how do I keep my plastic coldframe free of condensation?
March 29, 2011 3:57 AM   Subscribe

What can I apply to to the plastic sheeting of my cold frame to prevent it from fogging up?

Built a coldframe out of materials that I had lying around-- pine studs, 1mm clear plastic dropcloth for the sides, shrink-plastic window insulation for the top. Top is hinged and slopes 45 degrees towards the south. Both sides of every surface are covered-- two ply, baby!

The issue is that the interior walls get covered in heavy condensation, blocking some of my precious sunlight. What can I do to mitigate this? I tried the well-known "thin layer of shaving cream" and that worked for 20 minutes. I tried a spot with petroleum jelly, and that was only just as effective. Any other things I might apply that won't block more light than the condensation that I'm trying to beat?
posted by Mayor Curley to Home & Garden (15 answers total)
I think you're going to have to reduce the humidity in the cold frame. As well as fogging the plastic, you're also running the risk of having plants rot or develop mildew.

To reduce humidity you either need to supply additional heat or improve air flow. Air flow is the first thing I'd work on; fit a fan and get some proper circulation.
posted by le morte de bea arthur at 4:19 AM on March 29, 2011

Actually, a fan would probably be overkill. Just prop the cold frames open in the day, wipe the condensation off, then close them at night. That ought to fix it.
posted by le morte de bea arthur at 4:21 AM on March 29, 2011

Response by poster: Yeah, solid points, but I need to provide more info-- my seedlings are currently in the frame for just a few hours per day. Our house doesn't get much good direct sunlight (and the spots that do are crowded or vulnerable to a toddler and cats), so right now the frame is there to provide a very warm, very sunny spot for as long as the interior is both. So I'm not overly concerned about mold. And venting the frame will make it unsuitable because the sun is still weak in New England and it's still cold as hell.

I just want to know what I can put on clear plastic to make the fog run off rather than collect.
posted by Mayor Curley at 4:56 AM on March 29, 2011

Are you sure this is a big enough problem to worry about? My cold frame develops condensation sometimes, and seems to work well enough.
posted by jon1270 at 5:10 AM on March 29, 2011 [2 favorites]

I don't know how well it will adhere to flexible plastic, and I don't know if it'll be safe for the plants, but there's an automotive product called Rain-X that has the properties you're looking for. Water just beads off glass instead of fogging it up.

You might want to try it on a few inches to see how it works and then go from there.
posted by bondcliff at 5:47 AM on March 29, 2011

I came to say what bondcliff said, although I was going to recommend the Rain-X Anti-Fog.
posted by 4ster at 6:14 AM on March 29, 2011

I have a homebrew greenhouse, which is more or less like your coldframe - some wood, some clear plastic etc.

I get condensation and I've had no problems with mildew or rot. To me, it means the coldframe is doing it's job - gets warm during the day, plants respirate and water evaps from the pots. Then when it cools off, the water condenses on the plastic. As soon as it gets warm again, the water evaporates.

You can put vents on the plastic if it really bothers you.
posted by k5.user at 7:16 AM on March 29, 2011

Personally, I would avoid using a repellent like Rain-X, simply because you have no idea what, if any, off-gassing it might do in that warmer, moist environment. Or, in case the Rain-X would mix with the condensation and drip onto the plants.
posted by Thorzdad at 7:19 AM on March 29, 2011

1. The condensation indicates a release of heat into the cold frame, since water releases energy when it changes from vapor to liquid. Ergo, this is a good thing, from the POV of keeping plants warm.

2. Condensation forms on the plastic because it cools faster than the air inside the cold frame. Contrary to the suggestion of le morte de bea arthur (you're also running the risk of having plants rot or develop mildew), this will not be true of the plants, so they shouldn't acquire condensation. (Dew gathers on outside plants for the reason that they are cooler than the outside air.)

3. Given 1 & 2, it's more a "looks" issue than anything else. The water won't even block the sunlight from getting to your plant leaves.
posted by IAmBroom at 8:13 AM on March 29, 2011

this will not be true of the plants, so they shouldn't acquire condensation

I think you misunderstood my point. I agree that the plants won't have water condensing on the leaves. But if the humidity is too high, and if there is no air movement around the leaves, transpiration is reduced and the plants can overheat. Humidity and heat also provide conditions in which diseases such as damping off can easily take hold.

But as others have said, it's likely no big deal, really.
posted by le morte de bea arthur at 8:40 AM on March 29, 2011

Best answer: There's this product called Sun Clear: "Sun Clear, when diluted and applied to dry plastic surfaces, such as flexible or rigid greenhouse coverings, prevents the formation of water droplets. As a result, the plastic remains clear, admitting up to 50% more light. Dripping, which can be injurious to plants and is annoying to personnel, is eliminated."

No idea about the effects if it drips onto the plants.
posted by barnone at 8:52 AM on March 29, 2011

It looks like there are plastic films that have an anti-condensate additive that encourages the condensation to form droplets and drip down the sides of the greenhouse. It might be similar stuff as the Rain-X anti fog, but I'm not sure.

Or what about the stuff used for anti-fog in goggles?
posted by barnone at 8:57 AM on March 29, 2011

barnone's comment makes me wonder... is the lid of your cold frame sloped or horizontal?
posted by jon1270 at 10:01 AM on March 29, 2011

Response by poster: That is exactly what I was looking for, barnone (assuming it works). It's way too much and way too expensive for what and how badly I need it, so I'm not going to buy it, but such a thing exists.

I agree with everyone that says it's no big deal to have some condensation, but this is crazy "the pane is white and I can't see the plants" cloudy and I know that it's reflecting some of the sunlight (after all, I can see it and it's white). I'm sure plenty of light is getting through, but I want it ALL! I'm taking the Bill Romanowski approach, but to gardening: every advantage I can get, no matter how imperceptible, I want. (disclaimer: I am not going to hit my wife if my plants are not as big as I would like). And if I had Romanowski's money, I might consider actually buying the Sun Clear.

jon1270-- the top is hinged and slopes 45 degrees towards the south. (I mentioned it, but I miss stuff in others' questions too.)

barnone: what terms did you use to find that stuff? I googled my butt off before I blew my question on this.
posted by Mayor Curley at 11:51 AM on March 29, 2011

I'd say prop open the top a miniscule amount, maybe with a chopstick or something of that depth. If the ground is open, try propping up the cold frame a bit down there instead, so there's somewhere for the excess condensation to escape. It won't make it a huge amount colder.

Search terms: "spray prevent condensation greenhouse" worked, and "Prevent condensation plastic greenhouse" lead me here, towards the bottom of the page.

There are also a bunch of sites that talk about bottom heating of plants - keeping the plants above dew point. You might be able to do this with a seed tray warming mat.

But really, I think a tiny amount of venting will solve the problem. Good luck - I'm cheering on my little seedlings right now in frigid Boston.
posted by barnone at 1:24 PM on March 29, 2011

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