eps or epx foam under slab

[Highbeam]

Yes, that's a great site for this type of thing.

I used EPS and know the plant manager at the EPS factory so I was able to pick his brain. I am also a civil engineer and have used EPS as lightweight fill under roadways above gas mains and at railroad crossings. There is nothing at all wrong with putting EPS underground including those places where moisture may be present. 25 psi EPS is just as strong as the bearing capacity of most subgrades so strength is not an issue. Concrete, 6", only weighs 0.5 psi.

The trouble is when you wreck a truck and pour fuel into the foam. Then it shrinks. Probably happens with XPS too but I've never tried it.

 

[mikefrommaine]

http://www.gologic.us/videos/passive-house-foundation/

 

[tom in maine]

I derive some income from selling foam. I have been associated with a number of companies over the years.
A couple thoughts/comments from my experience:

Most foams are rated for at least 20psi. That is about 3000psf. There are very few applications that we would encounter that would not be well served with 20psi products. This would assume a reinforced slab, which is just common sense for any concrete project.
I believe even 1 pound density EPS would meet this criteria. There are many road projects that use EPS for filler to build up a substrate.
I am extremely nervous about bugs in foam.Once they get there it is scary. They are going to nest in it because it is easy to chew through. I would NEVER use foam that is contiguous from the ground to the wooden part of the house, especially where termites are common. I do not trust a flashing shield to keep them out. This goes for ICF's too.
There seems to be inconsistent results from treated foam in ground keeping bugs at bay.
I think the only real way to keep them out is baited traps and "nuclear" medicine if they show up.

Polyiso can be used under slabs. Realize that if you are doing a slab on grade, it should be very well drained.
Polyiso can pick up water, but so can the styrenes. If it is on a properly drained substrate, it will work.
I would use it under a slab, not on the edges where it sees daylight.
Polyiso can pick up up to 10 times its weight in water. At this point it will lose half of its R value.
And it will retain half of its R value. Once it dries out, it regains that loss. It can dry out, over time.
EPS and EPS can also pick up water. EPS more than XPS, but they both do with similar loss in insulation value.
They are slower to pick up and discharge water.

Regardless of what insulation I was using for under a slab, I would always use a reasonably good grade vapor barrier.
It only makes sense.
Joe Lstiburek felt that Polyiso made sense under slabs. He and I disagreed on where the barrier would go.
I would put it under the foam, he thought it should go over it.

My 2 cents worth from 30 years of dealing with foam as a buyer and seller.

 

[DaveBP]

If the vapor barrier protects the concrete from moisture, why not put it below the foam and protect the foam as well? I agree with you, Tom.

But an equally important factor, I think, is that if it's under the foam the film is protected from the mud crew.

 

[eauzonedan]

Tom in Main:

This is a copy of the design info for the Foamular products. What's your read on the long term dead load capacity? I interpreted this to de-rate the nominal ratings to about a third of the nominal load for long term dead loads like maybe a building sittin on it.....(see note 5) The table got a bit messed up when imported, but the last three items for each of the product items is Live, Dead and Ultimate. Here is a link to the original http://foamular.com/assets/0/144/172/174/c273fe0e-e0c8-4bb2-9ed3-6e3ba390a12c.pdf

I used only a couple sheets of the 60 psi foam, just under the grade beams of my building and everything under the center portion of the slab was 25 psi.......under a couple hundred bucks.......would like to hear your thoughts if I wasted the money? After 4 years no cracking, but that's not to say the foam was the reason...... I'm just a silly surveyor and not much a a structural kind of guy....

Dan


The Foamular Foundation Properties table provides engineering data for individual project analysis if needed. Foamular Foundation Properties
Foamular Product

Foundation Modulus (pci)1, 2, 3

Compressive Stress (psi)


Thickness (in.)

Live5
Recommended
Dead5
Recommended
Ultimate4


150

590
550
500
450
400
300
3
5
15
250

750
710
675
595
565
510
5
8.3
25
400

1100
1000
900
780
680
650
8
13.3
40
600

1520
1400
1275
1150
1040
790
12
20
60
1000

NA
--
2600
NA
NA
NA
20
33.3
100




1.
Foundation modulus is a measure of deflection at given loads, expressed as inches deflection per inch of thickness or "pci".
2.
For insulation installed in multiple layers, assuming the layers are identical, the foundation modulus for the system equals the foundation modulus for one of the layers divided by the total number of layers.
3.
For insulation systems that utilize a variety of thicknesses, the system foundation modulus is determined by adding the reciprocal of the foundation modulus of the individual layers. The total is the reciprocal value for the foundation modulus of the entire system.
4.
Ultimate compressive stress is measured at 10% deformation or yield, whichever occurs first. For thinner product (1"), yield typically occurs first. For thicker products (1.5" and thicker), yield typically occurs first with 3% to 4% deformation.
5.
Recommended stress (load) levels will limit long term compressive creep to not exceed 2% in 20 years.

