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Roofing/attic humidistat/temperature fan


I had my old roof torn off and a new one installed.  The fans were replaced with humiditstat/tempertaure ones.  I have two attics completely separate.  I noticed that one fan seemed to be running all the time.  I called the roofer and he took it out and put a smaller one in.  It still ran just as much so I figured it was just my attic needing it to run. Some time after that I had more blown in insulation put in.  It now seems that the two fans run 85% of the time, maybe more -summer or winter.  Both of the temps are set high.  85 for the temp and 90 for the humidity.  I am afraid to go any higher.  i would like to mention that I also had new gutter and soffits plus extra ventilation installed.  I thought I did everything I could to prevent any mold in the attic.  I did have mold because my house was very very tight and the attic took the brunt of it.  I had to have that removed at a high cost.  I check both attics often and they look fine.  However, my electric bill is really going up and I know it is the fans.  Is there a problem?  Should they really be running this much?  We were gong last winter for almost 4 months and our electric bill was quite high and the only things running  were the fans,a couple lights and the start up of the furnace as needed.  We live in Wisconsin.  We will be leaving again soon and I need to  know what to do.  Also, what happens if they burn out and we are not here?   Thanks for your help.

Sorry for the delay Sandy,

The temperature setting is not going to make much difference on the conditions for mold growth. Controlling the temperature of the attic is usually a summertime concern where a reduction of interior cooling costs are sought to be reduced.  Depending on the amount of insulation in your attic, you can find the right temp. setting by experimentation.  Don't be afraid of raising the temperature switch to 90 or 100 degrees Fahrenheit. Having an attic that is 10-20 degrees above ambient is not too bad.

What should be kicking your fan on in the winter would be the humidity level, and this is what will drive mold growth.  If you have the relative humidity switch set at 90% and it is still running all the time I would say you have an issue that needs troubleshooting.

Things to look for:

Make sure that your insulation baffles at the soffit are clear into the attic space, which means those soffit vents you put in are actually letting fresh air into the attic.  Often the insulator will bury them, or they dont exist, and the insulation is blocking the vents and air movement.

Make sure that you have either a ridge vent or an array of hooded vents through the roof sheathing near the ridge.  You are checking to make sure you have free air flow from the soffit to the ridge within the attic space.

Be sure the sensor/switch is in the attic and within reasonable proximity to the fan.

Test the fuction of the sensor by turning up the RH to max to see if it will turn off, there is a possibility it is wired wrong.

There may be an override switch somewhere that is in the on position defeating the sensors.  There are other electrical issues that may also be present.

Also check to make sure bathroom and kitchen ventilators have dedicated ducted exhausts to the exterior.

As far as the fans not running in the winter when you are not home:  You should not have a moisture build up scenario in the attic with a vacant home unless your passive ventilation system is not working (soffit and ridge venting).  Most moisture buildup conditions happen with heat and moisture generating occupancy issues like dense occupancy, showers, cooking, etc.  If you are away, your heat is down and no moisture generating activities are occuring.

If you are away in the summer and they stop running, your attic will get hot.  This should not be an issue if your passive venting is working as you wont be any hotter than most residences without fans. Chances are you are not air conditioning when you are away.

Good luck finding the issue!

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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