Fixing thermal bridging in old homes – GreenBuildingAdvisor

Interior-side polyiso was often done in my area in the 1980s for higher-performing homes. There’s nothing wrong or risky about it, in a cold climate. One downside is that it doesn’t address thermal bridging where interior wall partitions meet exterior walls, which were traditionally filled with framing lumber. In my area, I often see mold at these inside corners, because they stay cooler than room temperatures all winter.

As for whether continuous insulation is worth it: in a typical home, around 20% of the walls are framing, not insulation. Adding continuous insulation is worth more than 120% of using additional cavity insulation, due to the complexities of three-dimensional heat flow.

Walls–not including windows–typically account for roughly 15% of a home’s heat loss. So improving the wall by say 30% via continuous exterior insulation will result in roughly 5% energy savings–on the order of $100 to $200 per year for most homes. But–properly done–it also reduces the risk of microbial growth, which is worth far more.