Condensation in a Seattle Sunroom: Why It Happens and the Design Choices That Prevent It
The short answer
Condensation on a Seattle sunroom is usually the glass doing exactly what physics says it should. Warm, moist air meets a surface colder than the dew point, and the vapor in that air turns back into water on that surface. Single glass sits close to outdoor temperature, so on a 38 degree January morning it fogs long before anything else in the room does.
That’s normal, and it isn’t a defect. The things worth worrying about are water standing in the bottom track for days, mildew creeping along the frame or the ledger, and fog trapped between two panes of a sealed window. Those are separate problems with separate fixes, and we’ll get to each of them.
We build sunrooms and glazed balconies across King, Snohomish and Pierce counties, and from November through February this is the single most common thing homeowners call us about. Most of those calls end in a five minute conversation about airflow. A few end in a site visit.
Why glass fogs, in plain terms
Air holds water vapor, and warm air holds more of it than cold air does. Cool a parcel of air far enough and it reaches a temperature where it can’t hold what it’s carrying any more. That temperature is the dew point, and any surface colder than it will collect water.
Condensation therefore needs two things at once: moisture in the air, and a cold surface. Remove either one and the fogging stops. Everything further down this page is some version of that.
The table below gives dew points at ordinary room temperatures. Find your indoor temperature, read across to your humidity, and you have the surface temperature at which water starts forming.
| Air temp | 40% humidity | 50% humidity | 60% humidity | 70% humidity |
|---|---|---|---|---|
| 60 degrees F | 35 degrees F | 41 degrees F | 46 degrees F | 50 degrees F |
| 65 degrees F | 39 degrees F | 45 degrees F | 51 degrees F | 55 degrees F |
| 70 degrees F | 44 degrees F | 50 degrees F | 56 degrees F | 60 degrees F |
| 75 degrees F | 48 degrees F | 55 degrees F | 60 degrees F | 65 degrees F |
The 65 degree row is where most Seattle sunrooms actually live. Heat the room to 65, let the humidity sit at 60 percent, and anything colder than 51 degrees ends up wet. Single glass on a December night here sits in the low 40s, so it was never going to stay clear.
Bring that same room down to 40 percent humidity and the dew point drops to 39 degrees. Now the glass stays clear unless it gets genuinely cold out. Nothing about the glass changed; only the air did.
Why Seattle is harder on glass than Denver or Phoenix
Our climate lands on close to the worst combination for condensation. Damp enough that outdoor air carries a lot of moisture, mild enough that nobody treats it as real winter. Sea-Tac averages roughly 156 days a year with measurable rain and somewhere around 37 to 38 inches of precipitation, with November and December each accounting for close to 6 inches on their own.
Winter relative humidity around Puget Sound sits in the high 80s on a typical December or January day, against something nearer 70 percent in August. Average December lows at Sea-Tac run around 36 degrees. Put those two numbers side by side and you have the whole problem.
A cold, dry climate behaves differently. Denver in January has outdoor air so dry that warming it indoors sends the relative humidity straight down, which is why people there run humidifiers all winter. Here, air at 40 degrees and 88 percent humidity brought into a 60 degree room still lands near 50 percent, and that’s plenty to fog a cold pane.
The mildness makes things worse in a way most people don’t expect. Seattle rarely gets cold enough to stop anyone using a sunroom, so the space gets heated, occupied and breathed in all winter. A sunroom in Minneapolis sits empty and cold from November to March, and an empty cold room doesn’t fog. Ours get used, which is the entire point of building them, and use means moisture.
That’s also the reasoning behind building these as three-season rooms rather than heated additions, which we go into on our Seattle sunroom and balcony glazing page.
