Shutter BoysUpdated
The short version: condensation comes from moist indoor air meeting cold glass. Shutters don’t create the moisture, but closed shutters, like closed blinds or drapes, keep warm room air off the glass, so it gets colder and wetter. Keep indoor humidity in range, keep air moving, and tilt the louvers open on cold nights.
Why windows sweat in winter
Natural Resources Canada puts it plainly: condensation often forms “during colder weather when the moist air inside the home” meets the cold glass, and “the risk of condensation increases as the weather gets colder and/or the inside humidity rises”[1]. Everyday life adds the moisture. NDSU Extension points out that “each person in a house adds 3 pints of water vapor to the air every day just from breathing”, before showers, cooking and laundry[3].
Where the water shows up tells you something. Water between the panes of a sealed unit means the seal has failed, which is a window problem, not a humidity one[1]. Water on the room side of double- or triple-pane glass, in NDSU’s words, means “you most likely have too much humidity in your house”[3].
Where window coverings come in
This is the part shutter sellers tend to skip. Natural Resources Canada says “deep window sills, closed drapes and blinds, and even nearby furniture can block air movement and allow cool air to pool near a window, which can lead to condensation on the glass”[1]. It adds that “air flow across the glass surface helps to keep it warmer”, which is why heating vents and baseboards usually sit under windows[1]. Health Canada gives the same warning: “heavy curtains or blinds can trap the cold and moisture and cause condensation on your windows”[2].
A closed plantation shutter is a covering like any other here. It isn’t sealed against the glass, so moist room air still finds its way behind it, while the warm air that would have kept the glass dry is held back. An inside-mounted shutter with louvers shut on a deep sill is close to the worst case NRCan describes.
This isn’t a new finding. A National Research Council digest from 1960 says “drapes and curtains can affect both the air temperature adjacent to a window and the inside convection condition”, and that, depending on where they sit, they can raise or lower the glass temperature[6]. It also notes that “window stools interfere with air flow at the bottom of the window and tend to lower temperatures in this region”[6], which is why condensation so often starts at the bottom of the glass, just above a deep sill.
Where is the water? What it tells you
| What you see | What it usually means | What to do |
|---|---|---|
| Fog between the panes | The sealed unit has failed[1] | A window repair; no covering fixes it |
| A band of water along the bottom of the glass | Cold air pooling above the sill, where air moves least[6] | Tilt the louvers open at night; keep the sill and any vent below clear |
| Water on every window, every morning | Indoor humidity too high for the weather[3] | Bring humidity down: fans, HRV, humidifier off (below) |
| Water only behind closed shutters or blinds | The covering is holding warm air off the glass[1] | Leave the louvers tilted open on cold nights |
| Water only in the bathroom or kitchen | Moisture made in that room | Run the fan or range hood while you shower or cook and afterwards[2] |
Open or closed at night? The experts don’t agree
Two university extension services give opposite night-time advice for drapes, and both have a point. Missouri Extension says to “leave drapes open during the day to allow air to circulate freely over the windows” and, at night, to “close drapes to prevent warm moist air from reaching the cold window surface”[4]. NDSU says that if condensation forms behind your drapes, leave them open at night, and accept that you lose a little more heat doing it[3].
The window industry’s own advice sits closer to NDSU’s on the daytime half: “Open curtains and blinds during daylight hours”, because “keeping them closed increases the likelihood of condensation forming”[5]. It also suggests running ceiling fans, the way you’d run a car’s defroster[5].
Our reading, which is reasoning rather than a test: a tight, sealed covering keeps moist air off the glass, but louvered shutters aren’t sealed. So the practical test is your own window. If the glass behind closed shutters is wet in the morning, open the louvers at night. If it stays dry, close them for the warmth and privacy.
How to use shutters in a cold snap
- Tilt, don’t close, on the coldest nights. Louvers tilted open let room air wash over the glass. You keep most of the privacy, because the louvers still block a straight view in.
- Open the panels in the morning. Swing the panels back for an hour after you get up so the glass and sill can dry. That’s when the house is at its most humid after showers and breakfast.
- Keep vents and baseboards clear. Missouri Extension says not to cover supply or return registers with furniture[4]. If a heating vent sits below the window, make sure the shutter frame and any furniture leave it open.
- Dry the sill when it’s wet. Health Canada says to “dry your window frames and sills daily to keep water from dripping and causing mould to grow”[2]. Painted wood handles the odd wipe; it doesn’t handle standing water every day.
Fix the humidity, not the shutter
Health Canada’s target is to “maintain your home’s relative humidity level between 30% and 50%”[2]. NDSU suggests aiming for “around 40 percent in the winter”, and notes that even then, on double-pane windows, condensation “should begin to appear when the outside temperature dips below zero”[3]. A cheap hygrometer tells you where you stand.
