Why exterior insulation is superior to stud/cavity insulation

Most homes in Vancouver use stud insulation, meaning the wall’s insulation sits between the studs. Insulation, with its low thermal conductivity, resists heat transfer between the inside and outside of a building.

The problem with stud insulation is that a 1 1/2-inch wood stud interrupts the insulation every 16 inches. Wood is a less effective insulator, so heat moves more readily through the studs, reducing the wall’s overall thermal performance. This is called thermal bridging.

If you pointed a thermal imaging camera at such a wall from inside on a cold day, the areas of the studs would appear colder than the rest of the wall. You might even feel the difference by running your hand across the surface.

Unfortunately, heat loss is not the only problem. When insulation sits only between the studs, the plywood sheathing on the outside remains exposed to outdoor temperature changes. On a cold day, warm, moist indoor air can leak into the wall and reach the cold sheathing. As that air cools, its moisture can condense, creating conditions that may lead to mold. To make matters worse, a vapour barrier just behind the drywall limits the wall’s ability to dry inward.

So how can we reduce these risks?

One approach is to install continuous exterior insulation over the sheathing, in addition to the stud insulation. Continuous exterior insulation reduces thermal bridging at the studs, which means less energy loss. It also keeps the plywood sheathing warmer, reducing the risk of condensation.

Even an inch is a good start, though more insulation provides greater protection. I prefer a vapour-permeable material such as mineral wool because it allows the wall to dry outward.