Rigid insulation placed under continuous concrete footings?

[tgengr]

I have a client (homeowner) who wants to insulate under the slab on grade and wants to continue the insulation down under the continuous concrete footings (turned down footings 12" embed. The location of the project is in the west and there is not frost depth it is a one story wood frame. The slab is going to be heated (reason for insulation). Any advice or recommendations from the cold weather structural engineers since this is a first for me and not our typical construction method and does not appear normal practice of running the insulation under the footings.

 

[msquared48]

Under the slab and down the inside of the stem wall is normal, but not under the strip footing. What would be the R value in that? Better to extend the stem wall insulation further down vertically.

Moreover, I would never do that for settlement reasons.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

I agree with Mike ...no foam under the footing. I just cant see a reason for it at all. Maybe the Homeowner is watching too much "Holmes on Homes"

 

[BAretired]

Mike and Toad,

You may be right, but rigid foam has been used under footings for a long time. It does not seem to result in problems. What are your objections?

BA

 

[ToadJones]

no direct objection...just don't see the need. I thought Mike was on the right track extending the foam vertically. This is what we did on residential homes in my building days.
We used to put foam under sidewalks and slabs at times depending on where they were.
When we did home strip footings, I'd hand cut the last three or four inches of ground before forming the strip. Maybe I'm insane, but I wanted to feel every bit of bearing soil.

 

[ToadJones]

extending the foam vertically would accomplish more IMO, however, you'd be excavating lower than the strip....not very logical. It sounds like this footing wont be very deep.

 

[ToadJones]

Also, be sure to make a good thermal break between the interior slab and stem wall. This is the worst spot for cold penetration.
I have seen slab houses with ice cold floors around the perimeter from this!
Of course this makes having a good solid, dry base of paramount importance for the interior slab. I have also seen interior slabs move from heave.
But, it doesnt sound like you have temperatures that cold.

 

[tgengr]

My intuition agrees with not insulating under the footing due to potential settlement..The slab is tied to the footing so there would be no differential settlement there. I think I will recommend insulating exterior and interior vertically and under the slab but not under the footing, unless I can find further research that would show the insulating material would not compress over time..

 

[Ron]

If you want to insulate below the footing and not have a settlement issue, consider using Elastizell or similar as a geotechnical fill. Not sure you need it since the additional 8 inches or so of concrete in the thickened edge might be equivalent to your insulation below the thin slab anyway. Beware of moisture issues in your slab and barrier type floor coverings the owner might want to use.

 

[tgengr]

Thanks. A little more research has led me to a possible recommendation of using the insulation under the footing however it must have a density of min 25 psi and also have the mfr recommended or acceptable use for this application.

 

[csd72]

In my experience, rigid insulation comes with a maximum rated pressure that can be applied to it. If the footing pressure exceeeds this then do not use it underneath.

 

[msquared48]

Unless you are sitting on a permafrost, or similar foundation material, where you do not want to transfer heat to the permafrost, I just do not see the value of doing this.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Going a step further, if what you are trying to achieve is a thermal break between the slab and concrete stem wall, then most Architects will place a piece of rigid insulation between the edge of the slab and the stem wall.

This makes the slab floating at the wall with the possibility of local settlement, but that can be controlled with proper compaction.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

Mike- see my above bombardment of posts...

 

[msquared48]

Repetition for emphasis...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

I agree Mike- excessive settlement in this case could be disastrous.

 

[dik]

I've used Dow Hi-60 underneath footings for shallow foundations...

Dik

 

[ltlspf]

I am also working on a project where the foundation is a shallow frost protected and the rigid insulation is required below the footing for the design. I was leaning towards csd72's thoughts that if the allowable maximum pressure of the insulation should exceed the actual footing bearing pressure. For this project we will need about 2500 psf or I guess we could increase the footing dimensions as needed to reduce the pressure.
Anyone have any other ideas of how to distribute the footing pressures out across more of the insulation area without increasing the footing?

 

[Brad805]

tg, I am confused. Does the client have in floor heating or cooling?

Keep in mind the rated capacities of Rigid insulation are usually at 10% compression. I use this stuff in a vacuum press in my woodworking hobby, and trust me, 10% compression is true.

If you consider something like HI 40 or greater, you best discuss the cost realities with the client before preparing any drawings. I have had many a discussion with clients about the cost of HI 40 to 100.

Brad

 

[ACtrafficengr]

EPS lightweight structural fill may be an option. If its used for highway embankments with 80 kip (total legal weight) live loads, I think it should work under a house.

I'm still not sure I see the point. Granted, heat transfer was my worst subject, but I don't see how it would significantly reduce heat loss. How deep is the footing?

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928