I am considering installing a 24" whole house fan in my garage/shop celing for two reasons. To help ventilate my shop and to help ventilate the attic.

I have several static roof vents, gable vents, and soffit vents.

Would there be any issues with this?

Daniel,

If the garage/shop attic is connected to the house attic, yes, definitely some problems could occur.

The whole house fan will first and foremost violate the separation required between the garage and the house (you would need to install a wall with 1/2" gypsum board on it separating the house attic from the garage attic to maintaining the separation which should be there now).

Secondly, the whole house fan will be exhausting air from the garage/shop and all the contaminates (particulates and vapors) produced in the garage/shop into the attic while pressurizing the attic. This means that all of those contaminates will be forced into the house space, which is not pressurized (the house will be at a lower static pressure than the attic, meaning that attic air will be driven into the house).

The above action will pressurize the house as compared to the exterior, thus driving the condition air within the house through the walls and outdoors.

I really cannot think of anything good you would be doing by adding that whole house fan.

If you want to exhaust air from the garage (ventilate the garage), then install an exhaust fan from the garage to the outdoors and add sufficient outdoor air intake into the garage so as to not depressurize the garage, thereby sucking air from the house into the garage.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
EastWestData (www.EastWestData.com )

Thanks for the information. The responses are inline with my thoughts, but I wanted to verify.


This may be along the same lines of my ignorance, but I installed a powered gable vent in my attic. It is an exhaust vent. Would it be bad to make it an intake vent?

My reasoning is the gable is considerably lower than the static roof vents. And the gable vent seems like it would be fighting the purpose of the roof vents by creating the negative pressue at a lower level, thus holding the hot air higher in the attic in place.

If I turn the gable vent it would be pulling air from outside and help push the hot air out of the roof vents. Does this make sense?

Daniel,

As currently installed, that powered gable vent is depressurizing the attic, doing all the wrong things to the house area: pulling out conditioned air which you paid to condition and putting it into the attic; pulling out extra moisture from the house and putting it into the attic' depressurizing the house in the course of the preceding which then brings in more air from outside which needs to be conditioned; bringing in moisture and contaminants (allergens if you are a family member has allergies) from the outdoors; potentially pulling garage air through the house - if the house walls/ceiling leak more than the garage walls/ceiling does; just not much good comes from it.

Turning it around backward and making it into an intake would pressurize the attic and cause basically the same thing, only in reverse: outdoor air would be drawn into the attic which would then be pushed into the house which is at a lower static pressure which would then push the conditioned air out through the walls.

Depending on where the vapor barriers are in the walls, and where the insulation is in the walls (such as EIFS), you could be creating a moisture problem in the walls.

Whenever you install an exhaust fan, you will need to install sufficient intake openings to allow for the proper air flow with minimal depressurization of the space, with the reverse being true for installing intake fans - you need to allow sufficient exhaust openings to discharge that air for minimal pressurization of that space.

There are HVLP (High Volume Low Pressure) fans being used for large warehouse areas which are not as bad as they do not increase/decrease the static pressure in the space much while moving high volumes of air. However, those HVLP fans are frequently 8'-12' in diameter with slowly rotating blades ... I doubt those will fit in your gable end though ...

__________________
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
EastWestData (www.EastWestData.com )

Will these pressure conditions apply if I have adequate openings for the intake or exhaust?

If so, what is the best way to "help" ventilate the attic?

Yes, but to a lesser extent.

Add more natural air movement vents.

Increase the size/amount of soffit ventilation (not always possible) and gable vents (not always practical). If you have ridge vents and gable vents, the soffit vents may become less effective as the air may circulate gable-vent-to-ridge-vent, leaving the soffit vents doing little.

__________________
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
EastWestData (www.EastWestData.com )

My opinion ridge venting is one of the best with what you have, However some roofers do not recommend gable end venting with ridge venting. So eave and ridge seams to be the best way to go.



Rolland Pruner
Livermore, Cal.

Some of those contaminates JP, not all. Get it right!

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What is the circumference of a moose?

Thanks for the information and feedback everyone.

One more question - How do you know if you have adequate attic ventilation?

Daniel, the best type of attic ventilation to have is of the passive variety: continuous soffit vents along the exterior eaves and a baffled ridge vent along the full length of the roof ridge. Baffled ridge vents are better than mesh ridge vents as the mesh vents can be short-circuited if the wind currents drive directly against the vents. Baffled ridge vents deflect air currents over top of the vent and create a vacuum effect that actually pulls air out of the attic as it passes over the ridge (the bernoulli effect). When you have the the ridge vent and soffit vents in place, having any other type of roof ventilation system in place such as gables or power vent fans can short-circuit the air flow you get throught the soffits and ridge.