The Inside-Out Exterior Cavity-Wall Method

By way of introduction to this method, it was developed for moisture/mold remediation in an existing brick cavity-wall project where the back-up wall sheathing had become wet due to various air-borne moisture transport mechanisms. Since that project, it has been used in similar retrofit situations that had flashing and drainage plane material failures. This method is applicable to multi-story buildings and to projects with wood-framed construction.  The following case study demonstrates how the method was used in a 2004 Elementary School project:

The Inside-out method

The key to this method is to manage the water that passes through the brick cladding (the rain screen) as you would in new construction, while installing the drainage plane from the inside of the building. In new construction, the back-up wall is installed and the airtight, vapor tight, and waterproof thermal envelope is installed from the outside before the brick ties are installed and the brick is laid. In the reverse access scenario, the back-up wall is dismantled from the inside, exposing the back (inside) of the brick cladding. In most cases the through-wall flashing remains in tact.

The goals of a newly installed system are as follows:
Manage water in the cavity wall to prevent water intrusion to the inside.
Provide adequate insulation within the existing available cavity size.
Provide air leakage control.
Provide vapor control.

These goals are achieved by:
Installing a drainage plane material/membrane to assure a capillary break on the back surface of the brick and direct water to the outside of the through-wall flashings.
Sealing the drainage plane material/membrane against air, vapor, and water leakage at mechanical penetrations, fasteners, and brick ties.
Installing thermal insulation with a high R-value per inch to achieve the current minimum recommended performance.
Using an insulation material with low water, vapor, and air permeability.

Means and Methods:
Depending on the conditions on the back/inside of the brick and the insulation system to be used, a water resistive material is placed to hold the drainage plane membrane away from the brick.
A UV-stabilized water-tolerant film/membrane is installed on the inside of the brick, lapping down and onto the outside of the through-wall flashings.
A closed-cell polyurethane foam is used to seal penetrations in the membrane (at brick ties, fasteners, etc.) and to insulate the wall. This material provides the highest available R-value per inch and provides vapor and air leakage control within the available space.
The foam material is installed in a manner that provides the maximum thermal break possible at the metal studs and the recommended insulation value in the rest of the cavity.
The installation is then quality assured using infrared thermography and/or pressurized air leakage test protocols.
Finally, the interior sheathing and finishes are installed over the completed building envelope assembly.

After Katrina, I was asked to further explain how this method, previously used in mixed and cold climates, is applicable to the southern climate. The choice to use closed-cell foam is key to this discussion. The physics of the approach in Texas is not dissimilar to using this system in a mixed climate with extremes of hot and cold weather. In the north, the assembly must not only work in summer heat with air-conditioned buildings, but in the extreme cold weather where heating is required. The vapor drive of these two conditions is in opposite directions. In the summer, the drive is inward from the warm, high-humidity exterior; in the winter it is from the warm, moist inside to the outside. Southern climates include both of these vapor drive scenarios, but to a lesser extent in the winter months. Using vapor retarders with conventional insulation forces the designer to put the vapor barrier (usually polyethylene sheet membranes) in a location that will serve only one of the two possible drive scenarios. Using closed-cell foam incorporates the vapor retarder into the insulation so that the vapor retarder is in the correct location for a vapor drive in either direction. Using closed-cell foam without a drainage plane membrane in new cavity wall installations is common as the foam can also act as the drainage plane material, thus eliminating the additional material and installation costs. In the retrofit scenario, the membrane is necessary to maintain the capillary break on the inside of the brick. This drainage space allows water to flow down to the through-wall-flashings and out of the wall cavity through the weep holes.

The most significant problem with installing a drainage plane material in a retrofit situation like this is installing it after the fact where the brick ties and other penetrations (windows, mechanical systems, etc.) are already in place. They cannot be removed to install the membrane continuously, i.e. removing the brick tie fasteners to slide in the membrane would compromise the structural integrity of the exterior cladding. Another advantage of the closed-cell urethane is its ability to act as both a sealant and a drainage plane material. Thus the membrane can be installed with minor gaps at the brick ties and sealed in place with the closed-cell foam application. The only compromise to this system is the absence of a capillary break in the small area where the foam provides a seal around the ties. This is not a problem for the overall performance of the system as the foam is “water-tight.” Wind-driven rain can travel a very short distance to either side of the sealant around the ties and down in the drainage plane to the through-wall flashing and out through the weep holes as in the original installation. This is similar in concept to water traveling horizontally on the head flashings above a window or mechanical penetration to the open drainage plane on either side of the window.

Finally, the closed-cell foam is airtight, thus addressing air-borne vapor transport into the brick cavity from the inside in the colder months, or into the insulation and the conditioned space from the outside in the summer.


  1. It is key that the drainage plane/VB membrane lap onto the outside of the existing drainage plane surface(s) to conduct water down to the through-wall flashing and weep holes below.
  2. The drainage plane/VB membrane must be sealed to the outside of the existing drainage plane surface(s) at the side/end of the retrofit area to assure that water is conducted down to the through-wall flashing below without leaking inward at the transitions.