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Frosty in the Attics


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Hark building scientists. This may be a bit long, but I'll try and be succinct.

We have been having an unusually nipply (or is it nippy?) cold spell here in the great Northwest. Lows have been dipping to the low 20's and highs during the day are only in the high 30's. Crystal clear skies and my hygrometer reads anywhere from 60-75% relative humidity (exterior).

This leads to a situation I have been finding consistently in the attics this week - a coating of frost covers the underside of the sheathing - usually towards the north sides of the home (where the sun don't shine). Depending on the time of day, the roof sheathing is both frost covered and is dripping wet.

Two homes had visible mold / mildew growth under the frost - one home had frost but no mold / mildew.

I find the mold / mildew growth issue consistently on many homes. Sometimes its just a 'black' layer of mildew or ??. Sometimes its very evident that it is mold by the circular, splotchy growth patterns. I usually discuss this with the client as a lack of ventilation in combination with too much moisture but cant' provide a clear remedy or solution and defer to a building scientist. I think there's another thread somewhere that discusses theories about what the 'black' really is.

But the frost is new. Is the frost simply a more visible indication of the fact that too much moisture is entering the attic with not enough ventilation?

Any good theories on a solution or just keep deferring to the building scientists??

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Congratulations. You've discovered that condensation occurs on cold surfaces.

In addition to everything Mike said, you might want to consider the connection between crawlspace water and attic condensation. It's a common pairing.

As for BoraCare, it's a pain in the ass to work with. Mixing is up is like trying to mix honey and cold water. Even if you use hot water and agitate the sprayer constantly, when you're done there's still a slug of honey-like stuff at the bottom of the sprayer.

Since the mold is purely a surface feature, you don't need the penetration of BoraCare anyway. Timbor would work just as well and be cheaper and easier.

- Jim Katen, Oregon

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Originally posted by kurt

Would there really be a problem w/a solution of 1/4 cup clorox to a gallon of water being sprayed over the area?

It'd probably kill the fungus just fine, but it wouldn't have any residual effect.

I guess that could be a good thing or a bad thing depending on your point of view.

- Jim Katen, Oregon

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Originally posted by hausdok

The discoloration will stabilize if they air-seal the ceiling plane and seal duct/wire holes through the walls plates to minimize the amount of moisture getting up there. They can treat the underside of the roof with a light spray of BoraCare. It's a glycerin/borate mix. You cut it 1/1 with water and mist it on with a pump sprayer. It'll kill the organism and prevent future growth. It's expensive - over $100 a gallon, but simple to apply with a pump sprayer and a homeowner doesn't need a pest applicator's license to apply it.



Just did a house yesterday that had this done. Whole attic sprayed white - I have no idea what the product was. All the ceiling drywall had been removed, clear visqueen installed and the ceiling re-rocked. New fiberglass batts laid in attic.

Minor blotches of mold and mildew were re-appearing at random areas on top of the white coating.

I think the "science" of this whole phenomenon is pretty complex. I'm not sure there's a single definitive remedy that will work for every home.

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