[Murdy]

My attic is 1040 square feet. I have 2 large gable vents on either end of the attic and 3 static vents (mushroom style) along the ridge, but no soffit vents. My understanding is that 1 vent per 300 square feet is sufficient. Is this sufficient? If not, how many would be enough, in light of the other venting?

 

[Bud9051]

Passive attic venting (no fans) functions based upon the difference in height between the low vents and the high vents.

As for how many vents the calculation is based upon the NFA (net free area) of all vents. In general when a vent is covered with a screen to keep the bugs out its NFA is about half the actual area.

As for the calculations they want 1 ft² of NFA for every 150 sq ft of attic floor when the ceiling has not been carefully air sealed. That number drops to 1 ft² per 300 sq ft of attic floor when well sealed.

If you don't know the NFA rating of your vents search for something similar and use their listed NFA,

Whatever you calculate for total vent area is then divided helf high and half low, but don't panic if you can't meet that it originated as a wild guess 70 years ago.

Can I ask what your concerns are?

Bud

 

[Murdy]

My main concern is lack of soffit venting, so basically everything is high, so I suppose I should stick a few in there. By my calculations, 3 to 4 would be right. I'll see if I can find something similar to what's in there and get specs. It's an old house and the attic has always been dry, so I'm probably overthinking it. Thanks for the advice.

 

[Bud9051]

The explanation I gave you is for when the wind is not blowing, ie passive. When the wind is moving air it does so far better then than passive and as long as it doesn't drive rain or snow into the attic it is great.

By using the passive vent calculations you get the best of both circumstances and you make the inspectors and code people happy. But an old attic that is doing fine falls under the "if it ain't broke don't fix it" thinking.

Best,
Bud

 

[Murdy]

Thanks much.