What’s a renter-friendly way to keep sunny windows from heating up a room?

Shade the glass from outside if you can do it safely and without permanent fittings. Even a light-colored shade can stop a lot of direct sun before it warms the window. A board or fabric taped outside might work briefly, but wind, rain, and falling material make that a poor bet on an upper floor. Check with the landlord before attaching anything, and don’t lean out to install it.

If outside shading isn’t practical, a light-filtering blind or sheer curtain is an easier compromise. It won’t block heat as well as exterior shade, but you keep more daylight than with blackout curtains.

For film, look at the window’s glazing type and the film maker’s compatibility guidance before buying. Some reflective films applied indoors can increase heat stress on double glazing, so don’t assume a film is safe just because it’s removable. The product’s visible-light transmission tells you roughly how much daylight remains, while its solar heat performance gives you a better idea of how much sun it rejects. Those two goals trade off against each other.

Foam board faced with reflective material can block sun effectively if mounted outside, but it blocks the view and light too. It needs a secure, weather-suitable fixing, not just tape that could let go in heat or wind. For a rental, a purpose-made removable shade may be less hassle.

Don’t overlook the AC window opening. If the hose panel or temporary seal has gaps, warm air can leak back in, so a close-fitting removable panel around the hose may help. Keep the hose clear and follow the unit’s instructions.

When outdoor air is cooler, airing the room then and closing the windows and shades before the sun hits can also help. For windows you can’t shade outside, a pale blind or curtain is a reasonable low-commitment option, even if it won’t perform like exterior shading.