Gable Vents-with vented soffit and ridge?

[Mike from NH]

My house has vented soffit and ridge, should I get rid of my gable vents?

I heard that the gable vents cancel out the soffit and ridge vent.

The house is 24x40 with and has 2 12x18 gable vents. My vinyl sider will be finishing up the gables tomorrow so I have a chance now to leave out the gable vents. What do you think?

[Bill Robinson]

Re: Gable Vents-with vented soffit and ridge?

Interesting query, what does "cancel out" mean?

Here are a couple of links that address if not answer your question.

http://www.advancedenergy.org/buildi...ntilation.html
http://www.buildingscience.com/resou...fs/default.htm

My best guess is that unless you properly seal the ceiling the additional venting will not cancel out but instead exacerbate (word of the day) the problem of drawing conditioned air from the living space.
This is in reference to air circulating through the attic pulling air out of the house/living space/conditioned space through gaps in the air barrier.

Capice?

Bill R

[David Meiland]

Re: Gable Vents-with vented soffit and ridge?

Bill, I believe his question is about whether the gable vents will disrupt the convection that the soffit and ridge vents create. Maybe you already got that.

[Bill Robinson]

Re: Gable Vents-with vented soffit and ridge?

Kind of David, but I am asking the question to be sure and to maybe stimulate conversation.
My first thought was that whatever I said would be corrected by one of our resident building scientists.

I am still trying to grok the concept of what moves the air, how much air needs to be moved, and what is the preferred method.

It seems the answer, as it usually is, 'it depends.'

My next guess is that there is not much benefit in having too much ventilation in light of the issue of pulling conditioned air out of the conditioned space.
It is hard to imagine the two methods cancelling each other out.
Still no defining answer.
IMO, one of the hazards of getting specific answers w/o specific details.

Bill R

[Mike from NH]

Re: Gable Vents-with vented soffit and ridge?

yes david, that was the question.

I at some point plan to make a cathedrel ceiling in one side of the house, I will install proper vent from soffit to ridge, but the above the "flat" section of the ceiling will or would be just under the gable vents.

To clearify, since I have vented soffit and ridge, will it help or hinder having gable vents? As I said I have a 24x40 ranch so I have 80' of vented soffit and 40' of vented ridge.

[Bill Robinson]

Re: Gable Vents-with vented soffit and ridge?

If the soffit/ridge vent is adequate and detailed properly there doesn't seem to be any benefit from additional venting.
And, if the ceiling air barrier is leaky, additional venting would then be an un-benefit.

Bill R

[Mike from NH]

Re: Gable Vents-with vented soffit and ridge?

What other details would be helpful in giving a definitive answer?

This fall when its cooler I was going to install proper vent and r-30 from soffit to ridge in the rafter bays. To the insulation already that is on the floor of the attic. I was thinking the gable vents would vent the area where the insulation meet (attic space) and the soffit and ridge would vent the roof deck.

[TSJHD1]

Re: Gable Vents-with vented soffit and ridge?

What other details would be helpful in giving a definitive answer?...

Clarifying the ones you've already mentioned would help.

I had the same size house, and I eliminated the gable vents when I re-roofed. Ridge vent is a better choice imnsho, and I was told that gable vents just disrupt the airflow from soffit to ridge. Makes sense to me. I say take them out.

You're a drywall sub...What do the builders you sub to say?

Tom

[carpenter]

Re: Gable Vents-with vented soffit and ridge?

As explained to me when I asked the same question:

The gable vents will allow a path of least resistance at the ends of the attic and lessen or eliminate venting in the mid areas. Gable vents should be eliminated when changing to ridge vent.

I asked this question to my suppliers years ago and this is what I gleaned from the literature and conversation they supplied.

Good Luck
Dave

[Mike from NH]

Re: Gable Vents-with vented soffit and ridge?

You're a drywall sub...What do the builders you sub to say?

Tom-

FYI I'm not a drywall sub, I'm the owner of a drywall business who doesn't work for builders.

[TSJHD1]

Re: Gable Vents-with vented soffit and ridge?

Sorry Mike. Must have gotten you mixed up with someone else.

