Paint It Black - Your GH Wall!

ââ?¬Å?I See a Red Wall and I Want It Painted Blackââ?¬Â

Several months ago there was quite a bit of interesting discussion regarding passive heating for green houses. Lots of ideas and theories were put up and discussed and enlarged upon.
My GH (a former pig sty) has a concrete floor and high, double brick thick, south facing rear wall. Several years ago I painted the walls white in order to reflect light, but after the discussion on GC decided that in theory black might be a better colour as it should help the walls absorb more heat during the day and release it at night and/or when the internal air temperature fell below that of the brick. In spring I took myself off to B&Q (on a Wednesday which = 10% discount cos Iââ?¬â?¢m quite ââ?¬Ë?matureââ?¬â?¢) and bought a pot of black wall paint and painted half the floor and three quarters of the rear wall black. I didnââ?¬â?¢t finish the job because I ran out of paint!
As spring and summer came and went I had the feeling that the black paint was having the desired effect. To put it simply ââ?¬â?? on hot days the black wall absorbed heat thus helping to reduce over-heating and when the internal air temperature fell, the heat stored in the wall acted like a storage heater and provided some heat to the GH air.
I didn�t really have any way of verifying of quantifying the effect of the black paint until a couple of weeks ago when I managed to repair a small digital thermometer that has a needle probe. I�ve drilled one small hole in the black area of wall and another in the remaining white section and have been sampling temperatures over the last few days. I�ve measured outside air temperature, GH air, black wall and white wall. The results are shown below and as might be expected the black wall so far has always been at a higher temperature than the white area. Note that at present there is no bubble wrap up and that due to recently replacing part of the GH roof there are still some gaps where the corrugated roof rests on the wooden frame. Once I fill the gaps and cut down on draughts and put up the bubble insulation I would expect more heat to be available for absorption by the brick.
Now I�ve got the thermometer fixed I�ll continue to sample through the winter and hopefully should , in due course, have data that is more useful.


The data -

 

I`m impressed Dave. I don`t think I would have the patience.

 

Sampling only takes 5 minutes and I'm in and out of the GH several times a day Dave.

 

Dave, I'm still getting to grips with the whole thing but at first sight it looks interesting.
Do you have any idea as to the effect the black wall has at night on the temp of the greenhouse.
I supose its difficult to work out, but if the wall temp tends to be higher than the GH air temp, we could assume its helping to heat the greenhouse.

 

Hi Pete
Yes I'm going on the fact that heat moves from warm to cold, so that at night, or at any time the air falls below wall temperature the wall will heat the air in the GH until the air and wall reach the same temperature. It's a far from perfectly efficient heat source as the wall is also losing heat to the outside at a greater rate as the outside temp is even lower than the internal one. Anyway, efficient or not, every free degree of heat is worthwhile. If I was really keen I'd insulate the outer surface of the wall as it faces north and doesn't really gather any significant heat.

 

Free heat is always worth having.

 

You must be right , Dave W - that is why in sunny Countries you find more light colour houses, clothes etc and the opposite in cold Countries

 

What you could also try if you have the equipment and the room is store a large container of water in there. The water would absorb the heat during the day and release it at night. Though even an extra layer of bricks will also have an effect.
What you could do is make a note of which data is for sunny days and which is cloudy. The radiation on a sunny day would be more direct while on cloudy days it is more dispersed. That might affect its absorption properties. Black will of course cut down on light. White walls are used to bounce the light around within the structure.
When measuring the temperatures screen the sensor from sunlight so you measure the air temperature and not the temperature of light falling on glass or metal.
Its a pity its an outside wall at the back cause as you say much of the stored heat is going to go out that way and in fact that could end up colder on a cold night than the greenhouse.

 

Hi Geoff.
I agree about using water as a thermal mass as it is extremely efficient and about double the mass of brick or concrete (lost my tables but I do know that concrete is better than brick as is rammed earth). I watched the BBC "It's Not Easy Being Green" and couldn't understand why Dick Strawbridge used broken glass as a heat sink in his GH rather than small plastic bottles of water, which would have had very much greater thermal mass.
I am taking note of the weather conditions (see the table) and the variation between cloudy and sunny is already apparent. What for me anyway, will be interesting will be when I add the bubble wrap insulation to the GH and I can compare the effects of the added layer with the 'bare' GH.
Really wishing I still had my old BBC computer as I had software and probes that allowed me to sample temperatures over a long period at a variety of intervals. (I used it in school to show the kids how leaving doors open in winter made the temperature drop.)

 

Initial rersults seem to suggest that there isn't a lot of difference between white and black with dispersed light but there is much more so in direct sun. What do you think would be the best colour for water containers - black or clear plastic?

 

That question has got me thinking Geoff. My gut feeling is to go for black containers on the basis that black painted brick (or whatever) absorbs heat better. I do know that a black garden hose in summer scoots out a jet of pretty hot water until the fresh mains water starts flowing. I'll need to try another experiment and measure the temperatures of a transparent lemonade bottle full of water and a black painted bottle when stood in the sun and shade.
Another thing that has just come to mind is that if water is being used as a heat sink, is what effect would the introduction of a dark dye have? My guess is not very much, as thermal mass seems to be dependent on molecular structure. Could be totally wrong though, my degrees are not in physics!

As the chineese philosopher Mei Dung Pile said in 1927.
"Black pyjamas for the masses and stuff the frost and cold" :D

[ 20. November 2007, 01:29 AM: Message edited by: Dave W ]

 

Great stuff chaps, and well done Dave! - I do like to see a bit of science occasionally squeezing through myth and folklore!

I think there are three factors to be taken into account. Black is a good absorber of light and also a good radiator - they always go together. This means that during the day in the winter it will keep more of the sun's rays and the GH will be warmer. But it also means that on a clear night it will be radiating more heat out of the GH. If there is more heat to start with the extra radiation at night may not be a problem. But it is not quite so clear cut to me. After a cloudy winter's day with little heat being absorded, followed by a clear frosty night and you could lose out. The ideal thing would be to draw the curtains at night.

I don't think a black wall will reduce overheating on a hot day - rather the opposite. A black wall will absorb all the suns rays and the resultant heat will warm up both the wall and the GH. A white wall would reflect some of the rays back out of the GH, and the wall and GH would both lose out in temperature.

A heat sink, such as a wall or barrel of water will store the heat, and I would love to know how big the effect was in practice. I once went around the Northern Electricity Board building in Harrogate which used a swimming pool in the basement as a heat sink. The claimed that they could maintain 70C inside the building when it was about 40C outside - with no additional heating over that given out by the lights and office machinery. A black dye in the water won't increase the specific heat (thermal mass) of water, but it would make a transparent container act as if it were painted black.

The other aspect is surface area and heat exchange rate. Car radiators have a lot of fins to speed up the heat exchange. A barrel of water really also needs fins. Perhaps that was the attraction of the broken glass. Maybe surface area is more important than thermal mass.

 

Dave, I am sure that at one time there was an insulating material which was like kitchen foil only with one surface painted matt black. This allows heat to be absorbed through the black surface but the shiny surface would prevent heat-loss.

 

Thanks David. I think I've used similar stuff behind radiators - it's a thin black foam with a silvered foil coating. It can also be cut to make thermal in-soles for boots.

 

Peter, something that I may well be wrong on but troubles me, on what you said about the black wall radiating more heat out of a greenhouse, on a cold night.
Would the stored heat, not be released from the wall by convection, rather than radiation, and thus be trapped to some extent by the glass.
Surely the wall warms by radiation but cools by convection.