Spray foam under slab question - Compressive Strength ?

[grkmec]

So I know the basics about spray foam - open vs. closed, R-value differences, etc. I was meeting with an insulation contractor and was asking about using closed cell under slab in lieu of something like Foamular 250. The question I had was on compressive strength - what is the compressive strength of closed cell foam?

The contractor didn't know. She told me that closed cell is typically 2 pounds per cubic foot. She offered me the option of higher density 3 pound per cubic foot closed cell which she says is typically used to insulate below pools. But again, I asked her what compressive strength is for 3-lbs and she didn't know.

Anyone have this info?

 

[purplezr2]

Claims 15-25 according to google, obviously 250 is 25psi

 

[Randy in Maine]

Why are you not using the normal 2" of foam (as in a 4'x8' sheet) under the slab? It is only about $1 per square foot.

 

[purplezr2]

Why are you not using the normal 2" of foam (as in a 4'x8' sheet) under the slab? It is only about $1 per square foot.

Spray foam is similar in cost(installed) , assuming it has equal or higher compression strength, I would go with spray foam cause it is easier.

 

[matt_i]

Man that's high risk for a site-formulated (mixed) product...I would want to see manufacturer data to include direct soil contact (basically with the potential to be consistently wet from ground water) before committing to it under a floor slab.

Just saying the Foamular is pretty repeatable in my estimation. Used in lots of builds without issue. If there was to be a spot where the mix wasn't quite right in spray-foam, there would probably be a fail in the concrete. Could get expensive.

Tl;dr, I'd go with the Foamular.

 

[grkmec]

I was debating doing a 15ml vapor barrier + 2" foamular 250... But would need to tape all the seams and worried about it getting damaged when laying the re-bar for slab, etc. I figured a monolithic spray foam pour would provide better protection assuming the product was properly rated for application.

 

[grkmec]

Ok just found these pdfs off the insulation companies website:

https://www.ecologices.com/wp-content/uploads/2017/10/Gaco-Onepass-Closed-Cell-TDS.pdf
https://www.ecologices.com/wp-content/uploads/2017/10/BASF-Spraytite-SP-Closed-Cell-TDS.pdf
https://www.ecologices.com/wp-content/uploads/2017/10/Lapolla-Foamlok-2000-closed-cell-TDS.pdf

Looks the the answer is 25-30 psi. The Graco product specifically says 28.5 psi so that's plenty.

 

[Pluribus]

Even after you were to address the compressive strength and the direct, moist soil contact over a long term, there are other potential issues. From a practical standpoint, how would you get it flat? You can't exactly spray after the slab is installed, and that stuff expands very unevenly. Unless you're going extra-thick with the concrete, then you're going to have high foam spots leading to thin concrete in places.

 

[wssix99]

Ok just found these pdfs off the insulation companies website:

https://www.ecologices.com/wp-content/uploads/2017/10/Gaco-Onepass-Closed-Cell-TDS.pdf
https://www.ecologices.com/wp-content/uploads/2017/10/BASF-Spraytite-SP-Closed-Cell-TDS.pdf
https://www.ecologices.com/wp-content/uploads/2017/10/Lapolla-Foamlok-2000-closed-cell-TDS.pdf

Looks the the answer is 25-30 psi. The Graco product specifically says 28.5 psi so that's plenty.

No. This will NOT work well. It's a penny wise - pound foolish proposition.

Compression strength/PSI (like size) isn't everything. (There are reasons you don't see anyone doing this and reasons why your contractor hasn't done this before!)

Foam insulation has another property called "creep". Even though the spray foam has a high compression strength, it will slowly compress over time, compressing and possibly even leading to cracks in your floor.

Foam Board is good against creep and Spray Foam is not. There are links in this thread to some comparisons: https://www.garagejournal.com/forum/showthread.php?t=372259&highlight=spray+foam+slab

 

[JeremiahTRD]

The foam I spray is rated at 25psi.

I have personally done several slabs and foundations with 2lb closed cell foam.

I had to look into this phenomena known as creep? I understand the concern and personally feel it's a non issue. Just your usual skeptics and naysayers.

I couldn't find any data to support this claim.....

The main benefit of spray foam is there are no seams its a monolithic barrier. Its faster and much more thorough insulation job.

At 25psi the foam is actually stronger than most soils compacted properly.

 

[Leaflessshadetree]

I can't see it as being any easier than just laying down sheets. I did have to cut some but didn't have to be super accurate. I cut them a little large with a table saw or circuar saw and then pressed them infor tight joints. Why would you tape the joints?

