Super-Insulating a Basement?

We've just finished the foundation and basement of our little SuperGreen Laneway house.  This includes a "super-insulated" basement. 

Why do that you might ask?  

Additional perimeter R25 insulation on Insulated Concrete Form (ICF) basement ready for backfill.

We know from recent work by Mark Carver at NRCAN and Energy Solutions Centre on their cost optimization study that basement insulation has less bang-for-the-buck than above-grade wall insulation.  This makes sense because the temperature differential between inside versus cold winter air of the above grade wall is much higher than the basement to ground temperature differential.  Higher temperature differential means higher heat loss (which is why keeping your house cooler inside reduces heat loss).  

The challenge however with building a small SuperGreen home is the space.  Because of all the insulation, the walls get quite thick. This makes it hard to keep the home small and fit it on the property.  There are limitations to practically how much insulation you can install.  This leads you to find places that are convenient to stuff lots of insulation, but won't compromise the design of the building.  A common example of this is the ceiling.  When looking at where the heat loss in a house is, the ceiling/roof really is a small portion of the heat loss.
So why do we heap the insulation so high in the ceiling (i.e. R100 or more)?
Because it is a cheap and easy place to pile up a lot of insulation, even though its effectiveness is definitely on the upper end of the diminishing returns curve.
 

ICF Basement/foundation ready for concrete

In the SuperGreen homes we've built we've been using Insulated Concrete Forms, or ICF, foundation/basements.  This gives a very resilient concrete foundation below grade, but is relatively easy to build and and gives you and insulated concrete basement.  The standard ICF block we use have about 5.75" of expanded polystyrene insulation.  This gives about R26 (with no thermal bridging).  In our last building, we then used an interior stud wall with additional batt insulation to add another R22 (making the wall R48).

ICF Basement braced and ready for concrete.

The problem with insulating on the inside is it eats up floor space.  In small home, this is undesirable.  When designing our new SuperGreen Laneway home the light-bulb went on:  Why not insulate on the OUTSIDE of the basement.  A product called "Terrafoam HS-40" by Beaver Plastics (made in Alberta) is commonly being used now in the Yukon for below-grade insulation.  It is specifically designed for this perimeter insulation and sub-grade application. 

Eureka!  We found it - here is another place, much like ceilings, where we can stuff lots of insulation without taking up space.  Since this will all be buried outside of the building footprint, this this an easy place were we can put lots of insulation without affecting the design of the rest of the building.  Yes, it is an area on the upper-end of the diminishing returns curve.  But when you are pushing the energy-efficiency envelope, we need to find these low(er) hanging opportunities. 

So our design calls for 5" of HS-40 insulation outside of the ICF.  The manufacturer claims it is R-5 / inch at -10C.  (The weird thing they claim is its R-value goes up the colder it gets.  That's nice!)  So this gives an extra R25 ontop of the ICF's R26 for a total of R51 basement.  Not quite R60 of the walls above, but getting closer.

Once again pumping concrete OVER the "heritage" house to fill the ICF foundation.

Below is the footing detail that shows how the insulation is installed.  Note that the second 2" layer laps over the footing, insulating the footing too.  Now keen observers will note a thermal bridge in this design between the slab and the footing.  Yes, I know it is there, and for a variety of reasons (that I don't have the energy to describe), I designed it that way for this particular building.

Footing detail showing exterior EPS insulation around ICF basement.

 Below are a few additional pictures of the waterproofing and extra-insulation installation details.

Self-adhering waterproof membrane installed on ICF.  The membrane laps over footing and ties into the poly wrapping the footing as a capillary barrier.

1st and 2nd layers of exterior insulation being installed.  Note how second layer laps over the footing to insulate it. The little bits of Tuck Tape (red tape) are just there temporarily to hold the insulation in place.  Once it is backfilled against, the insulation won't be going anywhere.

Another view of the below-grade exterior insulation around the garage portion of the house, ready for backfill.  Note the interior PWF cripple wall to support the garage slab.  The stud-bays here too have been filled with 3.5" of EPS insulation.