Sunlight Vs Insulation
 

I`m not sure whether it`s best to leave the curtains/shades closed in cold and sunny weather for insulation purposes or to open them up, losing the extra air layer in favor of "greenhouse" heat. I guess it depends on temps, angles, how well sealed, and other specific factors, but maybe there`s some rule of thumb or general recomendation?

Sorry if this has already come up- I tried a few searches, but it`s tough to find a good search term to lead me to the info I`m looking for.

A green house, or even a dog house with south facing glass or even a single layer of plastic is going to heat up inside as long as the sun is out, how well it retains that heat is a question of how much mass it has and how well sealed and insulated the rest of it is.
if you have curtains then regular non low-e glass will also work better on the south side as it will allow more solar gain, it is designed for better gain then loss but the coating still blocks some gain.

Thanks, Ryland. I think that`s a suggestion to open the curtains when the sun is hitting the windows, even when it`s colder outside than inside?

Passive houses use the low sun to warm up in winter, and physically block the sun's rays from entering in summer by using overhangs above the windows.

So yes, open the curtains.

Yes, open curtains when its sunny out, you can also build solar hot air collectors, more or less a black box that fits your window with a flap opening at the top and bottom so it lets air cool air in at the bottom and lets hot air out the top, flaps close to prevent cold air from dumping out the bottom.

In the summers here, we try and keep the curtains closed as much as possible. With that said, it is also important for the insulation and sealing around doors and windows to be sealed well. However, in the winter, we leave them wide open to capture to keep the heat coming in! I'm going to ask an Indianapolis hvac worker if this is beneficial to get more details. I will share my info when I get it!

It'd be a lot more efficient to block the sun on the outside, i.e. before it hits the glass.