Why There Is a Film on the Inside of Your Windows

The greasy film on the inside of your windows, the kind you can draw a finger through, is mostly cooking oil, soot from candles and the furnace, and the oily residue that comes off vinyl, carpet and furniture. It collects on the window because glass is the coldest surface in a heated room. It is a film, not dirt, so it needs to be broken up before it can be wiped off.

Where it comes from

Almost all of it is made inside the house, and most of it is cooking. Frying or searing anything throws a fine oil mist into the air that travels well past the kitchen and settles wherever the air slows down. A range hood that vents outside cuts this down a lot. Many only recirculate through a filter, which does very little.

After that it is combustion and dust. Candles, a fireplace and a wood stove all put out fine soot. Forced air heat keeps furnace dust, lint, skin and pet dander moving around the house all winter and drops it where air slows. Vinyl flooring, carpet backing and pressboard furniture give off a trace of oily vapor that condenses on any cool surface. Smoking indoors is the extreme version of the same process and leaves a film that is hard to remove.

Why it lands on the glass and not the walls

It lands on the walls too. You just cannot see it there.

Glass is the coldest surface in the room by a wide margin in a Minnesota winter, and warm air carrying particles cools when it touches the pane, slows down and gives up what it was carrying. A cool pane also holds a very thin layer of moisture that oily particles stick to and spread into instead of bouncing off. Walls are warmer, porous and painted flat, so the same deposit disappears into the texture.

On a window you look through it rather than at it, which is why a pane that seemed fine in July looks awful at four in the afternoon in January, when the sun is low enough to rake straight across the glass.

Why a Minnesota winter is the worst of it

Five months of closed windows is the whole answer. No window gets opened between November and April, so there is very little air exchange, and whatever the house generates stays in it and keeps landing on the same cold surfaces. The furnace blower runs for months, which keeps everything circulating rather than settling to the floor.

A newer, tighter, better sealed house is often worse rather than better, because an older leaky one dilutes everything with outside air. The deep cold makes it worse too. The bigger the difference between room air and the pane, the colder the glass and the more readily anything airborne deposits on it. Interior glass is usually the dirtier side by midwinter, which is also why cold weather does not end the window cleaning season.

How to get it off without smearing it

The usual mistake is reaching for glass cleaner and a paper towel. Spray glass cleaner is mostly water, and water does not cut oil, so you end up pushing the film around the pane and polishing it into streaks.

Two passes fixes it. The first one is about breaking the oil, so use warm water with a single drop of dish soap and a cloth, and let it do the work rather than scrubbing. The second pass picks up what the first one loosened, with a clean damp cloth followed by a dry one. Skipping the second pass is what leaves the haze behind.

• Use two microfiber cloths, one damp and one dry, not paper towels.
• One drop of soap, not a squeeze. Too much soap is its own streak problem, covered in how to clean windows without streaks.
• Do the sills and the tracks while you are there. The same film settles in them and holds grit.
• Keep abrasive pads and powders off the glass.

When it is not film at all

Three things get mistaken for this. If the haze sits between two panes and comes and goes with the weather, the seal on that insulated unit has failed and no cleaning on either side can reach it, which is glass replacement. If it is water rather than film, that is humidity, and condensation on windows in winter covers what to do about it. If a real film is back within a few weeks of a careful cleaning, the cause is ventilation and sources, not cleaning, and cleaning more often will not fix that.

This is also the half of the job most people should just do themselves. Interior glass needs no ladder, no pole and no special tool, so there is no good reason to pay for it on its own. Where it is worth handing off is tall glass over a stairwell, a wall of windows nobody wants to spend a Saturday on, or wanting both sides done at once before company arrives. Window cleaning starts at $99, backed by the Gleam Guarantee, in St. Paul, Minneapolis and the suburbs out to about 25 miles. You can request a free quote online or book a free 10-minute consultation.