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How airtight does it need to be?

Posted by ekling211 5, NY (My Page) on
Mon, Aug 16, 10 at 12:27

So I'm building a greenhouse out of old windows and using some corregated plastic as well. It's turning into one of those nightmare projects that just keeps going and going but it will be fab when it's done. My plan is to use it from Sept-Nov and then March-May. I have no intention of heating it and I'm pretty sure getting it airtight will be impossible. I was thinking I'd probably put a sprinkler with a timer and have it come on once a day, like I do with my garden in the summer. My question is, if I'm watering it anyway why does it need to be so airtight, a little rain inside won't matter, right? Also, how much of the warmth will stay inside if it's not airtight (and by this I mean tiny little leaks here and there)? This is a weekend home so I won't be able to do anything on a daily basis besides the timer. Am I living in a fantasy world with my idea?

Follow-Up Postings:

RE: How airtight does it need to be?

My biggest concern would be putting a water supply on a timer. Seen them fail and you want water running for a week?

RE: How airtight does it need to be?

Many of us weekenders use them and I admit they are a little scary but from what I can tell from a random sampling, they mostly fail by not coming on at all. Without them I wouldn't have the most amazing crop of tomatoes this summer (some of my plants have 25 each one)!

RE: How airtight does it need to be?

Sprinkler irrigation during the short, dark days of fall is tempting the gods of fungal infections. Depending on what you plan to grow, you may not need to water more than once every two weeks in November.

How much heat conserved depends on many other factors. Size of GH, exposure, glazing (number of layers), soil temperature. If everything else is optimized, tiny air leaks will be okay.

You can get an idea by putting a max/min thermometer in there, shaded from sun. Place another outside at the same height. Compare the two minima. Also, get a soil thermometer and monitor how it relates to minimum temperatures inside the greenhouse. As a VERY general rule, I find that if
X is the soil temp.
Y is the outside low
Z is the GH low

Z = k(X-Y) + Y
where k is a factor dependent on GH type, location, exposure, weather conditions (clear vs cloudy, calm vs windy), soil moisture, mulch, and depth of soil temp. probe, among others. You should try and get a general idea of what 'k' is on clear nights, what it is on cloudy nights, etc. It will change a bit with season but not much, so start measuring now :)

RE: How airtight does it need to be? #2

Just to clarify what I'm saying:

for my poorly insulated, single layer PC greenhouse, k is about 0.25 to 0.3 on clear nights and 0.10 to 0.15 on cloudy nights.

Yesterday morning, the low was 38 degrees (!). Soil temp. at 6" depth was 66 degrees. G.H low was 46. k = (46-38)/(66-38) = 0.29. This approximate trend has held true since installing the GH.

RE: How airtight does it need to be?

Thanks Dr, I get the point, though my question pertains to the building of it so I can't really do that yet. I do hear you about watering though, good thought on that. I guess the bottom line is still get it as tight as possible. Since I'm using old windows, each window is a different thickness etc. No matter how much I try I can tell it'll be far from airtight.

RE: How airtight does it need to be?

You'd be better off to build your greenhouse at your "weekday house". Once a week tending to it will not be very productive no matter how well your timer works. It would be easier to use the timer two days a week when your not home than 5 days when you're not there. Things go wrong fast in a greenhouse!

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