# venting and moisture in attic



## Alto (Jun 17, 2010)

It seems to me that one of the principal functions of venting is to reduce moisture in the attic. People seem to make a lot of venting for the preservation of the shingles or to cool the attic. But I figure that if a roof is made to take 160 degrees outside having a hot attic in itself doesn't seem to be much of a problem. The more important matter seems to me avoiding mildew and mold up in the attic. What about a dehumidifier? Wouldn't that help (and just by virtue of the lack of humidity the air in the attic should feel more pleasant. Hooking up a dehumidifier should be a very simple matter; you just plug in the device and run a tube to the edge of the attic and out a soffit , or drill a small hole. I have a couple of them in my home already and a huge one under another home that i own. you can keep the dehumidifier running constantly. The lack of humidity --keeping it at the proper level, usually around 50%-- would also probably be good for the wood in the attic. The cost of running it is really negligible. Is there any problem with keeping a dehumidifier in the attic?



Also, i have read that natural vents (by that i understand holes cut in the roof covered by different products for this purpose --not a turbine-- )are nearly as good as a ridge vent. Do you folks with more experience in this concur? The Owens corning web site provides a calculator based completely on attic square footage which tells you how many linear feet of ridge vent your house will need, or how many natural vents of a given size are needed. Is this a fairly reliable figure.


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## Proby (Jul 17, 2010)

Vents installed on the roof still allow hot air to "puddle" up at the ridge. The ridge is the highest point and heat rises, so it stands to reason to put the vent up there. Ridge vents also have a lot of area, but are low profile and don't stick out like a sore thumb like many roof vents do.

I don't know what roof is truly able to handle 160 degrees outside, but an attic can get nearly that hot if not ventilated, and that is VERY bad for the roofing. It also raises the temperature of the living space of the house which will raise your energy bills to cool it off.

A dehumidifier would cost you money to run, and probably wouldn't be as good removing the moisture as proper ventilation. It also wouldn't help with the heat problem.


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## igneous1 (May 6, 2010)

The best solution I've found is here http://www.atticfoil.com
Easy DIY project, during winter that is. Adequate insulation and radiant barrier foil keep my bills ~$250 a month, and I have a 3500 sq. ft. house.
Also installing enough venting will help w/heat and moisture. I've got ridge vents over the long horizontal ridges, and passive square vents at the cathedral roof summit. Instead of powered attic fans, I use box fans I've rigged up on a timer to move hot air out.


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## Gary in WA (Mar 11, 2009)

Removing moisture is very important and where attic ventilation helps. This will stop any ice dams from forming, especially if you seal the attic/living space holes first. http://www.rd.com/how-to-seal-attic-air-leaks/article18158.html

A dehumidifier will help in a closed environment or partial air movement one like a crawl space. In an attic, the ventilation system should move the air constantly which would negate the dehumidifier. 

Turtle, or pot vents are not in the peak or large enough to exhaust all the air, as Proby said. The ridge vent, with baffles, is much more effective: http://files.buildsite.com/dbderived-f/airvent/derived_files/derived20936.pdf

A good read: http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation?full_view=1

Be careful when using attic fans: http://www.advancedenergy.org/build...entilation/Attic Ventilation Case Studies.pdf
http://www.ronhungarter.com/black_mold.html

Be safe, Gary


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