When windows run wet every morning from October on, sill mildew usually comes from that condensation, not a leak: warm indoor air meets cold glass, water runs down, and growth follows where it sits. Wipe glass daily, vent the moisture you make, clean sills on a schedule, and treat anything large, soft or fast-returning as a moisture problem.
That’s the whole of it. If you’d rather have someone else keep it up, tell us about your place and we’ll send a price for it — nothing is booked until you say so.
Why Seattle houses sweat from October to March
NOAA’s 1991–2020 climate normals for Seattle-Tacoma International Airport come to 39.34 inches of precipitation a year, about three-quarters of it falling October through March; November is wettest at 6.31 inches, and January’s average low is 37.7°F. All winter, glass is chilled from outside while indoor air stays warm and soaks up moisture from showers, cooking, laundry and breathing. Where that air meets glass colder than its dew point, vapor turns to water, runs down and sits on the sill, bottom rail and track. EPA’s guide is plain: no type of mold grows without water or moisture.
Housing decides where it shows first. Our page on house cleaning in Queen Anne describes the hill’s old houses: “Raised long before air conditioning, that stock holds damp in baths and on north-facing sills through an October-to-March wet season, and the west-facing view glass over Elliott Bay shows every streak in the nine hours of daylight January allows.” NOAA’s solar calculator agrees: about eight and a half hours of daylight on January 1, nine and a half by the 31st. A north-facing sill never gets winter sun to dry it, and original single-pane sash runs colder than double glazing. Newer Eastside builds sweat too, wherever glass is the coldest surface in the room.
Habits that cut window condensation in winter
EPA’s guide boils it down to one line, “the key to mold control is moisture control,” and four habits do most of the work:
- Wipe the glass first thing. Washington’s Department of Health says to check windows for condensation and water droplets and “wipe them up right away so mold can’t start to grow.” A squeegee and a towel take a minute; finish on the sill.
- Vent what you make. EPA’s list: the bathroom fan or an open window when showering, exhaust fans when cooking or running the dishwasher, and clothes dryers vented outside. Lid your simmering pots, too.
- Give cold walls air. The Department of Health notes mold also grows where warm, moist air condenses on cooler walls, behind dressers and headboards or in closets with things stored against the wall. Its fix: keep air moving along exterior walls and pull large objects a few inches away.
- Keep heat and humidity in range. EPA suggests indoor humidity below 60 percent, ideally 30 to 50, and lists raising the air temperature as a way to prevent condensation. A shut, unheated north room is the likeliest place for sills to sweat; a hygrometer shows where you stand.
Cleaning old sills, tracks and caulk without damage
EPA’s method for hard surfaces fits in a line: “Scrub mold off hard surfaces with detergent and water, and dry completely.” On a century sash window, go gentle. EPA notes that homes built before 1978 are more likely to have lead-based paint and that worn paint on friction surfaces like window sills can shed chips and dust; it advises wiping sills with a damp cloth, never steel wool, scouring pads or abrasive cleaners. If paint is flaking, stop scrubbing: for chipping paint, EPA’s advice is to pick up the chips carefully with a damp paper towel and then wipe the spot with a clean wet one, and it says anyone paid to disturb paint in pre-1978 housing must be certified and trained in lead-safe work practices.
Tracks catch what runs off the glass: vacuum the dry grit first, then wash, dry and clear the weep holes; the full method is in how to clean window tracks and screens. Both agencies say never to paint or caulk over mold. A caulk bead that’s black inside, not just on top, gets cut out and replaced once the joint is dry. Original bath tile gets the sill treatment: mild detergent, a soft brush, rinse, dry.
Bleach is the reflex, not the method. EPA says using a biocide such as chlorine bleach “is not recommended as a routine practice during mold cleanup,” and that dead mold may still cause allergic reactions in some people, so it must be removed, not just killed. If you use bleach anyway, follow the label, ventilate, and never combine it with ammonia or an acid; the full list is in cleaning products you should never mix. Then dry the sill.
