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tns1 09-11-2010 01:23 AM

low slope ridge vent concern
I like the idea of adding a ridge vent to my low slope bungalow. The only venting it has now are gable vents and a few soffit vents along the backside of the house. In the summer the attic is too hot to enter.

Ed's idea of adding a J-channel baffle inside the ridge vent seems like a great way to prevent the wind driven rain from entering from the problem side. The one-sided Smart Vent seems like a good solution too.

My main concern is when the decking is cut away on both sides of the ridge beam, there is no support on that edge. For the thin 3/8" plywood that I have, it seems like I would need to either add blocking underneath or not have a continuous cut. Otherwise it might be too weak.

What do you think?

steveel 09-11-2010 05:39 AM

First, make sure insulation isn't blocking airflow at your soffit vents, and consider adding more of them on the other side of the house. you may need to add some "baffles" to keep insulation from blocking airflow above the topplates of your outside walls. Also, no one has blocked off the soffit vents from inside the attic, right?

tns1 09-11-2010 01:25 PM

There is no insulation between roof rafters, only between ceiling joists. Out of 26 rafter bays along the back wall, only 8 have soffit vents (340sq in total), and then the two gable vents (290sq in total).
If I am understanding the vent calcs, the 1400sq ft of ceiling area over the living space would require about 5sq ft of vent with half as inputs at the eaves and half as outlets on the roof somewhere.

It seems like the main issue is lack of outlets. The ridge vent I have seen is roughly 12" of venting for 12" of material, so it looks like I would want 20 to 30ft of ridge vent. I have not seen any roofs with partial ridge vents, so I assume you cover the entire ridge but only cut the deck for the amount of venting you need.

The garage has no vents of any kind which explains why it gets so hot.

steveel 09-11-2010 02:32 PM


Originally Posted by tns1 (Post 499446)
There is no insulation between roof rafters, only between ceiling joists.

Yeah, I understand. But what about the few inches of vertical space above the topplate, and below the roof deck? Can air pass through there, or is it choked off by insulation? Soffit vents are useless if that narrow gap is blocked, and it is often impossible to tell from outside without popping the soffit or fascia boards. You could pull some of the soffit vent gaps, wedge a bit of foil-wrapped wood up the hole, and then go up in the attic with a light to see if you can see the foil. If not, that vent is choked off at the topplate and is useless until you insert a baffle.

Gary in WA 09-11-2010 03:06 PM

The ridge vent will strengthen the cut edge of osb or plywood. Follow the directions. You may be able to substitute a stainless steel screw center of bay to keep vent/ply together. Read instructions with vent as to fastener placement.

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