Exterior foam on unconditioned garage wall set into bank
I’m building a 28×30 garage in northern Vermont with conditioned space above, two bay garage below will be unconditioned. The back half of the garage will be set into a slope with an approximately 6′ concrete wall above grade at the back on the garage exposed to unconditioned space. The wall does extend 4′ below grade so footing is below frost depth.
My question is, how to prevent frost penetration and heave potentially bowing the wall because that concrete is exposed to air and ground could potentially freeze down to that 6′ depth with only concrete between the air and soil.
Applying foam on either in the interior or exterior doesn’t exactly make sense to me if there is no source of heat. I’ve considered a foam skirt similar to what would be used on a shallow frost protected slab but would appreciate any thoughts.
There will be full perimeter drain with gravel wrapped in fabric and planning dimple mat as well on the back wall.
Hope that makes sense, see attached pics.
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Brian,
What matters is whether the footing is below the frost depth. If that is 4 ft where you are, and there is four feet of soil between the bottom of the footing and the slab, the foundation is safe. Unconditioned structures that have a basement, or are set into a bank, are only risky if (as is typically the case) the distance between their interior floor and the footings is less than the frost depth.
Hi Malcolm,
Thanks. Yes, all four sides of garage have footings below the frost line (making the back wall 10’).
I guess my concern not of a heave at the footing level and more of lateral pressure of potentially frozen soil from the 6’ of fill against the back wall of the garage that is only separated from cold air by 8” of concrete.
Brian,
Lateral pressure is why our code limits the height of unsupported concrete stem-walls. At 6 ft high your design would require engineering here.
That aside, you can eliminate the problem by using free draining fill against the wall, or adding foam to either side. There is a heat source - which is the ground underneath. That's why FPSFs work in unheated buildings.
Thanks Malcolm. Was planning on free draining fill against the foundation. Will also include some exterior foam.
Am I correct that there was previously a code for rebar schedule in unsupported concrete walls with varying heights of backfill but recent changes suggest it be engineered?
Brian,
Unfortunately I'm in British Columbia, so my code advice isn't useful. It seems to me codes are increasingly suggesting or mandating the involvement of engineers - which depending on your perspective is either a good thing, or motivated by a desire to offload responsibility by the reviewing authority.
I like those two storey garages built into slopes. It makes the space above much more useful.
Yes, that was my impression on the change to engineering. Clear rebar schedule in the previous code which in addition to foam and drainage I’m feeling much better about my system.
Thanks for the guidance!