 

[wssix99]

I have personally done several slabs and foundations with 2lb closed cell foam.

We'll if it pancakes slowly down to a thin layer of goo and ends up having no insulation value, no one would ever know until the structure is demolished and the builder is long gone, right?

I had to look into this phenomena known as creep? I understand the concern and personally feel it's a non issue. Just your usual skeptics and naysayers.

I couldn't find any data to support this claim.....

Read the link above and you'll see some real links to specs/data on the topic.

Here's some general information for the non-initiated:
https://en.wikipedia.org/wiki/Creep_(deformation)

BTW - Epoxy also has this problem under sustained tension loads: https://en.wikipedia.org/wiki/Big_Dig_ceiling_collapse

Personal feelings are interesting, but if someone doesn't have the experience to find the desired data/information or to fully evaluate the design; I don't understand how that becomes a green light to take a wild guess. In the absence of fact, why not defer to commonly accepted practice and products that manufacturers market for the specific purpose?

 

[grkmec]

No. This will NOT work well. It's a penny wise - pound foolish proposition.

Compression strength/PSI (like size) isn't everything. (There are reasons you don't see anyone doing this and reasons why your contractor hasn't done this before!)

Foam insulation has another property called "creep". Even though the spray foam has a high compression strength, it will slowly compress over time, compressing and possibly even leading to cracks in your floor.

Foam Board is good against creep and Spray Foam is not. There are links in this thread to some comparisons: https://www.garagejournal.com/forum/showthread.php?t=372259&highlight=spray+foam+slab

I am actually not trying to be a penny wise / pound foolish. Spray foam is more expensive by my calculations. Closed cell is like $1.10 per board foot and I have approx. 2,000 sq ft slab so that would cost $4,400. Or I can get 65 sheets of Foamular 250 from home depot for around $2,200. Then need to add the cost of 15mil vapor barrier. Call that about $600. Then figure some labor. Spray Foam is about 1k more expensive.

 

[matt_i]

I would do the Foamular. I think you can back off to 10mil vapor barrier, you just need enough not to get torn by the foot traffic. I used this thickness over top of crushed stone and it was in good shape before the pour, and so I think that will do very well between the smooth foam and the fresh concrete. Assuming you use rebar chairs there really shouldn't be any serious damage.

 

[ConCretin]

I've seen more spray foam used under slabs in recent years and I like the concept but given the high cost of failure, I'd be reluctant to use it until it has been proven over time.

If you are going to the trouble of putting in a vapor barrier - and you should - I wouldn't use poly. It will not stop water vapor, it will only slow it down. Secondly it is very easily damaged and every perforation reduces it effectiveness even further. And finally it degrades over time. I'd stick to your original plan and use a 15 mil vapor barrier.

 

[stokefire7]

If it's any help, I went with two 1" layers of Firestone polyiso (factory rejects)
with my vapor barrier sandwiched between the layers.

 

[stokefire7]

$7.00 bucks for a 4 x 8 sheet.

 

[kj_mustang]

Polyiso is made for roofs and walls not under slabs. It is not rated for it.

 

[stokefire7]

Polyiso is made for roofs and walls not under slabs. It is not rated for it.

You can't bury polyiso ?
Nonsense.

 

[Big Bad Dad]

I think termites will eat certain foam products.....

 

[grkmec]

Well, if termites can get to my foam sandwiched between 6" of compacted crush stone and 6" of 4500psi cement slab, then I will be thoroughly impressed. Just ran across this article:

http://www.jlconline.com/how-to/insulation/insulating-a-slab-with-spray-foam_o

Obviously written by a SPF contractor. Getting lots of conflicting information here for what is the best option. Cost is a secondary issue for me.

 

[grkmec]

Ok, just found a data sheet on 3-lb closed cell foam. Says compressive strength is 40 psi.

https://www.demilec.com/documents/Tech-Library/High-Density-SPF/Updated/B-233-TDS.pdf

wssix99 --

So you think I am still better off with Foamular 250 + 15mil plastic than something like 3-lb foam? If I use 2" of 3-lb, that's 40psi compressive and gives me R14. The slab will be 6" of concrete laid on top of #4 re-bar 12" OC both directions. So a 1/2" variation in spray foam seems acceptable.

 

[kj_mustang]

You can't bury polyiso ?
Nonsense.

I guess the manufacturer's reps speak nonsense.

