Making a propogator

I've just put electricity in my greenhouse and want to make a propogator using one of those heated mats. Firstly the advice is to put the mat on polystyrene blocks (available in B&Q)then the mat, then tin foil, then a pane of glass. Are there any substitutes for glass (other than a layer of sand). Could I just put a layer of capillary matting on top of the foil instead of glass ?

Secondly, when I construct the walls, what material is best. I would rather avoid glass and have the options of plain 4mm polycarbonate or the insulating polycarbonate, constructed a bit like cardboard with multilayers. My concern is with the insulating polycarbonate blocking out some light. I live in Inverness and need all the light I can get.

There was a nice picture of someone's homemade propogator a while back. Does anyone have any more pictures of their own home made propogator for inspiration ??

 

I can't see the purpose of foil on top of the mat as it will reflect heat downwards.
When I built mine I put down polystyrene, then a layer of foil and covered the foil with sand in which I layered the heating element. The sand helps distribute heat, acts as a heat sink preventing rapid fluctuations in temperature and also holds moisture.
I used plain thin polycarb sheet for the sides, but the top is twin wall polycarb. I used the twin for the top mainly because it is rigid and I can stand pots on it at times (it was also all I had to hand at the time!)
I think that if you choose the heating element with care taking into account the surface area to be heated, single wall 'glazing' should be ok.
I've a good variable thermostatic control on mine and if I expect a really cold night I just drape the propagator with a bubblewrap blanket.

I've just been looking for pics of mine, which I posted last year. Can't find them at present, but will post them again if they turen up.

 

Mines getting a bit old now but its made in the old way.
A shallow box about three inches deep, lined with a piece of pond liner, two inches of sharp sand in the bottom then a soil warming cable attached to a probe type thermostat covered by another inch of sharp sand.
The whole thing stands on the bench in the greenhouse which is 18mm ply, the bottom of the box is also 18mm ply.

The sides on mine are 10mm twinwall polycarbonate left over from when I reglazed one side of the greenhouse, the top is just a loose piece of the same material.

I, like Dave, cant see where the foil comes in, or the glass for that matter.

 

Forgot to add. I lined mine with a sheet of agricultural polythene ( a leftover from polytunnel).Helps keep the moisture in and away from the wooden walls and base.

 

Also, if you keep the sand moist, it distributes the heat better.

 

Thanks for the replies. I got the information about foil/glass etc from a web site that sells the heating mats (which include probe and thermostat) here
http://www.jungleseeds.co.uk/HardwareOrders/index1.html

and here

http://www.biogreen.co.uk/heatingmats.htm

I presume the foil may act to distribute the heat more evenly by conduction (perhaps not much of the heat is transferred by radiation) and the glass protects the foil and acts as a solid heat sink (it is solid sand in a way - silicon).

I'll just bite the bullet and buy a mat build a frame and glaze with twin polycarbonate. I'll improvise on the details of layering at completion - otherwise I'll never build the thing.

 

I've always gone on the assuption that the heat is conducted up into the pot from the damp sand to the damp compost, so I tend to push the pots down slightly into the sand.
But maybe there are newer ideas.

 

I use the biogreen method of insalling the heat mat I used a .75" thick polystyrene slab to put the heat mat on and used sand over the aluminium foil,I don't keep the sand wet the heat transfers sufficiently through the dry sand.

 

What size of heat mat did you go for out of interest, and is it still big enough ??

 

Never big enough, mine is a 120cm long could do with one twice that size ,may consider doing a full bench with heating cable.