Enough Ventilation??

Hi all,

Do you think this roof has enough ventilation with just these two gable vents? (one at each end of the house) I am thinking roof vents should be added? Am I mistaken?


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Only based on your photos of the gable vent alone, the answer would be definitely not enough.

There must be an equal amount of NFVA from the intake vents to bring in fresh air as there are NFVA for the exhausted air. That would be the ideal balanced ventilation scenario. If a truly balanced scenario is not possible, then weight the value in favor of the intake ventilation at 60 % of the total with the remaining 40 % being applied toward the exhaust ventilation.

Take the entire attic floor space square footage and divide by 300, (or divide by 150, if only one ventilation system is in use or if their is no vapor barrier on the warm side of the attic insulation). This will give you the total amount of square feet of ventilation required.

All ventilation product are sold listing their total NFA or NFVA per product. As a guestimate of the gable vents NFVA, multiply the length by the width for the total square inces of each vent and reduce that figure by 50 % to account for the screening or filter contained on the product.

Remember, that one square foot = 144 square inches, so although the amount of venting may seem prepostorous, that will be the true required total needed.

Additional information about this subject can be found in the “Principles of Attic Ventilation” booklet available from the Air Vent Corporation, which is a Gibraltar subsidiary.

Ed the Roofer

I agree with Ed, not enough.


When I run see gable end vents, I always recommend ridge and soffit vents at roof replacement.

I’ve seen several houses were the gable end vents are working absolutely fine and not causing roofing issues, but when I get inside the attic, I see why…

There’s inadequate insulation in the attic, which gives the roofing material plenty of venting on the underside, as the inadequate insulation allows venting to happen naturally.

Warning, Bill Robinson!!! (from old Lost in Space for anyone too young)

If you follow dvalleys’ advice, you must block off and seal the gable vents so you do not create a “short-circuited” air flow. Basically, the path of least resistance would wind up occurring, which would mean that the gable vents would now act as the intake ventilation system, leaving the actual soffit vents non-functioning. As a result, the middle 90 % of the attic, (approximately), would wind up being non-ventilated. Also, by the gable vents functioning as intake vents in this scenario, weather infiltration may occur through them into the attic or at least minimally be allowed to run down the interior side of those exterior walls.


The first thing I look for in every attic is signs of moisture build up. If the sheathing is growing mildew, etc, then it is a clear sign of insufficient attic ventilation. If not, then I don’t normally recommend additional vents, until the roof covering is replaced. Which most roofing contractors in our area do when re-roofing.
So what age is the roof?, and will it need to be replaced soon?
If the roof is newer, but vents were not added, then I suppose the argument that the ventilation does not meet standards could be made, and roof covering warranties could be voided. Buyers need to at least be made aware of this.

Hey David
That is a good point. The older homes never really needed as much ventilation because of the lesser amounts of insulation. A home the age in the pictures had maybe 4" of attic insulation… here…(not sure in Colorado)
Modern homes however have 12" to 14", so the roof sheathing is cooler during the winter months, requiring more air circulation.
Insulation values and ventilation are related.

Oh and Greg; Are there bath exhaust fans?..I don’t see any roof mounted discharge for those. I find lots of those pointed at soffit vents, which creates moisture problems also…or pointed toward the gable end louvers…:frowning:

Yes it needs soffit vents.

See http://www.airvent.com/professional/education.shtml

Lotsa good information regarding attic venilation, and there a training course that may garner CEU’s.

Warning, Bill Robinson!!! (from old Lost in Space for anyone too young)

BTW…it’s “Will Robinson” !!!

And NO…there is NOT enough ventilation. Rule of thumb, when there are only Gable vents, there needs to be 1 sq ft of ventilation for every 150 sq ft of attic space.

Hot air rises…

Where is the hot air in this house going?

I tried to edit the “Bill Robinson” to Will after I saw my typo, but was unable to make a change to it.

Ed and Little eddie;-)

One of the easiest solutions is to add a mechanical fan to one of the gable vents. Inexpensive, usually requires only minimal modifications to existing framing if at all, a junction box near the gable of choice (preferably the end of the house furtherest from the bedrooms, typically over the garage is best). One note though; in the SOP, it states we are not required to comment or determine the adequacy of any system / equipment, just a cautionary note to keep in the back of your mind.

I must take issue with the idea that ‘older homes didn’t need as much ventilation’…please explain your thoughts to me, I would like to better understand your rationale for this statement.

The function of ventilating the attic serves 2 purposes, removal of moisture that has escaped from the building envelope into the attic and cooling of the roof deck and the attic space from heat that has built up from the relentless rays of the sun.

The insulation in the attic has no bearing on the attic ventilation per se, it does effect ice damming (by keeping the heat inside the living space), it does effect the transfer of heat from the attic to the living space. Improperly installed attic insulation can block the flow of air in, through and out of the attic.

Ventilation of the attic for removal of heated air in the space and cooling of the underside of the decking is an effect produced by the movement of air. The life span of a shingle roof can be greatly reduced by overheating brought on by improper attic ventilation and this can happen on a roof that is installed on a building that does not have any insulation at all, for example a roof installed over a garage.

As I state that I take issue with the statement I will stay open to the reasoning that may be stated to the contrary.

My take on that is…Older homes with less insulation will allow the attic cavity to naturally ventilate, due to less insulation. Free flowing air throughout the house can actually penetrate a thin blanket/layer of insulation in the attic.

Todays standards require more insulation, which envelops the attic cavity and restricts air flow which in turn, requires additional vents to be installed.

What I understand you saying reqards the flow from the living space too the attic…the ventilation of the attic is the flow of air into and out of the attic space from the out side which carries away heat and moisture from the attic space.

From what you are saying if a house with less attic insulation allows more air to travel from the living space to the attic then would it not stand to reason that a greater air flow would carry more moisture and heat, thus in this situation a greater rate of attic ventilation would be necessary rather than a lesser amount.


I never stated that the attic would need less ventilation in any situation.

Consider that many homes built before insulation was installed in attics (and walls) also did not require ventilation, or at best it was limited.

The heat from the coal, wood, or oil heat sources also heated the attic space. This is a crude warm roof system, but effectively it increased the temperature of the roof system above the dewpoint, and you did not need all the soffit, gable and roof vents we have today.

On the other side of the coin is todays home with R-38 and more in certain areas. This high amount of thermal resistance keeps the roof system cold, and with out air circulation, condensation occurs.

The original home for this thread probably only had R-11 in the ceilings, or a close approximate. Therefore more heat is transferred to the attic requiring less ventilation, to perform the same job of a modern home with R-38. How warm the underside of the roof sheathing is maintained is a key element to condensation accumulation in the attic.

Not saying less ventilation is acceptable. Just trying to explain that there has been a distinct correlation between the required addition of more ventilation, when there has been an increase in required ceiling insulation.

Attic evolution.


BTW-little different topic than above posts…but informational…
Here is a nice article on unvented attic systems from Journal of Light Construction.