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How to build a truly 'frost free' greenhouse

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  • How to build a truly 'frost free' greenhouse

    Where we live in France we often get winter temperatures as low as minus 20C yet in what we call our 'conservatory', temperatures never fall below freezing. That is because it has stone walls on three sides, all of which are buried in an earth bank, with the front facing south. I have capped the building with polycarbonate panels and fronted it with doors and windows from the tip.
    That is fine, but I have too little space to keep my tender plants and shrubs over winter. Putting heating in a greenhouse is very expensive and environmentally unacceptable.
    My next project is to use the south facing bank which runs across our garden to create, I hope, a frost free structure for overwintering my tender plants.
    My idea is to create a thick block wall filled with rubble as a 'thermal mass' but use sloping polycarbonate roofing panels and a front wall from windows and blocks at the front.
    As only the rear will be insulated by the earth bank, I wonder if this is going to work?
    What should I be considering to ensure adequate insulation elsewhere? Should I plan an insulated floor? Or make the front windows double glazed? The whole structure is likely to be around 30 ft long, 7 or 8 ft at the back, and as low as 4ft at the front, with a width of around 6 to 8 ft. Any suggestions about how I can make this frost free would be very welcome.

  • #2
    there's all sorts of info about this kind of thing @ Permies: a big crowd of permaculture goofballs

    If I ever re did my greenhouse I'd try to implement a rocket mass heater into it from the get go
    I'd have the staging as my thermal mass I reckon

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    • #3
      I agree with Urban, rocket mass heater is the way to go, assuming you have access to a supply of twigs/branches.

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      • #4
        Easy.......build it in the amazon!

        Loving my allotment!

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        • #5
          I haven't read this but it may help you The Earth Sheltered Solar Greenhouse Book by Mike Oehler Free Download - DownArchive

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          • #6
            Thanks for all the advice and links there. I hadn't thought about incorporating a wood stove or 'rocket mass' heater but will look into putting that into my design.

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            • #7
              I've got round to clearing a site for my solar greenhouse and found a very useful website The Solar Greenhouse That's Right for You
              This gives some useful information on roof angles and insulation, as well as heat masses. I've decided to build the rear wall (north) and front wall (south) in blocks with some extra insulation while the rest of the walls and roof (apart from the polycarb panels of course) will be insulated with fibreglass. For the thermal mass I am going to use 20 litre paraffin containers filled with water and painted red, which is supposed to be a better colour for heat attraction than black. They will be stacked in tiers along the rear wall, supported on planks between each tier, up to 1.5 metres high. The opaque roof will be lined with tinfoil on the inside and the rear wall will be painted either black to attract heat or white to reflect light, not sure which yet. The polycarb roof will be at 67 degrees, while the rear roof will be 45 degrees. Rear wall is 1.5 metres, and front wall 50 cm with a total length of house of 11 metres. This should give useful headroom.. over eight feet in the middle, and useful depth of around 3 metres.
              Ventilation will be from a door at one end, and a circular hole with an electric fan, near the roof on the other end.
              The end walls will be built of boards and timber with insulation built in, probably fibre glass sheets as I have a supply of second hand stuff.
              I will be taking the advice here and building a cylindrical log stove into the rear wall, as these are supposed to keep in overnight with a couple of large logs. I will try to find a second hand one, but otherwise they can be bought for a couple of hundred euros here: expensive, but probably indispensable to keep the place warm on cold winter nights when there is no daytime sunshine to heat the thermal mass.
              The main expense will be the polycarb panels... those with a ten year guarantee are very expensive, but will those with only a two year guarantee be worth the price?
              Or would electric heating with a thermostat be better value for the heating in the long run? Certainly easier to install and use?
              I will post more reports on my project as it advances. I'm not looking forward to transporting and shifting all the concrete blocks...!!
              Last edited by BertieFox; 27-01-2013, 12:45 PM.

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