rigid insulation under footers?

[jeff_gates]

Hello All,

I'm building a work shop and one leg of it is 30x60 feet and has a second floor.
We are doing rigid insulation under the slab with radiant floor heating.
Because of the 30 foot span we are required to put in a few 4"x4" support post for the second floor. Where the supports land we need to have 4' x4' - 6 inch footer under the 6 inch slab.

The question is:
I already picked up some 2x8 wood and made the forms for the footers or should I remake the forms out of rigid insulation? or dig a little deeper and put the rigid insulation under the and/or up the sides?

We are building out side if Seattle where is does get cold but not too bad.

Thanks,
Jeff

 

[BadgerBoilerMN]

I use rated XPS under the footers and they are usually more consistent than the ground they sit on.

It is hard to get builders to insulate the lip that a floating slab "sets" on, so good luck under a footer.

 

[Zeke]

I think I'd form the pads with insulation. You don't want to bury and leave wood under the concrete. AFA 'under' the pads I'd confer with the engineer or an engineer. Around here we don't use forms out in the middle of a slab but we have good soil. If it was all gravel fill I'd still just make the 12" excavation and place the rebar in it with no forms.

 

[Rookie2]

You may want to think about where you are going to cut the floor, 15'x15' will put a cut line right up the middle. I would do a circular 14" deep post/pad if your setting a support on top. Then wrap it with a thin expansion joint material. The floor will expand a little with heat and the insulation will shrink a little over the years, let the floor float.
I built a 40x60 and have one 18" crack in about 20 years.

The commercial buildings around me just rotate the post/pad 90 deg. and cut the floor to the corner.But the floors are poured around them not over them.

 

[BadgerBoilerMN]

Well said.