Window film, shading and insulation for cooling

Everything else on this site is about moving heat efficiently. This page is about the heat that never arrives.

Most articles frame shading as a comfort measure. The more useful frame is that it changes the equipment you need, and equipment size is the dominant term in running cost.

Look at what a single capacity step costs across the certified units:

Dropping from 12,000 BTU to 8,000 saves 187.5 kWh a year, or $34. That is not a one-off. It repeats every year the unit is in service, on top of buying a smaller and usually cheaper machine.

And solar gain is precisely what pushes a room up a size. The official sizing guidance says so directly:

Handle the sun and that adjustment does not apply. For a lot of rooms, that single line is the difference between two capacity steps. The full sizing method.

The single most important thing to understand about shading is where it sits relative to the glass.

Sunlight that passes through a window has already delivered its energy into the room. An internal blind absorbs it and re-radiates it — as heat, inside. It reduces glare and it helps a little, but the energy is already indoors.

Shading on the outside of the glass stops the light arriving at all. The energy is reflected or absorbed outdoors, where the air carries it away.

• External shutters, awnings and shade sails are the most effective option, in that order of permanence.
• Planting and trees shade seasonally, which is convenient: deciduous cover blocks summer sun and admits winter sun.
• Window film sits at the glass, so it intercepts before most of the energy is in the room. A middle option and often the only one available to renters or in apartments.
• Internal blinds and curtains are the weakest but the cheapest. Light-colored and reflective is better than dark.

Film is worth its own section because its trade-offs are less obvious than a curtain’s.

• It works year round, including winter. The same film that blocks July sun blocks January sun, and in a heating-dominated climate that is a genuine cost on the other side of the year.
• It is effectively permanent. Unlike a blind you can raise, film is a decision about the window rather than a setting.
• It changes how the room looks. Both the light coming in and, with reflective types, the appearance from outside.
• It can conflict with glazing warranties. Applying film to some double- and triple-glazed units affects manufacturer coverage, and thermal stress on the glass is a real consideration. Check before buying.
• Performance claims vary enormously between products. We hold no certified dataset for film, so treat the manufacturer’s own figures as the source and read them carefully.

1. Close blinds before the sun is on the glass. Free, and the timing is the whole trick — closing them after the room has heated does nothing.
2. Move what is in the sun path. A desk, a bed, a pet bed. Free, and it solves the comfort problem without touching the load.
3. Seal the obvious leaks. Gaps around a window unit, uninsulated loft hatches, and knee-wall access panels are the usual ones.
4. Add external shade where you can. Awnings, shutters, planting. The highest-impact step available to most homes.
5. Consider film if external shading is not possible. Renters, apartments, and windows nothing can be fixed to.
6. Then size the equipment. With the load reduced, you may be a full capacity step lower — $34 a year, every year.