If rooms in your home are exposed to direct sunlight, pleasant outdoor temperatures can turn your home into a sauna-like environment.

Darkening the windows to keep the heat out is your best option, but sitting in the dark with artificial lights on in midsummer is hardly appealing. Experts have assessed various ways of protecting indoor spaces from high temperatures and developed some tips.

External blinds, shutters

Solutions mounted outside the windows are the way to go, according to Peter Schick, an engineer with Germany's consumer protection foundation Stiftung Warentest. "Anything fitted in front of the windows keeps the heat from reaching the living area. The heat that is not in a room in the first place does not have to be cooled down again at great expense," he explained.

Various systems mounted outside the window can intercept around 60% to 75% of sun rays, while indoor systems only manage up to 30%. Schick's pick: Outdoor blinds with rotatable and movable slats. They allow you to choose how much natural light you want to let into the room, depending on the intensity of the sunlight.

Roller shutters offer just as much protection as outdoor blinds, according to Heiko Ziffer, from a German industry association.

"Today, most roller shutters are made of long-lasting aluminum and offer protection from both burglars and sunlight," he said. The cassette systems of modern roller shutters also help with the thermal insulation of buildings. The only drawback is that they cost more than blinds and must be installed by experts, who charge high prices.

Curtains, all kinds of blinds

Even though curtains, Venetian blinds, roller blinds, and pleated blinds also protect against sunlight, they are not as effective as outdoor systems as they allow the light to penetrate the window. However, they are easier to install; they are ideal for tenants of residential buildings and come in different sizes and a variety of materials, from transparent to opaque to completely darkening. Manufacturers frequently specify what percentage of the sun's rays they can keep out.

Ziffer recommends pleated or folding blinds that can be clamped or glued onto the window frame with rails and are moved up and down like an accordion. They are great if you still want a little natural light in the room. Honeycomb pleated blinds, in which the double-layered fabric is welded together like a honeycomb, are particularly effective as heat protection, Ziffer said.

Sun protection foil

Another clever solution is sun protection foil that can be stuck onto the windowpane. Under ideal conditions, the films manage to keep temperatures below 26 degrees Celsius in 76% of cases, according to Schick. However, the result depends on the respective product.

Although you can technically apply the foil yourself, Schick recommends hiring an expert to do it. "The window pane must be absolutely spotless before applying the film. Creases and tiny bubbles cannot be smoothed out and impair the effectiveness," said the product tester.

Sun protection film has some disadvantages. The foil remains firmly stuck to the window panes even in winter when it would be nice to get more sunlight to heat up the room and save energy costs. "Also, the nuances of colors in the room seem different, as if you were wearing huge sunglasses," Schick said.