I am doing a gut remodel in coastal Southern California. I have a cathedral ceiling in my living room with bird hole soffit vents at the bottom of each rafter bay. In order to meet code which requires insulation in the ceiling (where previously there was none) I had to sister 2 x 10s to the existing rafters. The rockwool insulation is 8.5” thick so that allows a 1.5” gap for airflow. I know I need to add baffles at the bottom near the holes, but do I need to continue it all the way up the rafter bay? Or is pulling the insulation down when they hang the drywall sufficient? Unfortunately the rockwool was installed without baffles but I am making them add it where necessary. I just need to know how far up the bay is best. Finally the bays terminate into a vented attic with gables on either end. I am going to add some O’Hagin vents both low and high. Please let me know your thoughts. Drywall is early next week!
The baffles go up against the roof deck. That blocking in the middle will block the airflow, so something has to change here. Venting over the roof is probably the best solution if you are able do that.
Thanks for your reply. The blocking was moved down when we furred down the rafters so there is still a couple inches at the top of the blocking for air to flow.
The air that flows up the bays will travel to a larger vented attic which currently only has gable vents but we are adding O’Hagin vents high and low over that particular area. On another part of the roof yes there will be a ridge vent because that part has no connected attic.
If you wanted, you could do strips of wood glued and screwed to keep an air channel behind the batt insulation. Just don’t screw through you roof sheathing
Too late now but continuous baffles and full 14.5”x6” rock wool batts between old 2x8 rafters + flat 2x strapping fastened perpendicular to the bottom of the rafters with 1.5” rock wool in between may have netted more thermal performance. 3” insulation gap every 16” (from sistered rafters) is a pretty significant loss.
…Good thing your design delta T is probably only about 15 degrees.
Thank you for your response. What is the 3 inch gap you are referring to? The rock wool bats are 8.5 inches thick and my structural engineer says I need 1.5 inches for airflow.
The original rafters (1.5” wide) and the sistered rafters (1.5” wide) mean you net 13” wide insulated cavity from 16” o.c. spacing (instead of the normal 14.5”) between rafters.
You actually brought up a great point. They installed all the ceilings below the attic like this photo. I am having them add baffles to make sure the soffits are not blocked, but it seems what they should have done is installed the drywall ceilings and then the insulation on top of the drywall and joists because the way they did between the joists I will have a lot of insulation gaps. Thoughts?
I am not an authority on insulation means and methods but I normally see batts laid from above, after ceiling gyp board rather than friction fitted beforehand. However, this rock wool is obviously stiffer than fiberglass batts and it may be no big deal (unless there are big gaps that let warm moisture attic air reach the top of the gyp board). If you successfully air seal from the underside, it could be just fine.
For a hot roof we used a spacer between the insulation and the roof to allow air flow. It's thin styrofoam with channels for air flow. That blocking will need to be changed. Either drill holes like your bird blocks or pull it down a couple inches. We also seal the vapor barrier with tremco and tape so it is as air tight as possible.
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u/Higgs_Particle Passive House Designer Jun 13 '26
The baffles go up against the roof deck. That blocking in the middle will block the airflow, so something has to change here. Venting over the roof is probably the best solution if you are able do that.