[stompy]

Im building a coldframe on a table 1mtr high.
The table will be 1mtr high and the cold frame will sit on top of it facing south with an aprox 20dgr angle to the lid.
i am doing this as the difference in temprature from ground level to 1mtr is aproximately 3dgr's.
The theory is (not backed by science  ) i will be able to bring on and store plants earlier giving me an earlier start to the season, i will also be using twin walled poly cabonate sheeting for the lids.

Has anyone tried this before and can anyone think of any problems/reasons why this may not be a good idea before i begin costruction?

[Robster]

Stompy,  A nice idea.  One thought- the thermal mass off the construction may be quite small and so the night time fluctuation may be greater than you think.  ie 3 degrees above ambient during the day and somewhat less at night.

[mumofstig]

I think that the earth acts as a heat source for frames on the ground 
yours will be cold on all sides, so you may not actually gain anything by raising it -
  

[stompy]

I think that the earth acts as a heat source for frames on the ground   
yours will be cold on all sides, so you may not actually gain anything by raising it -

One thought- the thermal mass off the construction may be quite small and so the night time fluctuation may be greater than you think.  ie 3 degrees above ambient during the day and somewhat less at night.

Hmmm i never thought about that.

What if i fitted concrete slabs in the bottom as a heat sink?
The sides/back and front will be made from scafolding boards, so quite a bit of mass there!

[Robster]

In my greenhouse I grow in the side borders and the path down the middle is full of broken rock chips and a lot of it.  This adds so much thermal mass that it acts like a radiator overnight in spring and autumn.  So your slabs will definitely do something.

As a theme on your idea- I recently constructed a cold frame against a south facing wall.  It is 1.2 metres tall and 1 metre back to front at the bottom and it slopes to meet the wall at the top.  Here I am trying to use the wall's thermal mass to keep it all snuggly.  Only works if you have a wall!!

[N.WalesIdealist]

Perhaps an easier idea would be to sink your coldframe into the ground a bit, that way the earth will act as an insulator, conserving warmth during the night. 

[tinlizzy]

Would a thick layer of polystryne in the base be easier than concrete?

[stompy]

Ploystyrene has very little thermal mass, it would only act as an insulator.