 

[eauzonedan]

sorry about that ........once I sent it - the table got completely corrupted for some reason.........best to follow the link to the Foamular site.........
Dan

 

[Bob Rohr]

I derive some income from selling foam. I have been associated with a number of companies over the years.
A couple thoughts/comments from my experience:

Most foams are rated for at least 20psi. That is about 3000psf. There are very few applications that we would encounter that would not be well served with 20psi products. This would assume a reinforced slab, which is just common sense for any concrete project.
I believe even 1 pound density EPS would meet this criteria. There are many road projects that use EPS for filler to build up a substrate.
I am extremely nervous about bugs in foam.Once they get there it is scary. They are going to nest in it because it is easy to chew through. I would NEVER use foam that is contiguous from the ground to the wooden part of the house, especially where termites are common. I do not trust a flashing shield to keep them out. This goes for ICF's too.
There seems to be inconsistent results from treated foam in ground keeping bugs at bay.
I think the only real way to keep them out is baited traps and "nuclear" medicine if they show up.

Polyiso can be used under slabs. Realize that if you are doing a slab on grade, it should be very well drained.
Polyiso can pick up water, but so can the styrenes. If it is on a properly drained substrate, it will work.
I would use it under a slab, not on the edges where it sees daylight.
Polyiso can pick up up to 10 times its weight in water. At this point it will lose half of its R value.
And it will retain half of its R value. Once it dries out, it regains that loss. It can dry out, over time.
EPS and EPS can also pick up water. EPS more than XPS, but they both do with similar loss in insulation value.
They are slower to pick up and discharge water.

Regardless of what insulation I was using for under a slab, I would always use a reasonably good grade vapor barrier.
It only makes sense.
Joe Lstiburek felt that Polyiso made sense under slabs. He and I disagreed on where the barrier would go.
I would put it under the foam, he thought it should go over it.

My 2 cents worth from 30 years of dealing with foam as a buyer and seller.

I argee with you Tom, about the bugs. I'm surprised it is also a concern in a cold northern climate.

Here is some foam ICF I put around my shop about 6 years ago. It was wrapped with an aluminum bug shields. I also sprayed around them when they were installed. I think the spray needs to be replensihed every few years. A fairly safer Borax product is what I used but it washes away.

I have also seen the BlueBopard product turn into a swiss cheese after 10 years from bug bores.

Years ago I recall that had a very toxic bug repellant in many of the underground foam prodcts. Since that was banned the bugs have found a nice warm nesting spot under, and around radiant slabs.

I met a contractor from Australlia years back. He told me all the foam foundations or foam insulation sheets needs to be wrapped with a fine stainless mesh product to keep the bugs out, down there. It is also available with a tar coating to act as a water shield and bug shield.

 

[tom in maine]

Dan,
I am not a structural engineer either. I do refer to them when I need to. Fortunately, we have a really good one who answers my questions at the University of Maine. He is a consultant for work NASA is doing on a moonbase(!) They built one in Orono, Maine!

There is the potential for foam to creep over time but I suspect the most important factors are a solid substrate (this is most important for any slab) and reinforcing the slab.
There is always the factor of manufacturers tailoring their specs for the best impression of whatever they sell. This is my first impression after only one cup of coffee this morning. I will look it over again. I think the average of 20 psi is real and stable.
Residential loads are minimal. If you have a reinforced slab over 4" thick(I believe mesh is adequate), I think the potential for impact on the foam is minimal.
This is really a good question for an engineer. I have a good friend who is a code guy and will run this by him as well.

 

[BoilerMan]

This is interesting indeed, I agree with you Tom about the vapor barrior placement as well, under the foam seems best. I have been cleaning up the yard as the snow has FINALLY MELTED up here. I found this, it's been on the ground for about two years as far as I can figure. Pink "foamular" Owens Corning 2" thick. These holes are a bit bigger than 1/4" scary as this is what's under my house.............. I have and will continue to spray pesticide 4' out around the entire house every month in the summer.

TS

 

[tom in maine]

So, I will throw out a pet theory on foam underneath slabs:
The Passiv Haus guys and others are using a LOT of foam under slabs. We all have seen or heard the horror stories of critters in foam.
If one built a well drained floating slab with no insulation underneath it, and then insulated on top of the slab, with either sleepers to carry the load to the floor or a tongue and groove deck over top, the insulation is inside the house and not in contact with the soil.
It would be more expensive, but simpler in some ways. It makes the slab install quicker. It would be critical for it to be well drained. It functions a bit more like a crawlspace but is filled with foam.It should not ever move because it is insulated on top and is on a well drained base.

What I think about in the evenings....

