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Community and Q&A

Liquid Applied WRB to Back of Siding?

Kevin S | Posted in Green Building Techniques on
Renovating my 1908 house in San Francisco, CA, the original redwood exterior siding is attached directly to the studs (no sheathing), we’ve removed the interior plasterwork and are preparing to insulate the stud cavities.
Apparently, it’s common practice here to paint the backside of the siding with liquid-applied WRB’s like Prosoco R-Guard, or even sometimes Redgard (eek! this is for showers…). My GC has suggested doing this, as well as caulking all inside corners of stud-siding connections. The thought being this assembly will prevent water from getting into the stud bay entirely.
From everything I’ve read on GBA about insulating retrofits, this approach seems wrong, I feel like this liquid WRB will trap moisture inside the siding, preventing it from drying to the interior side, and potentially rot the siding inside-out. And if any rainwater makes it through the liquid WRB layer, it’ll be stuck inside the cavity without anywhere to dry.
Any thoughts?

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  1. Daniel Allen | | #1

    I would be curious to hear about track record for this technique. Has your GC done this before, how long ago, can you talk to the home owners to see if this has held up well?

    Also does the house show evidence of water getting through the siding?
    What kind of insulation are you planning to install in the stud cavity?

    Alternatively you could try and create an air gap between the siding, and install a new weather barrier between the studs, or on the inside face of the studs, all of which gets complicated and expensive.

    I've got a similar question for a full interior remodel we're planning for a stucco house in Santa Monica. No sheathing, just assumed building paper on horizontal wire, which I'm sure is broken in many places by the stucco install.

    1. Kevin S | | #6

      Hi Daniel - This is something that not only GC's in the area recommend, I've seen building inspectors require it before passing a project for a close-up inspection. The thing is, the GC's I work with aren't 'building scientists', and the inspectors certainly aren't. I'm not sure who first touted this technique, but word has somehow spread that it's a viable option. The only way to know if this technique works is to re-open walls from the interior and see if there's any rot a few years down the road, since most applications are property line blind walls. The whole concept just seems super problematic to me.

      Yes, the house shows evidence of water getting through the siding. Small amounts only, which wet the back of the siding, usually where there are cracks in the old siding. No pooling on sill plates or anything like that. I imagine this has been happening for 100+ years, we are just seeing it now cause the interior plaster is removed.

      My plan is to install Delta-Dry against the siding to create a 1/2" air gap. Then install 2-ply building paper cut wide, stapled to the sides of the stud. Then Rockwool insulation in the cavities. Then drywall. This is a combination of Matt Risinger's 'How to Insulate When The Back of The Siding is Exposed' and the building paper technique in GBA's 'Sticking With Spray Foam for My Renovation'. Links below.

      I know this is heresy to say on GBA, but I'm not trying to make these walls air tight. We don't have any kind of HRV/ERV, we keep our windows open 80% of the year, and I just can't justify the extra install labor to tighten everything up in our super-mild climate. You might find the same for Santa Monica.

      1. Daniel Allen | | #10

        Kevin - great tips, sounds like a good plan.

  2. Expert Member
    Malcolm Taylor | | #2


    Before I'd go that route, I would staple a WRB like Tyvek Commercial into the stud bays - either against the siding, or if you want to put the extra work in spaced an inch away. That will help protect against any bulk water intrusion, and leave a drying path to the exterior. Among the less intrusive ways of dealing with this it's probably the best choice.

    1. Kevin S | | #7

      Hi Malcolm - I agree, see my reply above to Dan's comment with the assembly I'm planning on installing

  3. Daniel Allen | | #3

    Along with the Tyvek, I imagine it would be fairly easy to install a layer of mesh to create an airgap. Perhaps something like Cedar Breather by Benjamin Obdyke:

    1. Expert Member
      Malcolm Taylor | | #4


      A sort of inverted rain-screen gap. That's a great idea.

    2. Kevin S | | #8

      I was going to buy BO's Cedar Breather at the local lumber yard. I think it would totally work. But then I found the Delta-Dry for less $$$ online, so I went with that. I like the Delta-Dry, it's waffle-ing is very rigid, so will def maintain an air gap no matter how packed the insulation is.

  4. Expert Member
    Kohta Ueno | | #5

    Just for all of the folks recommending an air space behind the cladding (which I agree with)--Martin Holladay did a nice writeup of this topic.

    Insulating Walls in an Old House With No Sheathing
    You’ll need to create an air space behind the siding before you insulate between the studs

    Managing Water and Insulating Walls Without Sheathing
    A solution for detailing old walls to handle moisture and reduce energy losses

    BSC has a similar set of details for rebuilding a brick veneer wall from the inside-out; these details were developed due to flood-damaged houses in Houston.

    BSI-101: Rebuilding Houston

    1. Kevin S | | #9

      Thanks for sharing these. One more I'd add... GBA's 'Sticking With Spray Foam for My Renovation', this shows a good technique of cutting the building paper wide and stapling to the sides of the studs for WRB...

      1. Expert Member
        Malcolm Taylor | | #11

        What a useful thread!

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