Vented roof – can I OVERVENT? 1/75 instead of 1/150
Hello, we are framing our roof trusses as I type this and I wanted to know:
Is it possible to have too much airflow through your attic? I am going to have 60% intake,/40% exhaust proportion.
The ridge vent my roofer is using would result in 18′ of the 35′ vented, and 7 of my 17 bays with coravent intake (calculated with 1/150 rule).
I am wondering how to distribute the 18′ at the ridge. The product comes in 4′ strips.
Would it be easier to vent the entire ridge, and just have more eave vents? If I take this route, I will have about 11 square feet NFVA, while the 1/150 only calls for 5.6 square feet. Is it possible to over vent an attic?
R-60 blow in cellulose, building in Bellingham, WA.
Thanks!
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Replies
I am a builder and I ALWAYS maiximize the ridge vent. In your case you should be able to use 33-34 ft of your ridge and the more complex the roof design the more opportunity for dead spots where air does not move. There is a minimum amount of venting required to facilitate dry low moisture attics and that is called out in the building codes, but there is not a maximum. Personally I make sure that I have 1.5" in the soffit continuously around the building. Yes that is overkill to some people, but just remember that moving air through and out your attic is the goal and do whatever it takes to make that happen.
ArchitectJudge,
There may be some hot, humid climates where more roof ventilation causes problems, but the PNW isn't one of them. Depending on their length, you can vent the lower shed roofs either at the peak, or with gable vents.
The IRC allows a reduction from 1/150 to 1/300 venting area IF venting is balanced between the soffits and the ridge. If they are not balanced, you need at least 1/150 venting. The potential risk of oversizing the ridge vent is that it will be starved for air and could draw air from inside the home. If the attic floor is perfectly air-sealed it's probably not a concern, but on most houses it could be an issue.
If you meet or exceed 1/150 the floor area in net free vent area, you can do pretty much whatever you want--the idea is that there won't be enough negative pressure to cause problems.
Thanks for the feedback folks! I think venting the whole ridge is the way to go.
If I vent every eave, it will result in 67% intake and 33% exhaust.
Would that cause any issues?
ArchitectJudge,
No that will work well. It will help prevent the attic being depressurized and drawing air through the ceiling.