While on the road today, I had a thought so obvious there has to be a flaw. Please tell me what I overlooked.
Summer is here. Air conditioners are humming. As we all know, a hot attic leads to a hot house. So, how do you cool the attic?
Well, the coolest air is in the crawlspace. Crawl spaces also are usually vented so as to prevent moisture problems.
What if we used a duct to ‘pipe’ that cool, moist air into the attic – where it would cool the attic on its’ way out the roof vents? We could even give it a push by using a humidity-controlled fan.
Remember … the moist air would be getting warmer, so there would be no opportunity for condensation.
What have I overlooked? Why isn’t this done?
Replies
Ignoring any humidity issues ect,
How long you think it's gonna take before the crawlspace is filled with hot air?
Joe H
"a hot attic leads to a hot
"a hot attic leads to a hot house"
Not if the ceiling is insulated properly.
What Boss said.
Also, you can get convection-powered roof fans that will keep ambient air circulating through the attic space.
What, pray tell, is a "convection powered" roof fan?
Roof turbine, I think.
I think he's refering to those wind powered roof turbines.
If the attic space is properly designed, it shouldn't be a source of heat for the house:
1. Here, it's code for an attic to be vented at the eaves, and most have a series of roof vents too. On a breezy day, air enters the windward eaves, and exits thorugh roof vents and leeward eaves. On a calm day it enters both sets of eave vents, and exits the roof vents. My attic is seldom more than 10 degrees warmer than outside ambient.
2. If you live in a cooling environment, the only reasonable colour for your roof is WHITE. Indeed, try for a roof using titanium dioxide as the pigment, as it's both a good reflecter of visible light, and a good emmitter of infrared.
3. Cooling the ceiling at the expense of heating the floor is a dubious win.
Instead, put some fans on a timer or better on differential thermostats. The attic fan is mounted in a gable, and pulls air out of the attic whenever the interior temp is above the exterior. Similarly for the crawlspace.
We cool our house passively. At night we open the patio doors and the upstairs windows. By morning the entire house is cool. The house is massive enough that it never gets more than about 5-10 degrees above outside temps, and to do that requires a series of muggy days with no night time cooling.
We live in a climate with 10,000 degree days of heating. And from April to October the furnace never runs. (Although on a rainy day evening we will often build a fire in the living room stove.
Blow another 8" of insulation into your attic. I've got 16" currently. It takes two people an afternoon.
For folks with basements before AC and after gravity coal furnaces were replace by gas, that was standard operating proceedure in the summer.
Open the basement door, take the panel off that there newfangled gas furnace that has a blower, just turn on the fan - presto, cool basement air into the house. Common practice in the 1950 IIRC.
Fill the crawl with hot air? Well, you might have a point; such would make the system have very limited effectiveness. My model assumes that the earth below will be a source of infinite cooling capacity, reducing the tempreature of the outside air as it passes through the crawl space. If this heat transfer is ineffective, then the idea fails.
As for comments by others ...
As humid as our outside air is, I suspect that the ground under the house is even more moist. The whole reasoning behind venting crawl spaces at all is based upon this assumption. I am proposing to 'vent' this air out through the roof, by way of the attic - rather than out the leeward crawl space vents.
Against the 'natural' direction of air flow? I think not. Sun-heated air will want to exit the attic- and the cooler crawl space air will be drawn into the attic to fill the void. Where, it will be heated, and the cycle repeat.
A fan? Probably a good idea in any event. One can easily have the fan (below) controlled by a humidistat, or a temperature-controlled fan at the attic side. The second, in particular, would help address the question of what to do when winter comes.
I won't deny that there are code requirements for attic venting. The problems arise from the simple fact that so many code-compliant roofs don't get any but minimal ventilation- and there seems to be no downside to increasing that ventilation. I'm not even getting into the issue of blocked vents, etc.
Heat won't pass through a 'properly insulated' ceiling. Balderdash. I've experienced several instances where the addition of shade greatly reduced the temperature within even 'super-insulated' roof decks. Rather than heat the insulation, I'd rather heat the air within the attic - then get rid of that air (replacing it with moist crawl space air.) I see the moisture content as a 'plus,' simply because water can carry far more heat than air alone.
Folks used to let the basement air flow through the house for cooling? I did not know that. Maybe this idea has some merit ... only I'm not passing through the house, I'm proposing going straight to the attic.
FWIW, in a desert clime I did have great success venting the hotter air near the ceiling into the attic - and thence out through the attic turbine. Heck, even the addition of the turbine to the existing static vent made a HUGE difference.
I'm no longer in the desert. Quite the opposite; here summer air is so hot and humid the locals refer to it as 'breathing Jello." The water table is high enough to preclude basements.
My calculations say I need 42,000 btu/hr of cooling for this house. As I see it, anything I can do to reduce that load is a plus. Since my remodel involves opening the walls, adding a duct presents little additional effort or expense- something that can't be said for a new roof, new insulation, new windows, or an air conditioner. Thus, my skepicism that something that simple could be effective.
Unless someone comes up with a better explanition of 'what's wrong,' I'm quite tempted to give it a try.
Against the 'natural' direction of air flow? I think not. Sun-heated air will want to exit the attic- and the cooler crawl space air will be drawn into the attic to fill the void. Where, it will be heated, and the cycle repeat.
Think about it. You have two columns of air: One is the air (that you hope is) rising through the duct from the crawl, the other is the outside air. The outside air is warmer, hence lighter. The crawl air is colder and heavier. It's not going to rise up that pipe voluntarily.
My model assumes that the earth below will be a source of infinite cooling capacity, reducing the tempreature of the outside air as it passes through the crawl space.
Yes, the earth has an essentially infinite cooling capacity. It's just that the effective heat transfer rate is quite low. A bit like a water heater -- you've got a "tank" of cold air down there, but once that's used up you need to "recover" via heat transfer with the earth. That's not going to happen very rapidly, because the air will stratify in the crawl and because soil is a pretty good insulator.