Insulating the Greenhouse Follow-up

Seedlings in gh 3-18 compressed  For those of you interested in how much difference lining my 10’x 12′ hobby greenhouse with horticultural bubble wrap insulation made, here’s the follow up story and data so far.  The night following exchanging the packing tape with official greenhouse tape was not a great success.  The outdoor temp was 14 degrees.  The indoor temp was only 17 degrees, and maddeningly, the greenhouse tape failed to hold on one of the major seams and therefore two ceiling insulation panels were sagging nearly to the floor.  Contributing to this low result was also my having dragged inside two large bales of commercial potting soil that were absolutely frozen solid earlier that day.  I’m sure they did nothing to help that thermometer climb.

Gh pin compressed

In an effort to reduce the pull on the tape and keep it in place, I resorted to corsage pins through the tape and wrap in strategic places to anchor it.  Not extremely elegant, but seemingly efficient because so far, they’ve held the tape and wrap panels in place.  Since the night-time forecast was for upper 20’s, I took a risk by moving 12 flats of plants into the greenhouse about 3:00 that afternoon (top photo.)  The sun had actually come out after lunch, it was just above freezing and late enough that the seedlings wouldn’t get too much sun on their first time out of the basement, since some were newly transplanted.  The movie title “They Were Expendable” was appropriate as I selected the flats:  Sweet peas, snapdragons, broccoli raab and violas.  The violas were first choice, because I started them so late they may not bloom before the heat arrives…we seem to have such short springs now and I planted way too many.  Last year there were germination problems, so this year I planted more and of course, every seed grew. The snapdragons also because I sowed lots extra, and the broccoli raab & sweet peas because I want to get them hardened off and planted outside asap.  Plus the last two were getting too tall for the light stand unless I lowered the shelf, which I didn’t want to have to do!  Out they go!

Over the years, I’ve had 6 different sizes and styles of greenhouses, ranging from 2 different home-built, lean-to double poly on recycled 2 x 4’s styles; a homemade step-down in-ground double poly (these first 3 houses seasonally heated with wood stoves or kerosene heaters);  commercial 22’x 108′ double poly hoop house heated with propane salamander seasonally ;  commercial polycarbonite sided 22’x 108′ heated year-round with 2 commercial propane furnaces.  I’ve learned that the smaller the greenhouse, the greater and faster the temperature fluctuations.  Small greenhouses are tricky, and generally not very efficient, and this is the smallest by far that I’ve ever tried to manage.

On to that night.   At 9 p.m. when I scampered out to plug in the heater, the temp was 24 degrees outside, 36 inside.  We’d had a fairly sunny late afternoon  and those frozen soil bales had warmed quite a bit.  I felt pretty safe having the seedlings in the greenhouse, but all night I wondered what the g’h thermometer would read come morning.

I am delighted to report that instead of a ONE degree temperature difference between inside and outside (prior to the insulation, with heater) the outdoor temp was 33 and inside the greenhouse (Tah DAH!…trumpets sounding here) was a balmy 60 degrees, a 27 degree difference!  Happy dance, happy dance!  As  more and more plants are moved in, and thus more soil to help hold heat, it may even improve.   And, I’ll add a couple of 5 gal. buckets of water, which will also help.  Confident now that the temperatures will hold, I’ll move lots more violas, snapdragons, calendula and brassica today because artificial light just isn’t as good as REAL light, even if it’s not entirely sunny.  Here are two “Robin Hood” favas, planted same day, same conditions, except the one on the left stayed in the basement under lights while the one on the right has been in the greenhouse for two days.  Of those two days, maybe there was 4 hours of actual sunshine.  See how much bigger it is in that short a time?

Fava compare compressed

If the costs are amortized over several years (not sure how long the wrap will last) it should be reasonable.  Also, I think if I am more aware of spacing where the seams are and do a bit of repositioning next season, that may help the tape problem.  And I know if the tape were applied when it was warm, maybe 65 or higher, it would adhere better than in frigid temperatures.  So, the plan will be to put up the insulation late autumn, before it gets so cold.  Of course, I will have to purchase tape each year, because I used the entire roll.  I’ll keep monitoring, but enabling me to move plants into the greenhouse with confidence that it will stay above freezing is definitely worth the hassle of installing it.



About carolee

A former professional herb and lavender grower, now just growing for joy in my new potager. When I'm not in the garden, I'm in the kitchen, writing, or traveling to great gardens.
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7 Responses to Insulating the Greenhouse Follow-up

  1. Ruth jennerjahn says:

    Love the experiments and then I love your reports on the results .


  2. Yes, we use tape to attach ours.. and what bits we had left over we draped over plants on very frosty nights..
    You made a very neat job of yours


  3. Annie says:

    I’d say this is a genius solution!


  4. Cindi says:

    Hello fellow gardener. I realize this is a old blog but I’m hoping this message will get to you. I’m wondering how your bubble wrap insulation worked for you, do you have any updates? I also purchased the ACF greenhouse bubble wrap for my 12×10 aluminum frame harbor freight greenhouse and put it up last weekend. Our outside temp last night was 40 degrees and inside greenhouse was only 44 degrees, so only a 4 degree improvement. It’s been a rainy 2 days, no sun to help heat so that could be part of the issue , but I did run a heater to 66 and then turned it off before bedtime. So it does not appear the bubble wrap held much heat. What kind of heater do you use in yours? I have a greenhouse heater, also purchased from ACF but it’s not going to keep it warm enough and I am considering a propane heater. As I said, not sure this will reach you but I’m hopeful you can reach out and let me know. Thank you! Cindi


    • carolee says:

      The bubble wrap does make a difference in both winter (about 13-20 degrees) and summer, keeping it a bit cooler so my stored plastic pots don’t melt anymore! I just have a small portable electric heater, but I don’t try to keep the greenhouse going over winter. It’s just too cold here and not cost effective at all. I fire it up in March, when the days start getting longer and sunnier, and I’m running out of space for baby plants in the basement! Then the cool tolerant crops get moved to the greenhouse. I do have a fan mounted near the ceiling, angled to blow the hot air that collects there back down toward the plants. So far, knock wood, I’ve never had anything freeze or get damaged.


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