Suggestions for specifying air barrier and sheathing membranes
The first step is the selection of the appropriate air barrier and sheathing membrane based on the wall design parameters and the environmental differences between the interior design loads and the local climatic conditions. Most designers are familiar with their local climate, but caution is given to those who design away from their local environment. Even local micro-climatic conditions can have a significant impact on the cladding assembly’s performance.
A determining factor in the selection process may involve conducting a performance analysis of the wall assembly under various climatic conditions. Simple steady-state conditions can be easily determined for climate extremes such as worst-case summer and winter conditions. More complex situations may require two-dimensional modelling which will define how well a wall dries given dynamic environmental conditions. Once the appropriate design conditions are determined, the product best meeting the requirements may be specified.
The next step following product selection is the installation’s specification and detailing. Performance is everything and research is a critical element for product selection. When the assembly’s performance requirements are unknown, how can a durable design be specified? If dynamic modelling of a wall assembly is conducted, the product manufacturer should provide the water vapour transmission characteristics including the range of vapour transmission characteristics. A simple vapour transmission number from ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, is not enough. Wet-cup and dry-cup measurements can be different for the same product, and a range of vapour permeabilities under different humidity conditions are possible.
Products should be ‘characterized’ and the design professional should contact the manufacturer to review any special details prior to the project being sent out for bid. This can be accomplished by scheduling a meeting with the manufacturer or sending a copy of the plans and specifications to the manufacturer to allow for a plan review.
Continuous foam plastic insulation is not a new concept. The EIFS industry has pioneered it for the last 50 years, but the growing demand for improved building performance has transferred the requirement to all claddings. There are many durable insulation products suitable for use in an exterior wall assembly. As with air and moisture barriers, the characteristics of the selected assembly must be compatible with the wall design.
Questions to consider include:
- What is the thermal performance required for the climate conditions and the function of the building envelope?
- What are the vapour permeability requirements for the assembly?
- Has the construction detailing either eliminated or substantially reduced the effects of thermal bridging of the cladding elements?
These considerations are elements of a good wall design—casual mixing and matching of elements that often comes from the realities of bidding and ‘value-engineering’ can destroy the designer’s best intentions.
Insulation products are available in a wide variety of materials. Closed or open-cell configurations are available as expanded polystyrene (EPS) or extruded polystyrene (XPS) products with differing vapour permeability. The R-value of a 25-mm (1-in.) thick insulation ranges from 3.85 to 6.5, depending on the product’s chemical make-up and configuration. Recent studies have shown the value of continuous insulation installed outbound of the sheathing is 99 per cent effective as compared to 60 per cent or less for batt insulation installed in the stud cavity [source?]. In lay terms, a 50-mm (2-in.) insulation with an R-value of 8, has an effective R-value of 7.92 as compared to a R-11 batt insulation with an effective R-value of 6.6.
A whole building energy modelling study conducted by Medgar Marceau of Morrison Hershfield shows how much energy can be saved by upgrading standard building code requirements with added insulation and an air barrier assembly in the wall. The study compared the value of airtightness and increased insulation thermal value in a variety of North American locations. The modelling proves insulation with a barrier is more effective that just insulation alone, even with substantial increases in R-value.