Replies

1. GBA Editor
   Martin Holladay | Apr 21, 2009 10:59am | #1

   William,
   When I was recently asked to answer the same question for the Q&A column in the Journal of Light Construction, here's what I wrote:

   You may safely install exterior foam on most houses with a polyethylene vapor barrier, as long as the foam does not include aluminum-foil facing. In fact, exterior foam is a great idea, since it significantly improves the energy performance of walls.

   As most builders now realize, polyethylene is a double-edged sword. Its useful ability to limit the outward migration of water vapor into a wall comes with a downside, since poly also prevents useful inward drying of damp walls. In very cold climates ... many builders still use interior poly. However, in warmer regions — Ohio and Connecticut, for example — most walls perform better without any interior polyethylene.

   Back in the 1980s, when building scientists did not fully understand the disadvantages of interior polyethylene, its use was encouraged from North Carolina to Oregon. In most of the US, the routine use of interior poly was probably a mistake. Fortunately for builders, most older homes with interior polyethylene have not experienced moisture problems.

   The installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

   Dry and unstained sheathing may be safely covered with extruded polystyrene foam (XPS) or expanded polystyrene foam (EPS). One inch of XPS has a permeance of 0.4 to 1.6, while one inch of EPS has a permeance of 2 to 6; that means that walls sheathed with EPS have more ability to dry to the exterior than walls sheathed with XPS. Since aluminum foil is completely impermeable, the use of foil-faced foam is not recommended on walls with interior polyethylene.

   Walls sheathed with exterior foam perform better when they include a rainscreen — for example, vertical 1x3 strapping or a product like Cedar Breather. Of course, a foam retrofit job will require adjustments to window trim, door trim, and wall flashing, so be sure to research these topics carefully before tackling such a project.

2. 
   wg846 | Apr 22, 2009 12:59am | #2

   Good advice.
   The folks at DOW use the improved location of the dew point as one of their arguments. As does Lstiburek,Guide To Insulating Sheathing, 2005. So in my area, dealing with minus 40, 2" styrofoam would be better than 1", except less permeable (more double edged swords). Walls with significant stud cavity insulation (say 6" or better) could be more vulnerable than poorer walls.
   My current best practice is a program:
   -assume all interior vapour barrier is compromised
   -try to convince homeowner to control indoor humidity
   -seal major air leaks where accessible
   -2 layers 1" Cladmate, not tape sealed
   -drainage plane (felt, tyvek)
   -rainscreen
   -siding

   and hope for the best. I think we may not have all the answers on this issue in some climates and conditions.

   Thanks for your reply.