Replies

1. 
   kbentley57 | Jul 15, 2022 04:49pm | #1

   I'm not sure the first layer of plywood has any purpose other than air sealing here, is that right? The 2x6 T&G provides the structure, and the rest are the /water/air/vapor/thermal barriers, which is why he doesn't have plywood there.

   For that matter, there's lighter, more air tight, and more/less permeable materials that could be used here at cheaper cost, not to mention saving on the tape / nails / back. The self-adhered membrane is a foolproof way to accomplish this, but a good job on sealing the polyiso and eps could probably work fine too.

1. 
   jwschill | Jul 15, 2022 05:26pm | #3

   Good point Kyle. The structural engineer mentioned something about that bottom layer of plywood being "part of the diaphragm." I could delete that layer and replace it with the self-adhered membrane.

1. 
   brendanalbano | Jul 15, 2022 06:02pm | #4

   I would recommend consulting with your engineer before you delete a layer that they say is part of the diaphragm!!!

   It may be that the 2x6 T+G isn't adequate to create a diaphragm to transfer the lateral loads across the roof and that the plywood is an important part of the structural system.

2. 
   plumb_bob | Jul 15, 2022 05:01pm | #2

   It is the T&G that will give you problems for air sealing. The air control layer is above the T&G, so you need to figure how it will tie into the air barrier of your walls to make the air tight plane continuous.

3. 
   Malcolm_Taylor | Jul 15, 2022 06:11pm | #5

   Jason,

   If you can convince your engineer to forgo the lower layer of plywood, I agree with Kyle it's not doing much. I also agree with plumb_bob that the t&g is your most problematic layer from an air sealing perspective. Not in the field, where it can be covered by a membrane, but at the edges where it is continuous over exterior walls. You need to come up with a strategy to seal the boards there and make the air-barrier continuous with your walls.

   If your primary air-barrier is a membrane on the t&g (or the lower plywood), I don't think it's worth sealing any of the layers above that, nor would I worry about the roof fasteners.

   Are ice dams typically a problem in Wenatchee?

1. 
   jwschill | Jul 15, 2022 06:26pm | #6

   Thanks for the response Malcolm. I will check with the roofing supply company in town about ice dams. As for the T&G/Wall interface, I was planning on using Siga Fentrim tape for that on the interior and exterior.

1. 
   Malcolm_Taylor | Jul 15, 2022 07:29pm | #7

   Jason,

   The problem with the t&g as it goes over the exterior walls is stopping air-leakage through the grooves, and below the boards which usually have a mitered edge. The boards should be bedded in a flexible sealant, and a bead put in each groove during installation.

   A better solution is to make them discontinuous, by starting and stopping them at the walls, and leaving a gap that can be air-sealed, but sometime the structure makes that impossible.

1. 
   jwschill | Jul 24, 2022 01:35am | #8

   Thanks Malcolm. Is there a flexible sealant that you would recommend for this application?

1. 
   Malcolm_Taylor | Jul 25, 2022 02:18pm | #10

   Jason,

   It doesn't need to be particularly flexible. I'd use clear silicone, which is very durable over time.

2. 
   brendanalbano | Jul 25, 2022 02:26pm | #12

   In detail 8/S3.2, it looks like the T&G boards are continuous and provide the structure for the overhang. If they need to be continuous, you have to do the fancy sealant work Malcolm is talking about.

   If you can make them discontinuous, your life gets a lot easier, as you can just tape the roof sheathing to the wall sheathing.

   Instead of supporting the overhang with the t&g, you could use 2x outriggers above the roof sheathing air barrier to support the overhang, like in Figure 1 and Photograph 3 in this article: https://www.buildingscience.com/documents/insights/bsi-081-zeroing-in

   You may need to run this by your engineer to make sure it's all fastened appropriately.

4. Expert Member
   Dana Dorsett | Jul 25, 2022 01:38pm | #9

   >"plywood (seams taped with Siga Wigluv)"

   ==>AIR BARRIER

   You don't need another air barrier, as long as the edges and any penetrations of your taped plywood are properly detailed. You don't need to separately seal the t & g.

   Taping the seams of the top layer plywood as a secondary air barrier is good practice.

   What type of underlayment is there for the standing seam roofing?

1. 
   Malcolm_Taylor | Jul 25, 2022 02:18pm | #11

   Dana,

   You still need to seal the T&G below where it continues over the exterior walls. The plywood detailed as an air-barrier isn't continuous with the wall air-barrier otherwise.

2. 
   jwschill | Jul 26, 2022 01:04am | #13

   Dana,
   I was planning on using Siga Majcoat, unless you recommend something else. Is a vapor permeable membrane necessary for my situation? The standing seam metal roofing will be directly on top of the membrane.