7 ms·

If the roof assembly is decently insulated, then relatively little of that 10kW heats the house. So, while improvement is available, it’s not nearly that large.

by amluto 4y ago

hn_throwaway_99 4y ago
Still, though, I have to wonder what the equivalent "CO2 reduction" would be considering that a white roof is likely to radiate most of the incoming sunlight back into space, where if it is radiating in infrared then that heat is more likely to be trapped on Earth.
- throwaway3334 4y ago
  For now, everyone is free to pump out kilowatts of heat with their roofs, parking lots, air conditioners into the surrounding air. It is not considered pollution, even when it is already 100 °F outside. On the days when there is an inversion in the atmosphere, the hot air stays trapped. People have to run ACs in their houses, stores and offices. And that results in even more heat routed to the place, with ACs units, literally pumping energy from the solar arrays somewhere in the desert into the city.
  - rocqua 4y ago
    My intuition says that the volume of inside vs outside air is so big that it has little effect.
    - throwaway3334 4y ago
      You can look at how much space roofs cover in the suburb in the Bay Area. It'd be roughly 25% streets, 40% roofs, 30% backyard, 5% front-yard. Adding white reflective coating increases "Solar Reflectance" and decreases "Thermal Emittance" and absorption. Most dark roof materials reflect 5 to 20% of incoming sunlight, while light-colored roof materials typically reflect 55 to 90%. A white roof coating that you can purchase at your regular home improvement store would do that 90%. So effectively, in the suburban area there is an opportunity to change our average solar reflectance by ~32% [(90% - 10%) * 40%]. If we just abandon the idea that a good house should look like a house in Normandy and have a black roof. [edit: I've opened a section of sat map (in Sunnyvale, near Reed Ave/Sequoia Dr.) and estimated roof/street/backyard/front-yard by area, for a lot of 2 houses and adjacent street. The numbers above are from that estimate. Heat and noise of ACs is somewhat local (urban heat island effect, etc), so that local estimate is what counts. It is not surprising that we can have 40% roofs. Land is expensive in the suburb like that.] As per: https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate_engineering) https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate... - "If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. So it seems, there is not only local/urban heat island effect, but even some global effect. To put this into perspective, Tesla sold 2 million vehicles. No idea, if the Wiki numbers are correct...
      - uxp100 4y ago
        I suspect that there are good stats somewhere, but I think that 40% roofs might be a pretty high over estimate in say, Sunnyvale, or even San Jose.
        rch 4y ago
        I'm not aware of any stats, but I've experienced the difference between hot neighborhoods and cooler neighborhoods, right next to each other in Houston. It would be great to have a quantitative measure for this effect, similar to a walk score (which isn't perfect, but can be occasionally useful).
        kolinko 4y ago
        Are you sure it’s because of AC and not the amount of concrete pathways compared to green areas, vegetation and shade from trees? Also various thermal properties of the ground? Even if houses were 50% of the land area, they take up a miniscule amount of air volume. Also, hot air floats up, so there is little chance you could feel the heat from an AC outside. On the other hand, if the earth is hot because you have few trees, you will feel it everywhere when walking on a sidewalk. In Europe at least it’s common knowledge that you need to have trees and greenery everywhere to make heat bearable outside. If you replace trees and grass with concrete you get an island of heat that will be unbearable during summer to walk through. edit: thermal camera image of a walkway with trees in summer https://images.app.goo.gl/m2XN7rqodavDcwoE9 https://images.app.goo.gl/m2XN7rqodavDcwoE9
  - 8bitsrule 4y ago
    At this time last year, this region was subjected to a 'heat dome'. Luckily this building has a 'cool dome' in the basement. Spent a few days listening, reading and solving down there (12-16°C cooler). Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious. It's a low-cost, low-tech, zero-energy solution, used by native communities in many regions of the world.
    - smaudet 4y ago
      This and geo-thermal. The crazy megalomaniacs pushing solar are as bad as the rest of oil/coal kingpins, unsustainable and another ecological train-wreck in progress.
    - wcoenen 4y ago
      > Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious. When the London metro was built (over a century ago), the clay was at 14 degrees C. But over time, it heated up from all the power dissipated by the metro system. Now it is around 20 to 25 degrees C and doesn't really act as a heat sink anymore. The same happened in the NY subway. So soil can act as a great heat sink for a while, but not forever. I suspect the same would happen if you'd put a modern city underground and relied on the temperature of the soil instead of pumping heat out somehow.
- amluto 4y ago
  Indeed. https://en.m.wikipedia.org/wiki/Urban_heat_island https://en.m.wikipedia.org/wiki/Urban_heat_island https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate_engineering) https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate...
AngryData 4y ago
Yes this. It helps but it isn't a magic bullet because the roof is already airgapped from the roof and the house insulated from the attic. If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat. That is why there are roof vents, gable vents, and soffit vents, to let air freely flow into and out of the attic/roof cavity.
- DubiousPusher 4y ago
  > If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat. This is an old school carpentry trope and yet I've never actually seen any statistical evidence indicating that roofs over vaulted ceilings which are directly insulated must be replaced more often than non-vaulted.
  - cpwright 4y ago
    Anecdotally we have about 1/3 of our roof with a vaulted ceiling and the other 2/3 with an attic. The side with an attic is the side that would be suitable for solar panels (more sun at the right angle) or adjacent to the vaulted ceilings. The leaks that came up were all on the insulated vaulted ceiling side; starting about 25 years old, and we had another every couple years for about five years until replacing the roof. The attic parts never had any problems.
    - SoftTalker 4y ago
      We have a vaulted ceiling and there is still an air gap under the roof. The ceiling "vault" is separately framed; there is still a foot or two of air space between the top of the vault and the roof deck.
  - AngryData 4y ago
    In every vaulted ceiling ive ever seen or worked on it still has had a significant air gap between the insulation and the roof and it was still vented with roof and soffit vents. Also the areas that tend to leak before any other part of the roof because it is hard to vent enough air through such a small space. And nearly every roof ive worked on that had curled shingles lacked proper roof ventilation. That isn't direct evidence, but ive never seen curled shingles on a well ventilated roof either and the only common factor in curled shingles is heat buildup in the attic/roof cavity.

