I think there should be a factual answer, but Mods feel free to move this to IMHO is you disagree.
I live in Northwest Montana at the western edge of the Rocky Mountains. While it’s not as hot here as in other parts of the country, with our long summer days (the sun doesn’t set until 9:30 pm) and with a full western exposure my house heats up significantly in the late afternoon.
In fact, the hottest part of the day here if often 5 or 6 pm. Once the sun sets the temperature drops and it is usually in the mid-50’s when I go out at 8 am.
My question has to do with cooling our 3,500 sf house in the summer. We have a ground source heat pump that circulates fluid through plastic pipes that are buried 6 feet under ground where the temperature is always 65 degrees. This constant temperature provides a differential both in the summer and the winter. The only cost is the electricity to circulate the fluid around and the fans to push the cool air throughout the house.
I’ve been told my system is efficient, but given that water is free (I only have to pay for the electricity on my well pump) would if be more efficient to set up a system that streams cool water over my roof, collects the water and then sends it back into the ground? My house is fairly new and well insulated, but I assume most of the heat load is coming from my large roof.
Would cool water running over my roof provide better cooling than running the GSHP with fans pushing the air around?
I realize that it may not be practical to have running water on my roof for hours at a time, but if I choose to do that would it be more efficient than what I have now?
you would like your attic floor to be well insulated and your attic well ventilated day and night.
at night pull cool outside air through your house cooling the building and its contents. close the windows and home ventilation when the outside temperature is higher than the inside.
The water may be free…ish, but it isn’t unlimited. You’ll run your well dry trying to cool off your house with the water from it.
You definitely can’t send the water back to the well. Well water doesn’t work that way, and the water would be totally unfit for consumption after running over your roof.
You can install a small spray on the roof with the intention of it (mostly) evaporating… no need to collect the run off, just wet the roof and let it evaporate.
However because of the insulation of the roof, its not a way to cool the insides, like if you get home and the insides are hot, its a bit late… You’d have to spray all day to prevent the roof heating the insides.
If you were going to do this you’d want to spray a fine mist over the roof and west wall. The droplets will absorb infra-red and cool through evaporation, and you won’t need as much water. I’ve seen this attempted with simple spray misters, but I don’t know if that’s very efficient. Evaporative cooling usually requires a high pressure system that can spray very tiny droplets over a wide area. It all might still cost you more in electricity.
IMHO, if the sole method of cooling is cooling the roof only during daylight hours, then the short answer is no. During the day (say, 11 a.m. - 4 p.m.) with the sun at a high angle heating the roof, you would gain a cooling effect from water running on the roof. However, this process would have little affect on the thermal heating of exterior walls that heat during the day and transfer that heat to the interior of the structure.
As the day progresses, the walls become warmer from not only sunlight but also ambient air temp and this affects all sides of the exterior structure. Moving into the p.m., more heat is transferred to the interior because as you know, heat seeks cool and the walls would most likely overcome any cooling effect produced from cooling the roof. I believe the dwelling would be most uncomfortable during the late afternoon and evening until the outside air temp has lowered sufficiently to cool the exterior walls so as to reverse the heat transfer process.
Okay. Not a great idea I admit. I am seeing more a more restaurants with misting systems for outside dining during the summer.
A misting system would make my patio decks a lot more comfortable and usable in the summer and would be a lot cheaper than trying to cool the entire house by watering the roof…