Which side is wet tells you what is wrong
Before anything else, go out and put a hand on the glass. Where the water is sitting tells you most of what you need to know.
| Where the water is | What it means | How worried to be |
|---|---|---|
| Inside face of the glass | Humid indoor air meeting cold glass | Normal in winter, manageable with airflow |
| Outside face, early morning | Radiant cooling under a clear sky, same thing that fogs a car windshield | Harmless, burns off by mid-morning |
| Between two panes of a sealed window | The seal on that insulated unit has failed | The unit needs replacing, airflow will not help |
| Pooled in the bottom track | Runoff from the glass with nowhere to drain | Check the weep holes before anything else |
| On the frame but not the glass | Thermal bridge through an uninsulated profile | Expected on aluminum, watch for staining below it |
| On the ceiling or roof panels | Stratified warm moist air with no exhaust path | Needs air movement, worst case in the room |
Fog between panes is the one people panic about, and they’re right to. If you have insulated glass units in a sunroom roof or in fixed windows and the haze is trapped where you can’t wipe it, the seal has gone and the gas fill has leaked out. That pane will never clear, and no amount of ventilating the room will touch it.
Ceiling condensation is the one people underreact to. Warm wet air rises, and a glass or panel roof is the coldest horizontal surface up there. Drips land on furniture and flooring instead of running down a drainage channel, so the damage turns up somewhere other than where it started.
Single glass is a choice, and it comes with fogging
The frameless retractable systems we install use single tempered panes set in uninsulated aluminum profiles. Lumon states this plainly in their own technical documentation, and they’re just as direct that single panes tend to fog when warm, moist air hits the cold glass.
That sounds like a shortcoming until you understand what the system is for. These panels aren’t sealed windows. Lumon leaves gaps of roughly 2 to 3 millimeters between adjacent panes on purpose, and their own guidance calls that ventilation essential to preventing condensation and keeping the balcony structure dry. The first panel can also be locked partly open when you want more air moving through.
So the enclosure is deliberately not airtight. It blocks wind and rain while letting the space breathe. That design choice is also why these enclosures usually read as outdoor space for code and floor area purposes rather than conditioned living area, which is its own conversation and one we get into in our sunroom addition versus balcony glazing comparison.
If you want glass that genuinely doesn’t fog on the inside in January, you’re describing a different building. Insulated glass units, thermally broken frames, an insulated and conditioned floor assembly, heat tied into the house. That’s a four-season addition. It carries full energy code obligations and it usually costs several times what a glazed balcony does. Both are good products, and we build both. They just aren’t the same product, and most of the disappointment we run into comes from someone buying the first while picturing the second.
Design choices that prevent it
Each of these either takes moisture out of the air or keeps a surface above the dew point. Decided at design stage they cost nothing. Corrected after installation they cost real money, which is why this is the section to read before you sign anything.
- Keep the ventilation path clear. Those 2 to 3 millimeter gaps between panels are the system working as designed. Homeowners sometimes ask us to seal them with foam or weatherstripping to stop a draft, and we won’t do it. A sealed enclosure in this climate turns into a terrarium inside a week.
- Treat the house as a separate space. The biggest moisture source available to a glazed balcony is the heated, occupied house right next to it. Keep the existing slider or French door and treat it as a real boundary. Propping it open on a wet January evening pushes warm indoor air into a cold glass box, and that air has to give up its water somewhere.
- Keep the air moving. Still air stratifies, and the warmest, wettest layer ends up at the ceiling. A small ceiling fan on low reverse, or even a cheap floor fan pointed upward, stops the boundary layer at the glass from sitting still long enough to condense. The EPA lists increasing air movement among the standard ways to prevent condensation in its guide to mold, moisture and your home.
- Choose the roof deliberately. A glass roof gives you overhead light and also gives you the coldest surface in the room. Insulated panel roofs run warmer and fog far less, at the cost of that daylight. An acrylic cover sits somewhere between the two, and we walk through the tradeoff on our acrylic patio cover page.
- Slope the horizontal surfaces and check the weeps. Water is going to form. Where it goes afterwards is a detailing question. Sills, tracks and roof panels all need positive drainage to the outside, with weep holes clear and in the right places. A good share of the standing water calls we get turn out to be weeps packed solid with Douglas fir needles and moss.
- Watch the frame at the thermal bridge. Bare aluminum conducts heat straight through and will run colder than the glass in places. On a three-season enclosure that’s tolerable. On anything you plan to heat regularly, specify a thermally broken profile or expect the frames to sweat.
- Think about which way it faces. A north-facing enclosure under a tree canopy gets almost no solar gain to dry it out and can stay damp for days at a time. South and west exposures burn off overnight moisture by mid-morning. Siting matters more than people expect, and we covered the shaded-lot version of this in our post on building a sunroom under trees in Woodinville and Kirkland.
