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Roofing/attic fan with humidistat


I recently had an attic fan with thermostat and humidistat installed. The fan is running almost 24 hours per day. The temperature has ranged in my area between 30 and 40 degrees. Is this normal and what should the humidistat be set at. Will this be a problem in the summer if I use the humidistat and well as the thermostat to control the fan in the attic? Also wont this running all of the time cause the fan motor to burn out?


The humidistat should be set for around 85%-90%.  The purpose for the humidistat is to prevent condensation within the attic.  Generally condensation is most likely to occur when the moisture content of the air is greater than 90%.  As each attic space is its own microclimate, there is no golden rule, but your attic fan should not be running constantly either.  My presumption is that the humidistat or thermostat are set incorrectly.  

Once you verify and change the settings (Thermostat at around 90 Deg F.) make sure you periodically check your attic to be sure there are no signs of condensation within.  Check nail tips protruding through the sheathing and gang nails on truss work for water droplets, or the underside of the sheathing for frost on mornings in the 20s or below.  If these symptoms are present, and the fan is not running constantly, reduce the humidity level setting by 5%-10%.

The goal should be an automatically functioning attic fan that does not run constantly.  You should also not have to baby sit the Stat settings once you have found the sweet spot.  If the fan continues to run constantly I would check the wiring, sequence, or switch integrity.


Bruce Ryan II, RRO
Professional Roof Consultants, Inc.  


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Bruce A. Ryan II, RRO


I offer solid knowlege of all types of Commercial and Residential Roofing, Waterproofing, and, Building Envelope systems. Experience ranges across low slope and steep slope roof systems. I am also well versed in matters regarding condensation and ventilation. I enjoy donating some of my time and knowledge to the betterment of others.


Bruce Ryan has 20 years of roofing, waterproofing, and building envelope consulting experience with PRC, with 5 years of commercial roofing experience prior to joining the firm. He became Vice President of the company in 1998. Bruce Ryan plays a key role in the development of practical, long-term roofing and waterproofing solutions, along with implementation of on-site forensic studies. Bruce also has a high level of experience with regard to the impacts of roofing materials and construction for demanding clients with heavily occupied structures.

Oregon Construction Contractors Board Construction Specifiers Institute National Roofing Contractors Association The Institute of Roofing, Waterproofing, & Building Envelope Professionals Oregon Board of Investigators Installation Masters

University of Maryland BS Business & Administration Registered Roof Observer - RCI Private Investigator Certified EIFS inspector - Northwest Wall and Ceiling Bureau Certified InstallationMaster™ - The Installation Masters™ Training and Certification Program (developed by American Architectural Manufacturers Association)

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