Water on roof to cool shop?

[Bob Cooper]

All this talk about I insulation got me to thinking about something I've been curious about. If I were to put a sprinkler zone on the roof would that help a lot with cooling -- just the evaporation itself

[Jim Neeley]

If your roof gets hot and you have a lot of solar heat gain through it, spraying water on the roof can help to cool it, assuming the roof is sloped so it doesn't pool.

Unless your roof isn't insulated, is sloped and your water is cool, I don't know how much it would cool it once it was hot.

It wouldn't take too much to try it out.. use a garden hose and put a sprinkler on the roof to test it.

This assumes you aren't on a well (electricity to pump it) or pay by the gallon for water.

Jim

[Jim O'Dell]

Or on water rationing like we are, then you can only cool your shop with a sprinkler between the hours of 7 and 10 pm, one day per week. :rolleyes::D Jim.

[Joe Mioux]

Or perhaps plant some grass up there .... Green roof would be more environmentally friendly, no?

[Bob Cooper]

on a river so water is abundant. Much of the house roof = cathedral so there's really no attic. Yep it's insulated but i wonder sometimes how much ventilation flows through there. 12,12 pitch so no issues w/pooling.

[David Wong]

I have tried spraying water on the roof of my old dining room, which was an attached 9x12 structure. The roof was asphalt shingled and sloped, with southern exposure. The water helped very marginally. In my situation, it really was not worth the trouble.

Besides insulation, such as a radiant barrier under the roof covering, I would attempt to block radiant gain before it hits your building. On my current house, I have 90% solar block sun shades on the outside of windows with southern or western exposure. It makes a big difference. You could try this on your uninsulated walls. A light colored roof will help as well.

[John Hays]

So the interior of the house goes all the way up to the roof? Never seen that. :eek:

[Chris Parks]

I always thought that the best way of preventing heat build up in a roof cavity was to put the roof into shade. Not as hard as it sounds using the shade cloth available these days and some anchored posts.

[Charles Wiggins]

I always thought that the best way of preventing heat build up in a roof cavity was to put the roof into shade. Not as hard as it sounds using the shade cloth available these days and some anchored posts.

I've seen where folks did this with a mobile home. I've even seen where folks built a freestanding structure like a carport over their trailer.

[Jim German]

My dad did this back before we had central air. I don't recall it being particularly effective, adding more insulation or a ductless AC is probably a much better option.

[Phil Thien]

I switch to white shingles in the summer, then put the dark ones back on for winter.

[Thomas Bank]

So the interior of the house goes all the way up to the roof? Never seen that. :eek:

Really? Cathedral ceilings seem to be a standard selling point in new homes these days.

http://www.rogersrealty.net/wp-content/uploads/2010/07/5-vaulted-ceilings.jpg

I'm sort of thinking that the water cooling idea wouldn't be the most efficient option. Even if the water is free (river water, as you say) you still have to pay to pump it. Then there is the question of accumulation of sediment and bio-growth on the roof - or filtering to prevent it.

Chris' idea of a shade structure would likely be the more economical route. Basically the same principle as the heat shield above the muffler on your car - heat up the "shield" and then air passes between the shield and your roof so that the roof doesn't absorb the heat.

Phil brings up a point. White membrane roofs were getting a lot of marketing a number of years ago as the ultimate solution to solar gain - until they started doing some studies and realized that in northern climates you hit a point where the disadvantage of added cooling in the summer for a dark roof was not as great as the benefit in the winter of that same solar gain. Southern climates didn't have to worry about not getting the winter gain, so a light colored roof nearly always makes sense.

[Michael Weber]

lol, I thought I was the only one that had ever tried this. I did it a couple of years ago. I do data acquisition for a living so I had to the equipment to record all the temperature readings. Have the graphs here in front of me at work. Data was taken on two consecutive cloudless days in the summer. Both days the temperature outside was measured at 112 degrees.:eek: In addition to the outside temperature I placed thermocouples in the attic at the peak, and above and below the insulation as well as one at midpoint between the peak and insulation. I used a pulse sprinkler at the peak (dark grey shingles) turned down as low as it would go and continue to circle around. This was still more than needed to keep the roof wet as I had quite a bit of runoff down the gutters. Here is what I found.
Dry Roof
attic peak high temp. 133 degrees.
attic midpoint high 118 degrees
above insulation high temp. 94 degrees
below insulation 85 degrees.
Wet Roof
attic peak high temp. 105 degrees.
attic midpoint high 100 degrees
above insulation high temp. 88 degrees
below insulation 83 degrees.
In addition I placed a thermocouple on the bedroom ceiling. Also, one at a grill outlet register and one at the outlet of the AC coil mainly to see how often the AC was running. All this was upstairs in a two story house. AC size is delibertly small in order to dehumidify to the maximum. I have a split system with one 2 1/2 ton unit downstairs and one 2 1/2 ton unit upstairs.

