Venting under lookout rafters

[midrover170]

Here's kind of an odd one...

This weekend I started work on insulation and ventilation. Installed baffles in each rafter bay until I got to where the lookout rafters were installed. As you'll see in the photo, the baffles won't really work in those spaces due to the lookouts. What's common/recommended here? Was thinking I could "build out" the area between the lookouts with 1.5" foam board and adhesive to create a solid plane, then tack the baffles to that. However, I wouldn't want to create a weird vapor/air barrier issue with the foam... An alternate is putting 2x4's running along the rafters (with the 3.5" side against the roof deck) and fasten with 2" or 1 3/4" staples. Then, tack the baffles to that. Obviously, I don't want to penetrate the roof deck from below. Also obvious, I'll lose 1.5" of insulation space here, so the cathedral ceiling batts I'm getting will be slightly compressed here. Rafters are 2"x12"s, 12" OC.

Also, the cavity between the outer rafter and the first inner rafter is too narrow for a baffle to fit in there... Thoughts? Thinking about putting kraft faced insulation (paper facing the roof deck) and no baffle in this pocket. 



-D

[Redoverfarm]

The area where there is no heated space below are normally not insulated.  Insulation from the house to that point including on top of the wall sill.   The baffles are made to create an air space when installing insulation so that their is air from the soffit to the ridge vent or attic space.  If you are installing a ridge vent then you could place a lower soffit vent so that air would be drawn in at that point and exit via the ridge.  Didn't catch whether you were using shingles or metal roof.  With shingles it is more important for the air circulation to keep them from baking so to speak,  Metal not so much.

In the narrow area you can cut them down to fit but do so in a fashion that they will not collapse. Kraft paper (moisture barrier) always goes toward the heated space.

[midrover170]

Thanks, John.

Sorry if I wasn't clear (i'm using metal roof and it's a shed style roof). I'm more concerned about the three inner bays. Those lookouts make it tricky to install the baffles, i.e. if I installed them in between the lookouts, the air channel would just run into a stud. That's why I'm trying to think through ways of building the roof deck down to make another continuous surface to mount the baffles. Does that help?

In the narrow area you can cut them down to fit but do so in a fashion that they will not collapse. Kraft paper (moisture barrier) always goes toward the heated space.

That space is maybe 8" wide. Too narrow for a baffle to fit in there. That's why I was thinking of another method to create ventilation... Maybe if I cut a baffle in half lengthwise, and then overlap it with tape on the seam... that might work for that narrow space.

[Redoverfarm]

Thanks, John.

Sorry if I wasn't clear (i'm using metal roof and it's a shed style roof). I'm more concerned about the three inner bays. Those lookouts make it tricky to install the baffles, i.e. if I installed them in between the lookouts, the air channel would just run into a stud. That's why I'm trying to think through ways of building the roof deck down to make another continuous surface to mount the baffles. Does that help?

The lookouts are past the outside walls correct?  You do not need air baffles or insulation in the rafter bays.  The purpose of insulation is to keep the house warmer and prevent condensation in the interior of the house.  Same would be true for soffit areas.  Serves no purpose.  Just as long as you insulate over the sill plates where cold air meets warm air.  Clear as Mud 

[DaveOrr]

The lookouts are past the outside walls correct?  You do not need air baffles or insulation in the rafter bays.  The purpose of insulation is to keep the house warmer and prevent condensation in the interior of the house.  Same would be true for soffit areas.  Serves no purpose.  Just as long as you insulate over the sill plates where cold air meets warm air.  Clear as Mud 

If like in my construction the lookouts are joined to the rafters over the building and connect to the fly rafters in the unheated space.
He is trying to figure out how to vent the interior bays that the lookouts penetrate into without losing too much insulation space.

You can see in this pic of my place the lookouts connect to the first inside rafter. This is the area the OP is talking about.



I haven't figured out what I'm going to do yet but am looking forward to the answers.   

[NathanS]

My thought on a solution would be to use foam board to create an air channel (furring strips notched around the lookouts and another piece of foam cut to the bay width). This will help boost the overall R-value after having to compress the batts more.

Were you planning to use a poly vapor barrier on the inside? If so, EPS is the only rigid foam that is considered vapor permeable (less than 3" in thickness). If you're just using kraft faced batts, it shouldn't matter what kind of foam you use as kraft paper is considered a 'smart vapor retarder' that changes permeance at different relative humidities.

[midrover170]

He is trying to figure out how to vent the interior bays that the lookouts penetrate into without losing too much insulation space.

You nailed it Dave! That's a more straightforward way of saying it for sure. Right now, I'm leaning toward running the edges of the bays with about an inch width of 1.5" foamboard (minus where the lookouts penetrate). This will give me a "nailer" to attach the baffles to. Kind of a mix of what Nathan recommended. I'm all ears for other recommendations. That outermost rafter bay (not the fly rafter) is definitely going to be tricky...

Were you planning to use a poly vapor barrier on the inside?

I'm planning to do unfaced batts with a poly vapor and sheetrock under that.

- D

[Don_P]

You can bend the rafter chutes up and over the lookouts and then back to the deck. The insulation will be a little compressed in those bays. Thinking on your idea, if you used 2x2's you could nail them to the rafters rather than the deck.

[midrover170]

Bending a chute sure seems easier than nailing the 2x2's and probably works just as well... Thanks Don.

[Redoverfarm]

Sorry wasn't paying enough much attention to your original post.  I didn't catch that you had 1/2 & 1/2 bay area on both sides of the exterior wall. Like Don_P stated you can bend the baffles and insulate.  Not real easy getting the insulation in over the wall but it can be done.  I wouldn't put the kraft paper against the roof and then put another vapor barrier on the ceiling side.  It will trap what moisture there would be in between the two.  Yes it will not hurt to have the insulation protruding into the lookout bays I just didn't want you to think that all the lookout bays get insulation.  Good luck.

[reb]

Midrover, did you ever figure out a good way to insulate with the lookouts there?  Got any pictures?  I'm looking to do the same thing.