Floating slab concerns

[Bud Zeien]

I'm currently setting up shop at a house I purchased. The shop is separate from the house and is 24'deep x 40' wide. I'm about to insulate, and upon further inspection I noticed it's on a floating slab. Should I have concerns about irregular heaving when I have a constantly heated space? My fear is the center of the floor will stay put while the perimeter will heave come frost time.
Thanks for the input fellas!

[James Conrad]

There are a couple of approaches, essentially you want to isolate the slab. Where are you located?

[Mark Bolton]

Should I have concerns about irregular heaving when I have a constantly heated space?

I wish I could have put in a two character reply NO

[Bud Zeien]

There are a couple of approaches, essentially you want to isolate the slab. Where are you located?

James, I'm up here in Minnesota, so I get all the extremes!

I wish I could have put in a two character reply NO

Mark, can you expand a bit please? What happens as the frost moves under the perimeter of the shop while the center stays warm?
Thanks!

[Jim Andrew]

Does your building not have a foundation? Typically the perimeter of the building has a deep enough footing to be below frost.

[James Conrad]

If the slab base and soils have good drainage and there aren't any large rocks that will heave under the slab, it is likely you won't have any major issues. But, the slab will be a giant heat sink if you don't isolate it from your conditioned space which can lead to frost issues, and large heating bills. Where you are I would use at least 4" of min. 25psi compression rated rigid XPS foam over the slab and you can float an Advantech subfloor over it. This depends a little on what you want your finished floor to be.

I don't have my code books in front of me at the moment to tell you the likely R values you will need to meet, your town/ state may have other less rigorous requirements for accessory structures. In general if you are conditioning the space, and pulling a permit, you are required to meet the energy code.

[Bud Zeien]

Thanks James, I appreciate it!

[Mark Bolton]

Bud,
If you are conditioning the space, as James said, your going to have a good bit of a heat loss due to the slab on grade but if your going to keep the place heated (even minimally) year round your going to create a heat island. In my experience it would be unlikely for you to have issues other than the higher heat bill. While I know your in the extreme north, its not uncommon for the soils around a thin slab, close to grade, to never even freeze solid in cold temps because the buildings heat, as James also said, is making the slab the heat sink/island.

Its of course less than optimal but barring tearing out and installing a foundation to the frost line there isnt much of a remedy in my opinion. I would be more concerned with installing XPS and an additional subfloor because while you'll reduce your heat bill, your almost guaranteed to freeze under the edges of the slab (the buildings conditioned air will not warm the slab much if at all). I'd imagine this is already happening but it could happen to a greater extent. Its probably about as broad as it is long.

To me the real answer would be, how old is the current slab. What condition is the slab in currnetly (i.e. is it currently heaved or does it show evidence of it in the past?). Just because the space wasnt conditioned doesnt mean a slab wont self destruct if theres a heave'ing condition currently, especially a slab that large. If the ground beneath the slab is moving its moving and a slab that size will suffer regardless.

I dont feel heating the space is going to destroy the slab because the center is now warm (not freezing below) and the perimeter is colder (may likely never freeze).

I agree the situation is less than optimal from many perspectives, heat loss, lack of a vapor barrier, and so on. But the simple fact is many many buildings (my business for instance) were built that way and it pretty much just is what it is.

[Steve Rozmiarek]

The only time floating slabs fail is if they are too thin.

[Rick Potter]

Sorry, but what is a floating slab?

Rick P

[Bud Zeien]

Sorry, but what is a floating slab?

Rick P

A Floating slab is a slab of concrete with no foundation. Typical home construction up here in Minnesota is done with a footing below the frost line (4 feet) and then a concrete wall on top of that, then the slab is poured inside that wall. It's not uncommon to have out buildings to be constructed on just a slab of concrete, much cheaper that way.
I would guess my shop to be about 20 years old, and I gotta say, it's in great shape as far as settling and cracks. It has great drainage all the way around it. You guys have eased my mind mind as far as heaving so now I'll focus my worries on the tremendous heat bills Perhaps I will put some rigid insulation all the way around!?!?
I think I was mainly just really disappointed to find out my dream shop is on a floating slab, it's kinda like finding out your girlfriend slept with the whole football team

[Rick Potter]

Thank you,

I figured that's what it is, but wasn't sure.

Rick P

[Ole Anderson]

Around here code requires a "rat wall" under a garage or storage building slab. It is a shallow foundation about 12" deep and only 4-6" thick (vs a full 42" deep x 8-16" thick to be below the frost line). Why a rat wall? So animals are less likely to burrow under it and make nests. Ideally a heated building will have a full foundation with rigid insulation around the perimeter before the slab is poured. Whether your floor edges will heave depends on underlying soils. If they are well drained and don't retain water before the freeze you should be good. If they are clay maybe you will be ok, silt is the worst material as far as heaving. I don't think expansive clays are prevalent in MN, but it is possible.