Attic humidity fluctuations


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Old 02-02-21, 05:59 PM
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Attic humidity fluctuations

I live in a two story house in michigan with a vented attic. I noticed some condensation and frost on nails in the attic on colder days. I decided to put a hygrometer up in the attic to see what was going on. I have seen the following measurements.

7am
32of / 16% rh
4pm
34of / 60% rh
8pm
30of / 16% rh
I'm a bit perplexed by these numbers. It seems like when the sun goes down, the rh also drops- even to levels lower than outside. I was expecting to see an increase in relative humidity as the temperature drops at night. Can anyone take a stab at explaining what's going on and where that daytime humidity is going?
 
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Old 02-02-21, 06:07 PM
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I'd say that when its below 32F the moisture in the attic air turns into frost, effectively taking that humidity OUT of the air. Then when it warms up above 32F during the day that frost melts and the moisture goes back into the air.

To limit the amount of moisture that is exiting the house through the roof, you would need to air seal all penetrations (any holes for electrical, plumbing, hvac, etc) which is typically done with fire rated foam, and then a vapor barrier would be placed on the warm side of the attic floor. Or, if your existing insulation is minimal and you plan on adding additional insulation (you should have a minimum of R-49) you could likely cover the tops of the joists with a poly vapor barrier before adding additional insulation on top to bring it up to at least R49.

(See https://www.energy.gov/energysaver/w...ize/insulation and https://www.energystar.gov/campaign/...ation_r_values)
 
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Old 02-02-21, 06:15 PM
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I don't currently see any frost on the attic. Also, if the humidity was escaping the air by turning to frost, wouldn't the relative humidity increase to a point of saturation rather than decrease?
 
 

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