Vapour diffusion and wall assembly design
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In the design and construction of commercial walls in cold climates, it has been common practice to install a polyethylene (PE) sheet vapour retarder at the interior of the insulation to control vapour flow (and often airflow) and therefore limit vapour diffusion wetting while using vapour permeable materials to the exterior to promote drying.
Figure 1 illustrates how interior vapour control in a stud cavity insulated wall can help prevent condensation within the wall assembly in cold climates. Condensation can also occur if air leaks from the interior to the back of the sheathing. Often, this causes more significant localized damage than vapour diffusion alone.
In warm climates, the opposite approach is used, where vapour control layers are placed on the exterior side of the assembly to restrict inward vapour drive. This can include materials such as concrete, concrete masonry units (CMU), or exterior water-resistive barriers (WRBs) of 10 perm or less. Impermeable materials on the interior side should be avoided and permeable materials such as drywall and stone wool insulation must be used to allow for drying to the interior.
When insulation is added to the exterior of the walls, as in the case of split-insulated or exterior insulated walls, this insulation maintains the temperature of the stud cavity and exterior sheathing closer to the interior conditions, reducing the potential for the dewpoint and air leakage condensation to occur within the cavity. The more insulation is installed outboard of the sheathing, the closer to interior conditions the stud cavity will be. When possible, the exterior insulation ratio should be maximized, and fully exterior insulated walls work well in both cold and warm climates.
In cold climates, the type of insulation installed outboard of the sheathing (or as the sheathing) has an important impact on the vapour diffusion drying capability of the wall. Vapour permeable insulation such as stone wool allows for greater outward drying than can be achieved with vapour impermeable insulation such as extruded polystyrene (XPS), polyiso, and sprayed polyurethane foam (SPF) insulation. This greater drying ability results in improved durability of the wall assembly.
In some cases, the change in temperature profile due to the addition of exterior insulation means a vapour retarder may no longer be needed at the interior in cold climates, and alternate strategies such a latex paint may be used instead of PE sheet. When a vapour impermeable exterior insulation is used in cold climates, an interior vapour retarder should be avoided to prevent trapping moisture within the wall assembly, or potentially an adaptive permeance smart vapour retarder material could be used.
In warm climates, semi-vapour impermeable membranes (~10 perm), in combination with exterior insulation, will work to restrict the flow of vapour through the wall and prevent moisture accumulation within the assembly.
Other types of walls such as those with moisture- storing claddings create unique conditions with respect to vapour diffusion, and require careful consideration and design. When designing these walls, double vapour barrier situations are to be avoided so drying can occur.
Overall, the correct selection and placement of vapour impermeable materials within wall assemblies is fundamental to their durability in both cold and warm climates. Failure to correctly account for the impacts of vapour diffusion can lead to damage and premature failure of wall assemblies (The content of this article is primarily based or summarized from research and guideline development work undertaken for ROCKWOOL by RDH Building Science Inc.).
Alejandra Nieto is a building science project manager at ROCKWOOL North America. Her role includes providing unbiased building enclosure solutions to the architectural community, advancing the research of enclosure system performance, and managing projects related to energy-efficient, durable, and resilient enclosure design. Nieto holds a master of building science degree from Ryerson University. She is an active member in multiple American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) technical committees; and is fluent in different modelling and simulation software including AutoCAD, WUFI, Heat 3, and THERM. Nieto can be reached at alejandra.nieto@rockwool.com.
Excellent article. Just to be very clear, are you saying that we should be using a rockwool product on the exterior of upgraded OLD HOUSES, not a polystyrene product.