Summer turns windows into ovens.
You draw the curtains.
The goal is simple: stop the heat.
Physics gets tricky when you add a second layer.
Here is the scenario.
You have roller blinds installed right against the glass. Covering those are heavy curtains.
One layer stops half the energy. That’s the baseline. The fabric heats up, absorbs the solar radiation, and then radiates that heat. Half bounces back out the window. The other half moves into the room.
Now, layer the system.
Blinds on the glass. Curtains on top of the blinds.
Common logic suggests a simple multiplication.
If the first layer blocks 50%, it lets 50% pass. The second layer takes that remaining 50% and blocks 50% of that. 50% of 50% is 25%.
So, 25% of the solar energy gets into the room, right?
Wrong.
The math fails because it ignores where the heat actually goes.
Think about equilibrium.
Both layers get hot. Both radiate heat in two directions: outward to the window and inward to the room.
The blinds take the sun’s energy. Half reflects out. Half moves to the curtains.
But the curtains don’t just sit there. They are hot, too. They absorb heat from the blinds and from direct sun leaking through. They radiate 50% of their heat back out. And 50% in.
The key isn’t just the first pass. It’s the back-and-forth.
The heat trapped between the two layers gets trapped again and again, shifting toward equilibrium.
The answer is higher than 25%.
It’s actually 33%.
One third of the sun’s energy enters the room.
Why?
Because the inner layer (the curtains) radiates half its heat into the room. But that heat comes from the energy passing through the outer layer. The system creates a bottleneck that forces a larger portion of the total absorbed energy into the interior space than a single linear block would suggest.
Don’t take my word for it. Draw a diagram.
Start with 100 units of solar energy.
- Blinds : Absorb 100. Radiate 50 out, 50 in.
- Curtains : Receive 50 from blinds. Absorb it. Now they have 50 units. They radiate 25 out (toward the blinds) and 25 in (toward the room).
Wait.
If we stop there, it looks like 25 units enter.
But the heat radiating back from the curtains toward the blinds hits the blinds.
