More and more builders have realized the advantages of leaving stud bays empty and putting all of a home’s insulation outside of the wall and roof sheathing. If done correctly, exterior insulation can help produce a building that is almost airtight, very well insulated, and almost immune to water damage.
The construction method was first developed in the early 1960s by the National Research Council of Canada. In its purest form, the method is known as PERSIST — an acronym for Pressure-Equalized Rain-Screen Insulated Structure Technique.
Here’s how you build a PERSIST house:
To some builders and building inspectors, PERSIST details seem counterintuitive or dangerous. One typical reaction is, “You can’t install peel-and-stick over your wall sheathing! It’s a wrong-side vapor barrier! The membrane will trap moisture! The walls can’t dry out!”
Actually, the peel-and-stick works perfectly. The membrane acts as a combined air barrier, vapor barrier, and water-resistant membrane (WRB). Because the membrane completely seals the walls and roof, it produces an unusually airtight envelope.
Since the membrane is on the warm-in-winter side of the insulation, it’s exactly where it belongs. All of the home’s framing and sheathing is on the conditioned side of the membrane, so these wood components are maintained at indoor conditions. That means they aren’t subject to swings in humidity or temperature; the framing stays stable and dry in all seasons, in all climates.
On the exterior side of the membrane, there aren’t any components which are likely to suffer any moisture damage. Since the system includes a rainscreen behind the siding, any water that gets past the siding drains quickly from the walls.
PERSIST has a few disadvantages. It costs more than conventional construction, because of the cost of the peel-and-stick membrane, the two layers of…