Window Condensation and Mold in Post Falls

Condensation forms when humid indoor air touches a surface below its dew point. In a heated house in January the coldest surfaces are the window glass and the frame, so that is where water appears first.

Which means the window is a gauge, not a fault. If it is running with water, the air in that room is carrying more moisture than the house can handle, and the window is simply the first thing to report it.

The new-construction version, which is most of Post Falls

Post Falls has been among the fastest-growing cities in the country and a large share of its housing is recent. Modern envelopes are tight, which is good for energy and hard on moisture. An older leaky house exchanged its air several times an hour by accident. A new one does not, so the moisture from showering, cooking, laundry and breathing stays inside unless something removes it deliberately.

In practice that means the bath fans and the range hood are now load-bearing equipment rather than conveniences. A bath fan that runs for the length of the shower and shuts off has not removed the moisture. It needs to run twenty to thirty minutes afterward, and it needs to actually terminate outside rather than in the attic.

What else is collecting water, out of sight

This is the part that matters more than the window. If the glass is wet, every other cold surface in the assembly is also at or near dew point, and most of those are hidden:

• Rim joists at the top of the foundation wall, where the floor framing sits. See rim joist condensation.
• Exterior-wall closets, where a closed door and stored clothing block air movement against a cold wall. See mold in a closet on an exterior wall.
• Attic sheathing, if warm moist air is reaching the attic through leaks and bath fan terminations. See mold on attic sheathing.
• Behind furniture on exterior walls, especially in corners.

Wiping the sill treats the symptom you can see and none of the ones you cannot.

The source list, in order of how much water they add

1. A crawl space with bare soil or a torn vapor barrier
2. A dryer venting into the building rather than outside
3. Showers without adequate fan runtime
4. Cooking without a hood that vents outdoors
5. Unvented combustion appliances
6. Large numbers of houseplants or an aquarium in one room

The targets

Through a Post Falls winter, indoor relative humidity in the high twenties to mid thirties is generally comfortable and does not condense. As outdoor temperatures drop, that number has to come down with them. A cheap hygrometer in the room with the worst windows is the most useful ten dollars you can spend on this problem.

When to call

Call when black growth appears on the frames or the sill and comes back after cleaning, when it is on the drywall beside the window rather than just the frame, when a closet or a corner on the same wall smells, or when you have corrected ventilation and humidity and it is still happening. That last case usually means water is in the assembly for a reason other than condensation.

Pricing

A standard inspection is $425. That covers the visit, a moisture and visual survey, one indoor air sample, an outdoor control taken the same day and accredited lab analysis, with lab fees in the price rather than added afterward. Post Falls is about thirty minutes from our base in Spokane Valley and there is no travel charge for Kootenai County.

Why an independent tester

We test and never remediate. We take no referral fees from removal companies and keep no preferred contractor list, so there is no job at the end of the report that the report needs to justify. Idaho licenses neither mold assessment nor mold remediation, which means the separation is something a company chooses rather than something the state enforces. See Idaho mold testing rules and why we do not remediate.

Frequently asked questions

Is window condensation actually mold? Not by itself. It is liquid water, and water on a surface long enough grows things. Black speckling on the frame or sill that returns after cleaning means growth has established, and growth on the drywall beside the window rather than on the frame usually means the moisture is in the wall assembly, not just on the glass.

My house is brand new. Why does it do this? Because it is tight. New construction does not leak air the way older houses did, so moisture from showering, cooking and laundry stays inside unless a fan removes it on purpose. Bath fan runtime and a range hood that vents outdoors matter far more in a new Post Falls house than they did in a 1960s one.

What indoor humidity should I be running in winter? High twenties to mid thirties is usually comfortable without condensing, and the number needs to come down as outdoor temperatures drop. A ten-dollar hygrometer in the worst room tells you more than any amount of guessing.

Will new windows fix it? They move the problem rather than solve it. Better glass raises the temperature of that surface so it stops condensing, and the moisture then finds the next coldest surface, which is usually inside a wall or an attic where you will not see it for a year or two.