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Mechanically Challenged 10-13-2008 10:39 AM

Poor Attic Ventilation
I have continuous soffits and a ridge vent BUT there is insufficient air flow through the soffits due to builder error. There is no way to fix the soffit problem. So I don't have enough air coming in to vent properly. The result is a hot attic in the summer, ice build up in the winter. Also, my shingles are curled and damaged and will be replaced in the Spring.

I'm assuming I now need gable vents to allow air intake and the hot air will exhaust through the ridge vent.

One side of the house (North) is easily accessible. Can I get away with one large gable vent or do I need two? My home is 50 X 28. Any advice on how to install a gable vent on a cedar shingle exterior?


Slyfox 10-14-2008 05:16 PM

Gable vents provide minamal intake at best.
If your going to re-roof in the spring, ask your roofers about installing intake ventilation in the field of the roof itself,
google 'smart vent' and you'll find some information on what i'm talking about.

rredogg 10-14-2008 06:55 PM

Don't know if this will help you or not. On my house I found that the builder covered the soffit vents with the insulation in the attic. Took me a couple of days on my back in the attic but I cleared all the vents and added baffles (can purchase at any big box store), for a positive air flow.

Best of luck with the problem, rredogg

troyce1 10-20-2008 11:10 PM

Is there an easy way to figure out intake and outake?

I want to install a power ventilator but am
A) worried that there will not be enough soffit intake for the fan, and it will start pulling heating and or AC through any cracks it can
B) wondering if it will provide enough venilation in the winter when the auto thermostat is not going to activate.

I looked into using turtle vents also, but wanted more airflow.

Slyfox 10-21-2008 06:11 AM

My opinion on vent styles, 0 being the worse and 10 being the best,

Gable = 1
Power = 3
Turbine = 4
Turtle = 8
Ridge = 9

Unobstructed Flow:
If you don't have proper flowage space the best exhaust and intake will still not work properly.

Gable = 1
Power = 5
Facia = 5
Field = 8
Eaves = 9

As far as the csi's you need,
google roof/home ventilation requirements,
call a local roofer/hvac contractor 'free estimate',
go to lowes, home depot, etc., they will have the litature showing requirments.

troyce1 10-21-2008 09:02 AM

Is this what you mean by turtle vent?

marksf 10-21-2008 03:18 PM

Attic ventilation
Try this on for size, it's the 1/300 rule that will get you the amount of ventilation required by the bulding codes, BOCA & IBC.
Your home is 50'x28', subtract your overhangs (eaves), 2x50+2x28 = 156'
50x28=1400-156=1244 Now this is the where the 1/300 rule comes in, 1244 is the square foot area of your attic, 1/300 means you need 300 inches of ventilation for every square foot evenly divided between your soffits and your roof. Thus, 1244/300/2=2.073, you need 2.073 square feet of ventilation at your soffits and ridge. A square foot is 144 inches, so if you take 114x2.073=298.5 you need 298.5 inches of ventilation at your eaves and 298.5 inches of ventilation in your roof vents. Just remember that most ridgevents gets you 18 square inches per foot, so to have a balanced ventilation system you'll need the same amout at your soffits as you have in ridgevent.

Sorry to be so long winded but explaing that rule to someone not familiar with attic ventilation needs to have it broken down that way. I agree with rredog, you should check and see if your soffits are blocked with insulation. This is what happend to me also. Luckily the house I bought had been owned 5 years prior to my purchase by the guy who was running our insulation dept. turns out that the baffles hadn't been installed by his guys, just left up in the attic. :furious:

Sea ya,


troyce1 10-21-2008 04:33 PM

Mark, thanks for that info.

I guess Im trying to figure out if I have a thermostat controlled fan that pulls 1000 cubic feet per minute when on, how much soffit ventilation do I need?

I have about 15 cubic feet on each side, and was planning on adding more. (35 ft at 1/2 inch wide on each side)

I was thinking about adding 3 additional turtle vents as well near the ridge that would have 50 cubic inches ea.

The roof is extremely steep thats why I was planning on just instaling turtle vents at the ends.

Ed the Roofer 10-21-2008 06:06 PM


Excellent answer.

I didn't want to type in all of the information I gave previously and change the math, but as Mark pointed out, you need to do the actual calculations of your attic floor space.

I use the entire attic floor space though, versus him deducting for the overhangs, since air flowage still needs to be considered from those areas, even though they are not on top of the heated/air-conditioned portion of the home.

See my copy/paste response to someone else below:


45' x 26' = 1,170 square feet of attic floor space, also known as its "Footprint".

I am going to round that figure up just a little for simpler math.

1,200 square feet total.

