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by coryrc 152 days ago
Forced ventilation of the attic creates negative pressure in the attic, which pulls conditioned air from the house. The additional air movement (which you should minimize with air sealing) costs more than the additional loses by the mildly increased temperature differential through the insulation.

If you aren't using A/C and have the windows open, then it only helps, of course.
2 comments
WalterBright 152 days ago
The idea includes gaps where the roof meets the ceiling, so air is drawn from outside.

> If you aren't using A/C and have the windows open, then it only helps, of course.

I make use of the "stack effect" to cool the house down in the evening. Not even a fan is necessary.

One mistake I made was to not have the A/C pull from the basement, which is always 10 degrees cooler than the rest of the house.

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coryrc 151 days ago
> The idea includes gaps where the roof meets the ceiling, so air is drawn from outside.

Of course you should have soffit venting, but forced ventilation results in negative pressure, regardless of whether that venting is a cupola harnessing the wind or a motorized fan. That negative pressure will pull conditioned air out and building science research shows higher attic temperatures are better than forced ventilation.

Even better is larger amounts of passive ventilation, but if the only thing you change is adding forced ventilation, then you're creating negative pressure and this is worse if using A/C.

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scheme271 152 days ago
Unless you have really good insulation, in hotter areas, your roof will absorb a lot of heat and that gets transferred to the attic and then to the rest of the house. One of the cheapest and best upgrades in hotter areas is to have an attic fan and vents to send the hot attic air outside.
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WalterBright 152 days ago
> attic fan

The cupola does that using the "stack effect" and the acceleration of the wind as it hits the sloping roof, at zero cost!

That said, I do have an attic fan connected to a thermostat. I'd rather have a cupola, though.

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coryrc 151 days ago
Building science shows this is not true if you use A/C and have appropriate amount of ceiling insulation. Inadequate ceiling insulation is helped by this.
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