Where cleaning stops, and what we won’t claim
Size is the first line. EPA says a moldy patch under about 10 square feet, roughly 3 feet by 3, is in most cases a job you can handle yourself; past that, it points to its remediation guidance, and the Department of Health to restoration professionals. Soft or blistering plaster under the window, a musty smell you can’t trace, or growth back within days means water is reaching what a cloth can’t; EPA warns that mold will most likely return if the water problem isn’t fixed. Water inside the frame after rain, with dry glass, is a leak: a roofer, a window installer or your landlord.
A cleaning visit is not mold remediation. We don’t test for or identify mold or cut out drywall; no page of ours offers that work, so we won’t sell it. Cleaning owns the surface (sill, track, caulk line, tile) and the routine that keeps it wiped and dry. For anything deeper, the order of calls is in who to call for mold: a cleaning service or a remediation company.
Renting? Under RCW 59.18.060(14), Washington landlords must give tenants Department of Health–approved information on indoor mold and how to control it, in writing or posted in a visible public spot, and new tenants get it when the lease or rental agreement is signed. The department’s rule of thumb: “Generally, fixing leaks is the landlord’s responsibility and reducing condensation is the renter’s responsibility.” As the department asks, report water leaks or moisture buildup from building problems to the landlord promptly and in writing. This is general information, not legal advice.
A cleaning calendar for Seattle’s wet and dry seasons
Our page on house cleaning in Seattle describes the range: “Queen Anne and Magnolia hold century homes with tall sash windows, deep trim and original tile, while the Eastside tech corridor keeps adding new builds with wide glass and hard floors in Bellevue, Kirkland and Redmond.” Both kinds sweat on the same calendar:
- October through March: the recurring visit carries the load. The page names what this weather leaves: “mildew in baths and on north-facing sills,” “grit at every entry,” and “low winter light that shows every streak on glass.” Its standard visit includes “Dusting of reachable surfaces, shelves and sills” and “Bathrooms: toilets, tubs, showers, sinks and mirrors.” The morning wipe stays yours.
- April or May: the spring reset. Our deep cleaning in Seattle page describes the seasonal reset as “Twice a year, once before guests and once after the season that tracks the most into the house,” and here that season is the wet one. The deep pass adds “Blinds and window sills detailed” and “Tile and grout scrubbed in kitchen and baths.”
- June through September: the dry months. NOAA’s normals give July 0.60 inch of precipitation and August 0.97: open windows, a drying house, and the time for the second deep clean, before the rain and holiday guests return. That page notes “Appliance interiors and interior windows are priced separately so nobody pays for them by default,” so ask for the glass by name.
How often? The residential page answers: “Every two weeks is where most households land — enough to keep dust, floors and bathrooms from getting ahead of you. Weekly makes sense with pets, small children, or frequent guests.” In a sweating house, lean shorter through the wet months. On price, its rule is “Price follows the work, not a guess”: you describe the job, and the number comes back as “A price in writing before we schedule.”
FAQ
Is the black on my window sill mold?
Treat it as mold; condensation-fed speckling is the surface growth usually called mildew. EPA says sampling is unnecessary in most cases when mold is visible, and the Department of Health agrees. Clean it, dry it, cut the moisture, and past about 10 square feet, call a professional.
Should I use bleach on it?
Not first. EPA doesn’t recommend biocides like chlorine bleach as routine mold cleanup; detergent and water, then drying completely, is its method. The Department of Health lists diluted bleach only as an optional step. If you use it, follow the label, ventilate, and never mix it with ammonia or acids.
Will a dehumidifier help?
Yes, as one part of cutting the moisture. EPA’s target is indoor humidity below 60 percent, ideally 30 to 50, and it lists air conditioners and dehumidifiers among the tools; in a house without air conditioning, that leaves the dehumidifier. It pulls water from the air but doesn’t remove growth already there, so keep up the morning wipe and the cleaning schedule.
How often should sills be wiped in winter?
Every morning the glass is wet; the Department of Health says to wipe condensation up right away so mold can’t start to grow. Clean sills, tracks and caulk lines properly every week or two through the wet season, then dry them. A sill that needs more is pointing at the moisture source.