Venting a Rainscreen and Roof with Exposed Rafter Tails
I am installing a 1x strapping rainscreen and a roof with vent channels.
The eaves are open– exposed rafter tails. So there is no soffit.
I am trying to figure out how to detail ventilation at the eaves. The rainscreen will be vented at the base of the walls w/ coravent. Can I simply connect the rainscreen vent channels to the roof vent-channels behind the siding/trim and let the air flow up from the base of the walls into the roof and out the ridge vent? Or do I need air exit for the rainscreen and entry for the roof at the eaves? If the latter, how do I detail that? I want something more elegant and invisible than drilled out frieze blocks.
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Replies
bobww,
There are a few reasons I don't think it is a good idea to connect the two vent spaces.
- Our code precludes it for fire safety reasons.
- Most codes also require more clear area for roof venting that you would be supplying it from a 3/4" wall cavity.
- One of the main purposes of a rain-screen gap is to remove moisture from the wall system. You don't then want to send that air into your roof ventilation space, which is trying to do the same thing.
I wouldn't vent the top of the wall. Rain-screens have a number of functions - providing a capillary break, a drainage path, redistributing moisture, etc. Not providing an opening at the top moves you from a ventilated to a vented rain-screen, but still has all the other benefits. That's all our code here in Coastal BC requires and it works well.
I'd suggest two methods of venting the roof.
- Drill out the blocking, but install a nice pre-made vent. Something like the one I have attached, which you can get in white, or metallic finish.
- Install the blocking 1 1/2" shorter than the rafters and attach a U-shaped piece of perforated flashing in between the two. That's what I usually do.
Cor-a-vent nailed to top of frieze blocks, block between rafters, terminate siding into bottom of block. The Cor-a-vent becomes an invisible black line.
Thanks. Is the perforated flashing visible? Or do you hide it somehow? I'm having trouble visualizing that.
That's good to know about fire risk. How do you close off the rainscreen channels? horizontal 1x at the top?
bobww,
Yes you can see the perforated flashing. If you are already using Cor-A-Vent at the bottom of the walls, you might be better off going with the S-400 soffit vent as strausjw suggests.
A horizontal piece of furring closes off the top of the cavity. I should say though, that although our code precludes connecting the two concealed spaces, it's an open question whether in practice fire could spread through such a narrow cavity. Joe Lstiburek thinks not.
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Many thanks to both of you. This was very helpful.
I defer to the experts on whether or not it's worth it to ventilate the rain screen at the top. FWIW, this is how I vented the top of my rainscreen. Note that it is completely independent from the venting for the attic. Also note that this is just a toolshed and no experts were involved. Basically, I put an insect screen on the top of the furring strips (like I did at the bottom but flipped upside down.) I then terminated my top run of siding so that the top edge of the siding lined up with the edge of the insect screen. Finally, I furred out my frieze board so that it would overhang the top run of siding.