The title pretty much says it. In regards to the amount of heat that would be transmitted to the interior of a room from a window.
This is not a question of which will let more light through, as the window is already covered on the inside with a dark cloth.
I think that a window with aluminum foil covering the glass (on the inside) would reflect the majority of the heat back out the window. My SO thinks that ‘solar film’ would do a better job. Her reasoning is that the aluminum foil will ‘heat up’, and thus transfer the heat. So I guess the question is… Which would be better, foil or film (in combination with the dark cloth)?
Neither is a significant insulator, they are both radiant barriers.
Very shiny aluminum foil is among the best radiant barriers available. Installed on the inside of the window with the shiny side facing inside it will outperform the solar film in preventing heat from radiating into the room from outside.
Solar film will be slightly less efficient but it offers the added benefit of still allowing visible light to wnter the room and generally being a window instead of a very low r-value wall.
Note that this is already absorbing most of the heat from the sunlight, and keeping it on the inside of your room. You might want to consider replacing it with a white or light colored cloth.
The best thing would be to stop the sunlight from hitting the glass window in the first place. E.g. some type of opaque, reflective film or coating that you could apply to the outside. Or plant a tree in front of the window.
If you can only place things inside the window, the more reflective the better. Most of the energy in sunlight is in visible light, so you want to choose something that’s most reflective in visible light. Aluminum foil is very good, though I think aluminized mylar would be even better.
And I have to disagree with Emtar KronJonDerSohn; if you use aluminum foil, I think the shiny side should face the outside.
My information is correct. In radiant barrier applications the shiny side faces the airspace regardless of which direction heat is being transferred.
Why is that? I am interested because I want to insulate the server room I look after. (It has a south-facing window… stupid I know, but i have to live with it.)
Shiny finishes radiate heat poorly. If the foil is installed against a solid object such as the window pane the foil will heat through conduction and radiate less heat in to the room from the shiny side than it will from the dull side.
The critical property is emissivity. As noted earlier, aluminium foil has a very low emissivity (about 0.04) which is as good as you can do with any generally available material.
A perfect black body has an emissivity of 1. Now the interesting fact. Glass has an emissivity of about 0.9. Ie, not much different to a black body. If a plain glass window gets hot it radiates heat into your house very effectively. This is why reducing the emissivity of the inside face is so useful.
There are emissivity reducing coatings for glass. Essentially they make the glass behave like a mirror in the infra-red - where the heat radiating is occurring. Solar films will do this in addition to blocking radiation, and low-e glass applies a factory coating to the glass. It makes a quite remarkable difference. Not as effective as a pure aluminium foil however.
Thank you all for your responses. 
Allow me to ‘clarify’ the scenario, slightly.
Window with aluminum foil applied directly to inside of window glass. A cloth pinned to the wall on the inside which effectively provides an air space of approx. 2-1/2" between cloth and foil.
Would this be the best ‘setup’, as opposed to solar film and pinned up cloth?
I’m not concerned with light transmission, only heat.
The aluminium foil is likely to be better. As noted, it needs to shiny surface in.
The air gap is too big. You need to get the gap down to reduce the effectiveness, or if you can get really close, prevent entirely, convection cells from moving heat across. Ideally you want half an inch, which is hard to do. Multiple small airgaps will work better still.
Are you willing to put up an alternate product, like 2" thick polystyrene insulation? Assuming, of course that you don’t intend to open the window.
Interesting experiment you can try: Put a sheet of aluminum foil in the oven, long enough to bake something (350 degrees for 25 minutes is what I’m most familiar with). Take the foil out, and handle it. It’s not even warm.
I do this all the time when I’m making instant pizza.
But the amount of heat absorbed by the window+film is not constant. Most of the heat is in the form of visible light, so shiny side out should minimize the total heat absorbed.
Also, with the shiny side facing in, the matte side radiates towards the window, and since this radiation is IR, most of it is absorbed by the window and end up heating the air through conduction.
Yes, if we had a piece of glass with a semi-transparent, semi-reflective metallic coating on one side, I agree that the coated side should be on the inside. The total amount of heat absorbed is roughly the same for both cases. And you want the glass to radiate IR to the outside, and not in.
The situation with the aluminum foil is much more complicated. It’s a separate layer that we’re putting on the inside of the window. And the two sides have different absorptivity as well as different emissivity.
If both sides had the same absorptivity and one side had the higher emissivity, you definitey want the high-emissivity side facing out. If one side had higher emissivity AND higher absorptivity (which is what I’ve been assuming), it would depend on the exact values, but my guess is that shiny side out would be better - because the lower absorptivity will more than make up for the lower emissivity.
I think that’s a demonstration of low heat capacity (i.e. even when it’s hot, there’s not enough heat in there to heat up your finger enough to feel it). It has nothing to do with radiative properties of aluminum. You can do the same with parchment paper in the oven.
Well, the reflectivity of aluminum certainly helps, though I don’t know which effect is more significant. Either way, though, it’s certainly not true that aluminum in a window would absorb a lot of heat.