 

[BoilerMan]

So, I will throw out a pet theory on foam underneath slabs:
The Passiv Haus guys and others are using a LOT of foam under slabs. We all have seen or heard the horror stories of critters in foam.
If one built a well drained floating slab with no insulation underneath it, and then insulated on top of the slab, with either sleepers to carry the load to the floor or a tongue and groove deck over top, the insulation is inside the house and not in contact with the soil.
It would be more expensive, but simpler in some ways. It makes the slab install quicker. It would be critical for it to be well drained. It functions a bit more like a crawlspace but is filled with foam.It should not ever move because it is insulated on top and is on a well drained base.

What I think about in the evenings....

I like the idea, but you'd loose all the benifits of the huge thermal mass of the slab, which is one of the reasons I chose to build on a slab. I've seen it done as you say with PT sleepers and foam in a retrofit living space on an otherwise uninsulated slab. Like people converting a garage into living space.

TS

 

[Seasoned Oak]

WHat about using salt under the foam. I know some people use it to drive away termites.most if not all bugs DO NOT thrive in salty soil and actively avoid it. Salt does have a bad effect on concrete ,but if its under the foam it will not come in contact with the concrete.

 

[Chris Hoskin]

and some thoughts from Siggy here: http://www.pmmag.com/articles/91791-mountaintop-experience?v=preview

 

[Frozen Canuck]

So, I will throw out a pet theory on foam underneath slabs:
The Passiv Haus guys and others are using a LOT of foam under slabs. We all have seen or heard the horror stories of critters in foam.
If one built a well drained floating slab with no insulation underneath it, and then insulated on top of the slab, with either sleepers to carry the load to the floor or a tongue and groove deck over top, the insulation is inside the house and not in contact with the soil.
It would be more expensive, but simpler in some ways. It makes the slab install quicker. It would be critical for it to be well drained. It functions a bit more like a crawlspace but is filled with foam.It should not ever move because it is insulated on top and is on a well drained base.

What I think about in the evenings....

I have seen a variation of this on some of the newer net zero homes being built in Edmonton. No foam under slab. 2" high density foam on top of slab then pregrooved underlay with aluminum radiant plates in most cases but not all, on top of slab. Then tubing in gooves. Then floor covering usually 12mm laminate but other flooring options are doable like tile for instance. So far results are good, next to zero heatloss downwards, tubing is accessable in the event of a need, zoning is easy, accurate & perhaps most important given the life cycle of the building "changeable" for future owners with a different idea. All in all a better way to look at radiant floors IMO. Only real issue is that the buildings are getting so efficient that tubing in some cases does not yield that "warm floor" as the btu requirements of the building are too low. In most cases <10 btu/sq ft. When you think about it this is simply the old tried & true practice of insulating the warm side of a structure rather than the colder underside of the slab in this case. Makes sense, & takes the "bugs" out of the equation. A

 

[Frozen Canuck]

Sorry must have miss clicked. Last sentence was to be that all one has to do as a builder is plan for the increased height of the basement floor, not really an issue with many wanting 9, 10, or 12' ceilings in a modern home.

 

[mikefrommaine]

I have seen a variation of this on some of the newer net zero homes being built in Edmonton. No foam under slab. 2" high density foam on top of slab then pregrooved underlay with aluminum radiant plates in most cases but not all, on top of slab. Then tubing in gooves. Then floor covering usually 12mm laminate but other flooring options are doable like tile for instance. So far results are good, next to zero heatloss downwards, tubing is accessable in the event of a need, zoning is easy, accurate & perhaps most important given the life cycle of the building "changeable" for future owners with a different idea. All in all a better way to look at radiant floors IMO. Only real issue is that the buildings are getting so efficient that tubing in some cases does not yield that "warm floor" as the btu requirements of the building are too low. In most cases <10 btu/sq ft. When you think about it this is simply the old tried & true practice of insulating the warm side of a structure rather than the colder underside of the slab in this case. Makes sense, & takes the "bugs" out of the equation. A

Does the foam go down before the walls go up? --- is it under the exterior walls on a slab? Or is this just for basements.

 

[Frozen Canuck]

Framed as per normal Mike. Yes just for basement floors, thats +95% of market here (full basement), never seen foam under a load bearing wall, no reason though to not put it under partition walls in new construction, non load bearing of course & that's what I see in the newer net zero homes, total isolation of the living space from the concrete floor (conductor). In areas where a basement is not practical one could do this on a floor over a crawl space for example, or for a slab on grade. I for one have never got the flywheel thing. If I had a flywheel that was losing 10-20% of it's energy to the ground I would fire it for non performance issues.

 

[tom in maine]

We have done several floors with foam on top of the slab.
We (and some others) have grooved polystyrene foams and put tubing in the grooves. I had made a heated "groover" with some cheap wide soldering irons. It worked well and was quite smelly, if you have never heat cut EPS, it is very smelly! I was nervous about higher temps affecting the foam, and melting into the foam. I never did a post mortem since it is still in service. Nothing happened that seemed to be really bad.

