With the rapid evolution of building science, there is growing concern focused on constructing durable structures that effectively manage moisture in the building envelope. Significant problems, ranging from air quality to mold, are arising due to the confusion about the proper use, installation and compatibility with the variety of new materials available today.
The Membrane Drainage System
Most homes are built with a membrane drainage system (MDS) where stucco, masonry or siding (known as cladding) drain the majority of the moisture and protect the secondary layer(s) of building paper or house wrap. These layers of house wrap are known as the Weather Resistant Barrier (WRB) which prevents water intrusion while still allowing for efficient drainage.
The System
Weatherization of the building envelope should always be considered as a system dependent upon the weakest link. In other words, if a builder uses a state of the art building wrap but transitions the windows and doors with a marginal flashing product, the system is as good as the marginal product. Many builders are hiring weatherization consultants to better manage moisture as well as prevent costly compatibility and water intrusion issues.
Permeability in Weather Resistant Barriers
Permeability (perms) relates to a material’s ability to allow water vapor to escape. Traditional black paper or felt has very little (less than 5 perms). As a result, any incidental moisture that finds its way in, stays in and can lead to issues. These moisture issues include organic growth, premature degradation of materials, and termite infestation. Permeable house wraps have been in use for years. They not only prevent water from entering but also allow water vapor to escape (like Gortex clothing). Some have ratings of over 50 perms, making them a better choice for most applications.
Compatibility
Another major factor involves material compatibility. To provide high performance systems with self sealing capabilities, builders are using peel and stick membranes to transition windows and doors to the balance of the building envelope. These materials have self sealing characteristics and, when used correctly, provide greatly improved resistance to water intrusion. It is imperative that these membranes are compatible with the windows, the sealant and the balance of the weatherization barrier. These membranes are primarily asphaltic based or butyl rubber based and should not be used together. This fact has not been appropriately conveyed to many builders who do it on a regular basis and then add a non-compatible sealant to the mix.
The Solution
The best plan of action here is to use the products as a system. Pick one manufacturer that does it all and use their wrap, flashing materials and sealant. This allows these materials to perform as they were designed and prevents incompatibility issues. The best systems are butyl based as they typically provide better heat and UV resistance. They are also compatible with virtually all window and door frame materials. Not all asphaltic membranes are compatible with vinyl or certain frame finishes. Thus, if you decide to use these, you should check with the manufacturers to ensure you are within the approved installation guidelines.
A building envelope that is installed correctly manages water effectively and improves the structure’s durability dramatically. Breathable systems help to maintain the materials and prevent issues.
