1/300 or 1/150 air venting in attic with metal roof
I am building a new workshop … 3000 sq ft lower floor and 1500 sq ft attic.
16 ft high walls.
I had the trusses designed with a 24″ energy heel.
I am planning on metal standing seam roofing over 2 x 4 wood purlins.
It is a shop …. very little moister load (not washing cars or anything inside … metal and wood working).
I know I need to seal the ceiling well from air leaks … I will use a conventional poly sheet as a vapor barrier.
I will cover the ceiling with something easy and light to put up … say 1/4″ lauan or ????
I will put 12 to 18 inches of insulation on top leaving 6-12 inches of air gap minimum.
Anyway … I was going to go with the standard 1/300 venting.
Split two ways this gives me about 800 sq inches under he eaves and 800 on the ridge.
I am a bit worried about condensation on the bottom side of the metal roof.
Would it be wise to go with 1/150 venting … 1600 on the eaves and 1600 on the ridge ???
Thanks … Mike
Replies
Not much negative to more venting area.
But you do want balance between intake and exhaust of the vented area.
Your calculations seem a bit off.
3000/150=20 ft**2 venting area. Half that is 10 square feet, or 1440 square inches.
Will there be continuous air flow at the transition from the lower roof and upper roof?
(does your 12 inch clearance include the transition between the first floor only, and the second floor roof?
Yes, that is why there is only a 7 ft wall at the edge of the attic roof. I made sure I had 2 ft above any point.
That was basically my question ... any harm in have too much ventilation?
I have never had a metal roof ... just want to make sure it does not rain in my attic.
Mike
I've never heard of too much attic ventilation as far as eave vents. The only problems I've experienced were related to powered attic fans.
The section of the code that dictates the required ventilation for attics and rafter spaces is Section 1202.1:
https://up.codes/viewer/alabama/ibc-2021/chapter/12/interior-environment#1202.1
One must remember that the code is the minimum requirement and if you are at all worried about condensation, providing more ventilation is better.
There is also an option for unvented attics and rafter spaces, the requirements for this type of construction are in Section R1202.3:
https://up.codes/viewer/alabama/ibc-2021/chapter/12/interior-environment#1202.3
Thanks!
I had never looked at the chapter on unvented attics!
Mike