Condensation on Windows in Winter: Why Nova Scotia Windows Sweat

Every Nova Scotia winter the same thing happens in a lot of homes. The first real cold snap arrives, and the next morning there's water pooled on the bottom of the bedroom windows, frost creeping up the corners, sometimes a black line of mould along the bottom of the frame.

It looks like a window problem. It almost never is. The window is just the coldest surface in the room, so it's where your house shows you how much moisture it's holding. Treat it as a signal rather than the fault and it becomes very useful. Where the water sits tells you whose problem it is, and how much of it there is tells you how hard your house is struggling.

First: where is the water?

Put a finger on it. There are four places condensation can be, and they have completely different causes.

The sealed-unit case is worth a moment, because it's a genuinely different job. Natural Resources Canada says a sealed unit with a broken seal is "best corrected by replacing the glazing unit," and that refurbishing failed units is considered a temporary fix. That's the window people, and no amount of ventilation will clear it.

The storm-window case surprises people because it happens in dry houses too. NRCan's explanation: moist house air leaks past the inner pane and condenses on the cold outer one, and it's most common upstairs, where the stack effect pushes more air out through the windows. The fix is to weatherstrip the inner sash, seal it more tightly than the storm, and make sure the storm's weep holes are open.

The rest of this guide is about the first row: water on the inside of the glass. That's the common one, and it's an air problem.

Why Nova Scotia windows sweat at "normal" humidity

Warm air holds more water vapour than cold air. When room air touches a cold window it cools, and if it cools to its dew point it can't hold all its water any more, so the water lands on the glass. If the glass is cold enough, it lands as frost. That's the whole mechanism, straight from NRCan's Keeping the Heat In.

What makes it bite here is the temperature of the glass, and that's set by the weather outside. NRCan publishes a table of the maximum indoor humidity a window can take before it sweats, at a room temperature of 21 °C:

¹ NRCan's column heading reads "Older generation double-glazed low-E". Its values sit below plain double glazing, so we read it as older-generation double glazing. Source: NRCan, Keeping the Heat In, Table 9-1.

Now the local part. Environment Canada's 1991–2020 climate normals for Halifax Stanfield airport put the average January daily minimum at -10.1 °C, and February's at -9.7 °C. So the -10 °C row isn't a cold snap. It's an ordinary Halifax winter night, and it's the weather outside your windows on a typical January morning.

Put that next to NRCan's own advice that indoor humidity should sit between 30 and 50%, lower in winter. In a house with older double glazing, most of that healthy range is above what the glass can tolerate on a normal January night. Standard double glazing gives you until the mid-40s. That's why people are genuinely puzzled by it: the house doesn't feel damp, the humidity reading looks reasonable, and the windows are running with water anyway.

Two things follow:

• It gets worse on the coldest nights and better in mild spells. That's the table working, not your house changing.
• Better windows raise the ceiling a lot. Low-E double glazing tolerates 59% at -10 °C. But read the next section before you replace windows to fix this.

Why it often gets worse after you upgrade

It's a common pattern, and NRCan describes it directly: a homeowner replaces drafty windows, air-seals, or insulates, and the condensation gets worse. NRCan: "Many homeowners buy new windows only to find that the problem becomes worse because the old, leaky windows actually helped to reduce humidity."

A leaky house dries itself in winter. Cold outdoor air holds very little moisture, so every draft replaces humid indoor air with dry outdoor air, at a heating cost. Seal the house and that uncontrolled exchange stops, but the moisture keeps coming. NRCan's conclusion is blunt: if a house is tightened to that degree, "it will now need more ventilation."

Heating conversions can do the same thing. NRCan notes that switching to sealed-combustion equipment, which takes its combustion air straight from outside, reduces air changes in the home "and may cause humidity levels to rise." Our reading, not NRCan's words: removing an oil furnace and its chimney entirely, as happens in an oil-to-heat-pump conversion, takes away that draft too. A heat pump heats and cools the air already in the house; it doesn't exchange it. If your windows started sweating the first winter after a conversion, that's the likely reason. Our oil furnace to heat pump guide covers the ductwork side of that job.

