What causes condensation on windows in insulated New Brunswick homes? | Insulation IQ?

Condensation on windows is one of the most common complaints from New Brunswick homeowners after an insulation upgrade — and it's frequently misdiagnosed as a window defect or a sign that the new insulation is "not working." In most cases it is neither. Understanding what's actually happening requires a quick review of the physics of moisture and dew point in a cold-climate building.

When warm, humid interior air contacts any surface cooled below its dew point temperature, the moisture in the air condenses into liquid water on that surface. Windows — even good double- or triple-pane units — are almost always the coldest interior surface in a New Brunswick home during winter because glass conducts heat far more readily than an insulated wall. In Fredericton or Moncton, a standard double-pane window has an effective centre-of-glass R-value of roughly R-2 to R-3. A well-insulated wall beside it might be R-22 to R-30. The window surface temperature in a -20°C night can easily drop to 5–8°C at the interior face — cold enough to condense moisture from air at normal indoor humidity levels.

Why does insulation seem to make the problem worse? Before an insulation upgrade, heat moved relatively freely through the walls and ceiling, keeping surfaces warmer and also drying out the house by drawing conditioned air in and out constantly through leaks. After air sealing and insulating, the house retains more of the moisture generated by occupants — cooking, bathing, breathing, houseplants, and even fresh concrete or lumber in newer homes all add litres of moisture to the interior air daily. A family of four generates approximately 8–12 litres of moisture per day through normal activity. If that moisture can't escape and accumulates in the interior air, the relative humidity rises and condensation begins appearing on the coldest available surfaces: window glass, window frames, and sometimes exterior wall corners.

This is a healthy sign that the air sealing is working — but it also flags a need for controlled ventilation. The NBC requirement for mechanically ventilated homes (particularly airtight new or retrofitted homes) is specifically designed to address this: a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV) continuously exhausts stale, humid interior air and replaces it with fresh outdoor air, recovering 70–85% of the heat energy in the process. Without this mechanical exchange, a well-sealed NB home accumulates humidity to the point where window condensation is predictable and mould growth inside wall cavities becomes a real risk.

Surface condensation vs. interstitial condensation is an important distinction. Condensation you can see on the glass is a nuisance and can damage window sills, paint, and casings, but it's visible and manageable. Interstitial condensation — moisture depositing inside the wall assembly where you can't see it — is the more serious problem and eventually produces mould, rot, and structural degradation. A continuous vapour retarder (the 6-mil poly sheet behind the drywall in most NB homes) is supposed to prevent warm humid air from reaching the cold sheathing layer, but any penetration, gap, or improperly lapped seam lets moisture migrate inward.

Specific causes of increased window condensation in recently insulated homes:

Elevated indoor humidity from the tighter envelope not being matched with ventilation upgrades. Target indoor relative humidity of 30–40% in winter for most NB homes. Above 40% RH at -20°C outdoor temperature will produce condensation on standard double-pane windows.

Thermal bridging at the window frame. Even if the glazing unit performs well, aluminium or older vinyl frames conduct cold effectively. Interior condensation can be heavier at the frame edges than at the centre of glass — look at the pattern.

Blocked or absent window convection. Heavy curtains or cellular blinds closed tightly against windows in winter reduce the natural air circulation that keeps the glass surface temperature up. Keeping blinds slightly open at the bottom and top allows warm room air to circulate across the glass.

Cooking and bathing practices. In a tight house, running a bathroom exhaust fan for only five minutes after a shower is often insufficient. NB energy advisers typically recommend running bathroom fans for at least 20–30 minutes after each shower once the house is air-sealed.

The long-term solution is properly balancing the mechanical ventilation system to the airtightness level of the home. NB Power offers rebates under its Home Energy Efficiency Program for HRV installation, and the Canada Greener Homes Grant can contribute up to $1,000 toward ventilation equipment as part of a broader retrofit package. Professionals listed through the New Brunswick Construction Network can assess whether your current ventilation rate is matched to your home's post-upgrade airtightness and humidity load.

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

• Arctic Fox Construction Inc.
• Thirty Four Renovations
• moose luxury painting
• 3Tone Construction Ltd
• Brunswick insulation & roofing