How do I deal with thermal bridging through the studs in my Ottawa garage walls?

Thermal bridging through studs is a significant issue in Ottawa garages because our extreme cold amplifies the effect. A standard two-by-six wood stud has an R-value of only about R-6, compared to the R-20 insulation filling the cavity beside it. In Ottawa at minus twenty-five, those studs act as thermal highways conducting cold straight through the wall, and they can reduce the effective R-value of the entire wall assembly by twenty to thirty percent compared to the nominal insulation rating.

You can see thermal bridging at work on a cold Ottawa morning when frost or condensation forms in vertical stripes on the interior drywall. Those stripes correspond exactly to the stud locations because the surface temperature at each stud is significantly colder than the insulated cavity between them. Beyond comfort issues, these cold spots are condensation risks. When interior moisture hits the cold surface over a stud, it can lead to mould growth behind the drywall that you may not discover until the wall is opened up.

The most effective solution is a continuous layer of rigid insulation installed over the face of the studs, between the studs and the interior drywall. This layer breaks the thermal bridge because the insulation runs uninterrupted across every stud, plate, and header. The three common rigid insulation products used for this in Ottawa are extruded polystyrene at about R-5 per inch, expanded polystyrene at about R-4 per inch, and polyisocyanurate at about R-6 per inch.

For most Ottawa garages, a single layer of one-inch polyiso over the studs is the best balance of performance, cost, and practicality. One inch of polyiso adds R-6 to the entire wall surface, including over every stud, and costs roughly one dollar to one dollar fifty per square foot for the material. For a two-car garage, the material cost for all four walls is approximately three hundred to five hundred dollars. Combined with R-20 batt in the stud cavities, this gives you an effective whole-wall R-value of approximately R-22 to R-24, compared to R-16 to R-17 for batts alone when thermal bridging is factored in.

Installation is straightforward. After installing batt insulation in the stud cavities and before hanging drywall, cut the rigid foam panels to fit and attach them to the stud faces with construction adhesive and cap nails or screws with washers. Tape all seams with foil tape if using foil-faced polyiso, or sheathing tape if using XPS or EPS. This taped layer also serves as an effective vapour barrier, potentially eliminating the need for a separate polyethylene sheet depending on the product permeance rating and your local code interpretation. Foil-faced polyiso is a vapour barrier on its own when seams are properly taped.

The one practical concern is that adding an inch of rigid foam to the interior pushes your drywall out by an inch, which affects door jamb extensions, electrical box depths, and window trim. For a garage this is usually a minor issue since trim details are simpler than in living spaces. You will need electrical box extenders for any outlets or switches, which cost a dollar or two each.

An alternative approach for addressing thermal bridging without adding interior foam is to install rigid insulation on the exterior of the sheathing before siding. This is more common in new construction and provides the same thermal break benefit. However, for an existing garage it typically means removing and reinstalling the siding, which is expensive and disruptive.

For a budget-conscious approach, even half-inch rigid foam at R-3 makes a meaningful difference in Ottawa. It will not eliminate thermal bridging entirely but it reduces the cold stud effect enough to prevent most condensation issues and improves the whole-wall R-value by roughly fifteen percent. At fifty cents per square foot for material, it is one of the best performance-per-dollar improvements you can make to an Ottawa garage insulation project.

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Looking for experienced contractors? The Ottawa Construction Network connects Ottawa homeowners with qualified professionals:

• Homeupgraders
• JC Carpentry
• The Granite shop
• The Deck Store Inc
• True Fix Garage doors