Shutters, Blinds, Curtains: What Blocks Heat?
A closed curtain on a sunny window feels like protection, but by the time sunlight reaches the fabric, most of the damage is already done. The rule that decides everything is simple: heat must be stopped before the glass, not after. That one sentence explains why a battered wooden shutter outperforms the most elegant blackout curtain.
The greenhouse rule: stop the sun before the glass
Window glass has a convenient trick and an inconvenient one. It lets sunlight through easily — but when that light hits your floor, your sofa or your dark curtain, it’s absorbed and re-emitted as heat, and that heat is largely trapped inside. Your window works like a greenhouse: radiation in, warmth stuck.
So the position of the barrier matters more than its quality. A shading device outside the window reflects and absorbs sunlight in the outdoor air, where the heat blows away. The same device inside absorbs sunlight that has already crossed the glass — it then sits there as a warm radiator, gently heating the very room it was supposed to protect.
The ranking, from best to worst
| Option | Where it stops the sun | Verdict |
|---|---|---|
| Exterior shutters (roller or hinged) | Before the glass | Excellent — blocks most solar gain |
| Exterior blinds, awnings | Before the glass | Excellent, and you can keep some daylight |
| Trees, pergolas, balcony overhangs | Well before the glass | Great where geometry allows |
| Interior blinds, light/reflective face | After the glass | Moderate — reflects part back out |
| Interior curtains, dark and heavy | After the glass | Weak — absorbs and re-radiates indoors |
A dark curtain against a sunny window is the worst of the indoor options: it soaks up the light and turns the window bay into a heater. If you must shade from inside, pale or reflective surfaces facing the glass bounce a useful share of the radiation back out before it converts to heat.
Renting, or no shutters? What still helps
Not everyone can bolt hardware to the façade. In rough order of usefulness:
- Reflective films or panels against the glass reject part of the radiation before absorption.
- Light-coloured interior blinds closed during sun hours, ideally with an air gap to the window.
- Timing beats equipment: shade the east side in the morning, south at midday, west in the late afternoon — and keep the windows themselves shut while the outdoor air is hotter than yours, as covered in the heatwave open-or-shut question.
None of this matches exterior shading, but combined they take a real bite out of the afternoon peak.
Shading is half the job
Blocking the sun stops your home from gaining heat; it doesn’t remove the heat already stored in walls and furniture. Buildings have thermal inertia — they release the day’s heat slowly through the evening — so shading only wins if the second half of the routine happens too: opening everything at night, when the outside air is cooler and holds less water than the inside air, to flush the stored heat out.
Shut and shaded by day, wide open by night. Each half covers for the other, and together they’re most of what a heatwave survival plan without AC amounts to.
Let the app watch for you
Shutters you can manage by looking at the sun; the open-or-shut half is trickier, because it depends on temperature and on how much water the outside air holds. Open/Shut tracks both for your home and tells you the moment opening starts helping. Free, private, no account.