Thread: Attic Ventilation

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?

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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.

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?

Originally Posted by Daniel Palm

I installed a powered gable vent in my attic. It is an exhaust vent. Would it be bad to make it an intake vent?

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 ...

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?

Originally Posted by Daniel Palm

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

Yes, but to a lesser extent.

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

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.

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.

Originally Posted by Jerry Peck

... 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).

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

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.

Originally Posted by pablordstribe

try this type... Powered Attic Ventilators

That is one of the worst things one can to do a house regarding ventilation - those depressurize the attic which in turn draws air from the living space into the attic through all the many unsealed areas and holes through the ceiling and top plates.

That draws conditioned air (cooled or heated) from the house, unconditioned air from outside is then drawn into the living space to replace the conditioned air which is now passing through the attic to the outside.

Just not a good thing to do.

I agree with Jerry about attic venting.

The ceiling between the attic and the house (attic floor as I refer to it) is the leakiest part of most homes homes. Any pressurization or depressurization of the attic will cause either infiltration or exfiltration.

Powered attic fans will draw air out of the house. If a fan was put in the garage pressurizing the attic a lot of the air would make its way into the house.

I do not support the use of powered roof vents. Attic floors should be sealed and very well insulated. For energy efficiency DOE has air sealing at the top of the list. Air sealing is hard to do because it includes so many small leaks. Inspectors need to recognize that almost every home they are in will have a significant amount of air leaks between the house and the attic. If may include a few large leaks but it will include the seams along top plates and many small leaks. Don't be fooled by a thick layer of lose fill fiberglass or fiberglass batts. You wouldn't count on it keeping water out and it wont keep air from passing through either.

I think powered fans in the roof or gable can be useful on cool nights, when one would think to run a whole-house fan, that leaky and wasteful thing when in the upstairs ceiling. I make attics useful with excellent insulated and gasketed drop-down ladders. During perhaps-rare cool night-times, pull the ladder down part-way without step deployment, to let the powered fan draw cool outside air into living spaces. I like being able to tell a customer his powered fan has value, and am even accepting that I will install a powered fan, for proper use.

On hot days, daytime, one should never run a powered fan, but a good solar fan, e.g. Solar Star, works great to keep the attic reasonably close to outside temperature. Where a solar fan blows through the roof, up-high, gable vents or windows reliably admit outside air. Make sure there is plenty of admitted air, and carefully air seal the attic floor. Even a solar fan might otherwise expel conditioned air.

Natural circulation in summer, both daytime and at night, will work only with ample areas, well-placed, perhaps with re-learning concepts of central chimneys.

I will agree that powered attic ventilators are typically problem creators and not solvers--however they will work well under the right installation practices. It must be the only source of exfiltration venting in the attic space and MUST have sufficient low ventilation points to match the CFM of the fan's capacity. If any other static vents are present the fan will essentially play roulette with it and prevent (some) lower intake air from entering and ventilating the attic. Now if the power goes out or the fan seizes, you have inadequate ventilation needs for the attic and a problem. So, back to good ole gravity and pressure.

For ridge vents, we recommend these in our reports http://www.airvent.com/homeowner/products/ridgeVents-shingleVent.shtml and make sure that adequate ridge spacing is followed as many roofers do not allow enough space at the ridge sheathing for any air to escape.

Originally Posted by Daniel Palm

Thanks for the information and feedback everyone.

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

Where do you live? The types of problems you may see depend on your climate.

I've checked other posts and replies but I didn't notice an attic with the configuration I have. My garage is just under 2700 square feet, with 13 foot ceiling in sort of an L-shape. The garage is attached to the house however the attics are separated with plywood. Both attics are inuslated with the spray foam type. The air conditioner in the bonus room will not cool it and I think it is because of a heatsoak problem with the attic garage. The garage attic I know is not sufficiently vented at all since it has been foam insulated. My plan is to use a gable mounted power ventilator fan rated at about 3000 cfm and cut intake holes in the foam around the perimiter soffits. I'm prepared to install extra soffit vents in the event I don't get enough air exhange. I feel since the attic is sealed with foam I will be able to control the quantity and locations of intakes very well and should be able to adjust as required by filling unnecessary holes with spray foam. Can anyone adivse if this plan is sound or if I should go another route? Thanks In advance for any advice or comments. Emails are fine too.

sschas@yahoo.com

Charlie Romeo

Originally Posted by Charlie Romeo

I've checked other posts and replies but I didn't notice an attic with the configuration I have. My garage is just under 2700 square feet, with 13 foot ceiling in sort of an L-shape. The garage is attached to the house however the attics are separated with plywood. Both attics are inuslated with the spray foam type. The air conditioner in the bonus room will not cool it and I think it is because of a heatsoak problem with the attic garage. The garage attic I know is not sufficiently vented at all since it has been foam insulated. My plan is to use a gable mounted power ventilator fan rated at about 3000 cfm and cut intake holes in the foam around the perimiter soffits.

