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Damp attic insulation


Chris Bernhardt
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This time a year its not unusual to see condensation on the under side of roof decking sometimes even drenched wet and dripping however in just a very few instances I have it the other way where upper surface of lets say the loose fill insulation was damp but the roof deck above although damp was not drenching wet.

I am not quite sure how to write this up? Once the insulation is damp do you consider it damaged? If so do you try and dry it out in place? Remove and replace it? Do nothing?

And to the source I wonder in these cases is it really from the house or is just from a severe and sudden drop in temperatures?

In this case attic had approximately 14" of loose fill, soffited eaves, normal number of roof vents, one gable vent and kitchen and bathroom exhaust fans were vented up into roof vents and house is 1000sf and crawlspace was dry. Theres snow on the ground, very little on the roof and the temps lately have been low 30's.

Chris, Oregon

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Chris,

Is this and older home? say pre-vapor barrier era. It's possibly interior moisture migrating through the plaster/sheetrock up through the insulation and when it hits the top layer it condenses on the fibers and can form a sort of crust if it happens often enough. I see this often in WI.

Jim Weyenberg

HouseMaster Inc

Neenah, WI.

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The house was built in 1973. It had a black plastic vapor barrier over the ground in the crawlspace. It had a plywood subfloor although I don't recall if there was a vapor retarder there or not. And I believe there was no vapor retarder in the ceiling assembly other than the paint. Usually if the moisture is coming from say an exhuast fan venting into the attic its pretty obvious as condensation, mildew and mold tends to emanate from those points. The was general and every where thru out.

Chris, Oregon

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Bath vent or not, there's a fair amount of moisture in the home migrating up. People exhale a lot of moisture, cooking, etc. That could be condensing on the cold roof sheathing.

Did the blown-in insulation cover the soffit vents? That would prevent air from drafting through for adequate ventilation, leading to the moisture build-up.

In my own house my soffit vents were blocked from day one. Builder built the soffits solid, then covered the 1x8s with the aluminum vents. Looked like ventilation from the outside!

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Well, first you need to attempt to try and figure out the source of the moisture. If it is related to poor ventilation this is a fairly simple correction. If it is due to venting into the attic then that needs to be addressed and most likely the ventilation needs addressing as well.

As for the moisture laden insulation. If it is wet it will compact and will no longer provide the proper loft and insulation. Yes, it should be replaced. Sometimes batt insulation can recover from being wet, but blow-in does not.

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I have 2 really good books both of which I have read cover to cover and then reread them but like always its like reading the bible. Every time you read it you come up with something new.

The first book is (Water in Buildings - An Architect's Guide to Moisture and Mold) written by William B. Rose published by John Wiley & Sons copyright 2005. The other one is Listibureks Moisture Control Handbook which many here I am sure already familiar with.

I was studing about moisture vapor in particularly in both books when I read a paragraph in the moisture control book concerning fireplaces being really a big house exhaust system assuming you use it.

Well in this house that I started this post with it has an electric cadet heating system and upgraded windows and doors and more insulation. I imagine that the vapor pressure levels are literally thru the roof as has already been suggested permenating thru ceiling assembly and condensing on the upper boundary surface of the added insulation which is now colder then it would have been if they hed not added more insulation.

Has anybody noticed a correlation between the lack of moisture problems in homes with actively used woodstoves and fireplaces vrs ones that just have electric heating systems?

Chris Oregon

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