Vapour diffusion and condensation control for commercial wall assemblies

by nithya_caleb | December 20, 2018 3:39 pm

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by Alejandra Nieto

As building codes change in North America in response to the call for greater energy efficiency, builders, architects, engineers, and specifiers continue to see a move toward tighter building envelopes requiring increased insulation levels. While this comes with many benefits from the perspective of thermal performance, reduced energy consumption, and lower environmental impact, it also poses a number of challenges. Most consequential is the need to effectively control moisture. Yet, with a multitude of options for commercial insulation and wall assembly design, the guidance offered by energy and building codes can range from non-existent to confusing despite the fact moisture control remains a primary concern impacting occupant comfort, safety, and durability (or potential failure) of the wall assembly and/or its components. So, what is a best-in-class insulated commercial wall assembly? The truth is, it depends on a wide array of variables such as climate zone, insulation type, vapour permeability, and membrane selection. Yet, it is these exact variables that provide the answers to a more functional and effective wall system, its components, and the placement of each within the wall assembly.

Vapour diffusion

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An important consideration is vapour diffusion. When vapour diffuses, it moves from the warm to cold side because warm air can hold more water than cold air. Overall, the direction of the vapour drive has important ramifications to the placement of materials within a wall assembly. Improper placement of materials can affect a wall’s drying potential and can lead to condensation on colder surfaces, damaged materials, and fungal growth. To control vapour diffusion, vapour retarding materials are used. While all materials can act as vapour retarders to some degree, vapour retarders and vapour barriers are classified by their permeance or ability to let vapour pass through, measured in perms. Class 1 materials are the most impermeable, allowing the least amount of vapour to travel through the wall, while each subsequent class of vapour retarding materials is increasingly permeable, allowing for more vapour movement. Overall, the control of vapour diffusion must be a balance. The goal should be to minimize or manage wetting sources, while maximizing the drying potential, should moisture be introduced during construction or the materials get somehow wetted in service. This is particularly important, because while vapour diffusion is generally thought of as a negative process, in fact it is the only process through which the interior of most wall assemblies is able to dry in service. This is especially important with today’s highly insulated wall assemblies, as more insulation means less heat energy is available to dry moisture from within the wall assembly. Special consideration must also be given to placement of the vapour control layer, which will be highly dependent on climate and seasonal vapour drive where a given building is being constructed.

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Dewpoint and condensation

The dewpoint within a wall assembly can be described as the temperature at which the indoor air reaches
100 per cent relative humidity (RH), and usually occurs within the insulation layer of the assembly as it has the greatest temperature gradient. However, the critical layer is the condensing surface, the first cold surface within the assembly. In cold climates, this usually relates to the interior side of the exterior sheathing. The use of continuous exterior insulation reduces the risk of condensation as it will maintain the critical layers warmer and above the dewpoint. With that being said, potential for condensation is mostly critical if the assembly is not designed to dry out.

Air leakage

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Air leakage is an even greater concern than vapour diffusion and is a critical factor impacting wall assembly performance and durability. As a result, it must be carefully addressed because it can act quickly to deposit large amounts of water in a wall assembly in a short period of time. Difference in air pressure between the interior and exterior of a building drives air pressure through a wall assembly from the high pressure side to the low pressure side. To control air leakage in building enclosure assemblies, an air barrier is installed. Attention to connections and the use of tapes and sealants are important to ensure a continuous air barrier system. Unlike vapour diffusion, air leakage is not dependent on the material properties of the air barrier, but most often occurs due to holes in the air barrier. When air leakage does occur, it carries moisture. If the air then comes in contact with a surface below the dewpoint temperature of the air, condensation can occur,

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leading to potential fungal growth or degradation of the assembly and its components.

Wrong side and double vapour

Improper installation can also cause moisture problems. Two common conditions causing serious issues are vapour barriers placed on the wrong side and double vapour retarders. A vapour retarder placed incorrectly is one that is situated on the low pressure or cold side of the wall assembly, restricting vapour diffusion and subsequently creating a potential condensing plane in the wall assembly.

Double vapour retarders refer to when a vapour retarder is installed at two locations in a wall assembly, negatively impacting drying potential should water or any moisture get between them. Moisture between vapour retarders may be the result of air leakage, rainwater ingress, or built-in construction moisture.

Vapour diffusion and wall assembly design

The specified slider does not exist.

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.). 

[6]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[7].

Source URL: https://www.constructioncanada.net/vapour-diffusion-and-condensation-control-for-commercial-wall-assemblies/

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