Basement Walkout 2

[VictorAK]

A standard detail for new basement walkout requires underpinning the existing foundation. Can you avoid underpinning the existing foundation around the proposed opening by adding styrofoam insulation (3" thick) parallel to the existing foundation to avoid frost penetration?

 

[Lion06]

You can't provide the insulation below the existing footing elevation without undermining the footings. How do you propose that this will work? I don't see it working.

 

[concretemasonry]

There is a buried shallow insulation that is acceptable under most codes. In general, it is perpendicular or on an angle with the foundation wall.

Dick

 

[VictorAK]

Hello concretemasonry,
Thank you for the input.
Do you have any additional information on the insulation, ie maximum depth, thickness, application etc.?
The dwelling is in Toronto, Ontario.

 

[VictorAK]

Hello StructuralEIT
Thank you for the input.
I was thinking digging a shallow pit 1 foot out from the foundation and no more than 4 feet wide and 4 feet deep, placing the insulation, then backfilling.
This procedure would be repeated for the remainder of the footing effected by the walkout.
Any thoughts would be greatly appreciated.
Thank you.

 

[jike]

There were some studies done in Canada in the 60's about protecting less than frost depth shallow foundations from frost heave with styrofoam. I cannot put my fingers on the study but basically you run the insulation out horizontal (actually a slight downward slope away from the foundation for drainage) the same distance as normal frost depth.

Make sure that you use extruded and NOT bead board or any other insulation that would lose its insulating value due to water!

 

[beton1]

I believe the detail that you are referring to is a "TACBOC" issue (Toronto...Building Officials...), dwg B01. If it is a similar walkout, with a retaining wall and stairs, you will have to underpin and step down to meet the new footings. It is a structural (undermining) issue as well as thermal. If it is a walkout (lower level grade), the footings should already be deep enough, unless you are creating a new grade and 'berming' the sides. If it is the last case (bermed), it may work (insulation) but you should clarify your intent with the building department, and depending on who the plan examiner is, they may require proof the insulation will work. Also, you will likely need a drain, and how do you prevent it from freezing? Proceed with caution.

 

[VictorAK]

Thank you beton1,
Does anyone know of any design criteria for frost protected shallow footings?

 

[SkiisAndBikes]

In general, one inch thickness of extruded polystyrene insulation is assummed to have an R-value equivalent to 12 inches of soil. The placement of the insulation must be designed so that the path for heat loss around the insulation must be at least equivalent to the local frost depth.

If you are going to dig a 4 ft. deep trench within 1 ft. of the existing foundation, you will be undermining the undisturbed soil the footings bear on and you might as well underpin. The main reason for considering insulation is to limit your excavation and reduce costs.

I am assumming that you are not installing an access stair or retaining walls that would require footings to be excavated below frost depth. If so, you are better to underpin as you are excavating anyways.

To avoid the 4 ft. depth of excavation I usually specify excavating to the bottom of the existing footing (usually 1 ft below your walkout grade) and then place vertical insulation against the footing and then extend insulation horizontally for a length equal to the frost depth (Toronto = 4 ft)away from the bottom of the footing. Sloping the insulation away from the house at 20 to 30 degrees aids heat retention under footing and also aids drainage away from the footings. Don't forget to overlap the insulation at the horiz/vert. transition.

As the project is in Toronto, you shouldn't have trouble locating the original research paper (1970's) on foundation insulation by Robinsky & Bespflug, one of them was a U of T prof in the day. I have a copy somewhere but can't put my fingers on it.

 

[VictorAK]

Thank you SkiisandBikes,
I've tracked down the article at ASCE however the cost is pretty high.
I like your suggestion on placement of the insultation.
Hopefully the bottom of the footing will be low enough to accomodate the doorway.
4" of insulation should be adequate.
I imagine the construction should be done in the spring in order to prevent frost from penetrating below the exposed footings (or if winter construction is necessary then cover the excavation with straw bails or insulation).
Does the styrofoam have sufficient compressive resistance if the concrete walls are placed directly on top?