Footer and slab insulation

[mnpeterg]

For some reason I can’t make heads or tails out of this....



Starting the excavation of a 30x16 shed / outbuilding next week, building it on a monolithic slab. I live in MN, where the cold is obviously brutal at times. My property sits on sandy soil. The city requires footers to be 12’’ x 12’’. Some folks have suggested rigid foam on the sides of the poured mono slab footers to prevent frost heaving. Others have said if you plan to heat the structure than there should be no problem. I’m planning on keeping the structure at 68f. year round, and going to put a ton fo rebar in the footers as well as the 6’’ slab.

Do I need to worry about frost if I keep the inside warm?

 

[Fav Onefour]

All the rebar in the world isn't going to help with heating.
What folks are suggesting to heat the ground? It's nuts even with sandy soils. There is still a warm cold line.

Insulate under and around the "floating" or monolithic slab. It is considered standard practice. The gang further north even recommends apron insulation.

I'd be pissed if I was looking at frost on my wall bases in cold weather. The cold spots make it less comfortable and it's a waste of money to heat.

 

[billconner]

Frost protected shallow foundation is the simplest solution, though some cost in that foam. Something like under whole slab and an apron 2-3' out all around, minimum 10" below grade. You can go deeper and not as far out. Plus foam inside or outside the footer/stem wall.

I agree with fav that the rebar in the footer won't do much to prevent frost heave. The IRC doesn't even require it except in seismic areas. Far too much faith in rebar.

There is another way to frost protect a foundation - 2" crushed stone to frost depth and your slab on it. No real need to even thicken edge but I probably would a few inches. And drain tile to daylight under stone.

Google ASCE 32. I found one version out of date available for free download. The relevant parts haven't changed.

 

[mnpeterg]

thanks gents. this is for a golf simulator in my back yard. since it’s somewhat of a small space a mini split should heat it just fine especially with the wall insulation and the crazy amount going in the ceiling. what I want to avoid is any heaving from frost. That was the reasoning for the question not the the obvious waste of $ from heat loss through the slab. Does heating the structure stop heaving during the winter was the real question. my understanding is yes, but wanted others opinions

 

[billconner]

I'd say just heating and no under slab or slab edge insulation may not stop it from heaving. If you're building to code, both slab edge and under slab insulation is required.

 

[mnpeterg]

I'd say just heating and no under slab or slab edge insulation may not stop it from heaving. If you're building to code, both slab edge and under slab insulation is required.

Thanks for the reply I wasn’t going to insulate under the slab, but since it’s a shed albeit heated and on sandy soil but. . Maybe I should.

 

[ConCretin]

If the soil under your slab is reasonably free draining you don't really need to worry about frost. If the structure moves at all, it won't even be noticeable. I have several much larger structures on uninsulated mono-slabs up here in Maine where we get 48" of frost and they perform just fine.

If you do want to protect the structure from frost movement, you'll need to insulate under the slab and out or down a distance equal to your local frost depth. The insulation will trap the heat from the ground and keep the frost out.

The third option is something in between where you insulate under the slab to improve heating efficiency. Nothing wrong with this approach but I doubt you'd notice much difference in comfort or cost unless you have radiant heat.

 

[walrus]

Frost susceptible soil is the key. if the soil drains than frost isn't going to be much of an issue. If its clay, watch out. My shop in Maine is on several feet of well compacted gravel. Slab on grade, 30 by 34. Radiant slab by solar so inside temp varies. but never below freezing. Its also insulated.

 

[bb29510]

when the water in the soil freezes, it picks up the building and since it doesnt move uniform, slabs crack

 

[TurnipTruck]

Well-draining gravel on the left, still-frozen native soil on the right:

Two weeks ago the difference was around three inches.

 

[Ak Jim]

Like someone else said look up frost protected shallow foundations. Use the blue board “wings”. Ground has an R value of about R1 per foot. If you use 2” of blue board that is an R value of 10 which equals 10’.

 

[billconner]

You can have the soil tested to see if it drains well enough to not have to worry about frost - if it's No -Frosy Susceptible. I did - take a pail of dirt to a lab - about $200. "Non-Frost Susceptible soil. Defined by ASCE 32 as a granular soil with less than 6% by weight passing a number 200 sieve." Mine was nearer 12%. Ended up using a rubble trench.

 

[MadScientist3019]

All the rebar in the world isn't going to help with heating.
What folks are suggesting to heat the ground? It's nuts even with sandy soils. There is still a warm cold line.

Insulate under and around the "floating" or monolithic slab. It is considered standard practice. The gang further north even recommends apron insulation.

I'd be pissed if I was looking at frost on my wall bases in cold weather. The cold spots make it less comfortable and it's a waste of money to heat.

This is inaccurate if the insulation is done correctly. I just did this in my 40x60 shop in the last year. Look to frost protected shallow foundations for the keys to this as stated previously but the major take-away is this: Either in heating or cooling locations doing vertical insulation to the recommended R value down to the frost line (wings outward are dependent on location) allows the earth to help regulate the temperature in the conditioned space. The ground is pretty consistent in it's temperature year round down below frost line due to geothermal heating. This is why well water is almost always the same temp year round. So in the summer the earth helps cool the space being a heatsink to absorb heat from the space as thermal energy always flow from more to less. Then in the winter you gain some heat energy from the earth into the conditioned space due to the same physics. Lastly full under slab insulation is really only recommended if you're trying to build a near zero energy space or if you're using in floor heating.