Real-world tests by Germany's ADAC reveal that windshield sunshades have a limited effect on lowering cabin air temperature but significantly protect interior surfaces from heat damage.
The motoring association conducted the study at its Technical Center in Landsberg am Lech, Germany, in 2021, measuring how various sun protection methods affect parked cars in direct sunlight.
External Covers Perform Best
The most effective solution was a traditional half cover placed over all glass surfaces and the roof from the outside.
It kept the cabin at 43°C (109.4°F), a full 10°C (18°F) cooler than an unprotected car.
A reflective film mounted on the outside of the windshield came second, reducing internal air temperature to 45°C (113°F) — an 8°C (14°F) improvement over the baseline.
The common internal accordion-style sunshade ranked third, dropping cabin temperature to 49°C (120.2°F), a modest 4°C (7.2°F) reduction.
Placing a white cloth or towel over the dashboard was the least effective method, yielding an internal temperature of 50°C (122°F).
Interior Shades Protect Surfaces
While interior shades barely affect air temperature, they make a dramatic difference on surfaces.
A shielded steering wheel can be up to 26°C (46.8°F) cooler than an exposed one.
Rear-window tinting also proved valuable.
In the test, an untinted vehicle's rear bench seat reached 57°C (134.6°F), while a tinted car held that surface to 48°C (118.4°F).
Car color also plays a role.
Black body panels reached 65°C (149°F) compared to 44°C (111.2°F) for white, creating a 5°C (9°F) difference in cabin temperature.
Experts recommend combining a quality windshield shade with rear tinting and ventilating the vehicle before turning on air conditioning.
