If you can read long thread http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/34462/xps-versus-eps-underslab-foam

EPS soaked 238% of water. Type 2 EPS may be different, still... And you should have poly under xps or eps anyway.

Brian, I know it soaks up water. As does XPS. XPS soaks it up slower according to GBA. Either way, that is not much of a worry to me if it's below grade and not interior. I'm more concerned about the added energy efficiency, unless you are suggesting wet foam is worthless...water does take down the R-Value, but for the basement walls and above grade walls I think EPS is still viable?

Not sure if you are in a state that's adopted the 2012 IECC, but its probably a good investment even if they arent enforcing it locally yet. It calls for R10 under the slab for 2' around the perimeter. Going under the entire floor makes for a more debatable payback but most experts seem to recommend it to keep the slab at lower risk of condensation. Weve been using CC spray foam instead of messing with the rigid sheet insulation.

R10 under the slab for 2'? Along the footings vertically you mean, or just 2' along the perimeter of the slab interior? I don't know what the benefit of having 2' of foam all along the perimeter of the basement slab would do? This will be a basement, versus a slab built house. Code here is 2003 or 2008, not 2012.

Code will probably require full, under-slab foam some day in the relative future. Not that local codes will adopt and enforce it and not that builders/owners will still avoid complying. There are many ways to build cheaper than the minimum performance that laws are suppose to allow.

Yes, 2' around the perimeter. The 2012 IECC also requires R15 for the basement walls in your climate as a minimum. Higher performance builders are typically using more than code minimums are even going to great lengths to insulate under/around the footers.

Building in Toronto, a cold climate, I've used XPS as sheathing abovegrade and as basement interior insulation, followed by conventional fg batts. (I've also used CC foam on the basement interior--a much faster and tighter installation than with boards and hard to find experienced labour.)

Both uses bypass the contradictory claims of the EPS/XPS manufacturers. A recently released study funded by the EPS makers claims that over 15 years XPS underground is trumped decisively by EPS. An apparently independent study out of Finland comparing the products over four years said the opposite.

I've never used foam under a basement floor as I spec build and such upgrades don't seem to bring any benefits to the bottom line. I'm considering it for my next personal home, from the view of comfort (warmer floors) and the reduced need for expensive mechanical dehumidification. But I may well change my mind when I see the cost in black and white!

@SpringTime do you suggest one or the other (eps/xps)? Would you install it before or after the poly film?

@worthy that is what I come up with, contradicting studies. I'm all for EPS since it is cheaper, but it seems to be used much less than XPS. Local builders ONLY use XPS when they are using it for foundation walls. Around here it is uncommon to see new construction with any rigid foam on the walls...I'd really like to just find out which option makes the most sense financially, but I don't want to kick myself in the future for using the wrong product.

We use CC spray foam but given the choice between XPS vs EPS I would prefer XPS installed before poly. I would have no problem using the right type of EPS though. I think a more important detail is to have a well drained gravel layer, preferably to daylight. We typically do an interior PVC drain perimeter to daylight, which we add a stack to for radon control. Again, not required by code but darn cheap at this stage of the game considering the consequences of trouble down the road for both an active radon system or sub slab drainage issues. With a well-drained layer of gravel below your foam, it shouldnt ever be sitting in water for very long.

I don't plan on using spray foam. I will see what the concrete contractor says about using foam. Think it will be a 'never used it' answer.

Energy stretch code is now the norm in our area, and XPS is the most common solution. It holds its r value better when wet. It's a very small additional expense and has immediate payback in comfort. We did 4" under the floor. On the walls we used 2" of sprayed closed cell foam on the exterior, 2" of XPS on the inside of the concrete, then a 2x4 stud wall with fiberglass batts so the walls are close to r30. Most comfortable basement I've ever had. Heating costs for the whole house are extremely low for the region, I can't say to what extent the basement insulation is responsible for that.

Keep in mind that the earth below a house is warmer than the air temperature during cold months and the heat loss through the wall or slab is directly proportional to the temperature difference from on side to the other. In a cold climate the earth below a basement slab is generally the same as local well water or about 50 to 55 degrees. That's not very cold. The temperature of the earth against basement walls varies from the temperature of well water at the bottom to the temperature of the air at the top. Therefore, the farther below grade the less insulation is required to maintain energy conservation similar to above grade walls. So, it doesn't take as much insulation to make a basement comfortable and 2" of XPS rigid insulation is usually adequate below the slab and at the walls. Of course, the wall insulation should be integrated with the wall waterproofing and footing drainage systems (grooved XPS or a drainage layer against the wall, etc).

Building codes for multifamily housing usually require under slab insulation at habitable spaces but single-family residential codes usually let you make that decision for yourself. Of course, the biggest variable is how the space will be used.

The compressive strength of the lowest grades of XPS and EPS is adequate for use against foundation walls although XPS is tough and EPS is more fragile during installation. Only XPS should be used under a concrete slab not because of its compressive strength but because EPS is too fragile to be walked on.