If the roof assembly is decently insulated, then relatively little of that 10kW heats the house. So, while improvement is available, it’s not nearly that large.

by amluto 4y ago

If the roof assembly is decently insulated, then relatively little of that 10kW heats the house. So, while improvement is available, it’s not nearly that large.

hn_throwaway_99 4y ago

Still, though, I have to wonder what the equivalent "CO2 reduction" would be considering that a white roof is likely to radiate most of the incoming sunlight back into space, where if it is radiating in infrared then that heat is more likely to be trapped on Earth.

throwaway3334 4y ago
For now, everyone is free to pump out kilowatts of heat with their roofs, parking lots, air conditioners into the surrounding air. It is not considered pollution, even when it is already 100 °F outside. On the days when there is an inversion in the atmosphere, the hot air stays trapped. People have to run ACs in their houses, stores and offices. And that results in even more heat routed to the place, with ACs units, literally pumping energy from the solar arrays somewhere in the desert into the city.
- rocqua 4y ago
  My intuition says that the volume of inside vs outside air is so big that it has little effect.
  - throwaway3334 4y ago
    You can look at how much space roofs cover in the suburb in the Bay Area. It'd be roughly 25% streets, 40% roofs, 30% backyard, 5% front-yard. Adding white reflective coating increases "Solar Reflectance" and decreases "Thermal Emittance" and absorption. Most dark roof materials reflect 5 to 20% of incoming sunlight, while light-colored roof materials typically reflect 55 to 90%. A white roof coating that you can purchase at your regular home improvement store would do that 90%. So effectively, in the suburban area there is an opportunity to change our average solar reflectance by ~32% [(90% - 10%) * 40%]. If we just abandon the idea that a good house should look like a house in Normandy and have a black roof. [edit: I've opened a section of sat map (in Sunnyvale, near Reed Ave/Sequoia Dr.) and estimated roof/street/backyard/front-yard by area, for a lot of 2 houses and adjacent street. The numbers above are from that estimate. Heat and noise of ACs is somewhat local (urban heat island effect, etc), so that local estimate is what counts. It is not surprising that we can have 40% roofs. Land is expensive in the suburb like that.] As per: https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate_engineering) https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate... - "If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. So it seems, there is not only local/urban heat island effect, but even some global effect. To put this into perspective, Tesla sold 2 million vehicles. No idea, if the Wiki numbers are correct...
    - uxp100 4y ago
      I suspect that there are good stats somewhere, but I think that 40% roofs might be a pretty high over estimate in say, Sunnyvale, or even San Jose.
      - rch 4y ago
        I'm not aware of any stats, but I've experienced the difference between hot neighborhoods and cooler neighborhoods, right next to each other in Houston. It would be great to have a quantitative measure for this effect, similar to a walk score (which isn't perfect, but can be occasionally useful).
        kolinko 4y ago
        Are you sure it’s because of AC and not the amount of concrete pathways compared to green areas, vegetation and shade from trees? Also various thermal properties of the ground? Even if houses were 50% of the land area, they take up a miniscule amount of air volume. Also, hot air floats up, so there is little chance you could feel the heat from an AC outside. On the other hand, if the earth is hot because you have few trees, you will feel it everywhere when walking on a sidewalk. In Europe at least it’s common knowledge that you need to have trees and greenery everywhere to make heat bearable outside. If you replace trees and grass with concrete you get an island of heat that will be unbearable during summer to walk through. edit: thermal camera image of a walkway with trees in summer https://images.app.goo.gl/m2XN7rqodavDcwoE9 https://images.app.goo.gl/m2XN7rqodavDcwoE9
