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Attic fan. Thermostat or humidistat?

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Hi David,

Personally, I think that a humidistat makes more sense for an attic fan. After all, if you're in a hot climate the roof and attic will be hot anyway. Sucking a bunch of hot air into the attic under the eaves or through gable end vents doesn't make a whole lot of sense. The only way it will cool a house is if there isn't any insulation at all in the attic. If there is insulation, the insulation should already be reducing heat gain from the attic.

On the other hand, a humidistat would only operate when humidity is at unacceptable levels and run until it has been lowered. I think one question to ask is whether the humidistate should be sensing humidity in the air or moisture level at the surface of wood. If the soffit vents and high vents (ridge or jack) are functioning correctly, there should be good convective airflow from the eaves to the upper roof and that should be sufficient to remove moisture rising into the attic in air that's diffusing from the house.

However, what happens at night when the sun goes down, the roof cools and that convective flow slows or stops? That's when that air condenses on the underside of the roof. Over time and with the right weather conditions, that can allow the discoloration and fungi to develop on the underside of the roof. Would it still do so if there were sensors attached to the underside of the roof that were looking for a slight increase in surface moisture caused by vapor-laden air condensing on the underside of the cool roof?



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Often times in my area as well as yours I'm sure , The 70's style "shoebox" ranch only has two gable vents. Not the best design as it only works when the wind is blowing the right direction. and as Murphy prevails it's usually not that often.

As it is not always cost effective to install soffit vents or pay your local roof monkey "no offense intended, I personally am a ladder monkey myself..." to cut a bunch of holes in the rood for roof vents, it would seem that the logical and more cost effective solution would be to install a vent fan with a humidistat or thermostat.


If I'm understanding your statement correctly, by media you mean the stuff that triggers the switch like mercury in old therms? than I have no idea. what is it?

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Human hair to be more specific. I'll owe ya one. Meal or Long Island take your pick.

I was working on an old rural home with lots of hairy stuff in the crawl. The engineer added a bunch of piers to support the beams without replacing them. Before the contractor installed the fans I took the liberty of opening one of the stats. The manufacturer made a declarative statement to the effect of "Yes it's human hair. Don't touch it or you will need to purchase more"

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