If your ventilation is equaly balanced or you have a continuous non-penetrated vapor barrier under the warm side of the attic floor insulation, the calculation can be 1 square foot of ventilation for every 300 square feet of attic floor space. In this case, 1,200 divided by 300 = 4 square feet needed. Now convert 4 square feet to square inches.
4 x 144 square inches = 576 square inches required.
This is known as the 1/300 Formula.

Exhaust Vent Products:
Mushroom vents, standard size = 50 square inches
Mushroom vents, very large size = 65 square inches
Small whirly bird turbine vents = 150 square inches
Large whirly bird turbine vents = 250 square inches
Most ridge vent products = 18 square inches per foot, (ranges from 12 sq in to 19 sq in.)

Intake Ventilation Products:
Under soffit louvre vents
4" x 16" = 28 square inches each vent
8" x 16" = 56 square inches each vent
Continuous 3" strip vent = 9 square inches per lineal foot
Lanced or Perforated soffit panel = 6-9 square inches per square foot

Shingle Over Style Intake Vents, such as the Smart Vent from DCI Products, or TheEdge Intake from Airvent, or the InVent from Cor-a-vent. The Smart Vent and TheEdge Vent each provide a rated 9 square inches of Intake Ventilation per lineal foot installed, while the InVent is slightly less than that.

The Smart Vent is an excellent product to be utilized when you are removing the shingles and have no other options for increasing the amount of Intake Ventilation, either due to a lack or minimal overhang on the soffit areas, or due to clogged air pathways restricting the air flowage, typically due to improper insulation installation.

In your case, as it stands now, you will need to follow the 1/150 formula instead. (The 1/150 Formula is the preferred choice to ensure meeting more than "Minimum" Code Standards and Manufacturers Standards for Warranty fulfillment)

In your case, you not only will need to close off the gable vents to prevent the short-circuiting of the air flowage, you will need a grand total of 1,152 square inches of NFVA, Net Free Ventilation Area, to properly vent out the hot and humid attic air. These also are the "Minimum" requirements for the shingle manufacturer to enable your roofing shingle warranty to be valid.

So, if the ridge line is vented with a ridge vent, such as the Shingle Vent II, from Air Vent Corp., which provides 18 square inches per lineal foot, you wil achieve 45' x 18 square inches = 810 total square inches of exhaust ventilation.

Now, you need to provide an equal balanced amount of intake ventilation. The size of the holes cut through the soffit wood must be the same size as the vented panels to perform at their maximum efficiency. Also, if you do not cut them out continuously, you should space them out unifirmaly for an even distribution of fresh air intake entering into the attic cavity.

The most efficient proportion of ventilation distribution, is 60% intake ventilation to 40% exhaust ventilation, according to most experts.

Why didn't they cut out the wood soffit material away when installing the aluminum soffit cladding? It happens all of the time. They know the home owner does not pay attention to that and it looks like it is vented with those lanced or perforated soffit panels.

Why don't your neighbors houses have that many vent on them? Because between 90% to 95% of ALL HOMES, do not have adequate ventilation, and usually fall between 15% to 33% of the "MINIMUM" requirements.


Slyfox 10-21-2008 06:25 PM


Originally Posted by troyce1 (Post 174726)
Is this what you mean by turtle vent?

Yeah, there are a dozen different types/styles of roof vents,
and turtle, louver, pot, box, etc different names there called by.

Ventilation Calculator

troyce1 10-22-2008 08:28 AM

slyfox, thanks for the link to the calculator. Is the total NFA that it gives combined intake and outtake?

Slyfox 10-23-2008 06:27 AM

I believe it is intake and exhaust combined.

Take note that they are giving the minimum requirements.

On a shingle system such as Tamko, the minimum amount of ventilation
works "on and average/basic designed structure",
because of the shingles ability/flexibility to breathe and exspand/contract.

On a shingle system such as Owens Corning where the stronger self sealers are used, its better to over excede the amount of ventilation
because they don't have that same ability/flexibility.

marksf 10-23-2008 12:26 PM

Ed, you are entirely correct. I only deducted the overhangs to get a close proximity to the square foot measurement of his attic. Being an estimator I'm used to having people giving me the measurement of a roof over the phone to get a rough price for their roof. It didn't dawn on me that he was giving the actual measurement of his house.

Ed the Roofer 10-23-2008 01:06 PM

Most, if not all home owners do not know the square footage of their house as compared to the assessed square footage of living quarters.

They rarely account for the additional length of the soffit overhangs and the increased square footage due to the pitch factor.

Ventilation calculation guides are in the grey area as to whether or not to include the unheated overhangs, but since the volume of attic air space also permeates the additional areas, I feel it is more prudent to include the additional areas to be on the safe side.


troyce1 10-23-2008 06:35 PM

Well, I figured out my intake and out, take for standard static vents

Now I need to figure it out for an electric thermostat activated fan that pulls 1000 cubic feet per minute.
Amy ideas?

Thanks for all the helpful info guys.

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