Is the EPS preferable over the XPS for slab on grade insulation. My local supply house is pushing the R-Tech IV. I had originally planned on  XPS but looking at the specs its seems similar. I think I remember reading that the EPS is more moisture tolerant.

Also, any experience with Perminator or Stego vapor barriers?

Comments?

Thanks

Drew

XPS absorbs less water than EPS. Use 2.5lb density XPS on 6mil poly. Unless you have money to burn.

I've heard (somewhere) that EPS maintains better R value/$ underground. Even Dow's figures support this. Any other references?

Extruded takes up less or almost no, water and since no one wants to redo a slab 10 years down the road, get it right. Yes it is more expensive but so what.

If you get the higher density EPS, Type IX nominal 2# density, the compressive strength and water absorption is comparable to XPS. The initial R value of XPS is slightly higher, but aged R values are comparable. Another advantage of EPS for underslab is that you can get it loaded with borates for termite protection. It may also exist for XPS, but I haven't seen it. And even when comparing Type IX EPS with borates to standard XPS, you get more R for your $ with EPS.

I am always interested in a better way to do things but under slab is last place to experiment. SM has been around and used under slab for at least 3 decades and if you look at it only from an R-value and moisture absorption, long term, I would still stick with it. Especially with a slab on grade. It is pretty cheap and easy to get termite protection prior to the insulation going down anyway. I doubt I am the only one here who would like to see some real long term testing on type IX EPS. People promoted the bubble tarps for years under slab and that was a disaster.

Posted By MikeSolar on 06 Aug 2012 07:27 AM
Extruded takes up less or almost no, water and since no one wants to redo a slab 10 years down the road, get it right. Yes it is more expensive but so what.

Current Type-II (or denser) EPS is superior in long-term moisture rejection in sub-grade or sub-slab than XPS- you may be trading on dated information.

While EPS appears to take on water more quickly in short-term submersion type tests, the max-asorption is reached quickly (typically 3-7% of volume) with only a minor hit in R value.  When the submersion condition is removed, that volume of water is also rapidly released. This is be because the vast majority of that water was in the interstitial spaces between beads, and NOT in the cells of the expanded polystyrene.  By virtue of how they're processed and blown EPS has a higher ratio of fully closed cells than XPS, and although it takes it on more slowly, when water enters the broken cells of XPS, it's pretty-much forever. 

EPS has become the standard for sub-slab insulation in Europe.

XPS has a much heavier environmental footprint than EPS too, since it's blown with HFC245fa, with orders of magnitude more greenhouse gas potential than the pentane used for blowing EPS.

Also note that the price for EPS can be 1/2 that of XPS and that ICFs use it, even underground.

The best argument for using XPS under slabs comes from radiant heating contractors, because XPS has much better retention than EPS for the staples used for holding the PEX in place, which makes the heating installation easier. If it's not a radiant-slab, go with EPS and be happy!

Or attach the PEX to the steel mesh.

I like to put rebar on the pex. On a floor we just did, the homeowner was doing the pex, very few staples. We barely got the pex down with the staples, rebar then tied the pex to the rebar.

I would guess that if you just want to hold pex in position temporarily, a good tape would work.

Reclaimed xps is greener than virgin eps and cheaper besides. Check craigslist or insulationdepot.com. I got a number of saturated xps boards in the lot I bought, but they dried over a winter in storage. If you, like me, have no drainage issues where you are building, then xps/eps comes down to cost. I paid $5 each for 2x8x2" xps boards.

What toddm said, only to add:

Reclaimed EPS (even Type-I EPS commonly used as roofing insulation) is usually cheaper still, and has sufficient compressive strength to handle residential slab loads (but not footings, or slabs that bear the full load of the house.)

Under no circumstances should rigid iso (reclaimed or virgin-stock, any density or facer) be used for sub-slab insulation.

When you say "Facer" does that mean any EPS with facering material? I'm thinking of using the R-Tech by insulfoam form my underslab insulation. It has a facing on it.

Thanks

Drew

I've tried most of the systems for attaching PEX to XPS and I wasn't happy with any of them. Tried and true 6" mesh and cable ties works all the time.

I guess I just don't like the 40 year lifespans of some tract housing around here so I really want to make sure what I put in will last 200 years if desired. When OSB was first introduced, most good builders laughed at it, then said "only for walls", then it was "only for walls and floors but never roofs". Now I see it everywhere. I am still not comfortable with it and prefer plywood and, yes, I know foot print of ply over OSB. Take up a bathroom floor that swelled using OSb and you will never want to use it again.

I just want to avoid issues in places I can't get to in the future such as under slabs which is why i am conservative in this. That said...........if Dana says so.........maybe....

URGENT

Dana1 - you said don't use rigid ico..........  we are just getting ready to install 1.5" recycled insulation with some sort of dark facer (grayish black).  It says R-10 on it and it is 1.5" thick so I'm guessing it is some sort of closed cell foam, will this be a problem ?????  We are scheduled to pour in 2 days!!!

Posted By Drew Reed on 08 Aug 2012 12:19 AM
When you say "Facer" does that mean any EPS with facering material? I'm thinking of using the R-Tech by insulfoam form my underslab insulation. It has a facing on it.

Thanks

Drew

Having low-perm facers such as foil on EPS is fine in a sub-slab application, but  not necessary.  Some EPS sheathing materials have semi-permeable facers to make it a  class-II or low class-III vapor retarder without going as low as class-I  (poly or foil).  The particulars of those factors are more relevant in wall or roof assemblies, and have no bearing on sub-slab application.  Facers DO somewhat protect the sheet goods from damage in handling though, so it's not a bad thing.

If it is truly R10 and 1.5" thick, then that sounds like rigid polyisocyanurate insulation, or commonly referred to as polyiso or "iso" as Dana referred to it. This type of insulation generally should not be used where it is in contact with the ground.

Posted By jokin on 08 Aug 2012 12:47 PM
URGENT

Dana1 - you said don't use rigid ico..........  we are just getting ready to install 1.5" recycled insulation with some sort of dark facer (grayish black).  It says R-10 on it and it is 1.5" thick so I'm guessing it is some sort of closed cell foam, will this be a problem ?????  We are scheduled to pour in 2 days!!!

Iso is off-white to yellow depending on age & exposure.  Roofing iso comes with a number of different fiber facers- this sounds like an asphalted fiberglass or paper facer.

Iso is ~R10 @ 1.5" just like closed cell polyurethane, but unlike polyurethane it is highly permeable (permeability of sheet iso is controlled by the facers), and is hygroscopic- it will readily take on water in a damp environment, with detrimental effects on it's thermal insulating qualities.  While it's nominally a closed-cell structure, in rigid sheet iso the cells are oblong and not well differentiated, with a preponderance of open cells, and the material will in fact wick up water. (IIRC there are spray iso products with somewhat better moisture resistance, but it still takes on water much more readily than polyurethane.)

Bottom line- don't use it under a slab, even a well-drained slab, or any other sub-grade application where it doesn't have access to conditioned space air on at least one side. (eg: It's OK on the interior of basement walls, but not on the exterior where it's concrete on one side, soil on the other.)