- Get the floor off cold ground. A bare slab on grade in Seattle wicks moisture up continuously and stays cold into June. A vapor barrier under new slabs, or a ventilated deck surface laid over the existing one, removes a moisture source most people never think to look for.
Order matters here. Airflow and the door to the house solve most cases on their own, and the rest is refinement.
The moisture you bring in yourself
A glazed balcony holds a small volume of air. It doesn’t take much to saturate it, and most of what saturates it walks in through the door.
Hot tubs are the worst offender by some distance, and they’re a popular pairing with a glass enclosure. An uncovered tub evaporates continuously, and that water goes straight to the coldest surface available. If you want a tub under glass, keep it covered whenever you aren’t in it and plan for mechanical exhaust rather than hoping the panel gaps will cope.
Plants come next. People fill these rooms with tropicals because the light is good, water thirty pots, then wonder why the glass is running. Grouping them on trays rather than watering across the whole floor helps, and so does watering in the morning rather than at night.
Drying laundry out there in winter is common in older Seattle homes, and it’s a bad idea. One load puts a couple of quarts of water into the air over a few hours.
Unvented propane and kerosene heaters deserve a specific warning. They produce water vapor as a combustion byproduct, close to a gallon for every gallon of fuel burned, so they add about as much moisture to the room as they do heat. EPA guidance names kerosene heaters among the appliances that should be vented outside. For a three-season room we steer people toward electric radiant or infrared, which add heat without adding water.
People add moisture too. Six adults at a dinner party put a meaningful amount of vapor into a 200 square foot glass box. That isn’t an argument against dinner parties, just a reason to crack a panel before everyone arrives.
Four habits that make it worse
Sealing the gaps comes up often enough to be worth repeating. Taping, foaming or weatherstripping between the panels removes the only ventilation the system has.
Heating the room in bursts does more damage than leaving it cold. Running a heater hard for two hours on Saturday and letting the space sit cold the rest of the week lifts the air’s moisture capacity on each cycle, pulls vapor off every damp surface, then dumps it back onto the glass as the room cools again. Low and steady beats hot and intermittent.
Closing it up for the winter seems sensible and isn’t. Shut the door in November, open it again in March, and you’ll find mildew on the frames and a smell that takes a weekend to scrub out. These rooms want air exchange even when nobody is using them.
Wiping the glass and stopping there treats the symptom. The towel puts that water straight back into the room’s air. If you’re drying panes every morning, the room is telling you its humidity is too high. EPA guidance treats condensation on windows as a sign of excess moisture rather than a cosmetic nuisance, and recommends drying wet materials within 24 to 48 hours to keep mold from taking hold.
When to stop managing it and call someone
Fogged glass on a cold morning doesn’t need a contractor. The following do.
Fog sealed between two panes of an insulated unit means the seal has failed, and the unit needs replacing. Check whether it’s still inside the manufacturer warranty before anyone quotes you for the work.
Water standing in the bottom track more than a day after the weather clears points to blocked or missing weep holes, or a sill that was set dead level instead of sloped. Clear the weeps first, since that resolves it most of the time.
Dark staining or soft wood at the ledger, where the enclosure meets the house, is the one that costs real money. Condensation running down and sitting against framing will rot it, and a Seattle winter is long enough to do meaningful damage in a single season. If a screwdriver sinks into that wood, get someone out to look at it.
Visible mold on frames, fabric or trim means humidity has been high for a while. EPA guidance is to keep indoor relative humidity below 60 percent, ideally between 30 and 50, and to deal with wet materials within 24 to 48 hours. Cleaning the mold without fixing the moisture only resets the clock.
Condensation that turns up suddenly in a room that was previously fine almost always means something changed: a new hot tub, a sealed gap, a door left open, a weep that clogged. Work out what changed before you spend anything.
You can reach us at (206) 600-2225 or [email protected]. More often than not we can tell you over the phone whether it needs a visit.
What we tell owners to do each season
Two jobs in October, one habit through the winter, one check in March.
