You're not reducing the dewpoint. The dewpoint is the dewpoint. It's a function of air temperature and humidity. You can see it in your local weather report. It's a property of the air around the container (and you). Excuse my lesson here ... When you're wet, you feel cool, right? That's because as the water on you evaporates, it cools. The warm molecules of water depart, leaving the cool ones behind. The dew point is what the temperature would be if the humidity were 100%. See here ... http://www.dpcalc.org/ As you can see from that handy calculator, if the air temperature is 90F and the humidity is 50%, the dewpoint is 69F. So if you're at the pool, and you're wet, as the wind blows on you, it's going to feel like 69F. If the humidity is lower, you'll feel colder, right? Ever been wet in the desert? Ouch! In fact, that's why air conditioners in the desert southwest work inexpensively by blowing air over water -- "swamp coolers". That only works well when the humidity, and the dew point, is low, which it usually is in the desert. So the idea is to keep your container wet, so as the breeze blows over it, the container will be cooler. It's hard to keep a plastic container wet. It dries off fast. But if you cover the container with a thick layer of absorbant cloth -- a towel, or a piece of thick rug, you can soak it with water, and it'll stay wet for a while. When it dries off, just hit it with the hose. As the water slowly evaporates, the cooling cloth will cool the container. That's why a porous planter is a slick idea for this purpose. The moist soil inside keeps the outside a little moist, and so cooler than the outside air. With a porous container, you don't need to be wrapping towels or carpet! I've never really heard of someone trying to cool a planter, but if you're serious about it, this is the first thing I'd try.

Your situation is somewhat complicated since you are only getting direct sun for 2-3 hours per day. During those hours you will probably need at least 47%. ( my GH, in full sun all day, has 47% on the east side and 70% on the west). The 30% may not help enough during the sunny hours, and the 47% may be too much during the indirect sun hours. However, if you are currently experiencing burn on your plants, I would go with the 47% but then perhaps expect somewhat slower growth.

It may be the trees are planted in the ground as ours are and they are 12' tall. We are in Western Nebraska so are at about the same level for sunshine as NJ. I would look into the ventilation of the greenhouse. Are the vents close to the highest point in the house? We use geo-thermal to heat and some cooling our "greenhouse in the snow". Depending on the greenhouse size this is very economical but the house has to be pretty large to make it feasible to use geo as the underground tubes need to be at or over 200' in length.

Hi Sunny... ahha, the challenges of Colorado gardening. If it isn't snowing it's hot and windy. But don't we just love it? I have raised beds that I cover in early spring with row cover material that I get at a local nursery or through GardensAlive.com. When hot weather arrives I raise them with stakes and use them for shade. Great for lettuce and tender veggies. Of course, you still have to deal with the wind knocking everything down. This year I am also using small beach umbrellas in various places (the kind that have a pointed end to place in sand. They are cheap at discount or drug stores and you can easily take them up and down as needed. And they add a colorful touch to the gardens. Just watch the wind... although mine are holding up amazingly well. Hope this helps. Good luck.