Effective vs Nominal R-values

The City of Vancouver has recently implemented their new Vancouver building code. One small change affects the insulation of exterior walls. Previously, exterior walls needed to be at a nominal R22 insulation level. The new code requires effective R22 insulation levels. What difference does this make?

Nominal insulation is the R-value of the insulation batt we stuff between the wood studs. This is the R-value we see printed on the bag. Don’t be fooled, this is not the R-value of the wall. Wood has an R-value of R 1.5 per inch and makes up approximately 23% of a traditional framed wall. A 2×6 stud will have an R-value of 6 while windows range from R2-R5. Relative to the insulation property of fiberglass, wood and windows are poor performers.

What will it take to achieve Vancouver’s R22 effective insulation values in exterior walls? Here are three options.

1. The traditional 2×6 wall insulated with R22 batt will achieve R17.89 effective. Fail!

2. A 2×8 wall insulated with R28 batt will achieve R22 effective. Pass!

3. A 2×6 wall with R22 batt and 1” exterior rigid insulation achieves R22 effective. Pass!

There are many variables at play to achieve R22 effective. Make sure your builder considers the many options and doesn’t default to what is easiest. There is one wall assembly considered by the world experts to be the perfect wall assembly. Check back for my next blog post on the “Perfect Wall” and its many advantages.