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Plank sheathing with rain screen wood siding waterproofing on 1961 Seattle house

IvaKM | Posted in Expert Exchange Q&A on


First of all, I really appreciate this website and all the knowledge that you share!

My name is Iva and we have a 1961 MCM house located in 4c climate zone (Seattle). We are doing some interior renovations, but decided to redo the vertical siding.

We have drywall, old insulation of probably negative R-3:) in 2×4 exterior walls, plank sheathing with a lots of gaps in areas. The plan was to use rockwool insulation where we opened up the walls.  Then use 15 pound felt or similar, slicker classic and vertical siding with bug screens, etc.  The contractor is using two layers of Hardie Weather Barrier.  My question is if this above plan is a viable option. My understanding is that if using rain screen, the house should be water and air sealed. 

My research took me to three options:
1/ apply 1/2 or 3/8 plywood sheathing over the existing and proceed with the plan. (Don’t have the cost yet).
2/ apply self adhering vapor shield under the Hardie to seal the gaps. Insulate with rockwool. (Cost about 7k)
3/ open the entire interior to apply 1 1/2” of HFO spray foam with batt over (cost about 24k including re-drywalling).

Continuous R-5 on the exterior was rejected due to adding a depth to the assembly, causing problems with existing windows. Is that such a big deal?

Should we be concerned with water/air coming in or let the old house breathe?  We want to do the right thing, but it might not be feasible cost wise for us now. 
Thank you for any advice!

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  1. Expert Member


    You are right - you want the exterior wall to be air-sealed, and stop water ingress. You should definitely do those two things and not "let the house breath". No house benefits for air-leakage (breathing), and all the wall assemblies you are suggesting will be quite vapour-open which will allow moisture to dry to the outside.

    Air-sealing is most effectively done by either taping the new exterior sheathing in option #1, or using a self-adhering WRB (option #2). I don't see much justification for option #3. A fourth option would be to use a regular WRB like Hardie Weather Barrier or similar, and tape all edges for air-sealing. If you went that route, I'm not sure what two layers of that house-wrap buys you.

    Unless your existing board sheathing is in very bad shape, my own preference would be to use the self-adhered WRB with the Slicker, and forgo the extra layer of plywood.

    1. IvaKM | | #2

      Hi Malcolm,

      Thank you for your reply.
      Am I understanding correctly that one layer of Hardie Weather Barrier, overlapped and sealed, would work as an air and water barrier over the plank exterior sheathing with many large gaps?

      I’m too, leaning towards the self-adhered WRB, such as Vaproshield and then slicker. If we go this route, should we use a faced insulation oriented towards the interior, in this assembly?

      Thank you!

      1. Expert Member
        MALCOLM TAYLOR | | #3


        Yes, a sealed house-wrap will work, although not as effectively as a self-adhered WRB. If you go that route make sure it is sealed to the foundation, top of walls, and all penetrations, not just the seams between sheets.

        I would imagine that between 1961 and now the interior of your walls have had a number of coats of paint which act as pretty effective vapour-retarders. I don't think it matters whether you use faced batts or not.

        Although it would be nice to "to the right thing" by adding exterior foam, I think that between the air-sealing and fully filled wall cavities you will experience a big difference over that you have lived with until now. If I was going to go further I'd concentrate on air-sealing your crawlspace or basement, and the roof above. That's low hanging fruit with big returns.

        1. IvaKM | | #4

          Thank you! I think I have a clear idea what to do!

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