How much light do dirty windows block?

You will find claims online that dirty windows block 20 to 40 percent of your daylight. We went looking for the study behind that number and could not find one for house windows. Here is what the measurements actually say, and where dirt really does cost you light.

What the measurements show

The best field study we found measured 430 windows across a large city in the UK. Its conclusion: the light lost to dirt on an upright window “did not usually exceed 10%.” The biggest factors were how the building was used, how steeply the glass was tilted, and whether an overhang sheltered it from rain.

The UK’s daylighting guide turns findings like that into the allowances architects design with. For regularly cleaned glass, it assumes these losses:

Where the big numbers come from

The 20 to 40 percent figures are real measurements, of something else. Thirty days of tropical dust on greenhouse plastic film cut the light getting through from about 88% to about 76%. Greenhouse plastic in Riyadh lost 15% of the light plants use after three months. Those are flat-ish plastic sheets in dusty climates, not a vertical pane of glass that rain rinses a few times a month.

So where does dirt cost you light at home?

• Skylights and roof glass. Dust settles on anything close to flat, and the allowances above double.
• Windows under a deck, porch or deep eave. Rain never reaches them to rinse them, and research on glass sheltered from rain found it simply keeps getting dirtier.
• The side of the house facing a gravel road or a field. More dust, more often.
• Sprinkler-spotted panes. The white haze of hard-water spots scatters light and blurs the view, and it does not rinse off.

Why a few percent can still matter

Most of the year, a few percent is not something you would notice. In a Bozeman December it is a few percent of very little: the sun is up for under nine hours and never climbs far above the mountains. Winter light in Bozeman covers what that means indoors.

Plants notice too. Greenhouse growers have measured that, for crops short of light, each 1% more light brings roughly 0.5 to 1% more growth. That is a greenhouse finding, but a houseplant in a Montana winter is short of light in the same way. Houseplants in a Montana winter has the practical side.

And clean glass is about the view as much as the light. You can measure a 5% loss. You can see a sprinkler-hazed picture window from across the room.