Results of the test on the ceiling and register temperatures were interesting.
Dry roof:
Max. ceiling temperature 85 degrees
Max. outlet register temperaure 71 degrees
Wet roof:.
Max. ceiling temperature 83 degrees
Max. outlet register temperaure 68 degrees
Both days the AC ran continuously from Noon until 10PM.

During the test the indoor thermostat was set at 78 degrees and the AC failed to maintain that. As the evening wore on the room temp began to slowly rise. On the graphs its clear why the indoor temp rose in the evening. Its' because of the lag caused by the insulation on the temperature of the ceiling. The ceiling was acting as a heat radiator into the cooler rooms. Although it was only a few degrees difference the large surface area made up for it. At one time I calculated how many BTU's were being radiated but can't find that data.

At the time of this test the attic had only about 5 inches of insulation. I have since increased to a total of 15 inches and my AC cycles. Planning on adding additional venting to the roof with a solar vent and additional soffit vents. I have the graphs. If anyone is interested in looking at them I can try to scan them and post. It's interesting to see how temperature below the insulation climb long after maximium temperature have passed above the insulation.

[Bob Cooper]

Micheal...you are the man. I was thinking about this whole thing and what 'i'd need to do to really measure what kind of difference it would make' ... and you've already done it.

i'd thought about the 'cover over the roof' (an umbrella) but seems kind of impractical for a house not to mention my wife would probably rightly want to shoot me (this is encouraged in the South).

[Michael Weber]

Thanks Bob. I've never been "The Man" before.:cool: For me the only gain of any significance was a drop in the grill register temperatures of 3 or 4 degrees. I had less loss in my duct work which is in the attic positioned around the midpoint between the peak and attic floor. This was caused by a nearly 20 degree drop in temperatures at that level with the wet roof reflected in less heat gain to the air moving through the duct. Duct had typical 1 inch foil faced insulation. I didn't measure the air on the inlet side of the coils. But assuming a typical 18-20 degree drop across a coil, a 4 degree difference would be about a 20% efficiency increase. I was using city water and my lawn irrigation circuit. Sadly, I did not think to measure how much water I used so don't know if any efficiency gains would be offset by water charges. I was mainly just curious as I had been thinking about it for a good while. If it had really shown significant gains at the lower attic temperature readings I would have built some kind of

[John Hays]

Really? Cathedral ceilings seem to be a standard selling point in new homes these days.

No, I've seen cathedral ceilings before, just never knew there was no space between the interior and roof. I would think that there has to be enough room for an atic fan.

Btw, nice living room. :cool:

[Jim Neeley]

John,

That's what referred to (At least in the Arctic) as a "hot roof" vs one with an attic that's called a "cold roof".

[Thomas Bank]

No, I've seen cathedral ceilings before, just never knew there was no space between the interior and roof. I would think that there has to be enough room for an atic fan.

Typically the roof sheathing is on the top of the rafters and the drywall is on the bottom of them with only insulation in between.

[John Hays]

Typically the roof sheathing is on the top of the rafters and the drywall is on the bottom of them with only insulation in between.

Well I guess you learn something new every day, eh? Is the insulation effective enough compaired to having an attic with fan ventilation? And if not, how do builders get away with that in the South?

[Jim Neeley]

It is not as good at rejecting solar heat as an attic-style with a passive or active fan (or even good vents) but..

(and I'm from the North but I'll guess)

..What the little lady wants, the little lady gets! :-) It does make the room look larger than it is.

Jim

[Jim Andrew]

I've built several houses with a sloped ceiling less than the roof pitch. That way you have the advantage of an attic with the cathedral ceiling look. Usually just set the peak of the ceiling 3 or 4' above the walls.

[Peter Hartman]

I actually read a book on this subject. From what research I have done, you would need to spend considerable time setting this system up. You would want some sort of mister or drip irrigation on a timer. The goal is for all the water to evaporate before it hits the gutter. If it is not evaporating you are using to much water. I think if a system was laid out well this would really make a big difference. Extremely humid climates would make this less effective. I think some where like Boise where you get super cheap water for your lawn and the air is dry, this system would be awesome.

http://www.knowledgepublications.com/978-1-60322-005-7_detail_page.htm

[Jeff Erbele]

I switch to white shingles in the summer, then put the dark ones back on for winter.

LOL! Too funny :)

[Jeff Erbele]

Instead of irrigating the roof, I think one might get more bang for the buck from a variety of other ways including insulating the ceiling and maybe walls, installing one or more ceiling fans, and/or exhaust fan, or installing a swamp cooler or air conditioner depending on your typical humidity.

[Rick Potter]

Speaking of humidity, and considering where you live, wouldn't this add to your problem indirectly?

If the bottoms of your rafters are accessible you might consider stapling foil to them. The bottom of the roof sheathing on my addition has foil attached, and the building inspector says it lowers the attic temp about 15 degrees. I can tell the difference between the new room and the old.

Rick Potter