My greenhouse goes below freezing 45 minutes after it does outside, but in the midmorning, when it's in the mid 30's Fahrenheit, it gets up over 100F. It was 106 degrees inside with all four roof vents open while it was 39F outside. When it's 72, is it going to be 150F inside? When it's 95, will it be 180 inside? I don't get it. I should have just saved the money and built some nice cold frames.

What type of GH do you have, Rubi? Temperature regulation can definitely be a problem. What kind of plants do you (or will you) have in it?

Good venting and shade cloth can help keep things cooler in the summer. ("Cooler" being quite a relative term!)

Wow that's terrible. What size is the GH? It must be small to heat up that fast. I just installed a lean-to to my shed hope it doesn't heat up like yours. Like Weed said shade cloth and forced ventilation seems a must have. Shade cloth will have to be installed on the outside, probably 89-90%.

Welcome to the world of greenhouses, Rubi.

An unheated, or stored heat, greenhouse cools down just about as fast as the outside temperature. A greenhouse without shade protection and proper ventilation will heat up very fast once the sun hits it.

Unfortunately, no one who sells them tells you that! It is a steep learning curve when mastering greenhouse growing. Think of it as a challenge and opportunity to learn more.

@jpm995 Doesn't matter much if it's small or large, they heat up fast without the adequate safeguards.

I agree - and, going back to Rubi's original post, cold frames will also become plant cookers if not ventilated.

Two days ago, my Hi/Lo Temp was 113.4/25.9. In positive news, last night I left a 1500 Watt heater running and kept the temp about 9 degrees over the outdoor temp and I got to leave all my plants in there. It was getting to be quite a chore to put all the plants in the garage with a sprained ankle.

I kind of assumed shade cloth was for people in CO, CA or Arizona. It gets hot here, but the amount of time it's oppressively hot isn't very long. A couple years ago, I didn't even put my window air conditioner in the house. I couldn't have been more mistaken that the high desert States are the only place needing cooling. The other day when it was 113F inside, it was 41F outside. I had no idea the differentiation would be so great with the roof open.

I don't think I'm going to keep anything in it once I've got my seedlings in the garden. I might do shade cloth next year, but I'll have too much going on to install it this year. In the winter, I hope to try to keep some zone 5 perennials covered up in there, and on sunny days, be able to sit in my lawnchair without a snowmobile suit on.

You buy a greenhouse thinking it's going to keep your plants warmer. And it does, like 70° warmer.

I find the real challenge with a greenhouse or hoophouse is keeping it cool. I have removable ends, windows and screen doors on my hoophouses so I can open them up to let things cool off. But that's not enough. I also have 60% shade cloth and exhaust fans. That keeps them 90-100° most of the time during the hotter weather. For what it's worth, the ambient temperature where I live rarely gets above the low 90s .

Pro-tip: you can open all the vents and windows in your greenhouse and stick a box fan(s) in the doorway in a pinch. But its better to have dedicated fans.

Also, thermal mass works both ways, warmer at night and cooler during the day. I keep a row of cinder blocks in one of my hoophouses Nov-May

Do the roof vents open farther than what is shown in your photo (I hope)?

Watering down the floor of the GH can also help with moderating the temperature.

@Henderman Taos? That truly is high desert. The sun must be brutal there, and then it can be chilly at night. We have humidity here. Your plants will get grilled when it's hot and sunny, mine will get steamed.

@Weedwhacker Those are the auto-openers, and they open a lot further. I think when that picture was taken, it was about 10:00AM and about 30 degrees F. I didn't have any of the manual roof vents open yet.

It doesn't get that hot here, but with the low humdity (sometimes in the single digits) and mountain winds things can get dried out quickly. But the hoophouse helps hold humidity.

Might help to install some low corner vents so you have better airflow.



Here at 8000 ft. in the Rockies it's mostly cool but the sun is bright. I put on shadecloth in late May or early June and keep it on 'til September or so. You can order it cut to size and hemmed, with grommets. Easy to install.

There are several approaches to passive heatsinks, etc. that can keep nighttime temps from killing plants. Here's a thread I posted some time ago.

The thread "Low-tech Solar Radiant Heat" in Greenhouses forum

Im in the same boat here Chip. Very low humidity here as well. I installed a misting system to the celing of my greanhouse. I installed a water timer set to come on every 2 to 3 hours for about 15 minutes. It starts at 9am and stops at 6pm. It has made a big difference with my plants. Hope this idea helps.