Why do I need proper attic ventilation?
Attics can reach a stifling 165 degrees in the Midwest summers and even higher temperatures in the south. Ventilation exhausts this hot air to the outside and serves to keep the home more comfortable. Also, ventilation helps to protect other parts of the structure such as roof framing and sheeting.
I've found that standard passive ventilation like ridge vents and punched aluminum soffit vents do not work well. I've cut into enough roofs and felt the hot air rush out.
What about insulation?
Insulation prevents the conduction of heat from warmer to cooler areas.
The thicker the insulation, the longer it takes to equalize the temperature on both sides. In a 150 degree attic with a 70 degree second floor, the insulation will be somewhere between this.
At night as the attic begins to cool, the insulation will hold the heat and therefore will even be hotter that the attic air.
Ductwork in attic?
If you have a 2 story house, odds are you have ductwork somewhere in your attic buried beneath the insulation.
If your air conditioning is properly tuned, there will be a 20 degree drop against the A-coil. Meaning if the air returning to the blower compartment is 72 degrees then the air above the coil should be 52. Just a side note: having a 20 degree drop doesn't mean your system is dialed in, but not having the drop confirms it. Losing just 5 degrees of heat from the plenum (top of furnace) to the upstairs vents cuts your effective capacity 25%. The hotter the attic means the larger amount of temperature loss, leaving the insulation even hotter. A 12 degree temperature loss is not uncommon, meaning a loss of 60% of capacity.
Here is a typical scenario: The home empties out in the morning and the thermostat raises up to 80 degrees. As it gets warmer throughout the day the attic gets hotter and the insulation gets even more so. At 430pm, the thermostat is set to drop the temperature to 70 degrees. It is 77 degrees in the house, meaning that the discharge temperature at the coil is 57 degrees. And because your a/c is probably not tuned correctly, that 57 degree air might be 61 degrees when it hits the hot duct in the attic and could be 71 degrees at the vent, leaving your poor a/c to run constantly in a futile attempt to cool the house.
Though its cooling off outside, your left fuming inside because your house won't cool off for several more hours.
What's the Solution?
The solution is better ventilation, specifically, mechanical ventilation. Solar powered fans do not work. They simply don't move enough air around, possibly cutting the temperature by 5 degrees, but it won't be enough for you to notice.
A quality fan that is installed correctly should have at least a 1500 cfm rating. The fan should be installed in a gable wall preferably with a vent in an opposite wall. Thermostat controls in the attic will turn on the fan whenever the attic is above 90 degrees and the attic is hotter than the outside air. Look carefully at the picture: if the fan is installed like this it will not work and will probably curse me for leading you astray. The vent needs to be sealed completely with aluminum, plywood, ductboard, etc around the fan ducting to be effective.
