Author Topic: cold frames (Read 4439 times)

1066 · « **on:** September 18, 2009, 08:31:50 »

Hi, a bit of an off the wall question coming up! I've been looking at making a couple of cold frames, and everything I read on them says things like "paint them white" or "traditionally painted white". Now I know white reflects heat - so what's the deal?
Just curious really :D

Thanks 1066

Buster54 · « **Reply #1 on:** September 18, 2009, 09:34:18 »

Just a thought is you get better light with the lighter colour but better heat with the darker colour :-\

1066 · « **Reply #2 on:** September 18, 2009, 12:29:01 »

Buster thanks, maybe that's it

1066

Unwashed · « **Reply #3 on:** September 18, 2009, 15:52:01 »

The frame loses heat throuh conduction with the ground, and through convection and radiation from the surface. You can cut down the radiation losses by lowering the emissivity of the surface - so that's a shiny white surface, but as most of the heat is lost in the infra-red you need a paint that's reflective in the IR and there's no guarantee that white paint which is reflective in the visible is also reflective in the IR.

Insulating the frame reduces the surface temperature which reduces both convection and radiation losses so that might be a better strategy, and standing the whole thing on a bed of insulation will also cut the conduction losses through the ground.

What could make a significant difference is the angle of the glass. Glass is pretty transparent when the light hits the glass straight-on, but angle the glass and more of the light is just reflected until at a shallow angle no light gets through at all. When the winter sun's low in the sky none of its direct rays will get into a coldframe with a horizontal glass cover. To capture the best of the weak winter sun the coldframe should be oriented south and the glass should be almost vertical.

1066 · « **Reply #4 on:** September 18, 2009, 17:30:18 »

Unwashed, thanks for the reply its appreciated.
I'd seen that most frames were angled and figured that it had something to do with getting more light in, but wasn't sure, so thanks for clearing it up for me.
I have some left over insulation stuff so can use that around the sides, and maybe some straw on the ground.
I guess the final colour will depend what left over paint we have in the loft :) ::)

Psi (Pronounced 'Si'!) · « **Reply #5 on:** September 18, 2009, 20:06:20 »

my neighbour gave me a simple coldframe - aluminium structure with single glazed glass panels. My first experiments are rocket, lettuce and dwarf french beans sown 2 weeks ago - all coming on great - it's a real hot bed and things are growing quickly. I'm hoping to over winter some salads and then get things going early in the early spring. I dont think the white bit is THAT vital.

Psi

chriscross1966 · « **Reply #6 on:** September 19, 2009, 02:34:12 »

Technically the best angle to maximize solar collection efficiency is equal to the latitude you are at or a bit steeper (you want the extra efficiency in winter), but for me in Oxford that's nearly 52 degrees,almost a whole radian, impracticably steep.... so I'd go with the biggest slope you can give it.

chrisc

1066 · « **Reply #7 on:** September 19, 2009, 10:51:47 »

Hi, I'm not really that bothered I was just curious as to why ;D
And the plan is certainly to be able to try to do more stuff over winter and to start things off a bit earlier. I was also thinking of piling in a load of manure and popping a squash in there in summer. Lots of plans but as yet to build the thing ::)

artichoke · « **Reply #8 on:** September 21, 2009, 08:43:12 »

Very interesting about the angle of the glass, which I had not grasped before.

I have three large panels of transparent conservatory roofing, double walled plastic stuff off a skip that people tell me costs around £60 per panel (just to clarify what this stuff is).

Because it is so thick I wonder if it actually stops any heat getting through at all???

Does anyone know if I could use it for a cold frame?

I also have some glass double glazing from the same skip (with permission, indeed active encouragement) and have the same doubts.....

chriscross1966 · « **Reply #9 on:** September 21, 2009, 13:48:39 »

Quote from: artichoke on September 21, 2009, 08:43:12

Very interesting about the angle of the glass, which I had not grasped before.

I have three large panels of transparent conservatory roofing, double walled plastic stuff off a skip that people tell me costs around £60 per panel (just to clarify what this stuff is).

Because it is so thick I wonder if it actually stops any heat getting through at all???

Does anyone know if I could use it for a cold frame?

I also have some glass double glazing from the same skip (with permission, indeed active encouragement) and have the same doubts.....

