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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorJoeSmith
    • CommentTimeJun 19th 2020
     
    I'm about to embark on building a cob oven in the garden. The idea is to cook up large batches of food for the freezer so it needs to hold on to heat for a reasonable amount of time.
    I'm OK with the cob mix and design of the oven itself but the base it stands on is bringing up questions. Is a solid base (round stone wall encasing compacted rubble and earth infill) better than an open base (two brick pillars, hollow underneath for storage of wood etc)? My question relates to thermal mass. Presumably a solid base would hold more heat than an open base but what would ensure it holds the heat? And is enough heat lost through an open base that it matters? I've seen some people put a layer of old wine bottles under the oven floor - would they help insulate?
    If anyone's built one and can suggest anything, I'd be very grateful.
    • CommentAuthormuddy
    • CommentTimeJun 19th 2020
     
    After making quite a few of these, I would suggest that you make the base from whatever you want, gabion, stone wall, brick, pile of pallets etc. But always have a flat layer of thermalite or aircrete blocks then a top layer of refractory heavy tiles. The best are the tiles reclaimed from old electric storage heaters. Make the hearth layer at a comfortable cooking height, not too low. Put a roof on it to keep it all dry.
    • CommentAuthortony
    • CommentTimeJun 19th 2020 edited
     
    I would use a paving slab with a layer of aircrete blocks dry jointed on top. I never expected to recommend these to be used for anything, I hate them.
  1.  
    Over here the bases are filled with crushed scrap glass (nasty job to do) or dry sand both with fire bricks over. The sides are best with brick with cob or isinglass/sand mix as mortar The sides should be thick (1 layer of flat bricks with 3" of cob over or 2 layers of flat brick) and with mineral wool EWI to conserve the heat. The usual here is to light the fire in the oven and when the oven is hot scrape out the fire/ashes and put in the bread or food.
    Those ovens made with cob on a hazel stick form are considered a short term affair whilst those of brick are built to last. The kitchen at Hampton Court has a brick built oven that would still work if they dared to use it!
    Such ovens had a fashionable revival a few years back and the owners were always surprised to see how much wood was needed to get them up to temperature. A bit like central heating unless you have a supply of wood then gas can be cheaper, but the experience/life style is priceless

    And yes they need a roof, preferably extended to cover the operating area as well
    • CommentAuthorJoeSmith
    • CommentTimeJun 19th 2020
     
    Thank you all for the information. I won't worry too much about the base although I suspect it'll be pretty solid. I have some bricks from inside old storage heaters. I bought them in an auction for £1 as they looked interesting. Can anyone tell me what temperature they're likely to be able to handle? For instance, if I sandwich them between a concrete slab and some fire bricks will that be too much?

    My plan is to be able to cook pizzas when the oven's around 330c and then make use of the varying temperatures while it's cooling to cook other stuff. I appreciate the help here.
  2.  
    I would be more worried about the concrete slab than the storage heater bricks. It is not so much the target temperature of the oven that does the damage but the red hot embers sitting in the oven whilst it comes up to temperature.
    The one I use has a temperature gauge that goes op to 350C - very useful and much better than spitting into it and using the sizzel to guess the temperature.
    • CommentAuthorJoeSmith
    • CommentTimeJun 19th 2020
     
    Many thanks Peter_In_Hungary. Invaluable information. I think I'm favouring a "solid" base up levelled with sand then a layer of storage heater bricks then fire bricks for the oven base.
    • CommentAuthormuddy
    • CommentTimeJun 19th 2020
     
    The storage heater fire bricks will handle all the heat you need for a long session of pizza cooking, or tandoori or any other hot baking. Concrete is unstable at about 200C, don't use it. A good cob mix with plenty of saw dust or chopped straw in it has both insulation and heat retention at the same time. It needs to be a minimum of 50 mm thick, better double that. It's a good idea to make a door out of the cob while you're building it. I agree with Tony, an ideal use for aircrete blocks, dry laid, under the firebricks are a a cheap insulation that can cope with the heat.
    • CommentAuthortony
    • CommentTimeJun 19th 2020
     
    +1 thx a plan is born
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