Parked in the sun: how hot does the cabin get, and what actually helps? Pick your car, colors, and window crack, and compare four scenarios - closed, reflective windshield blanket, cracked windows, and both - across a range of sunny-day temperatures.
The diagram is labeled with the same symbols as the input fields below.
Four cabins warm from ambient simultaneously - closed, blanket, cracked, and both - racing to the exact steady temperatures the chart predicts.
This preview solves a handful of lumped nodes. The NovaThermal engine behind ThermalResults.com (coming soon) runs the same physics on tens of thousands of nodes - full transients with phase change, radiation, fluid loops, and Monte-Carlo design envelopes, GPU-accelerated at 400× real-solver speed - and hands you review-ready margin reports.
A parked car is a greenhouse with seats: sunlight enters through the glass, gets absorbed by the dark interior, and the resulting long-wave heat cannot radiate back out through glazing. The balance settles where losses through the shell and any ventilation equal the solar input - typically 25-30 C above ambient for a dark closed sedan, which is how a pleasant 25 C afternoon produces a 55 C cabin.
The two cheap countermeasures attack different terms, and that is why the comparison chart is interesting. The reflective windshield blanket cuts the INPUT: the windshield is the largest, most sun-facing pane, and bouncing ~85% of its share back out removes the single biggest gain before it becomes heat. Cracked windows raise the LOSS: hot air buoyantly vents out the top of the gap while cooler air feeds in below - but the driving force is the very temperature rise you are fighting, so venting alone can only shave a handful of degrees. Stacking both works best, and the model shows the blanket doing the heavier lifting in every scenario.
Honest limits: this is a steady, lumped estimate with no breeze, no clouds, no transient warm-up curve, and it reports average cabin AIR - a black dashboard under glass can exceed 80 C while the air reads 55. And the safety point outranks every number here: fatal heatstroke occurs in cracked-window cars at modest outdoor temperatures. The chart is for comfort and curiosity, never for judging occupancy safety.
Published measurements and this model agree on roughly 5-10 C for cabin AIR in bright sun - and considerably more for dashboard and seat touch temperatures, which is where a shade earns its keep.
A few degrees: buoyancy-driven flow through a 2-3 cm crack removes some hot air, but the flow is weak because it is powered by the same temperature difference you are trying to reduce. It never makes a sunny parked car safe for children or pets.
Yes, twice: a white or silver shell absorbs a third of the solar load a black one does, and a light interior absorbs less of what comes through the glass. This model shows a white / light-interior car running visibly cooler than a black / dark-leather one in every scenario.