Cooling Your RoofWe saw a question posed by a reader about spraying water on the roof in the summer time when it is really hot. The idea was to spray water on the roof cooling your roof down and this in turn may decrease the cost of cooling the house with AC. She did this at dusk when the sun was going down and it did cool her roof down significantly and the house was actually cooler as well. She succeeded in cooling her house and she also mentioned that the neighbors thought she was nuts. They wanted to know if this was a smart thing to do and if it would save energy.

Cooling Your Roof

The writer or the blog owner of the site that this idea was written on thought this was a really great idea. He had wanted to do this himself but did not have a hose with a sprayer that was strong enough. There is no doubt that spraying water on the roof will cool the roof. But is this really a smart thing to do?

The reason this has such a cooling effect is that the heat from your roof (and inside your attic) is being used to warm the water and then evaporate it. Evaporation is a cooling process (which is why sweating makes us feel cooler), so you wind up lowering the temperature of the roof itself substantially. This reduces the heat gain from the roof to the attic and from the attic to inside your house.

Concern About Spraying Water on the Roof

We are very concerned about this answer and also that many people might actually even try this. Not only is it a waste of water, but anyone who pays for their water by the gallon is going to get a huge surprise at the end of the  month if they do this very often. The money they save on cooling the house if any will be quickly used up by a higher water bill. Water is expensive in some areas and this action of spraying your roof can end up costing quite a bit.

Another concern is the impact on the shingles. When water is sprayed on the roof it will cool the shingles. There is no doubt of this, but now you have one side very cool and the other is still very hot. Over time this will cause the shingles to twist and crack and prematurely age the roof, which could cost you thousands of dollars in repairs. Any savings you received from cooling the roof in this manner, will quickly be lost with the cost of a replacement roof. A replacement roof will cost several thousand dollars  depending on the size of your roof.

What Should the Home Owner Do

The writer would be better off to make sure that the roof is well vented so that hot air which rises can vent out the roof vents at the top of the roof and cooler air would be drawn in at the bottom of the roof. Placing trees around your house to provide shade and also lowering blinds inside your home to block the sun coming in would be far more efficient. Not only is the air flow automatic when the vents are properly sized, you do not have to remember to spray the roof every day to cool it off.

Depending on the size of your roof and the construction of your home you may need to have more than one roof vent. Each gable and each roof area should have vents that allow the hot air to escape from your attic and dissipate the heat. There should also be sufficient openings that allow cooler air to enter the roof as well at the eaves. Hot air rises and this is what you are trying to take advantage of. Air will flow in at the eaves and out through the roof vents which will have the effect of cooling the roof with no damage to the shingles.

On a large roof you may need to have as many as three or four vents to ensure that you have sufficient air flow to properly cool the air in your attic which in turn will keep the rest of the home cool.

We do not think that spraying water on a roof is a very good idea and would really recommend that home owners not take this approach at all.

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