Roof ventilation Types

Question on how you would report on the following? See Poll

1966 Ranch with a basement.
Roof 3D architectural shingles. 5 years old 1 layer.

Venting is soffit/ridge and gable ends.

No moisture stains at roof decking, insulation is a fiberglass loose fill 12-15 inches with baffles.

Ridge and Gable.JPG

Ridge and Gable.JPG

Soffit venting.JPG

Ridge and Gable.JPG

No question about the poll on this one ah David!

It looks like the trim and soffit is covered in alum and vinyl. Most likely at a later date and was not orginal to the house. I would mention this and that you cannot see the condition of the orginal trim/soffit.

After lifting the attic hatch, I pop my head up into the attic with NO lights so that I can get an idea if there is adequate intake ventilation at the soffits. If there is no evidence of under ventilation then I simply report:
“No significant deficiencies observed at the time of inspection.”

The SOP does not require a description of the ventilation methods and I do not include this information in my reports.

I take lots of photos and include the following:
Limitations and exclusions of a visual insulation & ventilation inspection:
“Where continuously vented soffit is installed over pre-existing soffit, the presence of adequate ventilation behind was not determined.”

I would rather see gable end vents than soffits.

Not a big fan of soffit vents.


And although we prefer not to have gable vents with soffit vents, having both does not make it wrong. More information is required for proper ventilation evaluation, which you know is beyond the SOP.

Just sayin’.

Gable vents when used with ridge vents interfere with the air flow form the soffit to the ridge and can lead to mold on the roof sheeting.

I have closed My roof vents and My Gable vents hope to Close of my Soffit vents soon .
I will be recording my attic and the out side temp regularly .
I will also be checking my attic constantly .
Will try and post info regularly .
I am a constant reader and my feeling at this time is stop allowing my home heat get to the out side .[1].pdf

I will initially concede that living on the border or in Canada will have different building practices.

I am from New England (just a few miles from Peter) and I will be the first one to say that regardless of standards or codes, different parts of the country require different practices whether they are adopted by the AHJ .

Considering the laws of psychometrics and fluid dynamics, proper attic ventilation requires upper and lower openings to allow convection to occur.

The IRC and UBC both recognize that less ventilation is required per square foot of the attic space if more ventilation is located on the upper level of the roof than the lower eve.

If you suck air out, you must allow it to come back in somewhere.

The attic produces a significant stack effect and if there are not soffit ventilators, this air must be drawn in from ceiling openings on the upper floor of the building. This causes a major efficiency issue.

I realize that increasing airflow and making a cold roof causes ice damning and water damage issues so I am not arguing the point.

However, people that have a ridge vent and a gable vent are providing a greater source of air ventilation to the exterior at the upper portions of the attic space as required to reduce the necessary ventilation of the attic.

So where have we evolved into saying that if you have a ridge vent and a gable vent that you should close one of them up?

The more ventilation on the upper part of the roof the greater convection that occurs and except for our friends in the North that have to deal with ice damning, the more up high the better.

In the South, we like to stick HVAC equipment in the attic without increasing the attic ventilation. This is stupid on top of stupid.

So for anybody in the South that takes on the substantiated practices of the North, you are probably wrong.

I am open to suggestions and clarification and I specifically request clarification of perspective from my friend Peter Russell who obviously has a real good reason for controlling ventilation at the soffit.

My point is, just because Peter does it, does not mean you need to worry about it if you’re in Houston Texas.

Do I have to close off my gable vents when I use a ridge vent?

Yes, the gable vents (a type of exhaust vent) should be closed off whenever a ridge vent (which is also a type of exhaust vent) is installed because vents installed at the roof’s edge or in the overhang should supply the intake air needed by the ridge vents. Air should flow in through the intake vents evenly along the roofline and exhaust out the peak. Any vents in place between the ridge vents and the intake vents may interrupt or short-circuit that flow of air along the roofline. The gable vents will end up becoming intake for the ridge vent — an undesirable situation that could lead to weather infiltration through the gable vents and also could prevent the attic from being properly ventilated. The same is true with mixing wind turbines or roof louvers with ridge vents.

