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darren 04-08-2008 10:27 PM

roof venting via the soffit
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So after noticeing some moisture problems in my attic i started to check out my vent situation and noticed I had no soffit vents.

This is the material I have under my soffits(see picture) Is this going to let in enough air to properly vent it. I already have two roof vents and two gable vents.


Ed the Roofer 04-09-2008 10:55 AM

If the lanced soffit panels are not obstructed by any insulation or filled up and clogged with dust or cob webs and "IF" the wood soffit has been cut out to allow for the fresh air intake to actually flow through them then you probably have ample and more than sufficient intake ventilation.

A problem that you do have though, is you stated you only have two roof top vents, which I presume to be the small static air mushroom style vents, plus the side wall gable vents.

By having opposing exhaust ventilation systems, the air flow is being short circuited. The extremely less than adequate amount of roof top exhaust musroom vent are struggling to alow enough exhaust air to flow out of the attic and it is pulling air from the path of least resistance.

Since the gable end vents are closer to the roof top, they are now acting as your intake vents. Instead of air being pulled in through the soffit vents and completely "Washing" the underside of the roof sheathing, it is eminating from the gable vents instead. This does not achieve the proper air flowage required to cool down the attic interior and remove the hot and humid, stale air inside of it.

If you measure the entire length of your home, from edge to edge, not interior wall dimensions, I could calculate how much exhaust ventilation you wil need to add, plus advise you to seal off the gable vents to eliminate the short-circuiting aspect occurring in your attic presently.


darren 04-09-2008 12:31 PM

Hey Ed, the house is 45x26

You would be correct the two vents on the roof are two static vents, i was thinking about adding a whirlybird, do you think that would help.

The plywood in the soffits are not cut so i will have to go around and cut them out but why would they put up that soffit material without cutting some vents in when they had a chance.

If you could tell me how much venting i need that would great then i would knwo how much cutting i have to do.


Ed the Roofer 04-09-2008 01:10 PM

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

In your case, as it stands now, you will need to follow the 1/150 formula instead.

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 did'nt they cut out the wood? 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.


darren 04-09-2008 05:53 PM

HeY thanks for your help, its people like you that make this board a great resource.

So what your saying is the more i cut out under the soffits the better of i will be.

johnk 04-09-2008 06:12 PM

Ed,you must have answered this question 6 or 7 times this week.You should just put a permanent post on here just for ventilation!

Ed the Roofer 04-09-2008 08:09 PM


Originally Posted by darren (Post 115052)
HeY thanks for your help, its people like you that make this board a great resource.

So what your saying is the more i cut out under the soffits the better of i will be.

Thank you very much Darren.

Yes, the more intake ventilation, the better off you will be.

Be very careful cutting that wood out from inside the attic, by making sure you have the blade depth set to the minimum plunge, otherwise you will be chopping through your aluminum soffit panels.



Originally Posted by johnk (Post 115058)
Ed,you must have answered this question 6 or 7 times this week.You should just put a permanent post on here just for ventilation!

That would take the fun out of it. Sometimes, if they are nearly identical scenarios, I just post a link to the past thread, but usually, there are enough differences to justify a specific answer for each individual seeking information.


johnk 04-09-2008 08:52 PM

That's why you the man!:yes:

the roofing god 04-13-2008 09:29 PM

get a room !!!

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