Where the water actually comes from

NRCan estimates a family of four puts about 50 litres of water a week into the air of their house through normal living:

Look at the top line. Three-quarters of it is people breathing. You can't reduce that, which is the whole argument for ventilation: a house with people in it produces moisture continuously, so it has to get rid of moisture continuously.

The table also leaves out the big optional sources: an unvented or badly vented clothes dryer, laundry dried indoors, a humidifier on the furnace, firewood stored inside, aquariums, lots of houseplants, and a damp basement. NRCan adds one that catches people every autumn: in humid weather building materials and furnishings absorb moisture from the air and then give it back during the heating season. After a damp Nova Scotia summer, that's one reason the first weeks of heating can be the worst for condensation.

How to fix it, in the right order

NRCan's order is: measure, cut the sources, then ventilate. It's the right order because the first two steps are cheap and sometimes enough.

1. Measure

Buy a hygrometer, an inexpensive digital humidity meter, and put it in the room with the worst windows. Check it on a cold morning. Compare it with the table above for your windows. NRCan also suggests simply letting the windows be the gauge: if double-glazed windows other than the kitchen and bathroom have heavy condensation, there's too much moisture in the air.

2. Cut the sources

From NRCan's list, the ones that matter most in a Nova Scotia house:

• Vent the clothes dryer outside, properly, and don't dry laundry indoors. A dryer venting into the basement or a disconnected duct is a large moisture source on its own. Our dryer venting guide covers what the code actually requires.
• Run the kitchen and bathroom fans when you use those rooms. NRCan suggests a timer or humidity control on the bathroom fan so it runs long enough. Check that they actually vent outside. A bathroom fan dumping into the attic moves the moisture into the roof instead, where it condenses on the underside of the sheathing. See bathroom fan installation and cost.
• Check the range hood. A recirculating hood filters grease but puts the steam straight back into the kitchen. See range hood venting mistakes.
• Disconnect any humidifier if your windows are sweating. It's adding the thing you have too much of.
• Deal with basement water: leaks, open sumps, standing water, bare earth in a crawlspace, eavestroughs and grading that send water toward the foundation.

3. Ventilate

If the windows are still sweating after that, NRCan's next step is to increase ventilation. There are two kinds:

• Unbalanced ventilation is exhaust fans on their own. Air goes out and the replacement leaks in wherever it can. NRCan notes this can depressurise the house and ventilate it unevenly.
• Balanced ventilation brings in a dedicated supply of outdoor air to replace what's exhausted. NRCan calls an HRV or ERV "one of the best ways to ventilate a home and to control indoor air quality," and notes an HRV typically recovers 70 to 80% of the heat from the outgoing air.

For a house with a winter moisture problem, the usual answer is an HRV: it exhausts humid indoor air, brings in dry outdoor air, and warms the incoming air with the heat from the outgoing air. An ERV holds some of the moisture back, which suits a house that runs too dry in winter, not one with wet windows. ERV vs HRV in Nova Scotia works through that choice.

"I already have an HRV and my windows still sweat"

Usually the HRV isn't broken. Work through NRCan's own troubleshooting list for "excess moisture throughout the house":

1. Is it running? NRCan recommends running an HRV on low speed continuously, year-round. Units do get switched off, for noise or on the belief that they waste heat.
2. Check the dehumidistat. Many HRVs have one that kicks them to high speed when humidity passes a set point. NRCan says to adjust it seasonally, turning it down in the heating season to limit condensation on windows. If it's set for summer, it may never trigger.
3. Clean the filters and the core. A clogged filter or a dirty core moves a fraction of the air it should. NRCan's schedule is filters every one to three months and the core every six.
4. Check the outside hoods. NRCan suggests checking them more often in autumn, for leaves, and in winter, for snow and frost. A blocked hood starves the unit.
5. Check it's actually moving air. NRCan's guide reproduces a simple CMHC test: time how long a trash bag held over each register takes to fill.

If all of that checks out and the windows are still wet, NRCan's advice is to call a contractor, because "the HRV's minimum continuous ventilation rate may be inadequate." Common causes are one of three things: the unit was never balanced when it was installed, so it's moving less air than it should or pushing the house out of pressure; it's tied into the furnace ductwork in a way that only works when the furnace fan runs; or the ducting is long, crushed or undersized. Those are ductwork problems, and they're what we fix. If the unit itself is at the end of its life, our HRV installation and replacement cost guide sets out what replacing it involves.

