Controlling condensation from air leakage and vapour diffusion
September 15, 2017
By Vincent Chiu
Water is the most significant factor in the premature deterioration of our buildings. Excessive moisture accumulation on porous materials can lead to water penetration, freeze-thaw damage, efflorescence, cracking, and façade soiling. Further, water penetration or interstitial condensation may lead to the chemical breakdown of organic materials (e.g. wood), failure of structural systems and fasteners, mould growth, and damage to interior finishes and furniture.
There are three main sources of moisture that affect buildings:
- exterior moisture (e.g. rainwater, groundwater, surface runoff, and melting snow);
- interior moisture resulting from occupants and occupant use (e.g. perspiration, respiration, and activities); and
- built-in moisture (i.e. manufacturing, construction, and naturally occurring moisture).
The three strategies to manage water penetration are known as the Three Ds: deflection, drainage, and drying. For example, the bulk of exterior rainwater can be deflected away using overhangs, flashings, and drip edges. Any rainwater not deflected can be drained away using drainage layers within the wall assembly, and away from the foundation wall using drains. Water capable of bypassing the two previous strategies must be capable of drying.
Numerous techniques have been employed by designers and builders to deal with bulk water. However, the transport of moisture through air or vapour diffusion is trickier and less understood. Condensation from air leakage and vapour diffusion on cold condensing surfaces within a wall assembly can lead to significant structural and indoor air quality (IAQ) issues that may cost more to repair than to prevent.
Controlling air and vapour flow
More than a half-century ago, assemblies did not require vapour barriers. They dried both outward and inward, were capable of managing leaky windows without flashing, and could be rained on during construction. The wall assemblies were also poorly insulated—this was actually the key to their moisture management, as heat flowing through the assembly helped dry the assembly of any built-in or accumulated moisture. The interior moisture levels were also low from the high dilution rates due to stack effect and the leakiness of the enclosures. (See Joseph W. Lstiburek’s “Macbeth Does Vapour Barriers,” published in the November 2013 Building Science Insights [vol. 73].)
Figure 1: Quantity of water through gypsum board via (a) air leakage and (b) vapour diffusion during a heating season.
Images courtesy Building Science Corporation
A trend toward tighter enclosures with fewer air changes has resulted in lower dilution rates and higher levels of interior moisture. This, coupled with cavity insulation (which resists the flow of heat through the assembly), reduces the wall’s drying potential. Therefore, interior moisture should be handled at the source using ventilation fans and through the proper design, installation, and operation of the building’s HVAC system.
Interior moisture that cannot be eliminated must be controlled by employing an effective air barrier and vapour retarder/barrier. Despite the fact these two barrier systems are neither the same nor interchangeable, there is sometimes confusion in the industry, as a single material is often used for both functions.
Controlling the flow of air is more important than controlling vapour diffusion, as seen in Figure 1. During a heating season, air leakage through a sheet of gypsum board with a hole in the middle will result in 28.4 L (30 quarts) of water. Vapour diffusion through a solid sheet of gypsum board (i.e. no hole), however, will only result in 0.3 L (1/3 quart) of water.
Therefore, fastidious air-sealing of the wall assembly from the interior is critical in mitigating moisture-related problems in Canada’s colder climate. Caulked, glued, or gasketed gypsum board to framing can serve this purpose while still being vapour-permeable, allowing the wall to dry toward the interior. Coating the gypsum with a semi-permeable finish such as latex paint is usually adequate in controlling vapour diffusion in most buildings. However, vapour barriers such as polyethylene sheet are often required by codes (e.g. National Building Code of Canada [NBC]) and also used as the interior air barrier.
Exterior climate and interior conditions
Figure 2: The diagrams shown above depict a high-performance flow-through wall (left) and its high-performance plastic counterpart (right).
Building design and material specification highly depend on climate zone. Many of today’s building failures are caused by designers and builders taking a concept that worked in one climate zone and applying it to another. A good example of this was during the ‘leaky condo crisis’ in and around Vancouver in the late 1980s to early 2000s, which led to widespread building failures. A survey of building envelope failures in the area concluded face-sealed wall assemblies were unacceptable in the region. Once wetted, the drying potential for drying in these walls in the coastal climate of the lower mainland during the winter is very limited. (For more see, D. Ricketts and J. Lovatt’s “Survey of Building Envelope Failures in the Coastal Climate of British Columbia,” published by Morrison Hershfield and Canada Mortgage and Housing Corporation [CMHC] in 1996.)
