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Moisture in Attic


CNewhouse
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The home I inspected today (1980 build) had damp fiberglass in the attic floor and elevated moisture content in the plywood roof sheathing. The attic was well ventilated at the low and high sides. The bathroom exhaust fans had dedicated outlets in the roof plane and the kitchen exhausted through the crawlspace by way of a downdraft vent in the range. My best guess is that the issue is warm air rising into the cool attic and condensing, leading to moisture on all surfaces. Curious what others have to say. And what is the solution? Air sealing?

 

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I think you are on the right track in the PNW. Air sealing was poor in 1980.

Culprits are ceiling fixtures and wall plates. Also humans create a lot of moisture in their living space and try to get by with the heat turned down. Sometimes you can see dark patches on the top surface of the insulation where the worst leakage is.

But there are other factors and causes.

Edited by John Kogel
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Appreciate the response. Warm moist air rising and condensing was the only explanation I could think of considering the attic was well ventilated and the bath, laundry and kitchen were exhausting properly. My only confusion was that I don't see this very often. Most homes I inspect are not air sealed, especially older ones. Yet, I can only recall damp insulation a few times. 

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On 12/17/2021 at 12:55 PM, Bill Kibbel said:

Venting of all combustion appliances OK?  Even just an unvented space heater dumps loads of moisture into the air, then condenses in the cold attic.

The gas fired furnace and water heater were both venting properly. I did not see any space heaters during inspection.

20 hours ago, Jim Katen said:

Was the house in a soggy, wooded location? 

I'd say yes. Tualatin just off I-5. Fairly close to that swampy area below the Nyberg offramp. The crawlspace was pretty dry and the vapor barrier was in good shape. 

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I occasionally find very humid attics like this when houses are located in very humid micro locations. On one, I observed morning sunlight vaporizing water on the house's siding, where it rose like a fog and got sucked into the soffit vents. In these locations, the attic is being vented with sodden air from outdoors. 

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What Jim said, I have also seen, trees too close maybe, house in a shady hole, frost in the attic along the lower edges of the roof sheathing from moisture coming in thru the soffit vents.

A humidistat costs maybe $25, and is easy to set down for a few minutes in various parts of the house while you go about an inspection. Damp stuff in closets, cooking noodles without the hood fan, and just having a dog are all sources of indoor moisture. People don't heat all the rooms consistently to save a few pennies but that raises the humidity. Then the heat comes on and the water evaporates into the attic. You can charge more for doing 10 mins of air quality testing with the humidistat, or just do it as a bonus.

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