When to call someone

Wipe the windows and move on if it's a little fog on a few windows on the coldest mornings. That's normal, and NRCan says occasional condensation isn't a problem.

Call someone if:

• Most windows have water on them most cold mornings, or it runs onto the sills.
• There's frost building up on the inside of the glass.
• You see mould, staining or peeling paint around frames, in closets, or on the ceiling along outside walls. NRCan lists all of these as signs of a moisture problem.
• It started after a renovation, new windows or a heating conversion.
• You have an HRV and it's still happening after the checks above.

Where we fit in

Ventilation is air-side work, and air-side work is our trade. That covers bathroom fans ducted properly to the outside, range hood and dryer venting, and HRV and ERV installation, repair and balancing, including the ductwork that decides whether an HRV works at all. See our ventilation services.

What we don't do is windows. If the fog is sealed between your panes, that's a glazing repair, and we'll tell you so rather than sell you ventilation you don't need.

Sources

This page makes no cost claims. Every source below was opened and read at the original rather than taken from an article about it. Last verified September 28, 2026.

All sources on this page are Canadian. Condensation depends on outdoor temperature, which makes it one of the places where our rule matters most: a US humidity table is calibrated to a different winter.

• Natural Resources Canada: Keeping the Heat In, Section 2: How your house works. Source of the dew-point explanation; the 30–50% healthy humidity range ("lower humidity in the winter"); Table 2-2, household moisture production (49.8 litres a week for four people, 38.0 from respiration); the drying effect of air leakage in leaky homes; building materials releasing stored summer moisture in the heating season; and the statement that a house tightened this far "will now need more ventilation."
• Natural Resources Canada: Keeping the Heat In, Section 8: Upgrading windows and exterior doors. Source of the new-windows-make-it-worse passage, the storm-window and failed-seal explanations (weatherstrip the inner sash; replace a failed sealed unit, refurbishing is temporary), the stack-effect point about upper storeys, and curtains and blinds contributing to winter condensation.
• Natural Resources Canada: Keeping the Heat In, Section 9: Operating your house. Source of Table 9-1, maximum indoor relative humidity by outdoor temperature and window type (at 21 °C); the list of moisture sources and remedies; the signs of a moisture problem; the windows-as-indicator method and the kitchen and bathroom exception; balanced vs unbalanced ventilation; 70–80% heat recovery; and sealed-combustion equipment reducing air changes. A note on Table 9-1: NRCan's heading for the first column reads "Older generation double-glazed low-E"; because its values sit below plain double glazing (34% against 44% at -10 °C), which a low-E unit would not, we read it as older-generation double glazing, label it "older double-glazed", and reproduce NRCan's wording in the footnote.
• Natural Resources Canada: Heat Recovery Ventilators (Office of Energy Efficiency, revised February 2012). Source of continuous low-speed operation, the maintenance intervals (filters every one to three months, core every six, hoods checked more often in autumn and winter), seasonal dehumidistat adjustment "to help minimize condensation on cold surfaces such as windows," the troubleshooting steps for excess moisture, the CMHC trash-bag airflow test, and "the HRV's minimum continuous ventilation rate may be inadequate." It's a 2012 publication; we use it for how HRVs operate, which hasn't changed.
• Environment and Climate Change Canada: Canadian Climate Normals 1991–2020, Halifax Stanfield (Airport), climate ID 8202250. Source of the January daily minimum of -10.1 °C (February -9.7 °C). The airport sits inland, so treat it as representative of HRM rather than exact for any one street.

What is our reasoning rather than a source

Oil-to-heat-pump conversions raising indoor humidity. NRCan states the effect for sealed-combustion equipment. Extending it to a conversion that removes the chimney altogether is our inference from the same mechanism, and we've said so in the text. We looked for a Canadian source that measures it directly and didn't find one.

Condensation on the outside of the glass meaning an efficient window. That's physics rather than a citation: a well-insulated window lets little heat reach the outer pane, so it stays cold enough to collect dew. We didn't find a Canadian primary that states it and we don't build anything on it.