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Thermal curtains in greenhouse  RSS feed

 
David Maxwell
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We have wandered through the subject of thermal curtains in another thread, but it got mixed up with heat storage, roof slopes, wall insulation materials, etc.  Could I pick the collective wisdom specifically on insulating curtains, to block the huge "hole" left by the glazing in a greenhouse at night?  The excellent article by Roberts, Mears et al covers the various materials, and talks about the importance of sealing the edges of the curtain, but glosses over the details of just how one might achieve this seal.  Has anybody got any concrete experience to offer here?  (I was blue-skying about incorporating magnetic tape into the edges, and using metal rails over which the curtain could slide.  Has anybody tried this?)  Indeed, this is such an obvious thing that I can't believe that a number of simple solutions haven't been developed...
 
Rebecca Norman
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Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5
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I've heard of a very low tech low cost solution, which is bubble wrap, and it just sticks to the glass gently. You can pull it off in the morning. You can even leave it on in the daytime sometimes, since it lets a lot of light through.

Of course it's best to have tightly sealed edges and all, but in my passive solar heated living quarters, I've used simple curtains made of blanket-cloth for about 15 winters. They are just drapes with no special edges or anything, but make a big difference in the comfort of the rooms. Of course you'd get the warmest possible temperatures in your greenhouse with ideal curtains, but if you don't need the absolute best performance, you'll find that various cheaper solutions can still make a big improvement over no curtains at all.
 
David Maxwell
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I fully agree with you with regard to curtains in residential applications.  Just in passing, you may note that such curtains are often topped with a valance.  This is not just for appearance; the actual efffect is to "seal" the top from convective heat losses.  And this is the essence of the problem.  Warm air rises.  On a vertical surface the major issue is sealing the top of the thermal barrier, to trap the air, (which is the actual insulator in large part).  In a greenhouse, one is working with either a sloping or a curved surface, and sealing along the sides becomes more important, while sealing at the top is much more difficult than using a simple valance. Curtains will hang vertically with no further support; in a greenhouse they need support over large areas.  In areas where snow is not a problem this is easily done by rolling down some sort of insulating barrier on the outside, but if one may get 3 feet of snow on top of it, (Ladakh?), this becomes less feasible.
This is not a trivial technical challenge.  But I have also been wondering about the need.  As things stand in my passive solar greenhouse temperatures drop to just above freezing on cold nights, but do not drop below 0 degrees C. at night.  During te day the temps go up well into the high 20's C.  The growth of our leafy greens is very slow, but my suspicion is that the limiting factor is not the temperature at all, but the light.  If this be the case, no amount of fiddling with heat retention at night will make any difference.
 
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