 

[joe--h]

EPS with borate (?) keeps ants & termites from eating it. Generally cheaper than Formular too.

This stuff is made for underslab use. Type II is more than adequate density, 2000+psf.

http://www.thermalfoams.com/foam-control-eps/construction/perimeter-underslab/ is near you, give them a call.

I've bought it direct from the manufacturer, they will cut it whatever thickness you want. Comes in 4x4x8' blocks wrapped in plastic sliced to your size. Like a giant loaf of bread.

And no matter how much stone you have under it, bugs that live in the ground will get to it eventually.

Joe H

 

[6768rogues]

I have seen buildings with foam under the floor where frost went under the building and heaved the floor. It happens during a cold winter with little snow on the ground. The only way I would insulate the floor is to insulate vertically at the wall lines to below frost depth, not under the concrete floor.

 

[JeremiahTRD]

We'll if it pancakes slowly down to a thin layer of goo and ends up having no insulation value, no one would ever know until the structure is demolished and the builder is long gone, right?



Read the link above and you'll see some real links to specs/data on the topic.

Here's some general information for the non-initiated:
https://en.wikipedia.org/wiki/Creep_(deformation)


Personal feelings are interesting, but if someone doesn't have the experience to find the desired data/information or to fully evaluate the design; I don't understand how that becomes a green light to take a wild guess. In the absence of fact, why not defer to commonly accepted practice and products that manufacturers market for the specific purpose?

So your saying read the facts and yet the best you can come up with is a link to a wikipedia definition? Jeesh give me a break. I read the link above and there was nothing in it nor have I seen a study on it. So prove me wrong or your just another naysayer.....

 

[wssix99]

Well, if termites can get to my foam sandwiched between 6" of compacted crush stone and 6" of 4500psi cement slab, then I will be thoroughly impressed.

Forget **** fighting. I'd start a competitive termite breeding enterprise if I could come across some specimens like this!

Just ran across this article:

http://www.jlconline.com/how-to/insulation/insulating-a-slab-with-spray-foam_o

Obviously written by a SPF contractor. Getting lots of conflicting information here for what is the best option. Cost is a secondary issue for me.

lol You are correct. JLC is an interesting place where anyone can show up and be an expert journalist. Just like any other place on the internet... (Although I think they are in print, also.)

My wife (a licensed GC) used to read this periodical regularly and her blood pressure is much lower now that she doesn't read it any more. Too much whacked-out stuff.

^ It could be a much better publication if the articles were peer reviewed, kind of like scientific publications.

I've seen more spray foam used under slabs in recent years and I like the concept but given the high cost of failure, I'd be reluctant to use it until it has been proven over time.

It's an interesting situation. People could be thinking they are doing a "good thing", but a mistake would be almost impossible to notice without ********* data collection showing a small incremental reductions in insulating values over time. Even professional engineers miss planning for the creep phenomena. (Hence the big dig accident, etc.)

 

[wssix99]

So your saying read the facts and yet the best you can come up with is a link to a wikipedia definition? Jeesh give me a break. I read the link above and there was nothing in it nor have I seen a study on it. So prove me wrong or your just another naysayer.....

Welcome to the site. An easy definition like what's on Wikipedia is sufficient for one to go off and research the topic however they want. If you doubt the concept of creep, given what you can start to find there, we might as well start debating the moon landing.

Here are the technical specifications we discussed the the thread where we already discussed this topic (I linked that, too.) where foam boards are rated for creep.

http://commercial.owenscorning.com/assets/0/144/172/174/c273fe0e-e0c8-4bb2-9ed3-6e3ba390a12c.pdf
http://msdssearch.dow.com/Published...foam/pdfs/noreg/179-02548.pdf&fromPage=GetDoc

Spray foam is so bad at resisting this phenomena, I haven't come across anyone who can even find a rating for it on the spray stuff.

This is a concern. That's why the board is tested for it.

Please show us a spray foam manufacturer who has a rated product for this application. I'm sure that you can find one if this is really a good idea.

 

[wssix99]

Ok, just found a data sheet on 3-lb closed cell foam. Says compressive strength is 40 psi.

https://www.demilec.com/documents/Tech-Library/High-Density-SPF/Updated/B-233-TDS.pdf

This is for walking surfaces or, possibly (I imagine) roofs. I can't find anything about creep resistance or applications where it would withstand a sustained load on top of it.

wssix99 --

So you think I am still better off with Foamular 250 + 15mil plastic than something like 3-lb foam? If I use 2" of 3-lb, that's 40psi compressive and gives me R14. The slab will be 6" of concrete laid on top of #4 re-bar 12" OC both directions. So a 1/2" variation in spray foam seems acceptable.

