Wednesday, July 21, 2010

Footing Insulations

Structural connections are the hardest parts to design for thermal bridge-free construction.  I've decided to install insulation under my strip footings.  There are high-strength insulation products such as Foamglas and Pur-knit and others.  They are expensive, and Foamglas is friable (brittle).  Therefore I plan to use HI-load 60.  An XPS 60-psi compressive strength rated insulation under the strip footings.  This stuff is R-5 per inch.  For nearly twice the price, you can buy HI-load 100, and Foamglass is beyond that price.  As an added benefit, the HI-60 comes in thickness up to 3".

The DOW building solutions rep said footings are insulated routinely by a large production builder in the Ottawa area - they have clay/silt soils that are highly prone to frost heave, and the builders insulate under the footings to protect their foundations during construction.  Railway tracks are also insulated using these HI-load insulations.
More on this later.

In Europe, they are making Foamglas gravel - all with recycled glass - I think this is fantastic, but no luck getting it here.


  1. Is the compressive strength of the standard foam density's not sufficient? I seem to recall them as being between 15-25 psi... What is the soils bearing density?

  2. John is right in pointing out the soil bearing capacities. I wouldn't use a soil bearing capacity more than say 100kPa (about 15psi) in a normal house foundation design. However, I think in other posts I might have mentioned the XPS (and I suspect other foams) are subject to long term creep. In other words, they will crush down very slightly over time, in the amount of 5% of thickness at rated load. This is the reason to use the harder stronger foam insulations under footings. Straight comparison to soil bearing capacities is not the only determinant.