Tom

[Greg Taylor]

Re: Gable Vents-with vented soffit and ridge?

Hello Mike,

I recently attended an all day seminar put on by AirVent.com and Paul Sclesi. I learned more about proper attic ventilation from Paul that my head was spinning. I'll try to distill down what I learned. First off, let me just say that there are many companies that provide parts to ventilate the attic. AirVent is one of them but I think they are right at the top. During the seminar, Paul showed a number of "smoke tests" of different scenarios, like ridge vent combined with gable vents, etc., and also smoke tests comparing their product to other manufacturers. Obviously they were far and away the superior product. What I found funny is that the other suppliers also have smoke test videos comparing their products to their competition EXCEPT AirVent. Anyway, enough of a product endorsement.
What I learned. Proper attic ventilation is all about balanced NFA or balanced Net Free Area. On a typical roof with a ridge and two soffits on either side you have to have balanced air in through the soffit vents. In AirVent's case, their soffit vents provide 9 square inches of NFA per linear foot. So in this example each soffit is providing 9 square inches of NFA per foot for a total of 18 square inches of NFA into the attic. That means you must have a ridge vent that exhausts 18 inches or more of NFA per foot. It is better to have more NFA going out the top than coming in the soffit. Their products do this. Also, their ridge vent (the Flash Filter Vent) has a flange on it that is designed to take advantage of wind turbulence coming up the outside of the roof to help drawn hot air through the ridge.
Some other points from the seminar. A passive system as described above is the best. Powered ventilators which they make are not as effective as this passive system. BUT you cannot combine systems. The worst thing that you can do is put in gable vents in addition to a GOOD passive system. They do compete with each other and for lack of a better phrase "cancel each other out". This also goes for putting in a powered vent when their is a good passive system in place. Also, their soffit vents create 9 square inches of NFA per foot. Vented Hardie soffit is only 5 square inches. Check out their website at http://www.airvent.com/professional/index.shtml I would encourage you to contact them to see if a seminar is coming to your area. It is free and a great bit of info.
The only complaint I have about their products is that the aluminum is very thin and you have to be careful in handling is as it bends and dents easily. Still a great product.

[Bill Robinson]

Re: Gable Vents-with vented soffit and ridge?

Thanks for the detailed answer Greg.
Makes sense and I learned something.
Bill R

[tucker]

Re: Gable Vents-with vented soffit and ridge?

On a typical roof with a ridge and two soffits on either side you have to have balanced air in through the soffit vents. In AirVent's case, their soffit vents provide 9 square inches of NFA per linear foot. So in this example each soffit is providing 9 square inches of NFA per foot for a total of 18 square inches of NFA into the attic. That means you must have a ridge vent that exhausts 18 inches or more of NFA per foot.

I'm not sure if I am misreading this, but I believe that I have always read that if the desired 50/50 balance between soffit and ridge ventilation can't be achieved then it is recommended that there be more NFA of ventilation in the soffit rather than the ridge. So in the example above a wouldn't you want a ridge vent that exhausts 18 inches of NFA or "less"?

I have also read that having considerably more soffit NFA compared to ridge NFA will have no adverse effects. What say the good folks who frequent this forum...will a roof system that has twice as much ventilation at the soffit (compared to ridge) still perform as well as a roof system with balanced ventilation at the ridge and soffits?

[Roger P]

Re: Gable Vents-with vented soffit and ridge?

IRC 2006 states:
R806.2 Minimum area. The total net free ventilating area shall not be less than 1/150 of the area of the space ventilated except that reduction of the total area to 1/300 is permitted, provided that at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet
(914 mm) above the eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1/300 when a vapor barrier having a transmission rate not exceeding 1 perm (5.7 × 10-11 kg/s ⋅ m2 ⋅ Pa) is installed on the warm-in-winter side of the ceiling.

This was revised in 2009 to read:
As an alternative, the net free cross-ventilation area may be reduced to 1/300 when a Class I or II vapor barrier is installed on the warm-in-winter side of the ceiling.

As Greg pointed out, you do not want to add gable vents to the mix as this could (will) cause a short circuit in the air movement resulting in the center portion of the roof not being properly vented.