GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Vapor drive and porcelain tile exterior walls

tmallett | Posted in GBA Pro Help on

The nice thing about building a house on one’s own is that the building gives you feedback before the finishing touches lock things in. One such lesson for me is summertime vapor drive. I have had Durock CBUs installed over vertical 1×3 furring for some time now with plans to install veneer stone later. The rainscreen gap is ventilated below and above and the insulation behind is closed cell spray foam, so no moisture drives into the house. However, on inspection, I see that the furring inside the gap on the south and west walls is noticeably moist–likely from vapor drive through the porous CBUs. I am replacing the CBUs and doubling up on the furring and will be switching to large format porcelain tile instead of veneer stone. I am wondering if the tile will be a sufficient barrier to protect the long-term integrity of the furring, given that grout lines will probably let some moisture to drive though. Is it advisable to coat the CBUs with a paintable moisture barrier (like Mapei Aqua Defense) before installing the tile? or moisture sealing the grout lines? /EM

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    ARMANDO COBO | | #1

    I'm taking a guess here since you don't describe your total wall assembly (WRB, sheathing, insulation, framing, etc.) and location, so its hard to know which way your wall is drying out. Typically, a 3/4" gap for a rain screen, and with good wall moisture detailing. is more than sufficient to avoid problems.

  2. tmallett | | #2

    Sure, here are the add'l details. Southern Ontario location. Engineered steel frame construction, 3" walltite polyurethane spray foam over 3" eps panels. Horizontal z-bars encased in foam at 18" intervals support the furring. 3/4" is therefore the minimum rainscreen gap--most of it is much deeper. Here is a pic of the unfinished south wall (with small section of rainscreen gap exposed). I have since replaced the CBUs and incorporated 2 horizontal expansion joints. Again the question is whether porcelain tiles in thinset are sufficient to prevent vapor drive in sunny location

    I already tiled a lower, wider wall like this on the north side, which had no apparent moisture in the gap

  3. Stockwell | | #3

    What is that dark siding? It looks great.

  4. ohioandy | | #4

    I'm not going to answer your question about porcelain tiles, because I have no idea. Porcelain would surely slow vapor drive when augmenting 1/2" of plain concrete, right? Your wall design is an exotic novelty to most of us. Curious to hear more about it. Is the 1x3 furring wood? If yes, I wonder if switching to something non-organic would be in order. I assume it's not, since it would be the only vulnerable component in an otherwise waterproof stackup. Whatever it is, it's just the temporary site for condensation in a vapor drive scenario, and your generously vented gap should allow quick drying.

  5. tmallett | | #5

    Thx Kevin & Andy,
    The wood siding comes from 'Juste du Pin' out of Montreal.

    The house is a Bone Structure system. I had considered using PVC boards as furring, but felt that it had less screw holding power. Instead, I switched to 1x4s where I could and glued additional lengths behind in some spots to give them 1.5" thickness. I also switched to 8"-to-12" centres for the furring--well above what the engineers specified. I suspect things will be strong and dry, but just wanted to check before going past the point of no return.

  6. GBA Editor
    Martin Holladay | | #6

    I'll have to agree with Andy that your wall design is an "exotic novelty" for most residential builders.

    I was stumped by your abbreviation -- I have no idea what a CBU is. After some online research, I eventually discovered that the abbreviation stands for "cementitious backer units."

    Furring strips tend to be quite dry, because most rainscreen gaps are well ventilated. I have no idea what's going on at your house to cause your furring strips to be damp.

  7. tmallett | | #7

    Thanks Martin,
    Definitely non-standard, though if you're calling it an exotic novelty, I'm not sure whether to be honoured or scared. Anyway, I chatted with the technical people at a major thinset mfr and they sent me specs for that kind of wall. It involves a roll-on membrane and then their latex/high-bond thinset mix. It's probably overkill, but since it is a highly exposed wall, I wanted to be safe. Cheers

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |