More stupid fan tricks: Over-Ventilation Ruins Comfort and Efficiency

Posted on 12. Feb, 2012 by in General

Louvers open only when fan is operating

Imagine my surprise when, a couple of days after posting my previous blog, I came across a properly installed attic exhaust fan. 

This fan really sucks.  It has louvers that close when the fan is off.  The fan is bigger, it draws more air, about 3000 CFM (cubic feet per minute) than the standard 1000 CFM fan.  Must be better, right?

Fan viewed from inside attic

Well, not really.  The fan draws air from the conditioned area of the home through poorly sealed openings.  It quite literally sucks conditioned air out of the house, especially the 2nd floor.  This results in very poor cooling upstairs.

The homeowners response to over-ventilation was to double the amount of ceiling insulation and add fans in the upstairs bedrooms.  This did almost nothing to solve the problem.  The answer lies in a reasoned application of building science.

Boundary not Draftstopped

Part of the problem is that no sealed boundary was established between conditioned and unconditioned space.  Sealing the boundary with rigid materials is termed draftstopping. Think of it as an open window in a car with the heater running:  the heat escapes quickly.  Putting a blanket over the window (like more insulation on a ceiling) might slow the loss a bit, but it does prevent outside air from getting in.   Poor draftstopping leads to heat loss in winter, regardless of whether a fan is operating.

Black Insulation at attic kneewall insulation

Now turn the exhaust fan on.  Our car is moving, increasing the loss of conditioned (inside) air.  This “fix” for a hot attic has now compromised cooling efficiency for the entire home.  How can I tell?  Look at the black insulation in the photo.  The discoloration is caused by dust particles captured by the insulation.  It filters dust out.  You often see this in carpet as well:  the material is not airtight and the fibers capture the dirt.

Black strip on carpet=airflow

An interior door was used to the separate the attic from the house.  It was not draftstopped.  Black marks on the carpet indicate airflow in both directions.  The attic exhaust fan sucks cool air out, pressure pushes warm air out.

My recommendations:
 Disconnect the attic exhaust fans allowing passive vents to reduce attic temperatures.  Establish a boundary between conditioned and unconditioned space and draftstop it.  Install reflective materials over vertical (knee) walls in the attic to block heat from entering the conditioned space.

What is written here has just been verified in the field:

Fan OFF!

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