Greenhouse

[dhaslam]

The recent storm  and more wind yesterday finished off the greenhouse, the combination of light steel framing and very thin  polycarbonate  just doesn't hold in  strong winds.   

What I plan do  is build a more sturdy and better insulated  replacement.     Probably low concrete insulated walls at the front and sides and up to about two thirds height at the back.  The glazing in the back wall  to give some light in  the early morning and evening and  to make the building less imposing.   The glazed part to be all  25mm multi wall polycarbonate.    This should be sufficient to retain  quite a lot of heat overnight  especially with the existing underground  heat storage.      It would be nice to have some better way to preserve heat at night,  the polycarbonate has a u-value of 1.6 which means  about 50 watts heat loss per degree.         

[JohnH]

Hi dhaslam
I will be interesting to see how you get along with the 25mm polycarbonate on your greenhouse.
I fitted 6mm twin wall polycarbonate to my green house about 10-12 years ago. Although it seems to keep the heat in better than glass it has developed dozens of small holes on the outer skin. This is mainly where it is in direct contact with the sun. I plan to replace it but my green house is cedar wood so 25mm may be a problem fitting. I had also put 4 inch polystyrene slabs beneath in the ground to retain the heat plus solid Rockwool around the walls. My greenhouse is full of Birds of Paradise plants which for some unknown reason are starting to flower in this bitterly cold weather. Now apart from having to heat it, I have also had to line the whole inside with twin laminated bubble-wrap to help retain the heat.
Regards
JohnH

[MarkB]

This is mainly where it is in direct contact with the sun.

Wow, that's thick polycarbonate 

[djh]

Don't forget that each layer in a multiwall also cuts down the light coming in.

Here's two other additional ways to insulate. One is easy, the other adventurous:

(1) rollup blinds of plastic bubblewrap

(2) soap bubbles! sounds crazy but it works

http://www.tdc.ca/bubblegreenhouse.htm
http://www.solarbubblebuild.com/

I talked to the second guy but he's moved on to other things now.

[dhaslam]

The light transmission for  25mm polycarbonate is about 75%  and u-value 1.6.  The 40mm  only transmits about 50%  but u-value  down to about 1.   The greenhouse is in a very sunny position  so a little light reduction shouldn't matter too much.     It would be nice to have a  system that gave  better daytime light transmission and much better night time  insulation but it wouldn't be easy to automate.       I suppose a system using polystyrene beads between two layers of  polycarbonate might work but would be tricky  to set up in a reliable way.   I suppose the bubble idea does  make regeneration easier but it sounds expensive.   

[Tigger]

Folks,

How about a combination of 25mm and 40mm polycarbonate sheets for a nice compromise.  If the ridge of the greenhouse runs East/West then why not put 40mm sheet into the North facing side/roof and 25mm sheet into the south facing, that way you can get the best of both worlds....

Ian.

[dhaslam]

Still  thinking of the polystyrene idea.     I was thinking of a  long sloping roof south facing, the sides  could be the      thicker polycarbonate  but for the roof  to have a  controllable insulation.   The beads  are one option but also there  could be  a slatted  system  that could be angled during the day  in winter and removed in summer.   

[dhaslam]

Still  thinking about the plans for the greenhouse.   

I will probably build the structure from  wood and  25mm  polycarbonate, however the heating system  is the area  I want to improve.     There are two 25 metre 100mm land drainage pipes buried under the soil  and normally these would be used to store hot air from the greenhouse.   However I have thought of a different way of using them.    I could make  a long panel from  Correx, probably three  2.4X1.2 sheets painted black.   Air would be blown through these panels  and into the ground.    Like the solar mound the fans would come on when the temperature reaches  30C  or the ground temperature, whichever is higher.   In winter the circulated air could reach about 60C at times, much warmer than the greenhouse air would be.       

The  heat would be transferred back via the panels only.    This is the bit that would be a bit experimental.   The panel's surface temperature at night would be something like 20C, same as the store temperature,  and also the  ground surface temperature would be warm.   The air temperature could be quite low but the radiant heat should be able to compensate for that.    In summer the steep angle and vegetation would  reduce the  heat collected by the panels and conventional auto ventilators would be used to keep the temperature down.