Frost Wall at New Basement Walkout 1

[RFreund]

There is a remodel of an existing structure. The existing structure has a partial height basement wall and they will be excavating to install a walkout basement area. Therefore a new 'frost wall' is required. See attached sketch options. I'am curious what is the best way to do this. I suppose any of the options could work as the final grade is still above the bottom of the existing footing. Any comments are welcome.

Also I'm curious if anyone has used insulation to avoid having to excavate the full depth?

Thanks

EIT
www.HowToEngineer.com

 

[oldestguy]

I have the same thing here when I added an entrance to my half basement. I was concerned that the footings and slab for the entryway would heave. So I placed all units on a closed cell insulation blanket. It has gone through 4 Wisconsin winters and no problem. I might be at a slight advantage, since the entry is on the south side and sun shining on it made that opening quite warm on sunny days. Side walls were doweled into former basement walls. My advice is lay that new slab on such an insulation and avoid the deep digging stuff. Alternatively bury the insulation vertically, but why the concrete wall then? Regularly we recommend burying a 4 x 8 sheet about a foot under ground next to thickened edge slabs, as for a garage. Also insulate the edge of that slab. The object is to make the cold travel the same distance as if there were a "frost wall". The closed cell foams seem to have excellent compression resistance.

 

[RFreund]

Thanks OG. I was looking more into that option as it is an attractive route to go.

EIT

http://www.HowToEngineer.com

 

[RFreund]

If the concrete method is used, which would be prefered?

Is it acceptable to pour the concrete frost wall directly adjacent to the existing footing or would it need to be underpinned?
My reservation with placing the new wall next to the existing as opposed to underpinning is that -> is it possible for frost (well heat actually) to travel through the new wall so that the soils directly under the existing wall freeze?



EIT

http://www.HowToEngineer.com

 

[oldestguy]

I suspect you are assuming the term "frost wall" means that it stops frost action, maybe not? In the original construction it is "insulated" to some extent by the earth backfill and is set low enough that soil below the footing does not freeze and thus heave. In your three options you do allow "cold' to get under the floor slab and the old footing and possibly heave them. If you want to do "frost protection", you need to insulate. Remember concrete probably is a better conductor of heat than most soil.

Your left hand #1 option could use some support improvement, since your new "frost wall" can lean sideways from the effects of the footing.

Green sheets of insulation should be covered with mortar or other to prevent ulta V degradation.

So, I say, why all this grief, when you can set your slab on closed cell insulation?

 

[CarlB]

I agree with oldestguy, if the only reason for "frost wall" is to keep frost from penetrating below footing and slab, providing insulation would be effective and avoid lots of construction headaches.
I'd suggest a variation on the vertical insulation, using "wing" insulation, also mentioned by OG. The principles are pretty well established, are covered in ASCE 32, "frost-protected shallow foundations". Two-inch thick insulation would wrap around the footing, then extend horizontally about 4 feet at the level of bottom of footing.

 

[RFreund]

Yes I suppose the term frost wall is a poor choice. Also I do like the use of insulation and that is most likely what they will use. Really the point is for the wall to be far enough below grad so that frost cannot heave the footing. In which case I would say only the traditional underpinning is accomplishing this as I suppose frost can move laterally through the concrete "frost wall" and under the footing in options 1 and 2 (as OG noted).

Thanks again!!

EIT

http://www.HowToEngineer.com