Optimal House Color in Cold Climate

[ejensen6]

A house with AC in a hot climate should be painted white (or covered with mirrors) to reduce absorption of light from the hotter sun and surrounding objects. That seems obvious.

But what should cover a house in a cold climate? The house interacts with a system that is hotter than it (the sun) and a system that is colder than it (its immediate surroundings), so the analysis is more complicated. In order to minimize losses to surrounding objects, the paint should be a poor emitter of infrared. In order to maximize absorption from the sun, the paint should be a good absorber of visible light (black). I'd like to know a few things to answer my question.

1) Which effect is generally more important, absorbing as much visible light as possible or emitting as little infrared as possible?
2) Is it possible for a material to simultaneously be a good visible absorber and a poor infrared emitter? If so, could you give a specific example?

 

[turbo]

In cold climates, we don't worry about thermal gain from sunlight. The most important factor is to keep in the heat that we have paid for. With conventional studded walls, it is common to achieve this with fiberglass insulation faced with foiled paper (facing toward the heated area). Want to reduce heat-loss further? Use 2x6 studs (and more insulation) instead of 2x4s for better insulation, caulk and seal seams to reduce infiltration, etc.

When it's zero outside, the sun is low, and the wood-stove is cranked up for warmth, thermal gain from the sun is a very minor factor.

 

[Bob_for_short]

1) Which effect is generally more important, absorbing as much visible light as possible or emitting as little infrared as possible?
2) Is it possible for a material to simultaneously be a good visible absorber and a poor infrared emitter? If so, could you give a specific example?

I) The first one, of course. Use the black color. It absorbs everything and transmits to the wall that stays rather cold so no infrared radiation is re-emitted, as a matter of fact.

II) I have no idea.

 

[turbo]

Theoretically, the darker paint would be preferable, but in practical terms it matters little. The amount of work that you have to do to heat or cool a house depends on the temperature differential between the inside and the outside. If you want to keep your house at 70 deg F in colder climates, you'll have a few summer days with a 20 deg differential to overcome with the AC, and quite a few winter days with 70 deg or more differential to overcome.

Come to New England and look at all the old farmhouses - mostly white-painted clapboard exteriors.

 

[ejensen6]

I) The first one, of course. Use the black color. It absorbs everything and transmits to the wall that stays rather cold so no infrared radiation is re-emitted, as a matter of fact.

II) I have no idea.

Bob, what is your proof for your first assertion?

 

[ejensen6]

Theoretically, the darker paint would be preferable.

Turbo, what is your evidence?

 

[Dadface]

A house with AC in a hot climate should be painted white (or covered with mirrors) to reduce absorption of light from the hotter sun and surrounding objects. That seems obvious.

But what should cover a house in a cold climate? The house interacts with a system that is hotter than it (the sun) and a system that is colder than it (its immediate surroundings), so the analysis is more complicated. In order to minimize losses to surrounding objects, the paint should be a poor emitter of infrared. In order to maximize absorption from the sun, the paint should be a good absorber of visible light (black). I'd like to know a few things to answer my question.

1) Which effect is generally more important, absorbing as much visible light as possible or emitting as little infrared as possible?
2) Is it possible for a material to simultaneously be a good visible absorber and a poor infrared emitter? If so, could you give a specific example?

Different colours transmit or reflect some wavelengths better than others and if a colour is a bad net absorber of a particular wavelength when cooler than the surroundings it is also a bad net emitter of that wavelength when warmer than the surroundings.To reduce radiant losses from the walls they should be painted white (ideally silver).The savings,however,will be minimal since most of the heat reaching the outer wall surfaces is lost to the surroundings by convection.
You may be able to get more information by googling "spectral emissivity"

 

[Naty1]

Which effect is generally more important, absorbing as much visible light as possible or emitting as little infrared as possible?

I'm not so sure there is a one size fits all answer. In my own house for example, in NJ, winter sun from the south tends to warm the exterior rear of my house and together with heat gain through south facing windows results in my furnace hardly running. An hour or two after the sun sets, my furnace begins to run normal on/off cycles during the night.

Clearly one effect is the relative number of winter hours of daylight to the number of hours of darkness. The more hours of darkness the more important to emit as little heat as possible from inside where costs. As an extreme example, if there is no visible winter sun why bother trying to capture any solar energy?? Another factor would be average temperature differences between day and night.

Judging from typical passive solar systems, a dark interior mass is most useful for capturing and storing as much solar radiation as possible. Once sunlight disappears, using some type of reflective window covering to keep in the heat is most efficient.

You might check on low E glass to get an idea about heat gains and losses relative to plain glass...

 

[ejensen6]

Different colours transmit or reflect some wavelengths better than others and if a colour is a bad net absorber of a particular wavelength when cooler than the surroundings it is also a bad net emitter of that wavelength when warmer than the surroundings.To reduce radiant losses from the walls they should be painted white (ideally silver).The savings,however,will be minimal since most of the heat reaching the outer wall surfaces is lost to the surroundings by convection.
You may be able to get more information by googling "spectral emissivity"

Dadface, I'm not sure you understand my questions. Your answer looks at only one of the two interactions. Reducing emission makes sense, but this would also tend to reduce the amount of absorbed radiation from the sun. It seems it would be best to get a poor emitter of infrared and a good absorber of visible.

I agree that this effect is probably unimportant from a practical standpoint.

 

[Dadface]

Dadface, I'm not sure you understand my questions. Your answer looks at only one of the two interactions. Reducing emission makes sense, but this would also tend to reduce the amount of absorbed radiation from the sun. It seems it would be best to get a poor emitter of infrared and a good absorber of visible.

I agree that this effect is probably unimportant from a practical standpoint.

If I understand you correcly you need a colour that has a high emissivity for a band of wavelengths in the visible part of the spectrum and a low emissivity for a band of wavelengths in the IR part of the spectrum.I cannot think of an easy way to find a paint with the characteristics you need a major problem being that emissivity is temperature dependant.

 

[ejensen6]

OK, it looks like we have 4 different answers to question 1:
1) Paint it black to maximize visible absorption.
2) Cover it with foil to minimize IR emission.
3) It really doesn't matter.
4) It depends.
I'm going with 3 and 4 unless someone can provide evidence for 1 or 2.

Regarding the second question, the low E glass is a good example of transmitting visible while reflecting IR.