# Spray foam under a slab for insulation --



## Mike Finley (Apr 28, 2004)

Full article here: Insulating a Slab With Spray Foam



> _Why SPF?_ In cold climates, insulating under a concrete floor is just common sense. But it’s fair to ask why SPF should be used instead of rigid foam board. Perhaps the greatest advantage SPF offers is that it provides a monolithic layer of insulation, with no seams or other imperfections in the thermal barrier. Because it bonds aggressively and completely to just about anything it contacts, the foam creates a superior vapor barrier around plumbing and other slab penetrations
> 
> Detailing foam board to provide an equally tight barrier takes time and great care. Getting the substrate sufficiently smooth and flat to evenly support the board takes a lot of effort too. SPF, by contrast, is totally forgiving of irregularities in the substrate. And as soon as the SPF cures — within 15 minutes — it can be walked on without concern for punching through into a concealed void (*Figure 2)*. There’s also far less waste involved with SPF; there are no off-cuts or other discards. We can spray the exact amount required. “Extra” foam goes to the next job instead of into the dumpster.
> 
> _Cost comparison._ The material cost for spray foam is higher, but once the labor to properly detail rigid foam board is taken into account, the installed price becomes competitive. Our installed cost for a 2-inch layer of closed-cell foam runs about $2.20 per square foot. Two-inch polystyrene board costs around $1.80 a square foot installed, but it has a lower insulating value of R-10. Also, the bigger the slab area, the more labor is required to detail foam board around the perimeter and penetrations. On slabs 2,500 square feet and greater, we often reduce our price slightly, which ends up making SPF the less costly route. And because of the time saved, the slab pour can be moved up in the schedule.


How long has closed cell spray foam been around? No worries about it deteriorating under the slab and leaving a big void in 10 years?


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## rustyjames (Aug 28, 2008)

That's exactly my thoughts about any kind of foam buried like that. Even worse, when foam is used under footings. These applications just aren't time tested.


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## thom (Nov 3, 2006)

Mike Finley said:


> Full article here: Insulating a Slab With Spray Foam
> 
> How long has closed cell spray foam been around? No worries about it deteriorating under the slab and leaving a big void in 10 years?


I put 2" of extruded polystyrene under the slab of my own house 26 years ago, no problems yet. All the bearing walls sit on separate footings and stemwalls that are isolated from the slab. The slab is heated hydronicaly. Seems to work for me.


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## B.K (Dec 15, 2008)

Very Interesting, good article. 

The only question i have is why bother with SPF? Rigid foam board does have it gaps, but is there a big enough gap to cause a slab to raise? 

I have never seen rigid foam fail, if installed correctly.

The cost is very surprising as well. What would one use to spray it on with? I find it hard to believe it cost more for a few guys to lay out the rigid foam board.

Would i have to get a company to come and put it in for me? I would have to wait till the next day to have them come in, when i have 3 guys on site...i could have the job done that day and be pouring the next morning. 

The only place i can see it being quicker is in big jobs, were there would need to be a lot of rigid foam board.


Seems like one guy didn't have an easy time doing it, down in the comments section.

-Bill


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## Mike Finley (Apr 28, 2004)

thom said:


> I put 2" of extruded polystyrene under the slab of my own house 26 years ago, no problems yet. All the bearing walls sit on separate footings and stemwalls that are isolated from the slab. The slab is heated hydronicaly. Seems to work for me.


You're talking about rigid foam panels right? Isn't there a difference between them and a sprayed closed cell foam?


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## Framer53 (Feb 23, 2008)

Mike Finley said:


> You're talking about rigid foam panels right? Isn't there a difference between them and a sprayed closed cell foam?


 
Yep, one is made in a factory with computor control on the menu. The other depends upon a guy at the site to keep his eye on things and make sure the ingrediants are correct.

I don't trust site foam at all. I have 70's foam that I have removed from my house as I remodel rooms. That was sold as being the be all end all!