I would not use any spray foam at all due to the unknown/unconfirmed resistance to sustained loads. If that 2" of 3-lb foam compresses to 1/2" of foam over 5 years with a car parked on top of it, then most of the initial value in the product gets lost. (You could also get differential settlement in the slab.)

You can get the Formular 250 in a number of thicknesses and also lay down multiple layers. Depending on how you are going to condition the space and frost protect the slab, there are diminishing returns to the amount of insulation you use, so that could be a separate conversation.

The 6" of concrete pays off for you in many many ways. One way here is that it disperses the loads over a greater area by the time they will hit your foam. For a 4 ton truck on tires inflated to 30 psi, you are going to have less than 5 psi weighing on your foam. So, on a 6" slab, the Formular 250 would be perfectly sufficient.

^ If the Foamboard (known quantity) is a least costly option, then you are even more golden with that route.

#4 re-bar 12" OC on a 6" slab isn't very much steel. Do you have any specific goals for the reinforcing (ie: are you just looking for a better replacement for mesh? are you looking to stretch your saw cut joints?) or a plan for how far you are going to place your saw cut joints?

 

[grkmec]

#4 re-bar 12" OC on a 6" slab isn't very much steel. Do you have any specific goals for the reinforcing (ie: are you just looking for a better replacement for mesh? are you looking to stretch your saw cut joints?) or a plan for how far you are going to place your saw cut joints?

I am using 6" of 4500 psi cement w/ #4 re-bar 12"OC for a few reasons:

1) I plan to install a 4-post lift - maybe even two of them. The post I am likely going to use is a Direct Lift Pro Park 9 Plus - rated for 9k pounds. That's a lift you can move around with casters so it wont be bolted into cement. The heaviest vehicle I have right now is a F150 Raptor pick-up. I may in the future buy up to a 1-ton truck.

2) The garage will also be used as a gym for strongman / powerlifting / olympic lifting. I will be dropping large live loads. For example, a 300-lb barbell with bumpers from 6-ft high. Or maybe a 550-lb barbell from 3-4 feet. I will have rubber mats + a lifting platform when dropping loads like this, but it will nonetheless be a common activity

3) The dimensions of my garage will be 30' wide by 58' long - gable with doors on narrow end. My architect specified two saw cuts at 20' from each gable end. 20' in because the garage will have a middle 4' deep bump out that is 18' long to allow for a small foyer area that opens to a 6' double french door. So there will be 3 sections of cement: (2) 20x30 sections and a middle section of 18x34.

 

[ConCretin]

Rebar won't decrease the need for control joints, in fact rebar increases the likelihood of shrinkage cracks by decreasing the concrete's ability to contract. For a 30 x 58 slab, I'd suggest a minimum of one cut the long way and three the short way, which would result in 15' +/- panels.

At the risk of being branded a heretic, I'll just mention that none of the reasons you mention for using rebar justify it's use over wire mesh. Rebar won't significantly increase the load bearing capacity of a 6" slab. It provides the same benefit as wire mesh in that it holds cracks together - including the cracks in the bottom of your control joints - but it won't prevent them. I like using rebar because it is more rigid than wire and thus easier to support but other than that it's the same as mesh.

Focus your attention on a properly constructed base under your slab and you'll see more of a benefit that you will using rebar.

 

[wssix99]

+1 on Doug's guidance on control joints. The architect is WAY off!!! 15' squares would be a minimum unless you did something industrial with the floor. (Depending on your door layout, it may make sense to cut the sections down a little smaller to have them fall in preferred places between bays - but don't push past 15'.)

If the floor cracks to hell, the designer can walk away and say, "I'm just an architect."

2) The garage will also be used as a gym for strongman / powerlifting / olympic lifting. I will be dropping large live loads. For example, a 300-lb barbell with bumpers from 6-ft high. Or maybe a 550-lb barbell from 3-4 feet. section of 18x34.

This would be the only thing that would worry me. If you aren't going to do this on a sacrificial platform, the effect would be just like dropping a sledgehammer on the floor.

You could use an isolation joint to make a totally sepearate section of floor and over-reinforce it like an industrial floor for the weight area, but...

From personal experience - a temporary sacrificial platform would probably be best. You'll only need something for a short time. By the time you get done with your project, (they all take "2 weeks") you'll be a lot older and won't want to be lifting heavy ****.