Getting the details right: building control layers

September 5, 2022

Photo courtesy Ian Miller

By Ian Miller, P.Eng., LEED AP, CCCA

Good building design incorporates four key control layers to separate the indoor conditioned space from the exterior—the layers of water shedding/drainage, vapour control, air control, and thermal control. Construction specifiers must thoroughly detail these control layers in the design stage. Without them, the design for those elements is left to the contractor—and their role should be installation, not design.

Inevitably, changes occur, and site conditions vary, so contractors need to make decisions in the field, but good details and adequate levels of construction oversight can help make the best building possible. Errors in the design and construction of the building control layers can lead to moisture ingress, occupant discomfort, and damage to the building. Therefore, it is important to get the details right.

Water shedding or drainage layer

The first layer at the exterior is the water shedding or drainage layer. This is the building’s first line of defence from water ingress. Many might think of this simply as the building roofing and cladding, but there is much more to it. Good building design typically involves a rainscreen, whereby the exterior cladding system does not need to be 100 per cent perfect. Instead, any precipitation managing to get past the outer face of the cladding, be it through cracks, holes, or open joints, reaches a secondary layer which drains the water back to the exterior. This is typically accomplished through both membrane and metal flashings.

Sealants should be relied upon as little as possible, since they degrade over time and form a weak point which will require regular replacement. Designers must think in three dimensions—not just what will happen at the window jamb and sill, for example, but also at the corner where the two intersect. This common weak point should incorporate proper layering of the jamb flashing over the sill upturn, as well as an end dam, for example, to prevent water ingress at the corners of the window. Sill and through-wall flashings should incorporate significant-sized drip edges to direct water out and away from the building.

Vapour control layer

Figure 1 Poor rain shedding at the windowsill has resulted in water penetration and freeze-thaw damage to the brick masonry.

The second control layer is the vapour control layer, sometimes referred to as the vapour retarder or vapour barrier. This layer, as the name implies, controls the movement of water vapour through the wall assembly.

Physics demonstrates how water vapour always wants to move from areas of high concentration to areas of low concentration, seeking equilibrium. If one opens the door to the bathroom after steaming it up with a hot shower, the humidity from the bathroom seeks to escape to the rest of the house, increasing the humidity outside the room and lowering the humidity inside. The laws of physics also demonstrate how warmer air can hold more vapour than colder air.

Looking at the psychrometric chart, one can see how air at 5 C (41 F) can only hold roughly half the amount of moisture at 100 per cent humidity as air at 15 C (59 F), and approximately one quarter of the moisture as air at 25 C (77 F). Air at 20 C (68 F) and 40 per cent relative humidity (RH)—relatively comfortable indoor conditions—reaches 100 per cent humidity, also known as its dewpoint, when cooled to 6 C (43 F). Cooled any further, and the air can no longer hold the moisture, which is deposited as condensation.

Figure 2 Thermal bridging through the aluminum curtainwall framing resulting in major heat loss, with interior component temperatures below freezing and frost buildup at the interior.

During a typical Canadian winter, interior air often contains more moisture than the exterior air can accommodate, so the vapour drive is to the exterior. Without a vapour control layer, water vapour would be driven outwards through the wall assembly, concentrating in building materials as the dewpoint is reached. This of course can lead to mould, rot, and many other undesirable results. To prevent this, the construction specifier must provide a vapour control layer in the wall assembly—a building material whose composition prevents vapour diffusion. In house construction, this is most often polyethylene sheeting, but in commercial construction various available material options can meet these needs.

The vapour control layer must be placed at the correct location within the wall assembly. As noted earlier, warmer air can hold more moisture, vapour drive is typically from the warm side of the wall to the colder side of the wall. In Ontario, builders typically place the vapour control layer at the interior side since most of the year the vapour drive is outwards. However, this also means for some days in the summer, when it is hot and humid outside and comfortably air conditioned inside, the vapour drive is reversed. In most parts of the country, this condition is much less common than an outward vapour drive, but designers should still accommodate for these conditions with moisture-tolerant materials capable of drying out should vapour drive add to their moisture content.

Accidentally introducing multiple vapour control layers in the wall assembly is a frequent problem. Take, for example, a building with a polyethylene vapour barrier at the interior, insulation in a stud wall cavity, then a layer of exterior insulation with its joints taped using a vapour-impermeable tape. If this insulation layer does not allow vapour diffusion, such as a foil-faced polyisocyanurate (ISO) insulation, then the building is left with a second vapour barrier at the exterior side of the building. Since construction is never perfect, some moisture movement is likely, but if water vapour makes its way to the space between these layers it cannot escape—the two vapour control layers prevent drying to the interior or the exterior. Therefore, it is not only important to make sure a wall assembly has no more than one vapour control layer, but also to ensure it has one to begin with.