That stuff will be perfect, heat comes through as infra-red light from the sun, but it re-radiates as convected warm air heated up by the soil.... it's a wacky process but your biggest problem will be making sure it doesn't get too hot rather than vice versa :D.... IT will make a brilliant (and huge if they're full size) coldframe... I'm jealous....

chrisc

artichoke · « **Reply #10 on:** September 21, 2009, 22:58:15 »

Wow - that's wonderful news, thanks!

I was cycling down from allotment, spotted skip, peered in, man rushes out and says please, please, take anything....brought husband back, he chose loads of ancient photographic stuff, and also helped me to heave the glass and plastic to the allotment.

I have to say that some of the conservatory roofing went missing later, and I found it behind someone else's shed.... Pulled it out, took it back to mine, and weighted it down with bricks to make it clear it was mine and I wanted it.

He has said nothing.

It is possible that there is a sad story behind this skip, as it was full of quite valuable things - I guessed he had big plans for extensions and double glazing etc but maybe marriage went wrong or an early credit crunch problem. Got a lovely wooden metre rule there too. Use it to prove to council where our boundaries are.

1066 · « **Reply #11 on:** September 23, 2009, 08:54:08 »

that sounds like a great haul Artichoke lucky you! I haven't seen as many skips in the last year around here - no doubt all to do with building / housing market ::)

landimad · « **Reply #12 on:** September 23, 2009, 17:02:09 »

I have found that mine is just to big for the area, so I have decided to reduce the size of it and move it to some other place which gets more sunlight.
I think that the angle of the lights are important and would change this aspect of the frame in future.
Looking back on what has been produced thus ffar it has been good but could as always been improved upon.
Maybe next year I will get things right.
I will just have to wait and see.

1066 · « **Reply #13 on:** September 24, 2009, 08:08:36 »

that's the beauty of gardening and growing your own isn't it - there's always next year ;D

Landimad- how big was too big? Are we talking depth or length?
Was flicking through Joy Larkcom's book and she mentions for salad crops a shallow frame would be 7" high at the front and 9" at the back, a deeper one for cucumbers would be 12" at the front and 18" at the back.
I think I'm going to have a wander around our plots today and see what everyone else does ;D

landimad · « **Reply #14 on:** September 24, 2009, 18:23:15 »

1066,

Mine was 3m x 2m x 1m deep a bit of a beast as far as they go ;D

Still I think its gonna have to be reduced by at least a half.

jimtheworzel · « **Reply #15 on:** September 24, 2009, 18:43:55 »

LANDIMAD
IS YOUR COLDFRAME COVERED WITH FLEECE OR PLASTIC?

JIM

landimad · « **Reply #16 on:** September 24, 2009, 18:58:09 »

Jim,

I covered it with plastic and then got some fleece to line it with in the winter.
Old bags of hay from the guinea pig food and bedding are good for this as it helps me to recycle the stuff.

Psi (Pronounced 'Si'!) · « **Reply #17 on:** September 24, 2009, 20:13:01 »

All

Interesting convo here. My smallish freebie frame has been brilliant - 3 weeks and I have strong rocket, french lettuce and chicory - baby leaf at the moment but racing away. Also dwarf french beans are up and getting going - hoping that the weather hold enough to give me one crop! I've no doubt been helped by the great Indian summer down here in London but really pleased.

I am now resolved to grow more under glass in future and am very keen on a poly tunnel now. I have a space 11ft x 7 ft spare that I have earmarked so hopefully I'll get the time to make it!

Psi

1066 · « **Reply #18 on:** September 25, 2009, 06:55:53 »

blimey landimad that is a bit of a beast :D
I'm probably thinking along the lines of 2 smaller ones. Its great to hear what people are growing in them, gives me loads of ideas - thanks
1066

landimad · « **Reply #19 on:** September 26, 2009, 09:50:51 »

Trouble with me is I start off big then realise that it is too big and have to reduce it before it takes over.
Still you can always separate it and half the size and sell the other half to someone else.
If I can I will be sorting out a runner bean frame also, as the bean canes have had their day now. I have looked at using blue underground piping to make the frame and broom handles to use as anchors in the ground. and a piece of wood to lock the frame together at the top.
The frame will be moved to a better location as the light is an issue here.
Beans also will be moved so they do not interfere with the other veggie growing.
We shall have to wait and see how things turn out. ;)

News:

Author Topic: cold frames (Read 4439 times)