Exterior Insulation and Detailing a Rainscreen
I live in British Columbia where we make extensive use of rainscreens and are starting to use more external insulation and I’m flummoxed about how to handle the top and bottom details.
This is for a small insulated shed so I don’t have the luxury of working with an experienced local contractor or getting stuff custom made.
When I visit the big lumberyards (not Home Depot,) I see two things:
1) The furring strips are all PT plywood (2″*3/4)
2) I don’t see any Cor-A-Vent but I do see the Menzies perforated aluminum J-Channel. (https://www.menzies-metal.com/vent-flashings/perforated-j-channel-rain-screen-low-back/)
So I am left with two big questions:
1) What are people doing for keeping pests out of the Rockwool Comfortboard? I suppose I could block it with wood at the top and bottom.
2) If this is really what people are doing for rainscreens, isn’t it going to be a problem in 10 years to have all that unpainted aluminum in direct contact with the PT strips? I suppose I could wrap the end of each furring strip with TuckTape to avoid direct contact.
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Replies
Malcolm Taylor may chime in.
In the meantime, you might want to read through Eric Whetzel's blog (https://kimchiandkraut.net/2019/07/11/passive-house-continuous-insulation/). He offers some suggestions for keeping insects at bay.
On the furring, you can rip 3/8 (or larger) plywood into strips. That is often the most economical way to create an air gap.
Thanks. That is a great blog. The local stores have all the plywood pre-ripped for a reasonable price. However, local custom is to use PT for everything beyond the WRB. I'm fine with that as well but raises interesting questions about all the aluminum (flashing, bug screens, etc.)
https://hammerandhand.com/best-practices/manual/4-rain-screens/
Thanks!
Canada_Deck,
There is a distinction between vented and ventilated rain-screen cavities - ventilated being open at both the bottom and the top. Our code only requires a vented gap, because with that you get the great majority of the benefits of a rain-screen, without incurring the possibility of water intrusion that top-venting can bring.
All you need are the furring strips and perforated flashing. With 2" exterior insulation you may want to use 1"x4" as furring. If you choose plywood, use regular plywood strips. If you choose PT ones, get MPS which doesn't corrode aluminum, or hold them up 1/4" above the back leg of the flashing so they are not in contact.
Fasten a block of 1 1/2" x2" lumber to the base of the wall, and lap your WRB over it, then set your exterior insulation on top. Fasten the flashing to the block, and set your furring strips into it. You will need similar blocking at windows and doors.
Malcolm, always helpful, do you have a detail or image link for the flashing to block connection? I am having trouble picturing what you are describing. Thanks!
Aaron,
The J flashing just gets nailed to the outside of the block. Another option (which I prefer) if you are using exterior insulation, is to set the stud walls in several inches so the insulation is protected by the concrete foundation below, and then no block is necessary.
Got it, so the J flashing is to protect the furring strips and air space form bugs etc from entering. Not the furring and insulation. Thank you much!
Aaron,
Yes. You could get a custom-bent piece wide enough that it protected the bottom of the insulation too, but I don't see much downside to a solid block. Depending on there the WRB was you might still be worried about moisture hitting the top of the block. In that case you could cover it with a piece of peel & stick, or simply slope the top.
A block at the top and bottom can also take some of the loading off the fasteners attaching the furring through the insulation.
Thanks. For some reason, didn't cross my mind to put the WRB over the wood blocking at the bottom of the insulation but that makes a ton of sense.