Frost protection

[WWTEng]

Please see the attached for renovations to an existing building. Currently there is a partial basement. The Arch and owner want to re-grade the area so that they can put a door opening. This means that the shown existing wall footing would not have frost protection anymore. I am proposing the attached scheme to get frost protection for the building. The new stoop will be all around the new graded area. What I am unclear about is if there will be heat loss through the horizontal new pour?

The recommended frost depth in the area is a min of 3’-6”.

 

[CarlB]

For frost protection purposes, I don't see the new stoop proving any value.
I assume this a heated building.
Insulation should be provided that ensures the building's thaw zone (also the limit of frost penetration) does not extend under the building footing. You also want to prevent frost heave of the sidewalk which could block the door.
Where soil is removed against the wall, provide insulation on the wall inside or outside. Where you show a slab under the sidewalk, this could instead be horizontal 2" subgrade insulation. Would be good to have 1" insulation over top of footing also. Frost should penetrate only a few inches through insulation, so provide maybe 12" NFS gravel below insulation under sidewalk. Slope sidewalk away from building.

Insulation schemes such as this have been successful in subarctic climates where frost penetration is 8 feet and more.

 

[WWTEng]

Carl,

Thanks for the input. Though I am not sure if I understand your response fully. Placing insulation on the wall doesn't seem to do anything for heat loss under the footing? Would it be a good idea (and practical) to provide a line of insulation along the sloped cut line. I can't cut soil vertically flush with the outside edge, otherwise that would be the ideal place to put the insulation.

 

[CarlB]

Is suppose you're right, the wall insulation may not affect what's happening below the footing, except possibly keep the floor inside the building a little warmer. Insulating foundation walls does help keep footings warmer in the case of a slab on grade.

Placing insulation on the sloped line would definitely help, but even that may not be too practical. I think the horizontal insulation achieves about the same effect, below the footing anyway.

Google "frost protected shallow foundations" to see some typical insulation placements.

 

[concretemasonry]

The horizontal insulation helps retain the latent heat stored in the soil. Soil freezes from the top down, so it can be effective.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[WWTEng]

These are a few updated sketches I have come up with based on the conversations here and some other research so far. Personally I think option 3 (sloped concrete with matching insulation) seems to be a relatively simpler approach. Any input would be helpful.

 

[WWTEng]

I have another question about the same topic. Should conventional exterior "foundation wall on footing" always have insulation on the inside face of the foundation wall? I have seen details where the insulation is not shown.

 

[dik]

Place the insulation under the new slab approx 2" extruded polystyrene Type 4. This will permit the geothermal heat to rise up and reduce any freezing.

Dik

 

[msquared48]

Why not locally underpin the existing footing with a couple more feet of concrete to gain the 3'6" vertical frost dimension?

Mike McCann
MMC Engineering