Your gables are not subject to heat or moisture from inside the house and they are an enclosed cavity, not continuous into the attic, thus no venting.

I have never worked with aluminum, just vinyl, so may say something stupid, corrections welcome. The first step is to determine how much clear vent area you need. The guide most often used is 1 ft² of NFA (net free vent area) for every 150 ft² of attic floor. If the ceiling below is very well air sealed and has a vapor barrier then half that number is allowed.

That number is then divided half for high venting and half for low venting and then you divide the low vent number half for front and half for back. Unless your house is shaped differently.

All vents should have a NFA rating, but once you have that number then you can better decide which vent panel and how much you need.

Bud

 

Figuring how much venting needed

So let me see if I understand and figured this correctly.

Total square footage of attic floor space= 2112 sq ft

Divide 2112 sq ft by 150= 14.08 sq ft of ventilation needed

Now divide that number by 2= 7 sq feet of soffit vent space needed?

This does't make any sense to me looking at how many soffit vents we currently have. Which are ALOT
Our soffit is deeper at 34 inches.

If we picked out new soffit, each peice put up would be 12 inch wide by 34 deep. The center vent of that is equal to 1 square foot of ventilation.
If they say we only need 7 sq feet of ventalation wouldnt that mean only 7 of those type panels for the whole house to equal 7 sq foot of ventilation?

Again right now we have (2) 10 inch solid followed by (1) 10 in vented
and at soffits 34 inches deep seem to currently have way more than 7 sq foot of ventilation total

Sorry to be so confused

 

So let me see if I understand and figured this correctly.

Total square footage of attic floor space= 2112 sq ft

Divide 2112 sq ft by 150= 14.08 sq ft of ventilation needed

Now divide that number by 2= 7 sq feet of soffit vent space needed?

This does't make any sense to me looking at how many soffit vents we currently have. Which are ALOT
Our soffit is deeper at 34 inches.

If we picked out new soffit, each peice put up would be 12 inch wide by 34 deep. The center vent of that is equal to 1 square foot of ventilation.
If they say we only need 7 sq feet of ventalation wouldnt that mean only 7 of those type panels for the whole house to equal 7 sq foot of ventilation?

Again right now we have (2) 10 inch solid followed by (1) 10 in vented
and at soffits 34 inches deep seem to currently have way more than 7 sq foot of ventilation total

Sorry to be so confused

I believe he was saying that you need 14.08ft^2 total. So usually you would put half this area in the soffit on the front of your house and half on the back. Keep in mind this is just the minimum required, you can always do more. Also any gable end vents and ridge vents are included in this calculation.

R806.2 Minimum vent area.
The minimum net free ventilating area shall be 1/150 of the area of the vented space.

Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:

1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

2. At least 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located no more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by eave or cornice vents. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

 

Let's see if we can check your numbers first.
Your 7 ft² looks correct for a 2112 ft² attic floor, but note, it can be half of that if all air leaks from below have been well sealed and you have either a vapor barrier of in some cases certain paints will do. And that would then be half on one side and half on the other.

Then, you said "12 inch wide by 34 deep. The center vent of that is equal to 1 square foot of ventilation." For a 12" wide panel I would be thinking something closer to 1/3rd that amount. More like 15 in² NFA per linear foot. Do you have the spec sheet on your panels?

Just back of hand numbers here, but a 30x70 home would be 2,100 ft² with a 70' front soffit. Needing 3.5 ft² or 500 in², if each vented panel has 144 in² then you would need only 4 panels.

Double check the NFA or get the spec sheet.
Also, how would you rate the air sealing of the ceiling plane? Have you done any air sealing?

Bud
Sorry Mingled if I repeated, we crossed paths, me being the slower .

 

We have not personally done any air sealing, but it may have been done before we moved in. So I don't really have a rating to give you on that.

 

When you are up there just look around any plumbing pipes or dig down to the top of a couple of walls to see if electrical holes have been sealed. The plumbing pipes would typically have a metal flashing fitted around them or at least lots of can foam.

Bud

 

Continuous along the gutter lines.

 

I agree with previous post, the fascia area is best for optimum wind/rain/snow prevention performance rather than close to the building face. Check your manufacturer's site, as said, you want about 9sq.in. per foot to match your 18sq.in. ridge vent on a gabled roof. The 10 inch perfs may only be 6-7", not enough,and the hidden may only be 2.66sq.in. per foot, need a lot if same/close to this example (in vinyl); http://bluelinxco.com/Portals/0/doc...ocs/LiteratureLibrary/Siding and Trim/GP Vinyl Siding/Vinyl Soffit Brochure.pdf

Then the holes get reduced if you paint the soffit, further reduced when the wet air stream deposits moisture around the punched holes for dirt to collect over the years... the punched slots are a little better, not by much, lol.

Gary