· by Michael Blair, Nashville's Sunrooms
A sunroom has more glass than any other room in the house, and glass is the coldest surface in it once the nights drop into the forties and thirties. In Middle Tennessee that starts in October, often after a warm, humid day, and the result is windows that stream with water first thing in the morning. It looks harmless because it dries by lunch. Over a winter, though, that daily water soaks the sills, stains the trim, grows mold in the frame corners and can rot the wall below the glass.
Why the glass gets wet
Air holds more moisture when it is warm. When room air touches a surface colder than its dew point, the extra moisture comes out as water on that surface. A sunroom at a comfortable temperature, with glass chilled by a cold night outside, is set up for exactly this, and the more humid the room air, the warmer the glass has to be to stay dry.
The frames matter as much as the glass. Many older sunrooms have aluminum frames with no thermal break, which means the metal runs straight from outside to inside and gets as cold indoors as it is outdoors. Those frames sweat even when the glass does not, and the water runs down onto the sill and the knee wall.
Where the moisture comes from
The usual culprits are plants, a door left open to a kitchen or bathroom, a hot tub or a fish tank, and people drying clothes in the one sunny room. A big one is an unvented propane or kerosene heater used to take the chill off a three-season room. Burning that fuel puts water vapor straight into the air, and a room heated that way often sweats badly on every piece of glass.
A three-season room shut off from the house has its own version. It sits cold overnight, then someone opens the door and warm, moist house air rolls in and condenses on everything. That is normal and brief. If it happens with the door closed, moisture is getting in from somewhere else, such as a damp slab, a leak at the roof tie-in or a wet crawl space below the floor.
- Houseplants, especially many of them close to the glass
- An unvented gas or kerosene space heater
- An open door to a kitchen, laundry or bathroom
- A slab without a vapor barrier, or a damp crawl space below
- A hot tub, aquarium or clothes drying in the room
What actually helps
Move the air. A ceiling fan on low, or just keeping blinds up at night so room air reaches the glass, raises the surface temperature enough to cut condensation noticeably. In a conditioned room, keep the heat on at a steady low setting instead of letting the room swing cold overnight. Take the moisture out: a dehumidifier in a sunroom with plants, and no unvented combustion heater, ever.
If that is not enough, the answer is better glass and frames. Insulated low-E glass with warm-edge spacers stays much closer to room temperature on its inner face than single-pane glass, and thermally broken or vinyl frames stop the metal sweating. On an older aluminum sunroom where both the glass and the frames are the problem, that usually means reglazing or rebuilding the room, not adding a film.
Fog between the panes is different
If the moisture is inside a double-pane unit, between the two sheets of glass, no fan or dehumidifier will touch it. The seal around that unit has failed, humid air has leaked in, and the desiccant in the spacer is used up. It will come and go with the weather and leave a permanent haze over time.
The fix is replacing the glass unit, and on many frames that can be done without replacing the frame. If one or two units have failed on a room of the same age, expect others to follow, and it is worth checking whether the frames were designed for today’s low-E glass while the glazier is there.