Why is there window condensation after my Toronto reno?

Window condensation after a renovation is almost always caused by increased moisture in your home combined with changes to your ventilation and airtightness. It's one of the most common post-reno complaints in Toronto, and the good news is it's usually manageable once you understand what's driving it.

When you renovate — especially if you added insulation, sealed drafts, or installed new windows — your home becomes significantly more airtight. Older Toronto homes, particularly the century houses in neighbourhoods like The Annex or Cabbagetown, were "leaky" by design and naturally exchanged indoor and outdoor air. Once you tighten that envelope, moisture from cooking, showering, breathing, and even new construction materials (drywall, concrete, lumber) has nowhere to escape. That trapped humidity hits your cold window glass and condenses. New builds and fresh renovations also off-gas moisture from materials for 6–12 months after completion, which compounds the problem.

The threshold to watch is indoor relative humidity (RH). During a Toronto winter, you want to keep indoor RH between 30–45%. Above that, condensation on windows — and eventually mold — becomes a real risk. A basic hygrometer from a hardware store costs $15–$30 and will tell you exactly where you stand. If you're sitting at 55–60% RH in January, that's your culprit.

The Most Common Causes After a Reno

Insufficient mechanical ventilation is the top offender. If your renovation didn't include an HRV (Heat Recovery Ventilator) or ERV (Energy Recovery Ventilator), your newly tightened home has no controlled way to exhaust humid air. Ontario Building Code actually requires HRV/ERV systems in new construction for this reason. Retrofitting an HRV typically costs $1,500–$3,500 installed in the GTA, and it's often the single most effective fix.

Bathroom and kitchen exhaust fans are the second issue. Many older Toronto homes have undersized or improperly ducted fans that vent into the attic rather than outside — a code violation that's surprisingly common in pre-1990s homes. Proper exhaust fans should run during and 20 minutes after showers.

New windows themselves can appear to perform worse than old drafty ones. Your old leaky windows were constantly warmed by infiltrating air. New sealed windows are colder at the glass surface, so condensation appears at lower humidity levels. This doesn't mean the windows are defective — it means your ventilation needs to catch up.

What You Should Do

Start by measuring your humidity and running your exhaust fans more aggressively. If RH stays above 45% despite that, have an HVAC contractor assess whether an HRV is appropriate for your home — this is especially worthwhile if your reno included significant air-sealing or insulation upgrades. A TSSA-certified HVAC contractor can evaluate your existing ventilation and recommend the right solution.

If condensation is appearing between the panes of your new windows rather than on the surface, that's a different issue — a failed seal — and should be covered under your window manufacturer's warranty.

Browse HVAC and ventilation contractors in our directory at Toronto Construction Network to find qualified local professionals who can assess your specific situation.