I see that some greenhouse manufacturers are using bronze polycarbonate on their roofs, which cuts the sun exposure in summer and keeps the greenhouse a little cooler. The greenhouse plan (8 x 12) I'm thinking of building only has 4' sidewalls, with the roof peak at a little over 8' tall. Would bronze poly be useful or are the sidewalls so low that there just wouldn't be enough sun in there using bronze on the roof? Thanks...appreciate any comments or feedback!

I would use shade cloth if you need to shade from the sun during the summer. In early Spring and late Fall, you'll be glad you didn't get the bronze.

8mm double wall bronze light transmission is about 30%. 6mm is about 50%. Clear is around 80% for all thicknesses. Greenhouse design shoots for 80% to 90% transmission. Three-wall clear is about 75% and that is considered low enough transmission to limit type of plant or to long growing season.

There's some research that suggests that you give up 1% yield for each 1% of lost light transmission. There are other factors, like orientation relative to sun path. But high light loss is going to be felt, all things being otherwise equal.

My greenhouse is covered in heavy plastic panels that are heavily matted from exposure. My lux metering result isn't bad, but I still provide supplemental lighting timer controlled to about the daylight period.

I agree that shade cloth is the way if absolutely have to cut back on sunlight in summer. It's no better than tinted material, but it can come down in darker seasons when the heat is not a bad problem. But if I had power, I would actively ventilate, which if done with sufficient vigor will keep the interior at about ambient outdoor air temperature.

Since you're building it, you have all sorts of options to avoid overheating, like wide lend doors or walls.

With your short sidewalls I'd recommend not using the bronze polycarbonate unless you only plan on growing low light plants. An 8 ft high GH won't be too difficult to put up shade cloth as needed.

I have the bronze poly on mine and I'm happy with it, however the peak is almost 12 ft tall and would be a PIA for me to put up shadecloth. I'm pretty happy with it, though I could probably use even more shade on the roof in summer. I grow orchids and aroids only though.

I don't use it

It depends on many factors. First, some fruits and veggies demand more sunlight to grow better, so sometimes you need fewer materials to create a greenhouse for them. However, those who demand less of it or may suffer from too much sunlight are better covered. You can use bronze polycarbonate for that

MoonShadows said: I would use shade cloth if you need to shade from the sun during the summer. In early Spring and late Fall, you'll be glad you didn't get the bronze.

This!! Shade cloth can be removed and added as needed, more flexibility and wayyy cheaper.