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Old 07-30-2022, 10:33 AM   #34
DickR
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Quote:
Originally Posted by John Mercier View Post
It is the mixture of heat/humidity relative to the ambient air outside the attic.

If the heat/humidity in the attic is much higher than the ambient, the venting system is not functioning properly.
For humidity to be higher in the attic than outside most of the time, there must be a source of moisture from within that is migrating into the attic. This could be from interior moisture-producing activity (eg. showers, cooking, aquarium, laundry). It also could be from a damp basement, with air leakage migrating up to the attic through wiring penetrations in the walls and even open passages, such as around chimneys. The source of the moisture must be found and dealt with properly; attic ventilation is just the band-aid, not the solution for a humid attic.

As to attic temperature being much higher than ambient, "much" is relative. With the sun shining on the roof, the attic certainly will be substantially hotter than outside air. With proper soffit and ridge ventilation, it's the air density difference due to the temperature difference that drives ventilation air flow. Using a powered fan to ventilate a hot attic will reduce attic temperature somewhat, but that's a wasteful, less-effective, and sometimes counter-productive way to address the discomfort issue.

Besides the direct conduction of heat through an inadequately insulated attic floor, a hot attic also enhances the "stack effect" (like the draft up a chimney produced by hot combustion gases). Without good air sealing between attic and the living space, a hot attic draws some of the air flowing out the ridge vent from the living space below, and that increases leakage of hot/humid air from outside into the living space. That, in turn, produces discomfort and increases load on an AC system. Stack effect air leakage also produces discomfort in the way of cold drafty areas, or at least higher heating bills, in winter.

A good energy audit, by someone competent, is a good first start for an existing house. A blower door test with IR photography can locate sources of air leakage and even rain leaks in the roof or around windows. That's a good starting point for a thorough air sealing project, much of which can be done by the homeowner. It's the "low-hanging fruit." For a new house, it's much better to make the house well insulated, much better than "to code," and very tight, with mechanical ventilation for controllable interior air quality. Not doing so for a new house is a wasted opportunity.
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