One big ticket item that is still sitting on my to-do list is the exterior insulation around the basement. Well, I did get the chance to practice a little, because our brand-spanking-new porch foundation wall was asking for some insulation before I started with the backfilling.
I had researched the insulation strategy a long time ago and knew which direction to go and that I would need XPS boards totalling four inches in depth. The question about where to get the XPS boards was also solved, thanks to my snafu in the attic insulation strategy.
I had started to take down some of the salvaged four inch XPS boards we had mounted to the bottom of the roof joists. I was now ready to repurpose them as foundation wall insulation. That was a tiny bit of silver lining in my attic insulation dilemma.
We cut the XPS boards to size so that they fit from the top of the footing (or bottom of the foundation wall) to the top of the foundation wall. To help with the moisture management, we placed a drainage membrane over the insulation.
The drainage membrane is a HDPE sheet with dimples on one side, creating a ¼ inch air gap between the membrane and insulation. That air gap prevents hydrostatic pressure from building up against the foundation wall. Any bulk water that enters the air gap immediately drains down, where the membrane connects to the footing drain which is embedded in open graded aggregate.
This assembly should keep the foundation wall dry and reasonably warm (or cold for that matter). With four inch XPS boards in place, I can expect an R-value of about 20, not counting the thermal mass of the soil behind it. That exceeds the Chicago Building Code requirement of R-10 (Chicago Building Code, Chapter 18-13-102.1.1; Building thermal envelope insulation, Table 18-13-402.1.1) and the Chicago Green Homes requirement of R-19 (Chicago Green Homes Program Guide, Version 2.0 – 210 Energy Use Reduction [210.3]).