Paint It Black - Your GH Wall!

I am reading a book on permaculture at present and it describes materials that are useful as heat stores. Brick isn't that good cause the heat only penetrates a short distance. The concrete will be a much better heat store. Also any stone and of course water is a good store as well.

 

Reading all this has been fascinating. My 6x6 greenhouse has no masonry walls, but the base is 6" of concrete on top of 2" of expanded polystyrene. So painting the floor black could be beneficial in the winter. Anybody want to swap a tin of grey floor paint for a tin of black? Cash adjustment available.

Nik

 

I think the main benefit might be in spring and autumn when you can have quite bright days and cold nights. The black would absorb the radiation in the day and release at night so moderating the extrems of temperature. Depends what you want. Do you want maximum light or warmth? If you want light then you colour it white so the energy is bounced back as light, rather than being absorbed.

 

Cor Nik - 6" of concrete will hold quite a bit of heat particularly painted black particularly on sunny days. I'll post some more results in a week or two, but so far my black painted wall is showing a significant difference in heat storage compared with the white one, I'd like to measure floor temperature too but the concrete is too ruddy hard to get a drill into!
I'm also looking at the temperature differences between two 2 litre bottles of water in the GH. One is covered in black plastic, the other transparent.
I'm not sure about benefits of upwardly reflected light as per Geoff's suggestion as I've an idea that the 'sensors' in leaves are mainly in the upper surfaces.It may perhaps help reduce temperature on hot days in summer by reflecting heat outwards. I Could well be wrong - I'm no plant biologist! Must do a bit of Googling to check.

 

I think if its reflected back off a white surface it would probably still be short wave radiation so more likely to bounce out so you lose more heat. i think once it radiated out off a body such as water or a heat store it is a longer wavelengh and more likely to be reflected back in by glass so retaining the radiation which is heat.
Another thing that has occurred to me - if the light is reflected off a white surface such as a floor it will not be reflected perfectly unless you cover the floor with a perfect mirror. White concrete is not going to be even. The radiation will come in as direct sunlight. When that light is bounced off the white uneven surface it will be bouncing around all over the place, it will be a much softer light, dispersed light.It will probably lighten deep shadows that you get in direct sunlight in the way that photographers use a white reflector to fill in shadows.
So it will hit the upper surface of the leaves and affect growth.

 

Quite an interesting bit of info regarding light and plants here Geoff.

http://www.bcb.uwc.ac.za/ecotree/photosynthesis/spectrum.htm

Agree with your explanation of reflection from an uneven surface ( angle of incidence = angle of reflection), but can't see how upwardly reflected light from a floor can turn through 180 degrees onto upper surfaces of leaves unless reflected from a roof or sides.

Edit - Just realised leaves are not horizontal, they hang down at an angle so will get some upwardly reflected light.

[ 01. December 2007, 09:20 AM: Message edited by: Dave W ]

 

Once the light becomes dispersed it will bounce off everything, much more so than direct light. So you get it coming from all directions. That is why on a cloudy day you don't get such deep shadows as you do on a bright sunny day. The light gets dispersed as it comes through the cloud layer and bounces around all over the place.