I think you are correct in questioning this Mike.


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## mudpad (Dec 26, 2008)

Around here, this is done only at the slab perimeter,if it is done at all. I'm sure it is more important in colder climates.

It just doesn't seem to make sense to me to spend all that time compacting a subgrade, have a geotech do densities, probably rip up and recompact at least once, then spread 4" stone, compact, and then put a 2" layer of styrofoam under the concrete slab. What kind of density do you get with styrofoam?


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## jimmyw (Jan 10, 2010)

this method deserves more research. just not enough information related to trial and error. sounds promising though.


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## Mike Finley (Apr 28, 2004)

mudpad said:


> Around here, this is done only at the slab perimeter,if it is done at all. I'm sure it is more important in colder climates.
> 
> It just doesn't seem to make sense to me to spend all that time compacting a subgrade, have a geotech do densities, probably rip up and recompact at least once, then spread 4" stone, compact, and then put a 2" layer of styrofoam under the concrete slab. What kind of density do you get with styrofoam?


I'm not sure I follow you. It sounds like you are associating all that work because there is spray foam under the slab?


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## Tom Struble (Mar 2, 2007)

well if the foam is rated for the psi required isn't that all that is necessary?are we talking spray foam degrading or dissolving somehow? i think the only thing that affects cured foam besides heat is UV exposure


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## SLSTech (Sep 13, 2008)

I was skeptical when I first saw the title, but after fully reading it, that actually makes a lot of sense. Any divets or depressions in the ground are fully filled unlike the foam sheets placed directly over the ground. Complete seal around all pipes is easier. No seams for water leakage, or radon? to enter

The only catch I saw was operator error & calling in a sub, but that applies to anything we do. As for the comparison to the foam of the 70's - that is two completely different animals & as the saying goes, we have come along way baby.


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## loneframer (Feb 13, 2009)

No problems with this ridgid foam install. Steel needles were added to the mix before the pour. One layer of welded wire to attach tubing, then another layer on top to get into the pour.


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## Mike Finley (Apr 28, 2004)

Never heard of steel needles in concrete. Have had the fiber added.



tomstruble said:


> well if the foam is rated for the psi required isn't that all that is necessary?are we talking spray foam degrading or dissolving somehow? i think the only thing that affects cured foam besides heat is UV exposure


How long has closed cell spray foam been around?

Adding lead to paint sure sounded like one hell of a good idea at the time. :laughing:


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## loneframer (Feb 13, 2009)

Mike Finley said:


> Never heard of steel needles in concrete. Have had the fiber added.
> 
> 
> :laughing:


 These are the ones that were added, although I can't recall the ratio that they were added per yard. They don't produce the hairy appearance on the concrete finish like fiber mesh.:thumbsup:


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## Tom Struble (Mar 2, 2007)

Mike Finley said:


> Never heard of steel needles in concrete. Have had the fiber added.
> 
> 
> 
> ...


1954

http://en.wikipedia.org/wiki/Polyurethane


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## Mike Finley (Apr 28, 2004)

tomstruble said:


> 1954
> 
> http://en.wikipedia.org/wiki/Polyurethane


Polyurethane may have been around isn 1954, but sprayed closed cell foam used in the construction industry? 1954???? We know already that it deteriorates under UV, we found that out the hard way I'm sure. How else does it deteriorate still yet to be discovered the hard way too?

I think the spray foam method has merit, but I'm just cautious about being the Ginnie pig.

No what I mean?


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## Tom Struble (Mar 2, 2007)

actually i read the wiki and the UV degradation really only happens on the very outside,leaving the Pu just under it unaffected

possibly contaminated soil could affect it?

absolutely i no what you mean Mike.i wouldn't want to use something i wasn't sure of

but...what do you do,look at the science which most here don't really understand and go with your gut i guess


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## Mike Finley (Apr 28, 2004)

I think that's what we all have to do. It's always safer to do an incremental change, like just adding a little more oregano to your spagetti sauce, compared to switching to tofu meat balls instead of meat.