The colder it gets outside, the lower the humidity a window can take. FGIA, the window and glazing industry’s alliance, publishes this guide for a home kept at 70 °F (21 °C)[5]:
| Outside (°F) | Outside (°C) | Indoor humidity |
|---|---|---|
| 20 to 40 °F | −7 to 4 °C | 40% or less |
| 10 to 20 °F | −12 to −7 °C | 35% or less |
| 0 to 10 °F | −18 to −12 °C | 30% or less |
| −10 to 0 °F | −23 to −18 °C | 25% or less |
| −20 to −10 °F | −29 to −23 °C | 20% or less |
| Below −20 °F | Below −29 °C | 15% or less |
Set beside Health Canada’s 30–50% range[2], the table explains a lot of January mornings: a home that’s comfortable at 45% in November is too humid for its windows in a cold snap.
- Run the fans. Use the range hood when cooking, “preferably on the highest setting”, and keep the bathroom fan running “for at least 30 minutes after a shower or bath”[2]. NDSU adds: make sure fans vent outside[3].
- Turn on the HRV. If your home has a heat recovery ventilator, NRCan’s advice is to make sure it’s running[1].
- Turn down the humidifier. A humidifier set for comfort in December can be the whole problem in January.
- Look at the windows themselves. NRCan notes that high-performance ENERGY STAR windows show less condensation than ordinary ones[1]. If yours are old or failing, that’s where the money goes first.
Choosing and fitting shutters for a damp window
If one window always fogs, a few choices help. An outside mount sits on the trim instead of deep in the opening, which leaves more room for air to move across the glass. Check that the louvers clear the glass and the window handles when tilted, so you can open them for airflow. How to measure inside and outside mounts.
An outside mount is a little bigger, so it costs a little more. For a 36 × 48 in opening in our basic build (flat moulding, paint stage 1, 3½ in louvers), today’s prices are $320 – $400 inside the opening ($460 – $570 CAD in Canada), or $380 – $480 ($550 – $690 CAD) for a frame covering 2 in of trim on every side, 40 × 52 in. How far past the opening to go is your choice.
One more habit for a damp window: check the sill in the morning for the first few weeks of winter. If it’s wet under closed shutters and dry under open ones, you have your answer for that window. Rooms where moisture is part of the day, like a bathroom or a laundry room, have their own guide: shutters for humid rooms. For those rooms our 2027 aluminum line is worth a look: Aluminum doesn’t absorb moisture, so it won’t swell and shrink with the humidity the way wood does. Aluminum interior shutters.
Our shutters are painted or stained American poplar. Like any wood, they belong out of standing water, so the humidity steps above matter more than any finish.
For heat rather than moisture, see whether interior shutters save energy, and for the paint itself, how to clean and care for wood shutters.
Common questions
Do plantation shutters cause condensation?
They don’t add moisture, but like any closed covering they can stop warm room air reaching the glass, so the glass gets colder and water collects on it. Natural Resources Canada lists closed drapes and blinds among the things that block air movement at a window. Tilting the louvers open on cold nights helps.
What indoor humidity stops window condensation?
Health Canada recommends keeping relative humidity between 30% and 50%. NDSU Extension suggests about 40% in winter and notes double-pane windows can still start to fog once it’s below zero outside, so on the coldest nights you may need to go lower.
Will condensation damage wood shutters?
Water that sits on a painted sill or frame day after day will eventually lift paint and can lead to mould. Health Canada’s advice is to dry window frames and sills daily when it happens. Fix the humidity and the problem goes away; a shutter isn’t a waterproof part of the window.
How low should humidity be when it’s very cold outside?
Lower than you’d think. The window industry’s alliance, FGIA, publishes a table for a 70 °F (21 °C) home: about 40% or less when it’s 20 to 40 °F (−7 to 4 °C) outside, falling to 30% or less at 0 to 10 °F (−18 to −12 °C) and 15% or less below −20 °F (−29 °C).
Would aluminum shutters stop condensation?
No. Condensation is about the glass and the air, not the covering. A covering that doesn’t absorb moisture copes better with a wet sill, but the glass still sweats if the humidity is too high for the weather. Our aluminum interior shutters are coming in 2027; the fix for condensation is the same either way.
Sources
- Condensation (windows, doors and skylights), modified 2025-01-16, Natural Resources Canada (government)
- Guide to addressing moisture and mould indoors, Health Canada (government)
- Window condensation common problem (January 2011), North Dakota State University Extension (university extension)
- Energy management for home: condensation control (EM112), University of Missouri Extension (university extension)
- Understanding indoor condensation (homeowner resources), Fenestration and Glazing Industry Alliance (FGIA)
- Canadian Building Digest 4: Condensation on inside window surfaces (A.G. Wilson, April 1960; copy hosted at MIT), National Research Council Canada (government)