- 8bitsrule 4y ago
  At this time last year, this region was subjected to a 'heat dome'. Luckily this building has a 'cool dome' in the basement. Spent a few days listening, reading and solving down there (12-16°C cooler). Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious. It's a low-cost, low-tech, zero-energy solution, used by native communities in many regions of the world.
  - smaudet 4y ago
    This and geo-thermal. The crazy megalomaniacs pushing solar are as bad as the rest of oil/coal kingpins, unsustainable and another ecological train-wreck in progress.
  - wcoenen 4y ago
    > Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious. When the London metro was built (over a century ago), the clay was at 14 degrees C. But over time, it heated up from all the power dissipated by the metro system. Now it is around 20 to 25 degrees C and doesn't really act as a heat sink anymore. The same happened in the NY subway. So soil can act as a great heat sink for a while, but not forever. I suspect the same would happen if you'd put a modern city underground and relied on the temperature of the soil instead of pumping heat out somehow.
amluto 4y ago
Indeed. https://en.m.wikipedia.org/wiki/Urban_heat_island https://en.m.wikipedia.org/wiki/Urban_heat_island https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate_engineering) https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate...

AngryData 4y ago

Yes this. It helps but it isn't a magic bullet because the roof is already airgapped from the roof and the house insulated from the attic. If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat. That is why there are roof vents, gable vents, and soffit vents, to let air freely flow into and out of the attic/roof cavity.

DubiousPusher 4y ago
> If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat. This is an old school carpentry trope and yet I've never actually seen any statistical evidence indicating that roofs over vaulted ceilings which are directly insulated must be replaced more often than non-vaulted.
- cpwright 4y ago
  Anecdotally we have about 1/3 of our roof with a vaulted ceiling and the other 2/3 with an attic. The side with an attic is the side that would be suitable for solar panels (more sun at the right angle) or adjacent to the vaulted ceilings. The leaks that came up were all on the insulated vaulted ceiling side; starting about 25 years old, and we had another every couple years for about five years until replacing the roof. The attic parts never had any problems.
  - SoftTalker 4y ago
    We have a vaulted ceiling and there is still an air gap under the roof. The ceiling "vault" is separately framed; there is still a foot or two of air space between the top of the vault and the roof deck.
- AngryData 4y ago
  In every vaulted ceiling ive ever seen or worked on it still has had a significant air gap between the insulation and the roof and it was still vented with roof and soffit vents. Also the areas that tend to leak before any other part of the roof because it is hard to vent enough air through such a small space. And nearly every roof ive worked on that had curled shingles lacked proper roof ventilation. That isn't direct evidence, but ive never seen curled shingles on a well ventilated roof either and the only common factor in curled shingles is heat buildup in the attic/roof cavity.