We are going to build a 3 stall garage attached to the house by a breezeway. The ground level had to be built up 4' to match house entry level. The footings have been poured and the interior area filled with sand/gravel. The exterior footing wall banked with clay/dirt mix. The footings were not insulated. The garage is going to be heated. If we put down a water barrier and foam sheeting before the cement floor is poored will this be adequate? Or will the frost move up the banked footing wall and cause problems inside the garage?

Where do you live?

We live in NW Iowa.

Does get a bit nippy out there doesn't it. Your footings are in and the sand and gravel are down so your space is limited. You can add a sheet of rigid foam covered by your vapor barrier before you pour but you still want at least 5" of concrete, do you have that much space? Insulation in the walls and most importantly the ceiling will give you the most bang for the buck.

There is approx. 10 inches from the top of the sand to the top of the footing. And yes, it does get nippy here.

Almost sounds like you need a 2" foam board and some additional sand unless you need a beefy floor for heavy equipment. 5" is generally used for a residential garage. I'm just over the IL border east of Dubuque.

We are planning on 2" foam board and 5" concrete and packing a little more sand. We are undecided if we should put down a moisture barrier plastic sheeting. Do you have any thoughts on that?

You do not need a vapor barrier if the garage is detached and unheated, if there is a chance to may have living space above the garage or want to heat it, you should put in a 6 mil vapor barrier.


IRC - R506.2.3 Vapor retarder.
A 6 mil (0.006 inch; 152 µm) polyethylene or approved vapor retarder with joints lapped not less than 6 inches (152 mm) shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists.

Exception: The vapor retarder may be omitted:
From garages, utility buildings and other unheated accessory structures.
From driveways, walks, patios and other flatwork not likely to be enclosed and heated at a later date.
Where approved by the building official, based on local site conditions

The cost for a barrier is small compared to what you are doing. I'd put one down. Someday you or a future HO may button up that breezeway and find other uses for those 3 stalls.