPa) is the common range of polyiso compression strength. I've been walking around on 2 3/4" polyiso lately with no issue. With plywood over top I'm certain you'd be fine.
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Old 02-28-15, 06:07 PM   #3
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Can you go up to something over 1" thickness? Polyiso is a bit brittle at that thickness and there is not much r value in 3/4". 1.5" would be better.
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Old 03-01-15, 08:37 AM   #4
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I wasn't sure if it was less brittle or more bouncy as it got thicker. It's one of those things, generally nobody builds like that around here, 3/4" is the thickest in stock at the big stores. I need to special order thicker, which I do need for my walls and roof. I was thinking about also adding batts underneath but I don't want to give rodents a place to nest. Last time I did a manual j for a house this size floor heat loss was around 3000btuhr for this area. Because of this we tend ignore the floors around here. I'm trying to do better. It also seems with climate change as our weather patterns have more extreme swings the manual j becomes less helpful.
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Old 03-01-15, 09:16 AM   #5
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http://www.loadcalc.net/structure.php
Free man j calculator. It doesn't have all the possible options for insulation, but its free!
Looks like I'm starting with 3600 btu heating. R-4 gets me down to 2444. R-11 gets me down to 1587. Looks like I'll need more to really bring down the floor load.
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Old 03-07-15, 04:57 PM   #6
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Up here in the tundra I did the floor of my unheated crawlspace cabin floor as such. standard 2x8 floor joists with small wave gavl barn roof metal on top(this made up my rodent proof under floor layer. O n the metal I nailed 2x3 stringers perpendicular to joist at 2' cc spacing on edge giving me 2" subfloor and metal underfloor. I laid 2" hi density insul board between stringers. I went with 3/4" 4x8 T&G ply for subfloor because I was ceramic tiling the entire floor(porcelain tile). Its 25 yrs going and no issues with floor. Cabin is unheated except when there in winter so it has done well thru 100's of heatings and coolings. Even when it is subzero only a few hours warm it up with the wood stove. Over half is exposed to passive solar so sunny winter days it is toasty. I'm sure you would have no problems scaling this design to whatever your climate needs.
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Old 03-19-15, 05:39 PM   #7
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I put "click" flooring directly over 1.5" polyiso. it took some getting used to as it squeaked in one area by the door for the first year. but it's been down over 3 years now with no issues. do wish I'd used an additional 1/4" ply layer tho. no need to fasten. unless the 1/4" is the flooring.

My old very heavy wood stove sits on one of those big box heat shields; on top of the click floor; on top of the polyiso. there is no problem.

For another building I put 3/8" ply over top of 4" xps. feels just like a regular floor.

And on yet another building I put a layer of 2" xps with 3" polyiso on top. then used 1" white pine boards. It's still in the first year and I'm not sure if it's the boards flexing as they're not tongue and groove or the polyiso ... but there's been no issues.

your idea is solid. (sorry)

good luck.
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Old 03-19-15, 08:48 PM   #8
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Quote:
Originally Posted by gtojohn View Post
...I'm thinking for my finshed flooring putting a layer of 1/2 to 3/4" polyiso board with 1/4" plywood veneer on top and this will be all on top of 3/4" cdx plywood...
There is another kind of PolyIso that is reinforced with fiberglass to give it greater compression strength.

You might look into that. As I recall, the compression strength was about 40% greater.

I have also looked at HighLoad Styrofoam, which is quite dense, but it doesn't seem to be available in anything thinner than 2".

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Old 03-20-15, 04:55 AM   #9
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Most of the green gurus are moving away from XPS and to high density EPS due to the VERY high comparative embodied energy and that it is often blown with CFCs. Europe is all EPS, PU and PP foams now.

Fun with plastics.
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Old 03-20-15, 09:11 AM   #10
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i get most of my "foam" insulations recycled from roofing and demo contractors. this I believe mitigates the embodied energy

search craigslist/kijiji for da bargains.

polyiso is not blown with cfcs.

I have never seen polyiso that didn't have strengtheners. so check, but it shouldn't be hard to find. I've used Dow, JM, IKO, foil and glass faced. all have had reinforcement.

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