Air control layer

While a lot of attention is always given to the vapour control layer, in this author’s opinion even more important is the air control layer. It prevents air from passing through the wall assembly between the interior and exterior spaces. Air movement through gaps and cracks from the exterior to the interior can result in uncomfortable drafts, but even more serious is the movement of air from the interior to the exterior.

Remember, for much of the year warm, moist interior air would reach its dewpoint somewhere between the interior face of the building wall and the exterior face of the wall assembly. If air can exfiltrate out through the wall, when it reaches this temperature moisture will condense out, depositing water on the colder component of the building enclosure. If this colder component is, for example, wood framing, the condensed water can result in mould, rot, or other undesirable conditions.

If the colder components are, for example, the steel anchoring system of a precast concrete cladding system or the steel lintel supporting brick masonry, the moisture can promote rust deterioration of the anchor or lintel, potentially compromising the structural integrity of the heavy cladding system.

Vapour diffusion is equal across the entire wall assembly; therefore, a missing vapour control layer would result in diffusion across a wide area. However, in the case of a failed air control layer, the escaping air is typically concentrated in a few localized spots, so the water deposition by condensation is most often localized as well. This can result in excessive amounts of water deposition in small areas, leading to serious localized failures. In opposition to the vapour control layer, a building cannot have too many air control layers. The location of the air control layer(s) does not matter either—they can be located anywhere within the wall assembly if they are continuous.

Thermal control layer

Figure 3 Fiberglass thermal spacer to provide structural support for cladding avoiding thermal bridging of a material with high thermal conductivity. Photo courtesy Cascadia Windows and Doors

The final control layer for this discussion is the thermal control layer. This component reduces the amount of thermal energy (heat) from passing through the building enclosure. Not only does heat loss waste valuable energy which must be provided to the building in the form of electricity, natural gas, or a similar energy source, but it can create cold spaces inside the building which have a negative effect on occupant comfort. In the worst cases, localized heat loss can create spots where the interior surfaces can be cold to the touch, and even result in condensation at the interior. This moisture, sourced from the interior air and not a leak from outside, can cause water damage, mould, and/or rot to building components.

As building codes have evolved, the requirements for the thermal control layer have become more rigorous. Modern codes require continuous insulation (ci) in most cases, not just insulation placed between structural members like wood or steel studs. Typical glass fibre batt insulation at 150 mm (6 in.) thick has a nominal insulating value of approximately R-19, but when installed between wood or, worse, steel studs, the building’s thermal energy can bleed through the studs which form thermal bridges through the insulation.

In a 150 mm wall with steel studs at 406 mm (16 in.) on centre, the effective R-value drops from a nominal R-19 to an actual R-7.1. Just 38 mm (1.5 in.) of extruded polystyrene (XPS) insulation, continuous outside the stud wall, provides a higher R-7.5 insulation value. Adding a continuous layer of insulation is the better design, but the best design attempts to eliminate all thermal bridges.

One often finds these thermal bridges at areas where structural supports need to span to the exterior, such as at window supports or cladding fasteners. These details are more difficult to get right, but more manufacturers are bringing to market specialty components such as fibreglass cladding supports to mitigate this additional heat loss path.

Each year building product manufacturers are introducing new and innovative products capable of functioning as more than one of these control layers, either intentionally or unintentionally. Some rigid board insulation, for example, can act as a vapour barrier, but if placed on the exterior of interior insulation it can create a second vapour control layer within an assembly, whereby trapping moisture within the construction. Hygrothermal analysis can determine where the dewpoint is and if it will be a problem.


One tried-and-true method for assessing the design of the building enclosure is to take the design—plans, sections, and details—and to draw on with different colours each of the control layers. If one could draw each layer without lifting the pen, then it should be continuous in practice. If there is a gap in the air barrier line, then it may have a hole allowing air to pass through the building envelope. If one lifts their pen on the thermal control layer, it may have thermal bridging. All four control layers are critically important to any high-performance building enclosure design. Getting the details right is vital.


[5]Ian Miller is a building science engineer with 18 years of experience in the design, assessment, and repair of various buildings and their components. He is a graduate of the University of Waterloo and a licensed professional engineer. He is a partner at Pretium, a building science engineering consulting firm, and works in the technical role of a project principal, taking lead roles on many of the firm’s most important projects. Project work has included assessments as well as design, tendering, contract administration, and construction review for various building construction, maintenance, and repair undertakings. Miller served on the board of directors for the Ontario Building Envelope Council from 2011 to 2021, including as the organization’s president from 2017 to 2019. He is the organizing chair for the 16th Canadian Conference on Building Science and Technology, to be held in Toronto in October 2022.

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