BAD IDEA!

You would no longer have a sealed attic, which makes your insulation on the underside of the roof sheathing useless, and totally and completely violates all codes.

I'm prepared to install extra soffit vents in the event I don't get enough air exhange.

EVEN WORSE of an idea!

You will totally be screwing the sealed attic aspect of the house up!

Can anyone adivse if this plan is sound or if I should go another route?

Hopefully I have already answered that.

Where is the bonus room? Over the garage, next to the garage attic?

If next to the garage attic, then I suspect your heat gain problem is from the wall between the garage and the bonus room is insufficiently insulated, and that would/could be causing your problem (but so could an undersized a/c unit, among other things)

Thanks for the quick reply. The bonus room is above the garage so it's inside the attic. I insulated it with R-19. Venting a foamed attic would violate code? Can you elaborate? The bonus room is using a portable ac unit now. Should i just upgrade to a split system dedicated to the bonus room only?

Originally Posted by Charlie Romeo

The bonus room is above the garage so it's inside the attic. I insulated it with R-19.

Insulated the walls? The floor? Don't forget you have an oven under that room in the summer, and a freezer under it in the winter (commonly called a "garage").

Venting a foamed attic would violate code? Can you elaborate?

That sprayed on foam insulation on the underside of the roof sheathing is only allowed under specific conditions, one of which is that the attic be sealed.

As soon as you "unseal" the attic - which is what you are talking about doing - then you need to redo all of the insulation back to the 'normal way' (on the ceiling, walls, etc.).

The bonus room is using a portable ac unit now.

Ha-ha (maybe) ... where is that portable a/c unit venting too?

Should i just upgrade to a split system dedicated to the bonus room only?

Without a doubt.

the room is insulated on all sides including underneath the floor. The portable ac vents to the outside with its 5 inch duct. My thinking was the attic just stays hot all night and heat transfers to the garage overnite. I can go out there at 5 am and the garage temp is 89 degrees while its around 75 outside. I thought it would be best to exchange the hot stagnant air for fresh air. Oh well. I planned to install a split system regardless as I'm told they're much more efficient than those portable jobs. The room is 22 x 13. any suggestions on the sizing of the unit I should install?

Originally Posted by Charlie Romeo

the room is insulated on all sides including underneath the floor. The portable ac vents to the outside with its 5 inch duct. My thinking was the attic just stays hot all night and heat transfers to the garage overnite.

That is what I am not understanding: You keep saying "the attic" and that the "attic just stays hot all night", but you have also said that "the attic" has the spray foam insulation on the underside of the roof sheathing ...

Do you mean that one attic is not insulated with the spray foam insulation?

If both attics are insulated with the spray foam insulation (on the underside of the roof sheathing) and the attics are getting hot, that indicates to me that the spray foam insulation was likely either not installed properly and/or not installed to the minimum thickness it should have been.

I have been in attic with the spray foam insulation, in South Florida, in the heat of the summer, in the hottest part of the afternoon, and the attic is basically the same temperature as the living area of the house. The temperature of those attics basically almost reach equilibrium with the house temperature and humidity.

That is what I am not understanding about what you are saying.

I can go out there at 5 am and the garage temp is 89 degrees while its around 75 outside.

Right, that is what I was saying, that the garage is an oven in summer (and a freezer in winter) under the bonus room (which you said is above the garage.

With R-19 in the ceiling of the garage/floor of the bonus room, and with R-19 in the bonus room walls, and with R-19 (possibly even R-30 on the underside of the roof sheathing, the area above the garage should not be getting that hot (or cold in the winter) and you should be able to easily heat or cool the bonus room.

Is the entire attic over the garage the bonus room? Is there spray foam insulation on the underside of the roof sheathing over the garage/bonus room?

I thought it would be best to exchange the hot stagnant air for fresh air.

If you are talking about venting the garage to the outdoors, that will help cool/heat the garage to the evening temperatures outdoors, but if the bonus room is properly insulated as well as you say it is, then it really shouldn't matter that much what the garage temperature is as long as it is not real hot hot or real cold cold.

I'm not very far north of you and when it gets down into the 20s and 30s outside at night in the winter, the garage stays up around 50 to 65 degrees. If there was a bonus room in my huge attic above the garage, it should heat rather easily, I don't have any insulation over the garage or above it and the garage attic temperature is still not freezing on those cold nights. I've been in the attic some on those cold nights and the attic is cold, but not freezing like it is outside, and that attic is rather large, I can stand on the floor and the ridge is probably 12 feet (give or take) up from the attic floor (I've not measured the height, but it is high).

The room is 22 x 13. any suggestions on the sizing of the unit I should install?

Contact an a/c company and have them do an energy calculation, you need to factor in the insulation all around that room, the glazing, air leakage, etc., to be able to size the unit correctly. You will likely want a mini-split as they are available in smaller sizes that regular split systems. Make sure to provide heat too.