The Forestry Forum

Going to build a greenhouse using old windows. Any thoughts on whether to have a clear roof or can an opaque roof be used? Will windows only around the sides be enough light?

Clear will probably do you better but if things are getting sunburned they are sometimes covered with various weights of shadecloth or whitewashed to make them more opaque. Depends on the season, crop and your conditions. Venting is used more for controlling temperature, most of the time you want light hitting the leaves, just drawing a distinction between shading for sunburn vs trying to use it to control temp at the expense of photosynthesis.

The walls won't give enough light but used windows on the roof are likely to trap water rather than quickly shedding water off the panes. I'm envisioning a pool at the lower edge of every pane.

I agree that the windows would trap water and have no plan to use any in the roof. I have tin already and wondered if I could get away with that or should I try to scrape up enough green for clear panels.

You could grow some houseplants by the windows but without more light from the roof it won't be able to produce much.

I had a client that wanted one of those lean to prefab greenhouses, think McDonald's, built on the side of the kitchen. I tried to talk them out of it but ended up doing it. Without a way to close that room off it turned the kitchen into an oven, the light was harsh, a disaster. What they wanted in reality was that "Florida room", and that is what another contractor did several years later. The glass roof was too much solar gain for human comfort.

 I don't know your specific climate but here in Oregon at our nursery we use shade cloth (47%) on our houses during summer, it reduces the need for water and most of what we grow does better in a semi- sun or semi shade location.
With a clear roof you can add shade during the summer.
There are so many variables to your questions.  What you intend to grow and during what time of the year all play a factor.  If part of what you grow needs full sun you can create mini shade structures inside your greenhouse for plants that require shade.  We have an area for propagation that is almost "black out" it gets ambient light from the sides is all, but this is propagation not growing, and summer only.