×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Rigid insulation placed under continuous concrete footings?

Rigid insulation placed under continuous concrete footings?

Rigid insulation placed under continuous concrete footings?

(OP)
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.

RE: Rigid insulation placed under continuous concrete footings?

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

RE: Rigid insulation placed under continuous concrete footings?

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"

RE: Rigid insulation placed under continuous concrete footings?

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

RE: Rigid insulation placed under continuous concrete footings?

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.
 

RE: Rigid insulation placed under continuous concrete footings?

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.   

RE: Rigid insulation placed under continuous concrete footings?

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.  

RE: Rigid insulation placed under continuous concrete footings?

(OP)
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..

RE: Rigid insulation placed under continuous concrete footings?

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.

RE: Rigid insulation placed under continuous concrete footings?

(OP)
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.   

RE: Rigid insulation placed under continuous concrete footings?

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.

RE: Rigid insulation placed under continuous concrete footings?

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

RE: Rigid insulation placed under continuous concrete footings?

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

RE: Rigid insulation placed under continuous concrete footings?

Mike- see my above bombardment of posts...

RE: Rigid insulation placed under continuous concrete footings?

Repetition for emphasis...  

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

RE: Rigid insulation placed under continuous concrete footings?

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

RE: Rigid insulation placed under continuous concrete footings?

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

Dik

RE: Rigid insulation placed under continuous concrete footings?

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?

RE: Rigid insulation placed under continuous concrete footings?

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

RE: Rigid insulation placed under continuous concrete footings?

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

RE: Rigid insulation placed under continuous concrete footings?

For lateral loads, friction resistance (concrete to soil) is usually much greater than passive.  Putting a layer of special insulation with unknown friction properties between the concrete foundation and the soil doesn't seem like a good idea for high wind or seismic.
Go deeper.  

RE: Rigid insulation placed under continuous concrete footings?

ASCE 32, "Design and Construction of Frost Protected Shallow Foundations", is a good reference.

My experience is that most install insulation under the slab and inside face of the wall. Most are actaully extending the wall insulation from the footing all the way to the top of the concrete wall. This also creates a thermal break between the slab and wall.

RE: Rigid insulation placed under continuous concrete footings?

I am also skeptical that any real benefit will be realized here.

But if you're using 25psi rigid foam, that's 3600psf. Assuming that's an allowable load, that exceeds the performance of many soils we encounter around here... I don't see how 'disastrous settlement' could occur - although it might be smart to consider how the product breaks down over its service life. If you're providing 2" or 4" and over 50yrs it breaks down and is crushed to a small fraction of its original thickness then I suppose you would have disastrous settlement, albeit relatively even throughout the structure...

The only issue I see with a floating slab with a 1/2" fiberboard joint for thermal break is seeing the 1/2" fiberboard joint. Doesn't bother me if it's caulked cleanly, especially in a basement.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources