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

How can I calculate perm rating for a TILE floor with GROUT lines? .06 perms

severaltypesofnerd | Posted in General Questions on

I am working to retroactively permit an ADU.  The jurisdiction uses an ADU conversion to impose a .06 perm rating requirement for a slab floor.  But this floor has 12*24 glazed ceramic tiles probably over redgard.  Humidity is good, and it seems the place is doing fine.

The question is how could I convincingly and correctly document, calculate or measure the perm rating of this assembly, in a way that might seem convincing to the building official?  

Or would you recommend just hiring a crew to tear up the tile not breaking as many as possible, add another layer of redgard then reinstall the tile.

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. dan_saa | | #1

    I assume this is a moot question since it's been 2+ months since you posted, but Redgard has an ICC-ESR. City of LA accepts by default for ADU conversion. See this post for more info:

    (FYI feel free to email me questions [email protected] my domain on profile)

  2. Jon_R | | #2

    "Exception: The vapor retarder is not required for the following: ... Where approved by the building official, based on local site conditions."

    A tile floor isn't effected by moisture. So the perm rating pretty much doesn't matter unless it gets to several perms, where the amount of moisture entering the interior could be a minor issue. So you might get away with "tile is low perms" combined with "it also doesn't matter in this case".

    Unfortunately, I wouldn't be surprised if a building officials don't understand the reasoning behind the .06 perm requirement.

  3. Expert Member
    BILL WICHERS | | #3

    My guess is the glazed tile is basically 0 perms. The glaze is essentially melted glass, and should be about as vapor permeable as a glass window. The grout would have some amount of permability, but I don't know how much, and I'm not aware of anywhere that would be documented as I don't think anyone tests for that.

    Redgard lists "<0.5 perms" in their TDS document. If you think you have a redgard layer, you might try calling the Redgard people and ask them about what you need -- maybe they can help. You might try a "whole floor" permeability calculation similar to what happens with walls (your grout lines are the vapor equivalent of studs as thermal bridges). Essentially work out the total area of grout, assume the 0.5 perms of Redgard, work out the total area of the tile at 0 perms, and then work out the effective total permeability of the entire area of the floor. I don't know if this would fly with the building dept people, but it's the only thing I can think of.

    You might try sending an email to Allison Bailes over at Energy Vanguard. He's written some articles that make me think he might have some ideas that could help you here. It's worth a shot.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |