# Required Attic Vent Calculation

## Recommended Posts

Suppose I have 300 s.f. of attic,  how much vent area do I need?

I use one of two rules to calculate required attic vent area.  Area divided by either 150 or 300

300 s.f. of attic/150 = 2 s.f. of required vents or 288 s.i.

or

300 s.f. of attic/300 = 1 s.f. of required vents or 144 s.i.

See the attached pic from this web site  (roofing company calculator.  Does this make sense?  Am I missing something?

##### Share on other sites

I think the IRC book says 150.

##### Share on other sites

Scott,  Just looking at the math aspect I think you missed the "divided 50 / 50 between the inlets and outlets" part.

I should be clearer but these things always seem to be based on assumptions.

##### Share on other sites

50% high and 50% low vents and it should say what vents you are using and what size in sq. inches each one is.

##### Share on other sites

I think Perry is right , the website has split the result in half because of the 50/50 rule on inlet and outlet air.  ie soffit vents and roof vents

##### Share on other sites
6 minutes ago, Chopsaw said:

Scott,  Just looking at the math aspect I think you missed the "divided 50 / 50 between the inlets and outlets" part.

I should be clearer but these things always seem to be based on assumptions.

Chop,  I saw that,  so in the first example when it says 144 net free area,  that means 144 net free area in and 144 net free area out so a total of 288 s.f.?   And so when it says I need 3 panel vents,  does that mean I need 3 panel vents total or 3 vents in and 3 vents out for a total of 6?

##### Share on other sites
2 minutes ago, Kbird1 said:

I think Perry is right , the website has split the result in half because of the 50/50 rule on inlet and outlet air.  ie soffit vents and roof vents

So how much area do you think I need in example 1?  144 total vent area or 288 total vent area.

##### Share on other sites
5 minutes ago, dshall said:

So how much area do you think I need in example 1?  144 total vent area or 288 total vent area.

I think 288in ........   144in of soffit vent (Bottom)  144in of roof/ridge vent (top)

how many panels vents depends on how many sq.in each one is .... but in my book you can't have too many vent's.....

##### Share on other sites

Scott, Since that is put out by a roofing company I think they are assuming that intake vents ( soffit ) are not a roofers responsibility.

And for example 1 you would need 144 in and 144 out for a total of 288 but again that is just an opinion as venting in Canada is different.

##### Share on other sites

Thanks guys.  You confirmed what I was thinking.  I was starting to question my mathematical ability..... if they had been clearer for dimwits like myself,  we wouldn't be having this conversation.  Thanks again for the input.

• 1

##### Share on other sites

Scott,

Don't forget that vents may need to have auto fire dampers, depending on the jurisdiction.

##### Share on other sites
9 minutes ago, Joe_Carrick said:

Scott,

Don't forget that vents may need to have auto fire dampers, depending on the jurisdiction.

I use the O'Hagan's low profile type, I wonder if you need auto dampers with those, haven't had to yet, but I usually don't do much in heavy fire areas

##### Share on other sites
4 hours ago, DRAWZILLA said:

I use the O'Hagan's low profile type, I wonder if you need auto dampers with those, haven't had to yet, but I usually don't do much in heavy fire areas

Imuse ohagin too,  never needed the dampers.  On this particular job spewing out very specific metal roof.  Not sure ohagin would work in this case.

##### Share on other sites
15 hours ago, dshall said:

Imuse ohagin too,  never needed the dampers.  On this particular job spewing out very specific metal roof.  Not sure ohagin would work in this case.

Vaulted ceilings have been a problem for me as we have to vent each bay using eave vents and in some cities ,ridge vents to the high vents.

##### Share on other sites

Scott - Make it real easy on yourself:  https://www.owenscorning.com/roofing/components/vent-calculator

##### Share on other sites

From our residential construction guide for the county.

NOTE that your roofing material may ask for a minimum venting, or you

can void warranty

##### Share on other sites

You can't over vent an unconditioned attic space (especially in snow regions).  We vent every truss bay because we need every bay to have that "wash" on the bottom side of the shtg. to help prevent ice dams.

In your warmer climate you probably could get away with every 3rd bay. (but I would still reccomend over venting) :-)

##### Share on other sites

You split the 144 sq. in. 50/50 high and low.

So 72 sq. in. high and 72 sq. in. low.

Andy.