Can a Sprayed Metal Roof Act as a Cooler and Radiator for a Room?

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Discussion Overview

The discussion revolves around the feasibility of using a sprayed metal roof as a cooling mechanism for a room, particularly in humid environments. Participants explore the potential of cooling a metal roof by applying groundwater during peak solar hours and the implications of this method on indoor temperatures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that spraying a metal roof with groundwater could create a temperature difference of at least 10 degrees Celsius, potentially allowing a fan to blow cooler air into the room.
  • Another participant questions the effectiveness of the cooling process, emphasizing that the metal must lose heat faster than it gains it for the method to work.
  • A participant shares personal experience with roof cavities reaching high temperatures and suggests that cooling the roof with water could be effective, drawing parallels to their own practices of cooling outdoor surfaces.
  • There is a proposal that if the roof remains significantly cooler than the indoor temperature, ceiling fans could help circulate cooler air into the room, functioning similarly to a heat exchanger.
  • Concerns are raised about the potential for rust and oxidation due to water exposure, especially with hard groundwater, although some participants mention protective coatings that could mitigate this issue.
  • One participant expresses an intention to implement a solar-powered system to automate the spraying of water on the roof based on temperature conditions.
  • There is a discussion about the dual effects of evaporative cooling and thermal conduction occurring simultaneously when water is applied to the roof.

Areas of Agreement / Disagreement

Participants express a mix of support and skepticism regarding the proposed cooling method. While some believe it could work under certain conditions, others highlight the need for careful consideration of heat transfer dynamics and potential issues like rust. No consensus is reached on the overall effectiveness of the approach.

Contextual Notes

Participants note various assumptions, such as the need for the roof to lose heat effectively and the impact of environmental conditions on the cooling process. The discussion also touches on the importance of material properties and the potential for oxidation due to water exposure.

georgemdn
First of all i must say that i have lack of knowledge about physics. I have this silly idea to spray my metal roof in a humid area.

No lack of groundwater and I am intending to sprinkle my metal roof with groundwater during solar peakhours. The roof is total only 30m2. I understand that thermal conductivity of metal is such that my roof will be cooler underneath if on top flow of cooler water is realized. By cooler i mean a temp difference of at least 10 degree C. So if i put a fan under the roof can the roof function as a sort of radiator and the fan blowing cooler air to the room?

Hope my question is understood.
 
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You are thinking that the water carries heat off the top of the roof, cooling it down ... then the fan blows under the roof... um...
Perhaps you are thinking that there are holes in the roof - water flowing over the roof, and the fan sucks hot air through the cool roof, cooling the air in the process?

For the metal to cool anything, it has to lose heat faster than it gains it.
I don't think this process will have a noticable difference but you can easily rig up a proof-of-concept prototype on a smaller scale to see.
 
georgemdn said:
First of all i must say that i have lack of knowledge about physics. I have this silly idea to spray my metal roof in a humid area.

No lack of groundwater and I am intending to sprinkle my metal roof with groundwater during solar peakhours. The roof is total only 30m2. I understand that thermal conductivity of metal is such that my roof will be cooler underneath if on top flow of cooler water is realized. By cooler i mean a temp difference of at least 10 degree C. So if i put a fan under the roof can the roof function as a sort of radiator and the fan blowing cooler air to the room?

Hope my question is understood.

I've been in roof cavities that are over 50 deg C, (outside air temp ~30 deg C). Almost all that heat entering the cavity is conducted through the tin, which is too hot to touch. I often hose down my concrete patio & timber decking when it can gets too hot for bare feet, it works well and I think your plan could will work.

Coincidentally, I was installing simple heat recovery systems, which was just a fan or two and a series of ducts that would blow roof cavity air into the house. The fan would come on depending on the the set point, house temp & cavity temp. Generally, in summer that'd be at night when the cavity is cooler than the house, in winter it'd be the middle of the day if the sun has warmed the cavity above the house temp. I've lived in 4 houses with them, their effectiveness varies from house to house, depending on construction and environment, most of the commercial units (like the link above) are wildly overpriced for what they are.
 
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billy_joule said:
I've been in roof cavities that are over 50 deg C, (outside air temp ~30 deg C). Almost all that heat entering the cavity is conducted through the tin, which is too hot to touch. I often hose down my concrete patio & timber decking when it can gets too hot for bare feet, it works well and I think your plan could will work.

Coincidentally, I was installing simple heat recovery systems, which was just a fan or two and a series of ducts that would blow roof cavity air into the house. The fan would come on depending on the the set point, house temp & cavity temp. Generally, in summer that'd be at night when the cavity is cooler than the house, in winter it'd be the middle of the day if the sun has warmed the cavity above the house temp. I've lived in 4 houses with them, their effectiveness varies from house to house, depending on construction and environment, most of the commercial units (like the link above) are wildly overpriced for what they are.
Thanks for your reply. Interesting.
Continiously sprinkling metall roof with (cool) water during sunny hours, thus not on basis of avaporation, will certainly cool the metal roof. Here an example of someone in thailand :

If the temperature of that wet roof is substantially cooler than the inside temp of the room, can for example ceiling fan(s) under the roof cool down the room when there is no attic at all. Can the colder roof function like a radiator?
 
Forgot to say that the guy of the example youtube vid circulates water. My plan is watering the roof with cooler groundwater.
 
If the temperature of that wet roof is substantially cooler than the inside temp of the room, can for example ceiling fan(s) under the roof cool down the room when there is no attic at all. Can the colder roof function like a radiator?
In principle, yes - it will act as a heat-exchanger.
You just need to be carrying heat away from the roof faster than it arrives... you should do better than the guy in the vid since you don't need to recirculate.
 
Simon Bridge said:
In principle, yes - it will act as a heat-exchanger.
You just need to be carrying heat away from the roof faster than it arrives... you should do better than the guy in the vid since you don't need to recirculate.
Thanks again for your reply. I intend to repaint my metal roof with white reflecting coating. After coating i want to implement this silly idea of continiously spraying the roof during peak hours with a solar powered pump only when the panel gives a certain volt or amp, assuming thus its hot outside :)

If you think that the roof will work like an heat exchanger i will certainly give it a try.

Assuming outside temp with ful sun is 37c. Inside temp cause of passive cooling=31.
Groundwater is 24c. Roof is 30m2. What can the result be?? Volume of water not important for me cause its groundwater
 
Don't forget about rust. Water and metals = oxidation. Especially with hard ground water.
 
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Kevin McHugh said:
Don't forget about rust. Water and metals = oxidation. Especially with hard ground water.

I have zinc aluminium coated with i think waterproof paint. I am not sure. But when i will (re)paint it white there is a special waterproof paint available.

Ground water will be filtered first. I am pumping now ground water to my watertank which is filtered. Incoming pipe is at the same level as the highest point of my roof. With small adjustments i can sprinkle the roof :)
 
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OK, evaporative cooling it is then :cool:
 
  • #11
Im thinking of CONTINIOUS spraying the roof when its very very hot. Evaporation is a bit different. You sprinkle with time intervals.
 
  • #12
You will have both effects going on, evaporative cooling and themal conduction.
 
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