Interior conditions also dictate how the building assembly should be designed and constructed. For example, vapour diffusion can be a significant moisture transport mechanism in buildings with higher interior humidity levels (e.g. hospitals, museums, and natatoriums). However, most buildings will have an interior relative humidity (RH) level in the 20 to 35 per cent range during the heating season. In this condition, the potential for vapour diffusion condensation is small; it can be easily managed with the correct material selection, insulation, and construction practices.
Although vapour barriers are intended to prevent assemblies from getting wet, they also often prevent them from drying. Vapour barriers installed on the interior side of assemblies impede them from drying toward the interior—this is a problem for air-conditioned spaces, and can be an issue when there is also a vapour-impermeable material installed on the assembly’s exterior, creating a double-vapour-barrier wall assembly. Therefore, it is practical to encourage drying mechanisms over wetting prevention mechanisms—meaning one should avoid using vapour barriers if vapour retarders will work, and avoid using vapour retarders when vapour-permeable materials suffice.
A high-performance flow-through wall can be achieved by using materials that allow vapour to pass through it, as seen in the left side of Figure 2. This wall assembly works in any climate zone. The ventilated cladding—along with the vapour-permeable exterior stone wool insulation, vapour-semi-permeable oriented strand board (OSB) sheathing or permeable gypsum sheathing, and vapour-permeable cavity insulation (e.g. stone wool) between wood or steel framing—allows any accumulated moisture to dry in either direction.
Adding a polyethylene vapour barrier on the interior eliminates drying toward the interior, as shown on the right side of Figure 2. Air-conditioning the interior with this type of assembly may lead to moisture-related issues, as condensate may collect on the polyethylene sheet during the summer.
Exterior to interior insulation
Figure 3: This table, taken from the Building Science Digest 163, Controlling Cold-weather Condensation Using Insulation (John Straube), illustrates the ratio of interior to exterior insulation to mitigate interior air leakage condensation.
Despite most efforts to reduce interior air leakage, designing an airtight building and actually constructing one are two different things. Constructing a building to meet strict Passive House standards requires the building shell achieve 0.6 ach @ 50 Pa. Most buildings are between 5 to 9 ach, with the better ones being between 2 and 3 ach. Therefore, most buildings will have some amount of air leakage.
The key to preventing condensation from occurring within a wall is to keep the condensing surfaces in the assembly—typically the sheathing, as it is usually the outmost layer—above the dewpoint temperature. By placing an adequate amount of insulation on the exterior side, the sheathing remains warm and condensation is avoided.
Although placing interior cavity insulation increases the thermal resistance of the wall assembly, it also resists heat flow to the sheathing, lowering its temperature. To ensure the sheathing remains above the dewpoint, the correct ratio of exterior to interior insulation is required to control air leakage condensation for various outdoor and indoor conditions, as shown in Figure 3.
Understanding the fundamentals of heat, air, and moisture transfer, along with the properties of materials, is critical in avoiding moisture-related issues. Constructing walls with double-vapour barriers or a vapour barrier on the wrong side of the building should be a thing of the past.
When considering the realities that may impact a building’s moisture equilibrium (e.g. water intrusion, air leakage, and vapour diffusion condensation), it is important to choose materials that can control and limit the amount of moisture introduced into the system, but at the same time allow for adequate drying when needed.
The goal is to design buildings resilient to the realities of aging, leaks, and imperfect construction by encouraging drying mechanisms or by ensuring the drying potential is greater than the wetting potential in the enclosure. As heat loss slowly disappears with increasingly stringent requirements for continuous exterior insulation, so will the requirements for vapour barriers in the ‘milder’ cold climates of Canada. Only time will tell.
Vincent Chiu joined ROXUL’s building science team as a building science specialist in May 2016. He has a master of applied science degree in building engineering from Concordia, where he focused on building diagnostics and rehabilitation, building modelling, wind engineering, and building aerodynamics. Chiu now works extensively with architects, engineers, design consultants, and contractors, providing building enclosure solutions to the architectural community and promoting energy-efficient enclosure designs and sound building science. He can be reached at email@example.com.
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