system
Given the following post, respond as if you're commenting on Reddit:
human
What are the efficiencies of different window coverings?

I have large windows facing south west which during the summer heat up my apartment dramatically.  After spending way too much time deciding which window covering to get I ended up going for white blinds with 2" slates.  They help greatly but I am curious which type of window covering would have been more efficient and how various types compare to others.

Does anyone understand the science behind window coverings for reflecting solar heat?
gpt
if you want just a quick advice: stickytaping aluminium foil on the inside of at least half of the window works quite well (by experience of many students with west-facing windows in our dorm).

i tried to find a solution to similar problem before: given an isomat with aluminium foil on one side and black foam on the other, which side would be better to sit on to isolate your butt from cold ground. it turns out, that the problem isn't so easy to solve... the isolating properties should be different due to different thermal conductivity and thermal capacity of the materials, but at least using the first approximation calculation (the foam is absolutely black body and aluminium foil is absorbing nothing and reflecting 99% of heat) it doesn't seam to matter which side you sit on.

using the same approach to model the window and cover: window reflects ~80% (R_w ) of heat and the rest goes through. the cover _behind window_ reflects 90% (R_c) of heat back, absorbs remaining 10% and radiates it to both sides equally. hot air in between the cover and the window and the heat absorbed by the window glass are neglected... reflecting the heat there and back between the cover and the window in this setup reflects

R_w + (1-R_w)*[R_c*(1-R_w) + (1-R_w)*0.5*(1-R_c)] + (1-R_w)*{R_w*[...]} + (1-R_w)*{R_w^2*[...]} + ...

of the incoming heat back outside. for given numbers, 99% of the incoming heat is reflected back outside by the window and the cover. the above expression depends on reflective properties of the cover lineary with constant term for properties of the window. **this is just a complicated way of saying that the more reflecting and less-heat absorbing cover you have (behind the window), the better** (hence, alufoil with no space for the neglected air). it also says, that the properties of the window have much bigger impact on your heat comfort than the covers.