I have just finished the roof sheathing for a 3900 SF home I am building in northern Michigan. It has a 10:12 pitch, gable-style roof. I will be using continuously vented soffit under 18″ overhangs and a good ridge vent. Here’s the problem: The home-owner wants me to install additional metal “can vents” at the mid-point of the roof. I don’t want to for two reasons. I don’t want to have to look at them, and I understand they can actually decrease the overall efficiency of the ventilation process. Any thoughts?
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There's a great PDF document on ventilation at:
http://www.certainteed.com/pro/ventilation/pdf/PrinciplesAtticVent.pdf
Maybe that will help the homeowner understand how ventilation works.
Does an angry bride make veiled threats?
Why not install a continous ridge vent. In my opinion this is the best way to go for ny roof system and the most over looked.
They provide even venting.
The shingle over style disappears from the street.
If you install a continous soffit vent it matches and makes a system.
I installed a set on my sisters house on her gable roof with a gable dormer.As soon as I cut the slot in the roof I could feel a blast of heat escape like a slot on a toaster. It also lowered the temperture in the room below by ten degrees in the middle of summer. The bedroom are right under the roof, you know 6 foot of flat ceiling the the slope of the rafters then poney wall.
I want to put a set on my house. I don't like he type that is just a roll of fiber material that you roll out, I prefer the rigid plastic type you nail through.
wally
Did you read his post before responding?
He already HAS a ridge vent and soffit vent.We are born naked, wet and hungry. Then things get worse.
Sorry missed the ridge vent part caught the continous soffit vents and the can vents. Still the soffits and ridge vent are all that should be needed if it is done correctly as a system. With no blockage in the truss or rafter bays. Heat rises to the highest point the can vents at mid point will ony interfer with the system.
I agree with you - But keep in mind he's trying to convince a homeowner of this. That's why I referred him to the PDF document, which does a good job of explaining it. I don't lie, cheat or steal unnecessarily.
Thanks for the feedback, gentlemen. Also thank you for the link to Certainteed. AirVent has similar info out there. What I am looking for, however, is something out there that says not to install the can vents. In other words, could I be over-ventilating, or providing uneven ventilation. This is not a major detail in the construction of this house, but I hate to put any additional holes in the roof if I don't have to.
I was hoping that if you showed the homeowner the documentation, they would see that it was pointless.
Maybe you could tell them that the unnecessary vents were going to cost them $xxx - That might throw a scare into 'em.Save water. Shower with a friend.
Peter, sorry, I've looked but can't find any thing written. I guess you can only appeal to their logic. I know that I have read articles on the subject though. You are correct, that a roof can be over ventilated. I would argue it ( with my logic, such as it is) based on the balance between the ridge and soffit vents. If you have too much ventilation at the ridge, on windy days this will create a negative pressure in the attic drawing warm moist air from the house into the attic. I do know that this is a problem with roofs that use power ventilation. Generally however, roofs are over ventilated in the winter and under ventilated in the summer.
Too much air flow can also diminish the R value of fiberglass insulation. I don't remember where you are, but in my area we are required to put draft stops in areas (such as between the rafters above outside walls) to prevent air from flowing into the insualtion and making it ineffective. It stands to reason that you don't want too much air flowing over the top of the insulation either.
I really appreciate all the feedback I've recieved on this matter. It's kind of nice knowing we are all in agreement. Most importantly I did discuss the issue with my homeowners last night, and they agreed as well, so the cans are out.