In Europe, understanding the need for buildings to breathe is a given. Products for cladding and decorating the exterior of buildings are expected to be breathable. In North America, we read literature and test reports providing evidence that we should incorporate this principle of breathability into the design of our buildings; but unfortunately, we are slow in recognizing this issue.
In North America, the most widely used underlayment is 15- and 30-pound roofing felt. In the last two decades, ice and waterproofing membranes have become the norm in situations where one anticipates ice-damming problems. Since the 1950s, humidification in buildings has become increasingly popular, which adds considerable humidity in residential, commercial and institutional structures. Older buildings simply are not equipped to handle this additional humidity. As a result, water vapor drives through the walls and ceilings and eventually condenses on insulation or other building components.
Historic buildings, in particular, are at risk because they generally lack vapor retarders. Yet with the goal of providing a secondary roofing system, these buildings are wrapped in roofing felt, or worse, ice and waterproofing membranes.
Roofing felts are marginally breathable and certainly suffer from condensation through the winter months. Ice and waterproofing membranes that are self-sealing at the joints and at nail penetrations are almost perfect vapor retarders. The designer, with the intention of providing a high-quality secondary roofing system, inadvertently produces a situation where long term performance of the building may be catastrophic.
