Paint it Black?

[Stuart]

Was just wondering if anyone have painted any external walls black or dark colours in order to capture heat from the sun. Every little helps...as they say.

[martin]

I think the problem is the lack of "one way valve" - my understanding is that painting a wall black will enable it to take in more heat, but unless you say pull down a mirror-surfaced blind over it as soon as the sun goes in, the losses will be higher too...... And sod's law being what it is, the time you most need the heat is in midwinter, when there ain't a lot of sun - midsummer there's bags of it, and the last thing you want is an even hotter house.........

[billi]

sure dark blue was our choice  and an attached polytunel on the south façade

[dhaslam]

Can be useful  on the outside of the wall, particularly west facing.   The wall heats up and acts as a patio heater in the evening.   Difficulty is that a black wall on an all white house  might be a bit too much of a contrast.  

[Heinz]

as soon as the sun goes in, the losses will be higher too......

Why would the losses be higher? surely the dark colour increases the absorption of heat but doesn't affect the loss of it??

Heinz

[martin]

I remember my old physics lessons involving large black iron balls, blowlamps, and teachers muttering about "perfect black bodies" (to muffled sniggering from the back  -I still get thoughts of a starkers Halle Berry.....) Unless I misremember, if it's black it both absorbs and radiates more.........

[Greenbeast]

Yes google black body radiation, Black is a great absorber and emitter of radiation

[welshboy]

To have any effect the wall would have to be thin.
Heat travels at about an inch per hour which is why Trombe walls are only 4 to 10 max inches thick.

[EccentricAnomaly]

I remember my old physics lessons involving large black iron balls, blowlamps, and teachers muttering about "perfect black bodies" (to muffled sniggering from the back  -I still get thoughts of a starkers Halle Berry.....) Unless I misremember, if it's black it both absorbs and radiates more.........

Yes, but is radiation the main mechanism for heat loss from a wall at night, or is it convection?

One of quite a few reasons why roof insulation is important compared to wall insulation is that the roof can be exposed to a clear sky (with an effective radiant temperature of maybe -30°C) over most of the hemisphere it can "see" whereas a wall typically "sees" objects of only slightly lower temperature over considerably more than half of its hemisphere.

Another question is what the emissivity of black paint is, compared with natural bricks or whatever, at thermal infra-red wavelengths.  I did read of some observations a few years ago which I could only interpret as meaning that green paint would do better as a selective coating.  Unfortunately, the person reporting this seemed to think that green was good at absorbing sunlight otherwise plants wouldn't be that colour (whereas the green colour is just an incidental consequence of the way photosynthesis works (Google fodder: PAR = photosynthetically active radiation, though a quite a few of the hits are about growing cannabis) so I couldn't get more details from him.

Some experiments with Ivan's thermal imaging camera or just an infrared thermometer might help.

I suspect a better approach would be putting a layer of polycarbonate or greenhouse film on battens on the wall.

[DaveSnafu]

When I was a lad I used to tune 2 stroke bike engines, we always painted the heads and barrels black in the hope it would cool better, (and it looked cool) don,t really know if it worked or not, we still melted pistons left right and center.

[dhaslam]

My solar panels seem to go below ambient temperature at night but I am not sure whether this is just inaccuracy in the sensors.   Even in December they can still reach 70C  while being cooled by the fans.    I think that for a black wall the heating effect would be far higher than the cooling effect.    One problem with an old house would be that the heat would drive moisture from the wall into the house so  it would be better to  use a panel with insulation behind.    It would be very effective in the kind of sunny weather we have had recently but only suitable for a house that doesn't already have high solar gain. 

Has anyone , apart from Ivan, used the Navitron ones.  They would produce some heat on dull days as well.   

[skyewright]

One of quite a few reasons why roof insulation is important compared to wall insulation is that the roof can be exposed to a clear sky (with an effective radiant temperature of maybe -30°C) over most of the hemisphere it can "see" whereas a wall typically "sees" objects of only slightly lower temperature over considerably more than half of its hemisphere.

That effect can be used to add a remarkably reliable cheap nighttime cloud cover detector to a weather station based on the temperature difference between the standard outdoor temp sensor and another that is placed close by in an enclosed environment (e.g. jar) that can only "see" the sky (insulation in the bottom of the jar helps). On a cloudy night the temps are the same, and once calibrated on a good clear night to get the temp difference for 100% clear you can deduce cloud cover by comparing the measured difference against the difference for 100%.

I did read of some observations a few years ago which I could only interpret as meaning that green paint would do better as a selective coating

I'm not sure if it was for efficiency or for aesthetic reasons but the house wall inside the "collector" on this Irish 'Solar House' looks to be a dark green:
http://www.viking-house.ie/solar-house.html

[Dyslexicbloke]

I am sure I read somewhere that aluminium, despite being shiny, has a big difference between what it absorbs and what it radiates which makes it get comparatively hot in the sun.
Its better than some flat black paint apparently.

I only remember because it is one of the most reflective materials you can get when polished, hence PAR lamps ( Polished Aluminium Reflector )

That said I guess a silver wall isn’t overly aesthetic either and I don’t know if aluminium paint would behave like foil or sheet.

Possibly just the mis-musings of a confused mind …

Al

[Ivan]

It may have a difference between absorbance and emissivity, but it's not something I was aware of.

I can categorically say that shiny aluminium absorbs heat much less than a painted black surface, as I've done the experiment in school, years ago to prove it, with tin foil.

But we do certainly get colours wrong - shouldn't domestic radiators be black? Roofs should be white? I'd say walls need to be white - in the UK we use far more energy keeping houses warm than keeping them cool. So heatloss during the winter should be our over-riding concern. Heatloss is lower for light-colours than for dark colours, so unless we are going to paint the walls with selective absorbers (expensive), then light colours are to be preferred.

Wiki(heat)leaks says (http://en.wikipedia.org/wiki/Thermal_radiation ) that colours make only minor differences with radiation at everyday temperatures, but that Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature - which means that white houses might have 7% less thermal emissions than black houses - which would be a significant reduction in heating costs.

Silver is indeed a very poor emitter. I've seen this first-hand with my thermal imaging camera. A normal radiator looks hot. However, a towel rail does not look hot, except where a towel (ie a good insulator) is draped over it - which always seems a bit weird. The towel is being heated by conduction, and because it's not shiny or silver, it radiates better than the radiator itself. So towel rails with towels on give off more heat!

[A.L.]

which means that white houses might have 7% less thermal emissions than black houses

- black paints are not necessarily more like the perfect black body emitter than white paints - I would expect the difference to be very much less if any

I'd say walls need to be white

given the above are you not forgetting the heating effect of visible light absorbed at the wall surface of dark walls?