Hi,
What is the ventilation ratio between the number of sq ft of ventilation to sg ft of attic space? Our general contractor in Boston, MA told us code is 1:300 (1 sg ft ventilation to every 300 sq ft of attic space.) This would count ridge vents footage, soffit vents and side vents (on the side of the house).
This there any rule of thumb on this? Thanks! Joe
Replies
Code there may be different than rule of thimb here, but we aim for one foot of vent to every 150 sq ft of floor space.
Not always doable.
But a foot of vent is not soffit and ridge both. You have to have one foot of soffit incoming for every foot of ridge outflowing and they both together count as one. Sounds lik ehe is adding one and one and counting two when there is only one foot of airflow.
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>>one foot of vent to every 150 sq ft of floor space.
Pretty close to the 1 sq.inch of ventilation for every 1 sq.ft. of attic floor space rule that I have always used, 144 to 1
For a roof slope exceeding 2/12 ,a MINIMUM of 1 sq ft of ventilation for every 300 sq ft of attic floor area. This 1 sq ft is divided Equally between soffit and high vents Make sure the opening is FREE area. Some vents measure 1 sq foot , (12" x12" ) but with a screen on it this cuts it down to approx half some insect screens on roof vents cut as much as 50 % into the "free" area calculation.
With a proper R value insulation in it, an attic space cannot ever have too much Ventilation.
According http://www.airvent.com, if the roofpitch is a 7/12 or greater, i think you're suppose to increase the venting by 20%, and over 12/12 you should increase it again. Check out that website - they've got some good info.
Link provided by Mike Smith in an earlier post. Website mentioned by Dan-o.
http://www.airvent.com/professional/whyVent/evaluate.shtml