» How do I calculate my attic square footage for ventilation?

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  Post subject: How do I calculate my attic square footage for ventilation?
PostPosted: Wed Aug 25, 2010 3:29 am 
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I know the surface area of my roof.
I know the length of the walls.

How do I calculate the sq foot for purposes of intake and outtake?

I have soffit vent that are the hardi plank?

They are 4 inches wide and I have about 64 feet.
How much CFM do I need in my vents?


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PostPosted: Wed Aug 25, 2010 4:31 am 
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you go by attic floor space so that should match your interior floor space. then you follow the 1/300 or 1/150 rule. so 1 square ft of ventilation per 300 square feet of floor space. so then half of that 1 sqft should be split between the ridge ventilation and soffit. If you didnt have soffit it would all go at the ridge ventilation. to figure out the ventilation you have just calculate square inches first, total that then convert to square feet.

hope this helps. just remember that 1/150 is double the amount of ventilation then 1/300.

cfm really depends on air pressure or speed.

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PostPosted: Wed Aug 25, 2010 6:21 am 
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CFM is cubic feet per minute. You can get your square footage by your floor (length x width). Multiply by wall height and you have CFM. If you take that number and multiply by 1.3 to 1.5 (depends on the pitch of your roof), you'll get a reasonable approximation of the cubic feet of space in your attic.


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  Post subject: Re: How do I calculate my attic square footage for ventilati
PostPosted: Wed Aug 25, 2010 6:25 pm 
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dadcjd wrote:
I know the surface area of my roof.
I know the length of the walls.

How do I calculate the sq foot for purposes of intake and outtake?

I have soffit vent that are the hardi plank?

They are 4 inches wide and I have about 64 feet.
How much CFM do I need in my vents?


What is the surface area of your roof?
What is the pitch?


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PostPosted: Thu Aug 26, 2010 12:48 am 
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Authentic_Dad wrote:
CFM is cubic feet per minute. You can get your square footage by your floor (length x width). Multiply by wall height and you have CFM. If you take that number and multiply by 1.3 to 1.5 (depends on the pitch of your roof), you'll get a reasonable approximation of the cubic feet of space in your attic.




correct me if im wrong but.......LxWxH doesnt give you CFM. cfm is a rate of flow. that would simply give you cubic feet of air volume of the area being measured.


I dont think you need to know squared cubic feet of the attic . primarily you need just attic floor space and go off of that. if your really concerned just add more venting then how my original response figures work out to be.

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PostPosted: Thu Aug 26, 2010 3:40 am 
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You can download the Ventilation Calculator from Air Vent. It is a useful tool to help you determine the amount of ventilation you need.

http://www.airvent.com/professional/cal ... Form.shtml


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PostPosted: Thu Aug 26, 2010 6:14 am 
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Gatesroofing wrote:
Authentic_Dad wrote:
CFM is cubic feet per minute. You can get your square footage by your floor (length x width). Multiply by wall height and you have CFM. If you take that number and multiply by 1.3 to 1.5 (depends on the pitch of your roof), you'll get a reasonable approximation of the cubic feet of space in your attic.




correct me if im wrong but.......LxWxH doesnt give you CFM. cfm is a rate of flow. that would simply give you cubic feet of air volume of the area being measured.


I dont think you need to know squared cubic feet of the attic . primarily you need just attic floor space and go off of that. if your really concerned just add more venting then how my original response figures work out to be.



You are correct, the rating states how many cubic feet per minute the vent moves. Therefore, you need to know the cubic feet in the attic to determine which vent or how many are needed. The attic floor square footage could be the same for a attic that has a 9' peak or a 25' peak. The volume of air in those two instances, inside the attic, is substantially different.


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PostPosted: Thu Aug 26, 2010 6:38 am 
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attic 1200 square feet divide by 300 times 144 to get to square inches then divide by 2 then get your intake or exhaust products flow in inches rating and divide it by that so you would need rounded up 6 vents on this home with the 300 rule. 12 at the 150 rule.

