Lightweight foundations for a climate-battery greenhouse

I want to a build a climate-battery greenhouse. It is being built in an urban environment, on a completely flat piece of land with clay soil, well draining, facing south. I'm in the UK- so no frost-heave, no real snow load, no awful winds. I'm coming from a very standard-construction background. I am a single female and I have to be able to build this myself- or with hired help (but that is expensive!). I'm trying to build a tropical-ish greenhouse.

I intend to use the following materials:
- south wall will be old double-glazed windows
- roof will be 10mm twin-wall polycarbonate (single slope)
- north wall will be styrofoam insulation, with cladding of some kind (probably shiplap wooden boards)
- west and east walls will be similar to the north, only with small old double glazed windows in the top for vents. And in the east wall will be the door- an old upvc door.

It is only a small greenhouse, 6m long by 2.6m (up to 3.2m.. I haven't decided yet!). It is a prototype with the potential for me to build a much larger one in a community garden- I'd like to build it fairly cheaply! It helps that I already have most of the materials listed above.

For the climate battery I intend to dig down 3ft (from digging test pits I think I will hit clay at 2.5ft, but we'll see). I will line my battery with 4-inch styrofoam insulation boards around the sides. How do I build foundations in the disturbed ground to support the above-ground structure? I priced using hollow foundation blocks (these: http://www.fieldenfactors.co.uk/building-materials/blocks/215mm-hollow)- you stack them up, pound some rerbar in the holes and fill the holes with concrete. This would provide a very permanent and solid foundation, but is prohibitively expensive! And uses a tonne of concrete.

Any better ideas for foundations? I could probably manage lightweight concrete blocks instead- but I've never tried bricklaying before. And I still feel there should be a better way- the structure I'm holding up isn't going to be that heavy. Whatever foundation-style wall I build underground will stick up about 0.5m above ground- to form a stem-wall and be an outer wall for raised beds within the greenhouse.

A terrible sketchup image for the foundation-bit:
Showing the foundation wall, the path inside (will have insulation underneath it) and the internal raised-bed walls (which will be old scaffold boards).


Thanks for any ideas, Charli

One thing I would check is the level of the water table. You mention that you have dug test pits and you are on clay. Have you left those pits open? If not I would suggest leaving some and see if they fill up with water at all. I only mention this because up here where I am also on clay soil any hole in the ground just fills up with water and that may affect your decisions about your foundations.

It is remarkably well drained here- it may be clay 2.5ft down but we're on top of a large hill so the drainage is still really good. I've got a pond dug not too far away- didn't fill up at all and I had to line it!

Gosh I'm jealous. We are on a hill and it's still boggy.

Hope you get some knowledgeable people to help with your ideas.

Charli,

Concrete piers are much cheaper. To support them while the pit is excavated you build a temporary support frame to keep them stable and upright. I think we have a pic of it on our blogs on gh foundations with climate battery systems (we call them ground to air heat transfer):

http://www.ceresgs.com/#!blog/c1whn/tag/foundation

Settling is a very good concern -- wise thought -- you can either do that with a compactor (tool rental often) or with water and a good bit of time. It all depends on when you want to build, but the nice thing is the climate battery can go in well before the greenhouse is built (as long as you can dig in your soil).
You can check out our GAHT faqs. I think some of them pertain to the water table factor -- another good foresight.

Best of luck,
lindsey

Thank you so much! A whole post on greenhouse foundations!

Concrete piers isn't something you see much of round here- but a brilliant idea for a greenhouse! I could get some really big cardboard tubes and use those as my forms if I pour concrete (and just leave them in place to rot?), or make formers out of old bits of plywood.

Re settling- I have a 'whacker' machine, so will be using that- no rental needed!

How to build the 'ring main' on top of the piers? I hesitate to use wood- as it will be in the soil (in my head it would form the top of the rasied bed- but would still get wet). The windows will sit on top of it- so it would be really hard to replace without dismantling everything. I could probably use concrete beams meant for beam and block floors.

The other question is about roof slope... I'm at a latitude of 53.1- light will always be the thing I'm lacking most in winter. The weather here is almost permanently overcast for most of the year, would I be better off with a very standard greenhouse roof of two slopes (one north and one south) at 45 degrees. Or a single slope between 76 degrees (most efficient for summer sun here), to 31 degrees ('would probably still do'). Books seem to indicate that the standard double-pitch greenhouse rooves were invented by victorian gardeners in overcast europe.. but I am in overcast europe, so would this be better for me? Most 'solar greenhouse' books are aimed at the US.

Thanks, Charli

I think I could use concrete lintels for the base frame- they only cost a bit more than the timber.

I've seen the blog post on the roof angle (that's how I got the angles)- it's just do they apply in overcast britain? 'The Solar Greenhouse Book' by McCullagh says:
The roof of a standard-frame greenhouse is divided into two parts, usually of equal, and shallow low slopes ... this form [was] 'developed in the Low Countries of northern Europe as a response to the low levels' of predominantly diffuse light prevalent in the'region in the wintertime. '

It goes on to explain how to work out the best roof angles for the US continent. But I'm in overcast northern Europe, so a 'standard' roof design may well be better for me. Most Solar Greenhouse books/blogs/etc are all aimed at the US, so I'm unsure how to compare the two roof shapes between continents. Of course a single-slope roof is easier to build!

Charlie,

I see. In cloudy climates light is diffused across the sky dome. Instead of coming from one direction, it comes from many. McCullagh has a sketch of this. So the "best angle" discussion is less relevant -- those optimize assuming light is coming from one predominant direction.
The cloudy mild climate is one area that justifies a larger area of glazing. One or two slopes; I think you can probably adequately do either. I would just ensure that some has a shallow slope to be able to collect light from across the sky dome. Though it's against solar greenhouse design trends, I would use a small insulated wall area (only on the North wall, perhaps not even that), and maximize the area of the light collector.

Lindsey

Thanks for your help Lindsey.

The north and lower-west walls of my greenhouse will be insulated- they are next to structures that don't let light though.

I'm going to get the climate battery done first, then we shall see how to build the upper-bit. I suspect I shall run out of money and have to wait a bit! I think a single-roof slope will probably be easier to build!