I was reading Martin’s column in the Green Building Adviser regarding “All About Water Resistive Barriers” (January 2011). I have a specific question in mind which has to do with a siding replacement project.

We will be replacing vinyl siding that was installed over T-111 siding/sheathing on some two story apartment buildings built in the ‘70’s. There is no weather barrier between the vinyl siding and the original T-111 siding/sheathing. There is building paper installed between the siding/sheathing and the face of the studs. Is it advisable to install a Weather Barrier system (Tyvek Drainwrap or asphalt building paper) over the original sheathing/siding and under a new (fiber/cement) siding system? I have always heard that it is poor practice to install multiple vapor barriers within a wall assembly. Are the weather barrier systems (vapor) permeable enough to make this a non-issue? Are there any tricks to specifying that make it ok…. The project is located in Southern Oregon, so the climate requires the use of both heating and cooling inside the buildings.

Thanks for your thoughts…