I did another odd install that was installed at the University of Maine in a heating project about 20 years ago.
This one used polyiso foam like Tuff R, which was foam without fiberglass in it. Polyiso like Thermax used to have fiberglass embedded in it and would not work.
This gets weird, but it did work.
I used foam that was 2" thick, took 1/2" polybutylene tubing (PEX might work), taped it to the foil facer with 3" wide foil tape and beat the tubing into the foam with a rubber mallet until it was flush with the foam surface.
We then covered it with a product like cement board (this was before cement board was readily available.
The board was glued to the foam and then a second layer was installed over the first, with the joints offset.
It worked well and did heat reasonably well.
It is still in service. Dick Hill had us do it. It was tedious.
This was in a house that we did a lot of heating system tests called the "Parallel Systems House".
The guy who built it claimed we were trying to kill him.
He was a real character who was the most untechnical person I ever met. He was an English professor who taught Stephen King.
Although he was a complete Luddite, he wanted us to do this project and funded it. (He was so opposite Dick and I, he could not figure out how to plug in a telephone!)

 

[Frozen Canuck]

Sounds like fun Mike.

In future you can use a hotknife for grooving foam in a situation like that, we use them when we do a SIP home for wire grooves/elec boxes etc, which I am happy to say the younger generation is far more interested in than mine. It's hard to express just how impressed I am when a young 20 something couple comes forward & says we want to build a very energy efficient home to start our family in - how do we best do that? They will likely be close to a million dollars ahead of my generation just on the money saved in utilities by the time they reach my age. Spend it upfront then bank it for 50+ years. Not like my generation at all.

The idea of more (thicker) high density foam has been kicked around by a few of us old dogs & then just like you said flooring right over the foam. We know we can achieve no measurable downward loss, the only kicker has been at high impact zones such as the bottom of the stairs, that I am sure we can work around.

Sounds like an interesting project with folks you enjoyed Mike.

 

[arbutus]

Frozen Canuck, thank you for posting that.
I am planning to retrofit my new to me home with slab insulation in a similar fashion.

 

[Frozen Canuck]

No problem, hope it helps. The only consideration on a retrofit is the uneven tread height on the last step of the stairs due to increased floor thickness in the basement. If you have new stairs as part of that retrofit then problem solved.

 

[BoilerMan]

I for one have never got the flywheel thing. If I had a flywheel that was losing 10-20% of it's energy to the ground I would fire it for non performance issues.

Isn't storage kinda like the "flywheel" like a slab? I know you can't really regulate the emmisivity of a slab like you can dray or not draw from storage, but the concept is simmilar. I build one fire a day, the air temperature changes very little. Seems like I have some type of storage going on. I wonder what the real loss factor of a slab is with R10 foam under it. I'd think the 10-20% may be high. The critter issue is a good case for a floating slab inside and isolated from frost walls poured around the perimiter as load bearing walls. I wonder just how deep sub-terrainan bugs go, that is at what depth would foam be considered inherrantly protected if there is such a thing.

TS

 

[arbutus]

No problem, hope it helps. The only consideration on a retrofit is the uneven tread height on the last step of the stairs due to increased floor thickness in the basement. If you have new stairs as part of that retrofit then problem solved.​

Haha!

New stairs are in the works, though for a different issue!

 

[Frozen Canuck]

BM. 10-20% would be for slabs in direct contact with soil. Probably a conservative number & yes plenty of other variables as well, clay/sand/gravel, water table height & on & on.

Up here the local water well driller says down to 10 feet he finds ants in granular soils so it's probably safe to consider exterior foam as bug food/attractant in many areas, no termites here thankfully (yet).

Most homes burn fossil rather than wood so buffer tank of say 40 gallons to shield the boiler from the direct loads, but yes I get your analogy to storage in a cordwood system, way easier to measure that loss accurately though & control/limit it.

 

[tom in maine]

Interesting thread. I keep remembering things. Guess I am getting old...
The groover I made did about three feet wide with tubing slots every 6". The grooves were for 1/2" tubing. It went very quickly.
The other thing was another floor which was all foam. It was basically a structural panel that we did with pressure treated lumber. There was PT on the bottom, a core of foam and then plywood on the topside. It was glued and screwed together. It was basically a panel on its side. To try to mitigate the ant issue, I wrapped the slab in .060" EPDM underneath and up the sides of the slab. It was then covered on the sides with PT plywood. I used tapered foam since the site was not level and I was too lazy to grade it.
It was a 12'x12' building and it had never moved.
No heat in the building, but it never freezes.

The advantage of the foam floor is although it is low mass, it responds very quickly.
That being said, a concrete slab has a significant heat storage ability.