DD, Mum,
Please read the full thread where I explain that I am fully aware that the conditions are not optimal for these seedlings because it is too early in the season, the day light length is not enough yet. this is an EXPERIMENT to trial the efficacy of the heated propagator box I built, using sample batches of seed. Now that I am confident of its ability to germinate seedlings requiring good bottom heat I can prepare to sow the full batches at a more appropriate time. Most of the seedlings are in small cell strips containing 6 cells and I am quite prepared to treat the whole of this batch as sacrificial.
As for the foil, if you wish to use radiator foil then that is up to you, I am using thermal insulation foil as used in loft spaces etc which is available by the roll in Wickes or B&Q and suchlike. This was originally developed by NASA for use in the space shuttle to counter the dramatic cold of space and the searing temperatures of re-entry where space was at a premium but effectiveness needed to be optimum for the space taken.
I have studied pics of the shuttle interior and I failed to see any radiators in it. Where would they put the flue pipe for the combi boiler anyway?
Ally, the temperature is controlled by the probe thermostat, this protrudes 10 inches or so into the upper sand bed. This will enable you to preset a temp. But this is very slow to respond to any manual setting so must be altered only gradually and as necessary. Trial and error is the only way here.
A lot of what heat output is achievable is determined by the spacing of the soil cable runs, 4" spacing will run a lot cooler than 1" spacing, mine runs at approx 2" spacing and I can get 32c if require, 20c to 25c is usually sufficient for most seeds.
My unit is approx 40" by 15" and at 2" spacing I require a 6meter length of cable and 2x 25K bags of silver or paviour sand. So these boxes are very heavy when completed, make sure your staging is up to the job.
The sand in mine is sat on a 2" thick polystyrene base so no heat loss from the bottom and the inner sides are again lined with the silver space blanket.