Heaving slab with Insulation?

[Ryan Schultz]

Per the following quote from C402.2.5, it appears that we can wrap the insulation under the slab as illustrated in the images below.

"or downward to at least the bottom of the slab and then horizontally to the interior or exterior for the total distance shown in the table."

In this condition, do we need to be worried about the slab heaving? This project is in a zone 6 (Wisconsin). If so, why is this condition allowed in the code?

Would just like to get additional opinions. Thank You.

 

[RLGA]

Where do you think the slab would heave? I don't see an issue with the design or the IBC requirements, but, then again, I'm not from that region and don't have a lot of experience in extremely cold climates.

 

[Ryan Schultz]

Conceptually, it would seem what's illustrated below would happen.

 

[mp25]

The insulation required in the energy code section is not for frost protection of foundation - it is to limit the amount of heat loss thru the slab. Frost protection for the structure would have to be provided by a different method unless the code specifically does not require frost protection for this structure (self supporting deck, detached accessory structure of limited size)

 

[tmurray]

You can use engineered soils to prevent ice lensing, or use skirt insulation to protect the area under the slab.

 

[RLGA]

I think the problem you have with the code requirement is that the code requirement is based on the presence of a foundation wall and footing. Since you have neither, maybe a reinforced thickened edge may counter any upheaval.

 

[mtlogcabin]

R403.3 will give you the prescriptive requirements for insulation for a shallow frost protected foundation

R403.3 Frost-protected shallow foundations.
For buildings where the monthly mean temperature of the building is maintained at a minimum of 64°F (18°C), footings are not required to extend below the frost line when protected from frost by insulation in accordance with Figure R403.3(1) and Table R403.3(1). Foundations protected from frost in accordance with Figure R403.3(1) and Table R403.3(1) shall not be used for unheated spaces such as porches, utility rooms, garages and carports, and shall not be attached to basements or crawl spaces that are not maintained at a minimum monthly mean temperature of 64°F (18°C).

Materials used below grade for the purpose of insulating footings against frost shall be labeled as complying with ASTM C 578.

 

[RLGA]

Just FYI, mtlogcabin's reference is to the IRC and not the residential area of the IECC. Anyway, that is a good solution.

 

[Ryan Schultz]

Thanks Guys.

I plan on approaching it like this....