Mountainbiker – here is the most concise link I could find to explain about shade cloth in all of it’s forms.
Here’s a great article on growing tomatoes under shade cloth in the NE (tomatoes were a problem for you if I recall, correct?)
If the problem is leaf scald/splitting I would use shade cloth – and in your case I would use black (some heat will still be retained) as long as the house is well ventilated with air movement and start with 30%. With a short growing season at your latitude you would need heat and light but the shade cloth will help mitigate leaf scald I would think. The white shade cloth (I use that in high heat summers – but this summer it was much cooler so I used black) reflects sunlight and reduces heat inside the house. I would think that you will only need to use the cloth in the hottest part of the growing season – and perhaps only on certain days. Greenhouse growing is truly an art!
My shade cloth is attached to the roof line of the small greenhouse in a channel I rigged up/attached to it that allows me to pull the shade cloth much like a shower curtain and then secure it. Some days I don’t slide it over until shortly after noon-time.
it would be worth a call to a local grower or your ag agent and see what is working best in your climate. I used 30% this summer as it was much cooler – last few years it was much, much hotter (many upper 90 and 100 degree days) and I used 50 and 70%.