Q: I have a friend who has mounted soaker hoses on each side of the ridge row of his roof to help cool it. The hoses are connected to a sprinkler timer and come on six times during the hottest part of the day. They stay on long enough to wet the whole roof. Our houses are about the same size, but his house is two-story with a gabled roof and has a smaller roof area than mine.
My house is single-story with a hip roof. One day I tested cooling the roof with a sprinkler. I think the water covered about 60 percent of the roof area. The temperature in my attic was about 10 degrees lower than the day before, but the air conditioner seemed to run just as much as usual. So my questions are: What would be the long-term effect on a composition roof of wetting it to cool it, and do you think there would be any savings by doing so? –Larry T.
A: This is an interesting question, and I’m glad you did the experiment.
Your home is going to gain heat from a number of different sources, and that’s what your air conditioner is trying to offset. Those sources include heat from the sun through your roof and exterior walls; ambient heat from the outside air, through the roof, walls and glass; and interior heat-makers such as appliances, electronics, and, of course, occupants.
The walls, ceiling and floor of the house make up what’s known as the home’s envelope, and we live inside that envelope of cooled or heated air. The better the envelope does at keeping heat out in summer (and cold out in the winter), the more comfortable the occupants are.
There are many ways to improve the performance of the home’s envelope. What you’re trying to do is cool the attic through an external means — applying water to the roofing. While in theory that’s going to be helpful, it has an awful lot of drawbacks as well, and as you’ve seen through your experiment, it has only minimal benefits to the actual attic temperatures.
First of all, this technique wastes an awful lot of water. And assuming you have to pay for the water — even if you’re drawing it from a well, you have to pay for the electricity to pump it — you’re wasting money. Those costs are going to offset any potential savings you’re going to realize from reducing the attic temperatures.
All that water also has to go somewhere, so you’re also increasing the potential for problems in your basement or crawlspace.
Another issue is what the constant flow of water across the roofing is doing. It could cause premature aging of the shingles and washing away of the granules, as well as the formation of mildew. Unfortunately, if you ever have a problem with your roofing, this would almost certainly void the warranty as well.
What I’d like to see you do instead are passive techniques to improve the overall performance of the envelope. These require a single, upfront expenditure of time and money, but no ongoing expense, so they’re better for your wallet and better for the environment. Many will also pay benefits during the winter as well. These include:
- Increase the amount of attic insulation so that the heat that’s in the attic has less opportunity to get into the house.
- Verify that you have the proper amount of attic ventilation, which is the best way to cool your attic. You should have about 1 square foot of vent area for every 300 square feet of attic space, roughly split between high and low, or with a little bit more on the low side. Add more vents if needed.
- Plant trees that will shade the roof of the house, especially on the hot sides of the house.
- Install insulated window coverings or solar shades, and keep them down during the hottest parts of the day to decrease heat gain.
- Install exterior awnings or pergolas that are slanted to block summer sunlight over windows that get the most heat gain, especially west-facing windows.
- Whenever possible, run appliances at night, such as dishwashers and clothes dryers.
- Consider installing a whole-house fan to take advantage of cool outside air, and take the load off your air conditioner.
- Replace older, single-pane windows and skylights, especially those with metal frames, with energy-efficient vinyl- or wood-frame insulated glass windows.
Q: What about a Tung oil finish (for wood floors)? Where do I get it? That’s what our out-of-business finisher used. –Karhoff
A: It really depends on what you mean by a Tung oil finish. Pure Tung oil is exactly that — an oil. It’s sometimes used for finishing floors, but it’s not one of the more common floor finishes. You can read a good article about pure Tung oil floor finishes at realmilkpaint.com/floortung.html.
More common is a Tung oil blend such as Waterlox. It has greater durability and water resistance, and is typically what floor finishers are referring to when they talk about a Tung oil finish. You can get their product through a variety of retailers, but their website is waterlox.com.