Window fan does the trick for me on all but the most humid days. I close up the house early in the morning and close/open pleated shades throughout the day according to the sun's travel. It stays at least
10 degrees cooler than outside. Once the temperature drops outside so that it's cooler out there, I open one upstairs window in an unused bedroom, put in the window fan blowing out, and open the downstairs windows. Shortly before bed, I close the downstairs windows and open my upstairs bedroom windows. BTW, this is a cheap window fan, ~$20, in a 1600+ sq ft house. I have a very hot attic but, like yours, a lot of insulation. Lots of shade trees that keep the yard too dark to garden, but none of them shade the house (great planning on the part of whoever planted them, eh? Yes, the house is old enough that it's been here longer than the trees.)
Even at 80 deg., a ceiling fan would make you feel cooler..won't cool the house but would increase comfort level. Perhaps an exhaust fan in the upstairs window that would draw cooler air into the house with a window open downstairs/shady side. For windows on the sunny side, cheap white or reflective window shades would keep a good deal of heat out.
I have one of those remote deals built into an alarm clock - it's a neat little gadget I picked up at Target. comes with a remote thermometer and the clock has an atomic clock receiver built in. I might see if I can get another remote to go with it, as I don't want to move the one I have inside; I use it to determine when to open the upstairs windows.
I also have one of those point and shoot infrared thermometer deals; I might have to retrieve that from my buddy's garage next time I make it over there. Not much use in the attic, but I could shoot the ceiling in the bedrooms to see how much heat is really coming down from above.
I suspect that the eventual solution will be a fan in the attic, but that would involve probably having someone install it, which isn't about to happen this year. I'm envisioning a fan set up at one of the vent openings blowing out, controlled by a thermoswitch somewhere in the middle of the attic to come on at some set point (100 degrees? 110 degrees? Something hotter than any normal ambient outside temperature, anyway.) However your point to actually collect data before committing to that course of action is well taken.
I realize it would be just as easy to install a window A/C unit in the bedroom, but the brute force method offends my sensibilities as an engineer. I'd rather try the more efficient solutions first...
For those of us who's most recent engineering, physics, thermo, fluids, etc courses were a LONG time ago (early 60's for me), please expand your very-interesting text with perhaps a few definitions, therefores, etc, so we can better *understand* your surely-excellent points.
Now that I finally think of it, re those hot ceilings, hot via the attic, don't forget to consider the heat you yourself receive via RADIATION from that ceiling.
Still air is a poor heat conductor, about R5 per inch. A square foot of still air 1" thick is like a 5 ohm resistor, for heatflow through the 1" thickness. DEFINITION: Ohm's law for heatflow is just like Ohm's law for electricity with different units, thermal resistances vs ohms, Btu/h vs amps, and Fahrenheit temperature differences instead of voltage diffs. For example, 4 Btu/h flows in this circuit, viewed in a fixed font:
THEREFORE, if a person generates 300 Btu/h at 100 F internally with a 91 F skin temp (measured with a $50 Raytek IR thermometer), we have something like this, with an internal resistance Ri and a skin surface resistance Ra in 70 F slow-moving air:
91 F Ri | Ra ---------www-------------www------- | --------------------> | | I = 300 Btu/h | | 100 F | 70 F --- --- - - | | | | --- --- - -
300Ri = (100-91) makes Ri = 0.03 and 300Ra = (91-70) makes Ra = 0.07, no?
If the slow moving air has a film conductance Ua = 2 Btu/h-F-ft^2 and Ra = 1/(UaAs) = 0.07, the person might have As = 1/(RaUa) = 7 ft^2 of exposed skin, out of 20 ft^2 of total average Dubois skin surface.
Now if the room temp rises to 85 F and everything else is the same, the skin temp rises to 85+300Ra = 106 F, very uncomfortable. What to do? Rest vs work, eg a siesta to lower heat generation, which can vary by 10:1, depending on activity, or lose weight, since heat output increases with body mass, or decrease Ri (some people in Arizona adapt to heat with more blood flow near the skin and higher skin temps), or evaporate sweat, or stand in a bucket of water, or increase the airspeed near the skin, and/or remove clothing.
To be equally comfortable with a 91 F skin temp, we might reduce Ra until
300Ra = (91-85), ie Ra = 0.02 or Ga = 1/Ra = 50 = AsUa. Increasing airspeed to 6 mph (enough to blow papers off desks) makes Ua = 2+6/2 = 5 Btu/h-F-ft^2 and As = 10 ft^2, with more exposed skin, and so on.
DEFINITION: fans are thermal conductors. A 1000 cfm airstream with a dT (F) temperature difference moves about 1000dT Btu/h, ie it has a conductance of about 1000 Btu/h-F.
And 1 ft^2 of 1/2" drywall is like a 0.5 farad capacitor...
DEFINITION: an RC time constant is a measure of how fast a circuit reacts to change.
The wall starts at Tsurf = Ti and becomes T(t) = Tout + (Ti-Tout)e^(-t/RC) t hours after the fan starts. Initially, T(t) = Tout + (Ti-Tout)e^(-0/RC) = Ti. After a very long time, T(t) = Tout + (Ti-Tout)e^(-oo/RC) = Tout. Nice, huh? Most of this happens in 3 or 4 time constants. In between, the exponential function e^(-t/RC) gradually squashes the initial temp diff (Ti-Tout) to 0 over time.
Adding resistors in series, RC = (1/500+1/(2x880))440 = 1.13 hours.
RC = 20ft^2-F-h/Btux0.5Btu/F-ft^2 = 10 hours.
... as before, with a final temp and a negative initial temp diff that gets stomped down to 0 over time by the cruel but patient exponential.
Same old stuff, with slower stomping.
Basements have lots of thermal mass. If we don't live in them, we can cool them below the comfort temp, but we have to be careful to avoid blowing moist outdoor air through a basement, with condensation.
The beauty police would write a ticket for awnings, and they're pretty much unnecessary today with low-e thermopane glass. Awnings only keep the sun out, they don't stop the heat from radiating through the glass, nor do they stop the ultraviolet rays. Low-e thermopane glass does all of that, and in the winter the low-e coating keeps your heat inside the house.
Even with top of the line Milguard low e, awnings are a big necessity when the sun is on the window glass for hours at a time.
Low E only reduces the transfer of ambient outside heat, it is insufficient at reducing the transfer of direct radiation when the sun hits the glass. And awnings most certainly reduce ultraviolet rays as well as low e.
It HELPS to retain heat, but is not an absolute barrier. It certainly does allow some heat to radiate out.
Ok, I'll buy that. Here, on my ranch-style house, my roof overhang is about
2.5 feet which is great in the summer. The sun is high enough in the sky that the roof blocks the sun from hitting the windows. In the winter when the sun is much lower on the horizon, the sun does hit the windows, but who cares in the winter.
I would put up trees if I could - that would be the best solution - but my front yard which faces south is only 10 feet deep.
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