Both XPS and EPS are made of closed-cell polystyrene foam but XPS is monolithic foam and EPS consists small beads of the foam. XPS absorbs very little water but EPS absorbs considerably more between the beads when submersed in water. How much water is absorbed in normal use below grade and how much R value is lost is a mater of contention between manufacturers.

The dry R value of XPS is 5 for 1" and 10.6 for 2". The dry R value for EPS is 4 for 1" and 8 for 2". EPS is cheaper but it takes more of it to reach a given R value.

I would not be concerned about the loss of R-vlaue due to aging of polystyrene insulation under ground. Polystyrene deteriorates in air and sunlight.

@renovator8 - good information. I am planning on 2" of XPS for sub-slab if I use it. In zone 5, I think ground 8' below is 55-60 degrees. My basement would not be used for now, but if I had a family it would be used for a family room and additional bedrooms, and of course the spare bathroom. Therefor, I think it is a toss up in terms of whether to invest and convince my concrete contractor I want to use XPS foam when he is not familiar with that technique. I would be installing the foam, and hope he and the crew are careful with it when walking ontop of their wire mesh. It is 30x44 basement size so about $1,500 in XPS foam if I went with 4x8 priced @ $34 a piece. Not *a lot* but also a considerable amount when this is not above grade walls. (Though the walkout basement walls will benefit more than the slab with insulation)

The temperature difference between the basement air and the earth below will be about 10 degrees. In that condition heat will not move very fast through the slab so the purpose of the insulation is to make the floor more comfortable for occupants and reduce condensation. Therefore, 1" of XPS should be adequate but if you want to jump ahead of the construction industry make it 1 1/2". 2" or thicker XPS would be needed for radiant heat in the slab.

Since the insulation board will protect the vapor retarder from punctures, the contractor should be happy to have it. Don't be surprised if your contractor chooses to use it for his future projects. I've seen that kind of change in contractor attitude occur many times.

While 1" of XPS is certainly better than industry standard at this point, its still less than what current international code requires under slabs at the perimeter for climate zones 4 and 5; R10 for 2' and jumps to 4' at climate zone 6. I think BSC recommends R10 under the entire slab for northern climates these days. Delta T of typical basement air and ground isnt the whole story. Completely agree that reluctant contractors are more on board when they know all the benefits and experience that its not that difficult.

Serious heat loss can occur at the perimeter of a slab on grade but this is a basement slab where insulation under the slab is required at the walkout side but not under the entire slab.

Technically 1" of XPS versus 2" of XPS would be about half the cost. So, is that worth the savings ultimately? I will not have in floor radiant heat for the basement. When you say slab I keep thinking you mean a slab built house, to reiterate, this is a basement and local code *here at least* does not require ANY rigid foam for the basement slab. Not sure if it even is required on the foundation walls. I guess this zoning ordinance is behind on code updates. -shrug- I see myself as one in a few that will be considering rigid foam for any basement application in new construction here.

Is there any benefit to using XPS over the EPS for the foundation WALLS? I know I can get some EPS cheaper per R-Value but it will be thicker. I was thinking over 2" of EPS foam would be troublesome to fasten and then cover. Any recommendations on what to cover the foam with before backfill? Or should I only worry about covering the exposed foam on the walkout basement portion? I intend on using a waterproofing prior to installing rigid foam to the foundation walls.

If you are looking to put foam under the slab and are concerned about cost check out places that sell recycled foam. I am about to begin building in MA and I am using the Pretty Good Housing standard for insulation:

R-5 windows

R-10 slab

R-20 basement walls

R-40 above grade walls

R-60 roof

All the rigid foam will be recycled xps. If you have a recycler close to you expect to pay about .50 on the dollar or even less and you are helping out the environment by not using new foam.

Dang, R40 walls? How much of that will be exterior XPS?

Unfortunately there does not appear to be any nearby recyclers. Small town, and low populated area of the state.

2 layers of 2" XPS on the exterior and 2x6 cellulose on the inside. Most places will deliver but it may not be economically viable for you.

Aledoux, where do you intend on having your WRB? Before or after the foam? What product are you planning on using?

Wall detail from interior to exterior:

drywall

2x6 dense pack cellulose

Zip sheathing with windows flashed to zip (innie windows)

Tyvek Drainwrap

2 layers of 2" XPS staggered and taped

1x4 furring strips

LP Smartside and Azek trim

I know the drainwrap is overkill(zip doesn't need housewrap as it is considered a WRB) but I really worry about water sitting between the foam and sheathing. You can get more information by searching for "Remote wall system."

I am going to be using fiberglass in place of cellulose & osb instead of zip sheathing. My windows will be outties and I am not too excited with trying to screw through the foam and into the 2x frame. Waterproofing them will also be another interesting detail since I plan on having my WRB after my OSB and before my foam.

Here are DOE R-Value recommendations, including for basements. Input the first three digits of your zip code so it pulls the data for your climate zone.

http://web.ornl.gov/sci/roofs+walls/insulation/ins_16.html