Thickened Edge Slab Insulation

[JStash]

Building a 30’x30’ garage in the spring. It will be situated on a thicken edge slab. I’ve done a decent amount of research on this topic, but there’s a lot of conflicting info, it seems mostly due to different areas have different requirements.

I’m in Canada, the frost depth where I am is 4’, so I plan to do something along the lines of a frost protected shallow foundation. Full foundation walls is insanely cost prohibitive since I wouldn’t want to take that on myself.

My plan is to vertically insulate the slab walls (2ft) and have insulation “wings” out another 2’ - 4’, this should eliminate frost heaving. I plan to heat the shop in the winter, but only to about 5C, maybe 10C (50F) when I’m in there. My thought is I could skip insulating under the slab as the temp difference would be minimal. (Constant from the ground (50F) to the air temp I’d be going for)

I will be building this to code, and have run most of this by the inspector, they don’t have any slab insulation requirements, so it will all be above and beyond what’s required.

Thoughts on my above plan?

Keeping in mind cost savings is absolutely a factor. Costs up here are crazy these days. For example, insulating under the slab, would add $2500 in materials.

Second part of this. What’s the best way to vertically insulate the edges of the slab? Put them in the forms? Attach them after? Do you have the 2x6 walls overhang the insulation? Ect, any details here would be great.

Thanks in advance!

 

[billconner]

I think your plan is solid. There are tables on line that will let you calculate and minimize amount of foam required. I found a version by ASCE (American Society of Civil Engineers) free online, and not hard to work through tables and formulas.

You might consider ICFs for form and vertical foam. Not as sure how it compares cost wise with traditional forming and adding foam. I think I'd put the foam in the form with some kind of anchor.

 

[JStash]

There maybe some specifics for the states, but I'm sure you can get the jest of the code requirements- ICC/IRC is just that...

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This is super helpful, thank you. I'm about 100km north of the boarder between Idaho and Montana, so I could make some assumptions on the amount. Our code (for in town) just says "R12 and 600-1000mm "wings" but not technically required since I'm out of town.

 

[JStash]

I think your plan is solid. There are tables on line that will let you calculate and minimize amount of foam required. I found a version by ASCE (American Society of Civil Engineers) free online, and not hard to work through tables and formulas.

You might consider ICFs for form and vertical foam. Not as sure how it compares cost wise with traditional forming and adding foam. I think I'd put the foam in the form with some kind of anchor.

Thanks, I think the previous comment linked that table, i should be able to work a few things out from that.

Another question, how would you deal with the foam along the front edge where the bay doors are? I'll have 2 doors. i assume you just take the heat loss where the garage door sits back from the edge of the slab.

Any suggestions on protecting the edge of the foam in front of the doors?

 

[billconner]

Iirc the ASCE standard gives you more options for depth of apron versus extent. It also has the requirements for unheated buildings, which HUD and IRC don't iirc.

It seems most common is to bevel top edge but you're probably fine just stopping it 4" below apron.

When I was considering this, I planned on a short stem wall on a shallow footing, with foam under footing and inside stem wall. I'm just not a fan of exposed foam.

 

[JStash]

Iirc the ASCE standard gives you more options for depth of apron versus extent. It also has the requirements for unheated buildings, which HUD and IRC don't iirc.

It seems most common is to bevel top edge but you're probably fine just stopping it 4" below apron.

When I was considering this, I planned on a short stem wall on a shallow footing, with foam under footing and inside stem wall. I'm just not a fan of exposed foam.

Sorry, what apron are you referring to?

 

[TurnipTruck]

I did what I’ve seen a lot of up here: 2” foam against the slab with 2 to 4’ of foam horizontally, then galvanized ”Z” flashing protecting the foam.

 

[billconner]

Sorry, what apron are you referring to?

apron = wings

 

[Blk88GT]

I offset my bottom 2x6 plate and my outside sheeting (treated) covered the vertical foam on my thickened edge. I then applied aluminum flashing to it and backfilled with 3/4 down crushed limestone.

I insulated my entire slab with 2" XPS and then did 4" around the outside 4' where I planned to clear snow (front overheads) The rest got 2" (ie: back of the shop)