Insulation not working!!!

[Mrs Ava]

Good idea with the mini greenhouse Richard.  I think some cannas are tougher than others as I have had some outside forever, and they are fine, then others pop of on a nippy autumn day!! 

[john_miller]

There is no arguing with data like that ... they are the results I would have expected ...

It occured to me afterwards that I probably hadn't made it clear that the problems I had described only occured in a tiny percentage of plastic greenhouses (although large enough for part of a college lecture on greenhouse materials to be given over to the subject) sited where a confluence of factors come together. Remembering back to the discussion Richard and I had last winter (?), and his later photos, concerning the positioning of his greenhouse it seemed possible to me that he may be encountering such issues. These conditions are very rare- I have never encountered them or heard of them directly.
One solution that Richard may be able to adopt, and springs from Jenny's comment regarding Specific Heat Capacities, is the use of containers filled with water to absorb heat during the day and release it at night. A grower friend of mine had a lean too plastic greenhouse (the 'lean-too' wall was cement/breeze blocks which would also have absorbed heat) in which his benches were stood on 220L drums filled with water. Even though there was no other heating involved he was able to produce a merchantable crop of basil right into December even though night-time temperatures are commonly as low as -10C. Being so far south of the U.K. helps of course but I would imagine that water filled drums in the U.K., even with Richards sheltered location, could absorb enough latent heat to keep tender plants from freezing during the night.

[Garden Manager]

Hello John. Yet another interesting posting there. Yes I do remember the discussion about siting the greenhouse last winter (after a bit of memory jogging).  Should i be 'honoured' to have such rare conditions in my greenhouse? I mean where the inside temperature can actualy be lower than outside? You think the positioning is the cause of this?

Reading back over your earlier post it seems to suggest to me that i may actualy have made things worse by insulating the thing, whereby the bubble wrap may be blocking out the heat energy. I have noticed the 'house is staying cooler in th morning when the temperatures would normaly soar on a sunny day. However with the addition of heat on a cold night the insulation works well and creates a significant temperature difference.

I wonder whether under such unusual conditions the bubble wrap insulation might be as good if not better than shading come next summer? if it keeps the greenhouse cooler during the day and warmer at night then why take it down in the spring?

Water containers as a heat storage method? Sounds a good idea. Its only a small greenhouse so would  a couple of watering cans or buckets full of water help?

Just one more thing, can you (or someone else) explain why positioning my greenhouse the way i have has caused this effect? I dont really understand.

Many thanks for the advice.

[Derekthefox]

I agree on the use of water containers as storehouses for thermal energy. The large capacity is necessary because of the slight temperature difference between the water and the ambient temperature (inside or outside, it matters not). Small containers would not hold enough heat to make a significant difference.

Derekthefox

[MikeB]

The subject of using water containers was covered in October under "early tomatoes" Tim supplied the address to a web site that covered the subject.

http://www.yale.edu/ynhti/curriculum/units/1983/1/83.01.13.x.html#h

Basically you need 1 gallon of water for every (1) square foot of glazing

MikeB

[Derekthefox]

That equates to over 200 gallons for even a small greenhouse !!! However, I presume that is without the bubblewrap on? That would be adjusted for the change in heat transfer coefficient ... fascinating article by the way.

Derekthefox

[john_miller]

Hello John. Yet another interesting posting there. Yes I do remember the discussion about siting the greenhouse last winter (after a bit of memory jogging).  Should i be 'honoured' to have such rare conditions in my greenhouse? I mean where the inside temperature can actualy be lower than outside? You think the positioning is the cause of this?

The positioning is the only explanation I could come up with as your experiences are so atypical. I would imagine that when it is windy you probably have a wind tunnel but when it is calm the air is really still and is contributing to excessive heat loss. The bubble wrap is probably also blocking the energy radiated from the surrounding structures at night from also helping to raise the temperatue inside your greenhouse. 

I wonder whether under such unusual conditions the bubble wrap insulation might be as good if not better than shading come next summer? if it keeps the greenhouse cooler during the day and warmer at night then why take it down in the spring?

It would certainly provide some shading. The problem I envisage is that the bubble wrap is probably not treated with UV inhibitors and will break down quite quickly. You will end up with an unshaded greenhouse and a mass of brittle plastic flakes

[chrispea27]

that seems like a brilliant idea

[Rose.mary]

When/if I ever get my greenhouse up I think I might try to insulate just the north side, and then during the day the sun should warm the soil etc. and that might work. Providing we get any sun.

Rosemary