2 ft x 4 ft x 15 inch cold frame thermal mass question

18 years ago

I just created a 2 ft x 4 ft x 15 in tall cold frame to start/grow seedlings and to grow early/late season lettuce and spinach. I used 1 x 3's to frame it and I used plexi glass for the sides and lid. The coldframe will be in sunlight from sunup to sundown in zone 6. Will adding milk jugs of water to the inside of the box buy me some added heat during the night? If so, how many should I add for my 10 cubic feet of airspace inside my structure? Remember, I only have 8 sq ft of ground that I want to maximize for planting.

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Comments (12)

weebus
18 years ago
If I were you, as small as it is, I would cover it at night with an insulated blanket.
tall_cool_one
Original Author
18 years ago
Good tip. Do you think it would also help to staple in a thin piece of sheet plastic to add an insulating layer of air?
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weebus
18 years ago
It wouldn't hurt...
bobb_2002
18 years ago
And don't forget the sun will run the temperature up to 150 or more on a sunny day unless you vent the coldframe. Even short periods of 90 or 100 degree temperatures can hurt spinach and lettuce. They love cool weather and can survive temperatures down to 25 without damage. An automatic opener is best but if you are scrupulous about watching it you can manually open it.
tall_cool_one
Original Author
18 years ago
Well, I finished my cold frame on Saturday. It was about 40 degrees outside. I put it out in my garden. It immediately went up to 80 degrees. I was shocked. Then Sunday I went out to see what it was doing. It was 30 degrees outside and it was 110 degrees in the cold frame. I figured it would heat up pretty well, but I had no idea it would get that hot in 30 degree weather.

What is my cheapest option for keeping the cold frame cooler during the day(outside of me going out and opening it every morning of course :))? Would some sort of shade help? Would using window tint on the top piece of plexi help? Or should I go directly to shelling out the $50 for an automatic opener? Is there a cheaper automatic opener option than $50?
nathanhurst
18 years ago
I believe that in the US there are things called basement vents which use a bimetallic strip to open a flap. Haven't seen them here. I heard them priced at $20.
tall_cool_one
Original Author
18 years ago
Nathan,

That was a great idea. Here is what I found on-line. It looks like a great inexpensive option. It don't think it would drop the temps like an automatic opener, but I think it will help quite a bit.

Here is a link that might be useful: automatic foundation vent
paws4pets
18 years ago
That looks like a very useable vent. Will 1 work for your cold frame? I will be making similar ones soon.
Paws
nathanhurst
18 years ago
It looks like a good solution. You might be able to adjust the set point using a screwdriver. I think from the size of your cold frame one or two would be enough. Try and arrange a flow from one end to the other so you don't get hot spots. Remember that hot air rises, so having one vent higher than the other will produce a better flow.

You might also find just leaving a hole open right near the bottom and using one auto vent at the top works better, as the air can flow once the vent opens, without having to wait for the second one to heat up. A hole at ground level protected from the wind will let very little heat out, as long at the top is well sealed.

Have fun!
tall_cool_one
Original Author
18 years ago
I just installed my 2 foundation vents. I am waiting for some silicone to dry before I screw the frame back together. I am anxious to see what it does to the daytime temps in the cold frame.

When I get it put it back together I'll take a picture and post it on here.
Mary4b
17 years ago
tallcoolone, how is your vented cold frame working? DO those vents work? THanks!
stevei48
17 years ago
I'm new to the gardening forums, and just finished a coldframe 4ft by 6ft by 2ft high in the rear. there are two windows on the top useing 5/16 lexan mounted on a 2x2 frame. i used a fishing scale and determined that the windows each required about 12 lbs of force to open them up, harbor freight sells automatic vent openers very simular to the $50 ones at charlies for $19.95 it took a few changes to make them work but they lift the windows just fine. the biggest change was taking one of the hinge pins out and replacing it with a cotter pin so you can open the window farther than the opener will travel hope this helps

2 ft x 4 ft x 15 inch cold frame thermal mass question

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2 ft x 4 ft x 15 inch cold frame thermal mass question

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