5 ms·

Can a coating on the outside of a material really have an appreciable impact on heat transfer from inside to outside, at night? I get that a coating can block o

Can a coating on the outside of a material really have an appreciable impact on heat transfer from inside to outside, at night? I get that a coating can block or admit light or radiation coming from somewhere else, but surely if the material itself is hot then thermal conduction within the solid will make the coating itself hot, and shed heat, too? Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? Or if not, why are we still covering homes with big chunks of styrofoam, polyurethane and mineral wool? Among architects, I'm seeing a lot more hype about high thermal mass, e.g. sand-lime bricks and woodfiber insulation, to design houses that respond very slowly to outside temperatures.

londons_explore 4y ago
There are benefits to all these approaches, and combining them all gives best results. But specifically on coatings, it's true that conduction and convection also play a role, and radiation is only part of the picture. However, it is theoretically possible to design a coating that can cool a material by 45C (~80F) compared to ambient at night, and practically a cooling of 4C (8F) has been achieved [1]. 4C is pretty substantial, considering it stacks on top of the effects of insulation, high thermal mass, and efficient heating/cooling appliances. Remember that in the daytime the effect is far more substantial - the difference between the best and worst thermal coatings could easily be 100C - when people say 'that rock is so hot I could fry an egg on it', it's simply due to the absorption properties of the rock under sunlight. [1]: https://www.mdpi.com/2079-6412/10/2/144 https://www.mdpi.com/2079-6412/10/2/144
- shanebellone 4y ago
  Are they attempting to generate electricity from these coatings yet? Sounds like both the "keep warm" and "keep cold" coatings offer that possibility.
  - londons_explore 4y ago
    Sadly it's very hard to generate much energy from small differences in temperature.
    - shanebellone 4y ago
      "Sadly it's very hard to generate much energy from small differences in temperature." Nighttime electric power generation at a density of 50 mW/m2 via radiative cooling of a photovoltaic cell https://aip.scitation.org/doi/full/10.1063/5.0085205 https://aip.scitation.org/doi/full/10.1063/5.0085205 Sure, but it does provide some interesting opportunities for embedded technology (LEDs, security cameras, Bluetooth, etc.).
- stdbrouw 4y ago
  That's 45C "with the absence of nonradiative heat exchange" though, but you're right, that is still impressive. Thanks for explaining.
shanebellone 4y ago
"Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? " It sounds like this paint is a "heat shield" that modifies the rate of thermal transfer. Shouldn't that mechanism improve the performance of the existing substructure and insulation?
londons_explore 4y ago
> why are we still covering homes with big chunks of styrofoam The energy loss through foam depends on the temperature on each side of it. So if you can use a fancy coating to keep the outside of your house cool/hot in the day/night, then the energy losses through the styrofoam are reduced, and eventually your heating/cooling bill is reduced.
Bluestrike2 4y ago
> Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? Or if not, why are we still covering homes with big chunks of styrofoam, polyurethane and mineral wool? If a home is well-built, the wall assembly already includes a physical gap between the structural sheathing or exterior insulation (if present) and whatever cladding materials you're using.[0] That gap allows for airflow and drainage behind your cladding, so that the water that gets behind it--and it will--can drain out and everything can dry. That gap would help create something of a thermal break (the cladding isn't attached directly to the house but to furring strips with a very low surface area), but it would still heat up the air in the gap itself. In that case, reflective paints would probably still be beneficial. That said, I think their greatest benefit would probably be in roofing. Metal roofs, for example, also require an air gap[1] behind they can get extremely hot. 0. https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-wall-advanced-frame-mineral-fiber-board https://buildingscience.com/documents/enclosures-that-work/h... 1. https://www.youtube.com/watch?v=yesjnBnPt0A https://www.youtube.com/watch?v=yesjnBnPt0A

londons_explore 4y ago

There are benefits to all these approaches, and combining them all gives best results. But specifically on coatings, it's true that conduction and convection also play a role, and radiation is only part of the picture. However, it is theoretically possible to design a coating that can cool a material by 45C (~80F) compared to ambient at night, and practically a cooling of 4C (8F) has been achieved [1]. 4C is pretty substantial, considering it stacks on top of the effects of insulation, high thermal mass, and efficient heating/cooling appliances. Remember that in the daytime the effect is far more substantial - the difference between the best and worst thermal coatings could easily be 100C - when people say 'that rock is so hot I could fry an egg on it', it's simply due to the absorption properties of the rock under sunlight. [1]: https://www.mdpi.com/2079-6412/10/2/144 https://www.mdpi.com/2079-6412/10/2/144

shanebellone 4y ago
Are they attempting to generate electricity from these coatings yet? Sounds like both the "keep warm" and "keep cold" coatings offer that possibility.
- londons_explore 4y ago
  Sadly it's very hard to generate much energy from small differences in temperature.
  - shanebellone 4y ago
    "Sadly it's very hard to generate much energy from small differences in temperature." Nighttime electric power generation at a density of 50 mW/m2 via radiative cooling of a photovoltaic cell https://aip.scitation.org/doi/full/10.1063/5.0085205 https://aip.scitation.org/doi/full/10.1063/5.0085205 Sure, but it does provide some interesting opportunities for embedded technology (LEDs, security cameras, Bluetooth, etc.).
stdbrouw 4y ago
That's 45C "with the absence of nonradiative heat exchange" though, but you're right, that is still impressive. Thanks for explaining.

shanebellone 4y ago

"Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? " It sounds like this paint is a "heat shield" that modifies the rate of thermal transfer. Shouldn't that mechanism improve the performance of the existing substructure and insulation?

londons_explore 4y ago

> why are we still covering homes with big chunks of styrofoam The energy loss through foam depends on the temperature on each side of it. So if you can use a fancy coating to keep the outside of your house cool/hot in the day/night, then the energy losses through the styrofoam are reduced, and eventually your heating/cooling bill is reduced.

Bluestrike2 4y ago

> Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? Or if not, why are we still covering homes with big chunks of styrofoam, polyurethane and mineral wool? If a home is well-built, the wall assembly already includes a physical gap between the structural sheathing or exterior insulation (if present) and whatever cladding materials you're using.[0] That gap allows for airflow and drainage behind your cladding, so that the water that gets behind it--and it will--can drain out and everything can dry. That gap would help create something of a thermal break (the cladding isn't attached directly to the house but to furring strips with a very low surface area), but it would still heat up the air in the gap itself. In that case, reflective paints would probably still be beneficial. That said, I think their greatest benefit would probably be in roofing. Metal roofs, for example, also require an air gap[1] behind they can get extremely hot. 0. https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-wall-advanced-frame-mineral-fiber-board https://buildingscience.com/documents/enclosures-that-work/h... 1. https://www.youtube.com/watch?v=yesjnBnPt0A https://www.youtube.com/watch?v=yesjnBnPt0A