View Full Version : OK, Why do I need soffit venting?

Matt Wilson

04-08-2011, 10:57 PM

I have old 6"x12" aluminum square "ugly" vents, with plywood and it's saggin' bad, and was doing some reno's so, I want change them out with T&G soffiting, I found these continous wooden soffit vents, and I'd like to know what the proper amount of airflow that is required between the ridge vent and the soffit venting. And any tips of hints would be welcome too.

Wood Soffit Vents-Whitehaven Wood Products-Continuous Soffit Vents, Handcrafted in Canada (http://soffitvents.ca/)

Tom Thompson

04-08-2011, 11:42 PM

1. Determine the square footage of attic area to be ventilated. To do that, just multiply the length of the attic (in feet) by its width.

Example: For this and the following calculations, we’ll assume the home has a 40' by 25' attic area.

Calculation:

40' x 25' = 1,000 square feet of attic area

2. Determine the total net free area required. Once attic square footage is known, divide by 150 (for the 1/150 ratio). That determines the total amount of net free area needed to properly ventilate the attic.

Calculation:

1,000 sq. ft. ÷ 150 = 6.6 square feet of total net free area

3. Determine the amount of intake and exhaust (low and high) net free area required. For optimum performance, the attic ventilation system must be balanced with intake and exhaust vents. This is a simple calculation: just divide the answer from Step 2 by 2.

Calculation:

6.6 ÷ 2 = 3.3 sq. ft. of intake net free area and 3.3 sq. ft. of exhaust net free area

4. Convert to square inches. The net free area specifications for attic ventilation products are listed in square inches. Therefore, let’s convert our calculation in Step 3 from square feet to square inches. To do this simply multiply by 144.

Calculation:

3.3 sq. ft. x 144 = 475 sq. in. of intake net free area and 475 sq. in. of exhaust net free area.

5. Determine the number of units of intake and exhaust venting you’ll require. To make these calculations, first refer to the Net Free Area Table below. The table lists the approximate net free area, in square inches, for common intake and exhaust ventilation units. To perform the calculations, divide the net free area requirement from Step 4 by the appropriate figure from

the Net Free Area Table3. For our example, we will use the figures for ShingleVent II and shingle-over The Edge Vent.

Calculation:

(for 4-foot length of ridge vent) 475 sq. in. ÷ 72 = 6.6 pieces of vent (or seven 4-foot lengths of ridge vent) (for 4-foot length of shingle-over intake vent) 475 sq. in. ÷ 36 = 13.2 pieces of vent (or fourteen 4-foot lengths of shingle-over intake)

Scott Patterson

04-09-2011, 07:19 AM

Or just used continuous soffit venting by James Hardie!

Ted Menelly

04-09-2011, 08:03 AM

Or just used continuous soffit venting by James Hardie!

That's the best way to do it. It may be continuous but it is just perforated so it is not like all soffits are wide open.

Not to mention the fact is looks a lot nicer the vents cut in that wind up getting painted over time and the paint

peeling off.

Matt Wilson

04-11-2011, 01:19 PM

Or just used continuous soffit venting by James Hardie!

but...that wouldn't fit with my T&G that I'm using, plus not as much ventilation as with the wooden soffit vents

Scott Patterson

04-11-2011, 01:23 PM

but...that wouldn't fit with my T&G that I'm using, plus not as much ventilation as with the wooden soffit vents

Well then, don't use it.

Matt Wilson

04-11-2011, 01:28 PM

I have old 6"x12" aluminum square "ugly" vents, with plywood and it's saggin' bad, and was doing some reno's so, I want change them out with T&G soffiting, I found these continous wooden soffit vents, and I'd like to know what the proper amount of airflow that is required between the ridge vent and the soffit venting. And any tips of hints would be welcome too.

Wood Soffit Vents-Whitehaven Wood Products-Continuous Soffit Vents, Handcrafted in Canada (http://soffitvents.ca/)

So I was looking at the Hardie soffit boards, and there is no way that these soffit panels allow an adequate air flow...does anyone know the sq. inches of flow per 8ft. piece, assuming of course that I am using ridge top venting as well. I know these wood soffit vents I have been looking at, allow approx. 1sq. ft. of airflow per 8ft. piece, I do not want to have any ventilation "issues" further down the road. I want this to be the last time I ever get up under the eaves, and tackle this project.

Matt Wilson

04-11-2011, 01:32 PM

Well then, don't use it.

Is that your friendly inspector answer, wow, thanks.

Matt Wilson

04-11-2011, 01:36 PM

1. Determine the square footage of attic area to be ventilated. To do that, just multiply the length of the attic (in feet) by its width.

Example: For this and the following calculations, we’ll assume the home has a 40' by 25' attic area.

Calculation:

40' x 25' = 1,000 square feet of attic area

2. Determine the total net free area required. Once attic square footage is known, divide by 150 (for the 1/150 ratio). That determines the total amount of net free area needed to properly ventilate the attic.

Calculation:

1,000 sq. ft. ÷ 150 = 6.6 square feet of total net free area

3. Determine the amount of intake and exhaust (low and high) net free area required. For optimum performance, the attic ventilation system must be balanced with intake and exhaust vents. This is a simple calculation: just divide the answer from Step 2 by 2.

Calculation:

6.6 ÷ 2 = 3.3 sq. ft. of intake net free area and 3.3 sq. ft. of exhaust net free area

4. Convert to square inches. The net free area specifications for attic ventilation products are listed in square inches. Therefore, let’s convert our calculation in Step 3 from square feet to square inches. To do this simply multiply by 144.

Calculation:

3.3 sq. ft. x 144 = 475 sq. in. of intake net free area and 475 sq. in. of exhaust net free area.

5. Determine the number of units of intake and exhaust venting you’ll require. To make these calculations, first refer to the Net Free Area Table below. The table lists the approximate net free area, in square inches, for common intake and exhaust ventilation units. To perform the calculations, divide the net free area requirement from Step 4 by the appropriate figure from

the Net Free Area Table3. For our example, we will use the figures for ShingleVent II and shingle-over The Edge Vent.

Calculation:

(for 4-foot length of ridge vent) 475 sq. in. ÷ 72 = 6.6 pieces of vent (or seven 4-foot lengths of ridge vent) (for 4-foot length of shingle-over intake vent) 475 sq. in. ÷ 36 = 13.2 pieces of vent (or fourteen 4-foot lengths of shingle-over intake)

Thanks for this answer Tom, cleared up a few questions, for sure.

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