Why South and West Windows Get Dirty Faster

If some windows in your house look bad months before the rest, it is usually not the glass and not the washing. It is which way they face. South and west windows take more sun, more wind driven rain and more baked on film than the north and east side of the same house, and on a lot of Twin Cities houses that means two sides want cleaning twice a year while the other two are fine with once.

Weather here mostly arrives from the west

Storms cross Minnesota west to east, so the west wall of a house takes the most wind driven rain, and the south wall takes a good share of it too. Rain by itself is not the problem. What is in it is. Wind picks up road grit, field dust, pollen and whatever is blowing off the neighbor's roof, and the west side of the house is what it hits first.

That is also why rain almost never leaves a window better than it found it. There is more on that in the piece on whether rain cleans your windows.

Sun does not clean glass, it bakes things onto it

A south or west pane spends hours a day in direct sun. Anything sitting on it when the sun comes around, tree pollen in May, sap, dust, dried sprinkler water, gets heated and dried in place rather than staying loose. Do that a few hundred times and the film stops rinsing off and starts needing to be worked off.

Here in St. Paul the two sides get their sun at different times of year. The south wall is in sun most of the short winter day, because the sun stays low and to the south. The west wall gets the hot end of a summer afternoon. So the south glass takes the winter beating and the west glass takes the summer one, which is part of why those two sides rarely look good at the same time as the rest of the house.

The north side has the opposite problem

North windows get very little direct sun, so nothing bakes on. They have the other issue: they stay wet. Dew and rain sit there into the morning, and in a shaded spot under eaves or a big tree the glass and the frame can stay damp most of the day in spring and fall.

Slow drying is what organic growth wants. North glass tends toward a dull greenish or gray cast in the corners and along the bottom of the pane rather than the hard, sun baked film you get on the south side. It usually comes off more easily, but it comes back on its own schedule, which has nothing to do with how sunny the summer was.

The street side counts as its own exposure

Whichever way it faces, the side of the house along the road is doing something different from the other three. Traffic throws up a fine grit and oily film, and from about November it throws up salt too. On a corner lot or a busy street, that side can be the dirtiest on the house even if it faces north. Winter is the worst of it, and there is a separate guide to salt stains on windows and siding.

What to do with this

Stop thinking about the house as one job. Most places here settle into a rhythm like this:

• South and west, twice a year. These are the panes worth doing in spring and again in fall.
• East, once is usually plenty. Morning sun dries the glass early and the afternoon leaves it alone.
• North, once a year, and watch the corners. The growth starts at the edges of the pane, so that is where to look rather than in the middle.
• The street side on its own schedule, which in practice means after the roads dry out in spring.

One caution before you decide a side needs doing. Low fall and winter sun rakes across a pane and shows every bit of film that was invisible in June, which fools a lot of people into thinking the glass got worse overnight. It is worth checking that before you get the bucket out, and there is a whole piece on why clean windows look dirty in sunlight. The general timing question is covered in how often to clean windows in Minnesota.

If you would rather hand off the two hard sides and keep the easy ones yourself, that is a reasonable way to do it. Ask for a quote or pick a time on the booking page.