In early October, before the rain settles in, clear the tracks and weep holes. Fir and cedar needles, moss, and the grit that washes off a mossy roof all collect in the bottom channel over the summer. A shop vac and a bent piece of wire handle it in about ten minutes.
While you’re at it, buy a hygrometer. They run 10 to 50 dollars at any hardware store and they turn this whole topic from guesswork into a number you can read off a display. Put it in the sunroom rather than the house. Anything consistently over 60 percent needs attention.
Through the winter, open a panel to the ventilation position whenever it’s dry out. Even an hour of real air exchange on one of those rare clear January afternoons resets the space. If there’s a fan out there, leave it running on low rather than switching it on and off.
In March, once things dry out, look hard at the ledger and the lowest 6 inches of any wood. That’s where a winter of condensation shows up first, and March is when it’s still cheap to deal with.
Moss and algae on the glass is its own maintenance question in this climate, and it’s worth doing on the same schedule since you’re already out there with a ladder.
Common questions
Is condensation in a sunroom normal?
Yes. On single glazing in a damp climate it’s expected. Condensation forms whenever a surface drops below the dew point of the air touching it, and single panes sit close to outdoor temperature. It only becomes a problem when water pools, stains, or sits against wood long enough to rot it.
How do I stop my Seattle sunroom from fogging up?
Lower the humidity or raise the surface temperature. In practice that means keeping the panel ventilation gaps clear, keeping the door to the house shut, moving the air with a fan, and keeping hot tubs covered and wet laundry out. A hygrometer reading over 60 percent tells you which lever to pull first.
Does condensation mean my sunroom was built badly?
Usually not. Fog on the inside face of single glass is physics. Fog trapped between two panes of a sealed unit is a failed seal, and water pooling in the track is normally a blocked weep hole or a sill that wasn’t sloped. Those last two are workmanship issues worth raising with whoever installed it.
Will a dehumidifier fix it?
In a ventilated enclosure, not really. You’d be dehumidifying outdoor air that keeps arriving through the panel gaps, which runs the machine constantly for very little gain. Dehumidifiers make sense in a sealed four-season addition. In a three-season glazed balcony, airflow and controlling the moisture sources do the work.
Why is there condensation on the outside of my sunroom glass?
On clear, still nights the glass radiates heat to the sky and drops below the temperature of the air around it, so dew forms on the outer face. It’s the same effect that fogs a car windshield overnight. It’s harmless and clears on its own once the sun reaches it.
Can a four-season sunroom still get condensation?
Yes, though far less often. Insulated glass units and thermally broken frames keep interior surfaces much warmer, so the dew point is rarely reached. A four-season room with a hot tub, poor ventilation or a humidifier running can still fog, because the physics hasn’t changed. Only the margin has.
Does condensation damage the glass itself?
No. Tempered glass is unaffected by water sitting on its surface. The risk is to everything underneath and around it: wood framing, fasteners, flooring, fabric and anything organic that stays damp.
Should I run the heater to stop condensation?
Gentle continuous heat helps, because warmer surfaces stay above the dew point. Avoid unvented propane or kerosene heaters, which release water vapor as they burn and add roughly as much moisture as warmth. Electric radiant and infrared heaters add heat without adding water.
Talk to us before you design it, not after
Most condensation complaints trace back to a decision made before anything was installed: the roof type, the door arrangement, a sill that wasn’t sloped, a north-facing siting nobody questioned. All of them cheap to get right on paper, all of them expensive to correct once they’re in glass.
Star Construction WA designs and installs sunrooms, glass enclosures and balcony glazing with our own crews. We handle the measure, the permits and the installation, and we’re the ones who come back if something fogs that shouldn’t. We work across Seattle, Bellevue, Mercer Island, Kirkland, Redmond, Renton, Issaquah, Everett, Mill Creek, Lynnwood and Tacoma, covering King, Snohomish and Pierce counties.
Free on-site measure and design visit anywhere in that area, with a written itemized quote within five business days.
Call (206) 600-2225, email [email protected], or request a free quote.
Worth reading next:
- Seattle sunrooms and balcony glazing, the main service page with specifications and project photos
- Can you turn a balcony into a sunroom in Seattle?
- Screened porch conversion or sunroom addition
- Sunrooms for condos and townhouses under King County HOA rules
- Project gallery and service areas