I'd like to find out how long closed sell spray foam has been around being used in the insulation business. Seems they are also mixing up a lot of new versions of it now aren't they? Aren't they doing soy based now? To me that's another big change and variable added to the equation. That is like changing to tofu.


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## loneframer (Feb 13, 2009)

Mike Finley said:


> Polyurethane may have been around isn 1954, but sprayed closed cell foam used in the construction industry? 1954???? We know already that it deteriorates under UV, we found that out the hard way I'm sure. How else does it deteriorate still yet to be discovered the hard way too?
> 
> I think the spray foam method has merit, but I'm just cautious about being the Ginnie pig.
> 
> No what I mean?


 The problem is, construction technology is an evolutionary process. If you don't get in at ground level, you end up playing catch-up with the guys who are vastly experienced because they were on top of the "fad". Unfortunately, they are always going to be the Guinea Pigs in the process. Dri-Vit is a great example. Also, 25 years ago, I would never have imagined that vinyl siding would revolutionize the marketplace. Now vinyl is old technology but PVC trim is all the rage. Cementitious siding is an old technology that is seeing a resurgance because it was and is a great product. I guess you just gotta roll with it or get rolled over by it.


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## Mike Finley (Apr 28, 2004)

All I know is this closed cell spray foam is really getting to be the new rage in building. It's showing up more and more and in more and more applications. I just hope it doesn't end up being another asbestos or similar super product that turns out to be a huge mistake.



> The human race has an interesting way of meddling with all sorts of things that make up our world to try and make new stuff. Sometimes it leads to poisonous substances that cause more harm than good and other times it leads to useful substances that have a wide range of uses in many different areas. One of those ‘inventions’ is polyurethane foam. Most people know about it, but most people do not really know what it really is. This article is not about to try and explain just what it is, but rather one its many uses – especially as regards closed cell polyurethane foam. Closed cell polyurethane foam is often used as insulation in houses and other buildings. There are many advantages as to why this type of foam makes such a good insulator aside from the fact that it is easy to install if you have an experienced foam contractor doing the job. It can be installed in nearly any building and any location and it does not require a lot, if any maintenance.


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## mudpad (Dec 26, 2008)

Mike Finley said:


> I'm not sure I follow you. It sounds like you are associating all that work because there is spray foam under the slab?


No, I'm just saying you spend all that time making a firm bearing service for the slab and then you put styrofoam on it. Seems like it defeats the purpose.


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## loneframer (Feb 13, 2009)

Mike Finley said:


> All I know is this closed cell spray foam is really getting to be the new rage in building. It's showing up more and more and in more and more applications. I just hope it doesn't end up being another asbestos or similar super product that turns out to be a huge mistake.


 I've always had my reservations as to the safety of fiberglass insulation as well. The jury is still out on that one as far as I'm concerned. I believe that inhalation of the airborne fibers is more than a just a "nuisance". Look at the sattelite issues that insulation in general has brought to the forefront such as moisture and mold conditions. The balance between insulation and ventilation is a science unto itself. As a kid, I remember Tyvek being marketed as an air infiltration barrier for homes with little to no insulation. Now it has evolved into a new species of which I've witnessed many issues with due to insulation/ventilation imbalances.


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## SLSTech (Sep 13, 2008)

Mike - they had issues with the foam formulas back in the 70's, so they have been doing it since at least then

Demilic is the one company that I use and they have been in business in 1983. The Soy Foam formula was started by them as I recall and has been in use for at least 4+ years. 

The reason that it is becoming more of the rage is people are slowly catching on to all the positive aspects of using it. They have been putting foam panels under the slabs for at least 10 years, utilizing basically the same product used in the spray. 

The lead comment doesn't fly as it was being used by the Egyptians and everyone kept using it after they knew there was problems with it.


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