The gable vent is at the roof line.

Who cares if the air is coming out of the gable vent or a ridge (which was most likely improperly placed)?

Air out = air in.

It doesn’t matter how much air goes where, it’s where it happens.

That is the point.

I think what you may be missing is that the gable vent short circuits the air flow from the soffit to the ridge vent.

This means that less air is drawn in from the soffit vents especially near the center. That interferes with moisture removal due to lack of sufficient air flow.

Reduced air flow mens warmer sheeting and increases the possibility of ice damns.

You are certainly entitled to your opinion but it would be nice if you would explain it.

How does the moisture get in the attic in the first place?

I must agree with Michael even though I hate too! LOL


Sorry, but I don’t think so.

I used to think that way when I was a young guy, but now that I’m old and cranky I’ve come to realize the basics of physics.

There are a few things required for air to flow from the interior to the exterior of a building.

#1 there must be an opening.

#2 there must be a pressure differential from one side of the opening to the other.

#3 resistance of air flow regulates the direction of flow.

Have you checked the coefficient of friction between a gable vent and a ridge vent?

Air flows through the path of least resistance.

Air is rising and wants to get out of the attic.

Is it going to try to go through the ridge vent with a higher resistance or the gable vent with practically none?

If air is going to be drawn through the gable ventilator backwards with very little restriction, through a ridge vent which has a huge restriction coefficient, must we not be able to pass more air through the ridge vent than the gable vent?

Check out the “net free area” of a gable vent versus a ridge vent.

Air pressure is regulated by temperature differential and elevation.

What’s the temperature differential and height elevation between a ridge vent and a gable vent?

Is it possible to generate the condition that you mentioned? Yes.

Does this exist under normal construction techniques? You tell me. I don’t live where you do.

Agreed and pretty basic for everyone to know.

I also agree with Michael.

On a recent episode of this old house hour, long timer Tom Silva explained about the gable vent interfering with the high low venting of the soffit and ridge. Was one of the better episodes.

I saw that one also.

I think I am older and crankier than you David. :wink:

I idea is to have even air flow in each rafter or truss bay to control moisture and temperature.

The goal is to get hot air out of the attic.

As hot air rises the pressure of the air to leave the attic is greater at the upper elevation than it is at the lower elevation.

The net area of the upper elevation openings is a sum of the ridge vents and the gable vents. You can measure these differential pressures with a micro manometer and compare them with the differential pressures at soffit vents.

The net result will tell you whether or not air were short cycle from the gable vents into the ridge vent rather than coming in from the soffit vent.

I don’t care how many people from Holmes on Homes or This Old House, you cannot change the fact that if the pressure differential for one opening is greater than another that air will not flow in a particular direction.

I am not going to do the mathematics.

I agreed that there is a potential that an improperly design system where the ridge vent has a higher net opening than the gable vents installed (i.e. 100 feet of ridge vent versus one gable vent).

In most cases there are at least two gable vents installed in every house that has an aftermarket ridge vent installed.

The purpose of this convection application is to allow hot air to exit the upper part of the roof and drawer in cooler air from the bottom. If the total net area and pressure differential at the top of the roof exceeds that of the lower soffit ventilators than air will be drawn in from this offense rather than the gable ventilator at the same elevation as the ridge that.

I’m not talking about agricultural tobacco barns! They have total free area space at the ridge. I’m talking about a residential ridge vent that has little tiny slits of all kinds of fiberglass mesh to keep the bugs out.

You have 2 inches of open space on either side of the ridge board totally blocked with all this restriction.

If you care to impress me, show me the math.

Now if you want to talk with me about installing a power gable ventilator in a gable vent and its effects on inflow of air at the ridge and soffit vents we have another conversation.

If nothing less, tell me how many linear feet of ridge vent it takes to equal one gable vent…