Soffit Vents
#1
05-20-13, 08:04 AM
Soffit Vents
Realized just this weekend that we have a ridge vent in our house but no soffit vents. Several questions since part of the attic has been finished so there is ceiling, insulation, then roof with only the plastic things to allow ventilation. Should I calculate this part as attic space? Also is it better to error on less soffits per ridge vent or to have extra soffit vents. Thank you.
#3
05-20-13, 09:43 AM
The calculation process is a wild guess to start with, so attic floor area is fine. On an existing house that hasn't been extensively air sealed use one sq ft of vent area for every 150 sq ft of attic floor. When the attic to house has not been well sealed, then as Joe stated, more low venting is better.
Also, be sure to use "net free area".
Bud
Also, be sure to use "net free area".
Bud
Last edited by Bud9051; 05-20-13 at 09:45 AM. Reason: typo
#5
05-21-13, 06:12 AM
A one sq ft hole has a net free area of one sq ft. Cover it with a bug screen and some louvers to keep the rain out and it reduces the air flow. All vents will have a NFA (net free area) rating some where well below the opening in which they are installed. NFA ratings will range from 90% to 10%. A soffit vent that covers 4 sq ft but is actually just a lot of small holes might have a 30% NFA. When that number is not available, common sense has to be substituted (best guess). I rarely allow more than 50% when I guess.
Bud
Bud
