I like the capillary barrier rather than the foam. I just can't get my arms around bearing on foam. I have seen too may breakdowns under slabs.

I have seen straight foam that broke down before, But have not had a problem with Geofoam in the 1.5 to 3 lbs density. It is made for this function. Density used depends on the load applied by the building. Boric acid keeps the bugs out. A waterproof coating is a good Idea especially in a frost zone. This product was and is designed for use under highway and bridge construction where it sees more demanding loads.

Posted By zehboss on 30 Apr 2015 07:18 PM
I have seen straight foam that broke down before, But have not had a problem with Geofoam in the 1.5 to 3 lbs density. It is made for this function. Density used depends on the load applied by the building. Boric acid keeps the bugs out. A waterproof coating is a good Idea especially in a frost zone. This product was and is designed for use under highway and bridge construction where it sees more demanding loads.

If you do a search you will see some problems with highways that used foam underneath. Although the highway sees a lot of loads the repercussions of failed foam is usually a cracked up road surface which can be fixed relatively inexpensively and easily. When it comes to a house, things are different because you can't go back and dig out the foam and footing as you can on a roadway. Failed house foundations are bad news.

Posted By emmetbrick on 30 Apr 2015 03:48 PM
I like the capillary barrier rather than the foam. I just can't get my arms around bearing on foam. I have seen too may breakdowns under slabs.

Nobody really knows the LONG TERM effect when people put footings on foam. This is a new phenomena that started just recently. I have a feeling that 10-20 years from now people will be looking back and saying "boy that was a dumb idea" but time will tell.

The risk vs reward is just not worth it in Zones 1-5. Maybe in a Zone 6+ there can be justification for it but foam under footings is really majoring on the minors. There are ways to insulate and prevent thermal bridging that don't involve risking your homes structure.

All most all homes and buildings built to the passive house standard are and have been built with the foundation on foam. Several sky scrapers have used foam for dampening and isolation in earth quake areas. Their are not a bunch of horror stories about those homes that are pushing 15 years of age in Europe. Most highway failures I know of are in conjunction with water drainage and freeze thaw issues. I think it comes down to doing it correctly in the application. Properly water proofed, properly drained, properly specified foam, and you have a successful implementation.

Have it done wrong in the design and application and you have a catastrophe on your hands. I think not doing it if you do not plan on taking the care to do it right is the take away, and not that it is generically risky or bad. It is the same with vapor barrier's in the 70s, diffusion walls now, ventilation and other construction processes, done correctly and you have successful projects, done wrong and you have mold, unhealthy living environment, rotting buildings etc.. That does not lead one to believe that proper application is bad. It leads to incorrect application is bad.

People need to understand how to do it right and avoid the problems that arise from doing it wrong. A small number of ICF failures does not mean ICFs are bad, A small number of SCIP failures does not mean SCIPs are bad. Over sized HVAC equipment does not mean HVAC equipment is bad. The list goes on. The issue in all these cases is do it right from the beginning. It also is a fact that if someone is a DIYer they need to be careful to do the due diligence to understand what they are tackling. It is easy to do a lot of things wrong if you do not study and understand the issues upfront. That is also why it is usually better to at least consult with an expert on all of the above before trying to tackle it on your own.

This is a great forum where many experts freely advise and help people. I have found great value in several of the experts on this forum especially when it comes to application of new ideas, products, assemblies and best practices.

I work with people that are building passive house, net zero, utility free homes that have ROIs that create a positive net cash flow situation. Not using foam in the foundation assemblies would make it more difficult and costly to accomplish the high passive house standards we aspire to in the buildings we work on with clients.

Posted By zehboss on 30 Apr 2015 09:09 PM
All most all homes and buildings built to the passive house standard are and have been built with the foundation on foam.

I work with people that are building passive house, net zero, utility free homes that have ROIs that create a positive net cash flow situation. Not using foam in the foundation assemblies would make it more difficult and costly to accomplish the high passive house standards we aspire to in the buildings we work on with clients.

I think you are misunderstanding. I am NOT talking about placing rigid foam underneath a slab. I am talking about placing rigid foam underneath a FOOTING. I am not against foam under slabs (when properly done with termticide treatments, etc).

The term "foundation" is somewhat loose and general as it can be referencing a slab on grade, in which the slab is not supporting the home.

Having the weight of the home bearing down on the footings and then the footings bearing down on rigid foam underneath is the UNKNOWN variable which has not been done until recently on residential homes. 99% of footings bear down on raw undisturbed soil/earth. Issues of creep are still unknown in the long term in regards to foam under a footing.

The other issue is pests. Termites can and have tunneled through borate treated foam, so can rodents, moles, pack rats, gophers, etc. Placing borate treated rigid foam underneath a SLAB requires termiticide pre-treatment of the soil as an extra cautionary measure. When I called the rigid foam company, that is what I was told, and they said "read the fine print" on their legal disclaimer about putting foam underneath a SLAB.

Now as far as foam underneath a FOOTING goes:

As per an engineer on a forum who responded:
<!--[if gte mso 9]> Be careful w/ the "25 psi foam". For EPS, I believe that advertised number is at 10% compressive resistance, ie, deformation. Beyond 5% compression is where you start to get out of the elastic limits of the foam, and by 10% the foam will not rebound. The "creep" referred to above is mentioned on the DOW web site, and they recommend a 3:1 ratio to avoid it (static loads; 5:1 for dynamic loads). I got info on this from the Insulfoam (EPS) web site, and by talking to a cat who does compression testing of foam. For example: EPS foam with a 40 psi compressive resistance (I believe that is what is called "40 psi foam") supports 35 psi at 5% but only 15 psi at 1%. I would not be happy building on something that I know will creep 10%.

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