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PostPosted: Fri Aug 27, 2010 7:48 am 
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well granted the over all volumes might be different but i know for ohagin vents they dont call for calculating attic cubed feet. just floor space.

my main point is cfm applys more to a fan then it would a static vent because a fan moves a certain amount of air consitantly at the given Rpm. a static vent can move more or less air pending the air pressures inside and out of the attic. so thats why they only require you to figure out flat squared area of venting.

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  Post subject: Re: How do I calculate my attic square footage for ventilati
PostPosted: Fri Aug 27, 2010 8:16 pm 
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neville wrote:
dadcjd wrote:
I know the surface area of my roof.
I know the length of the walls.

How do I calculate the sq foot for purposes of intake and outtake?

I have soffit vent that are the hardi plank?

They are 4 inches wide and I have about 64 feet.
How much CFM do I need in my vents?


What is the surface area of your roof?
What is the pitch?


dadcjd,

Let's start back at your first question - how to calculate your sq.ft. for ventilation purposes. It is your entire attic floor space. You said you know your roof sq. footage. Take that number and divide by the appropriate number from the following table for your roof's slope/pitch.

3 pitch factor: 1.0308
4 pitch factor: 1.0541
5 pitch factor: 1.0833
6 pitch factor: 1.1180
7 pitch factor: 1.1577
8 pitch factor: 1.2018
9 pitch factor: 1.2500
10 pitch factor: 1.3017
11 pitch factor: 1.0704
12 pitch factor: 1.4142
13 pitch factor: 1.4743
14 pitch factor: 1.5366
15 pitch factor: 1.6645
16 pitch factor: 1.6667

Then, you will have your attic sq. ft.

Then you can use the calculations mentioned above to get your net free vent area requirements. You will need to look up or estimate the amount of NFVA per foot you get with your continuous vent soffits. As a rough guess, I'd say about 24" NFVA per foot, so each foot would handle intake duties for roughly 50 sq. feet of attic floor space.

Based on that 24" NFVA/ft assumption, your 64 feet of soffit vent is good for about 3200 sf of attic floor space. You would need to match that to your exhaust vents. You can find the NFVA for any vent in it's spec sheet. Most ridge vents, for instance, will have between 17" and 20" of NFVA per linear foot, and will handle exhaust duties for about 37 sq. ft. of attic vent (per linear foot of ridge vent). Your theoretical 3200 sf attic would need about 85 feet of ridge vent to go with your continuous-vent soffit.

You can scale up or down, linearly, from these numbers.


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  Post subject: Re: How do I calculate my attic square footage for ventilati
PostPosted: Fri Mar 04, 2011 11:48 am 
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dadcjd wrote:
I know the surface area of my roof.
I know the length of the walls.

How do I calculate the sq foot for purposes of intake and outtake?

I have soffit vent that are the hardi plank?

They are 4 inches wide and I have about 64 feet.
How much CFM do I need in my vents?


Hi dadcjd, you can split the surface to several regular piecs, then you can calculate it with square footage calculator


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  Post subject: Re: How do I calculate my attic square footage for ventilati
PostPosted: Sat Jul 27, 2013 4:02 am 
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Yes, Authentic_Dad........CFM DOES mean cubic feet per minute BUT, that's a measurement of the speed of MOVEMENT of a given amount of air, as in, for example 500 CFM, meaning 500 cubic feet of air per minute is FLOWING through an opening or a space.

It has NOTHING to do with the cubic CAPACITY of a certain space.
That's CUBIC FEET. Period!

(With all due respect CFM is used to measure the air moving capability of bathroom fans, etc.)


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  Post subject: Re:
PostPosted: Sat Jul 27, 2013 4:03 am 
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Gatesroofing wrote:
If you didnt have soffit it would all go at the ridge ventilation. to figure out the ventilation you have just calculate square inches first, total that then convert to square feet.


If you didn't have a soffit vent........there'd be no point in HAVING a ridge vent!!


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