By: Saphron Skinner-Willson, P.Eng. QCxP

As a building enclosure engineer working in both design and commissioning capacities, I spend considerable time on construction sites. What makes this field particularly rewarding to me is that the building enclosure performance is visible—you can literally see it happening in real time if you know what to look for.

Working in a cold climate where temperatures swing from below -30°C to above +30°C (-25F to 85F for you American friends) annually, I primarily observe thermal energy moving from warm interiors (high energy potential) to cold exteriors (low energy potential). Think back to high school, where a high concentration of energy or molecules moves to low concentrations to reach overall equilibrium. Air, vapour, heat, and moisture all strive to move from areas of highest energy to lowest energy. If we simplify this too much and leave out nuance and minor forces that cause unusual behaviors, we can understand the fundamentals and why building enclosures are required. 

Understanding the Physics

Building enclosures must resist four primary forces, each following the same basic principle of energy moving from high to low concentration:

Moisture moves from highest to lowest kinetic energy using the path of least resistance—typically from high to low elevation, pulled by gravity.

Air moves predominantly through pressure differentials, mechanical forcing, or temperature-driven convection, where heated air rises and cooler air sinks, creating the stack effect (also called the chimney effect) in extreme situations.

Vapor moves through the air's carrying capacity (relative humidity) and vapor diffusion, where high concentrations move toward low concentrations to reach equilibrium.

Heat transfers through three mechanisms: convection (through gases and liquids), radiation (transfer of energy through electromagnetic waves, which does not require a medium - how the sun’s heat travels through the vacuum of space), and conduction (through solids).

This means the enclosures we design must resist these forces and maintain separation between differentially conditioned spaces. The best time for quality control is during construction, when assemblies and control layers are exposed and can still be remediated. A terrible time is after building occupancy, when remediation requires removing finishes and overlapping layers—opening areas orders of magnitude larger than the original discontinuity (hole). The WORST time is in the life of the building occupancy after a significant failure has occurred, whether from prolonged exposure or acute and severe moisture intrusion through that discontinuity (hole). Understanding building enclosures this way—even without grasping all the nuances of this highly complex system—allows you to catch most failures before they become expensive problems.

Given this physics-based understanding of how energy moves through building assemblies, we can now focus on the specific conditions that create vulnerabilities. When conducting on-site reviews, four critical areas deserve dedicated focus—each directly related to the fundamental forces we've just discussed.

Critical On-Site Review Focus

Negative Laps

Think of fish scales, duck feathers, or roof shingles—each creates an impenetrable barrier through properly nested individual elements. The installation sequence matters critically. Fish scales nest with forward scales over rear ones, allowing forward movement while keeping water out. Roof shingles work identically, with higher shingles overlapping those below, creating a continuous water-shedding surface as water moves down via gravity.

Water-resistive barriers (WRB) require the same approach: top layers must lap over bottom layers. Negative laps (bottom over top) typically occur due to poor site coordination or poor attention to detail, creating a potential path for future water ingress.

Holes and Discontinuities

Any hole in the WRB dramatically increases the risk of moisture ingress. Common problems include:

  • Fishmouths: Open bumps at membrane edges creating tunnels into the assembly interior, typically occurring during installation of self-adhered weather-resistive barriers
  • Mechanical damage: Holes and punctures in any weather-resistive barrier material—sealants, asphaltic membranes, plastic or metal flashings, silicone membranes, etc.

If water can find a way, it will. Every surface or material intended to control water ingress must be fully continuous, overlapped in a shingle fashion, and completely sealed to penetrating components through the envelope.

Thermal Bridges

Heat always moves from high to low energy, and while we can slow this process significantly, we cannot stop it entirely. Our goal is to ensure consistent heat flow without uncontrolled areas and to mitigate vapor condensation on surfaces that are too cold due to improper or discontinuous insulation.

On-site, we verify that insulation is installed tightly with no gaps between insulation and adjacent sheathing or structure, nor between insulation boards themselves. During design, we also try to reduce the amount of uninsulated structural elements penetrating the insulation (like how your arm sticking out when you’re wearing a winter coat is warm, but your arm sticking out when you’re wearing a winter vest is still cold).

Continuity of Control Layers at Transitions

Every control layer—whether controlling air, moisture, vapor, or heat—functions like a relay race. Each layer needs to pass the baton smoothly from one material or part of the assembly to the next, without holes or gaps. Insulation must be installed flush to the sheathing with no voids. Wall air barriers must transition seamlessly into roof air barriers. This is made more difficult by changes in plane or transitions between trades. Those transition points are where I’ll be paying extra attention—because that’s where problems with air or moisture barriers are most likely to show up.

These four review categories will catch a significant percentage of building enclosure deficiencies during construction. However, understanding how these failures manifest in completed buildings provides valuable context for why these on-site reviews matter so critically. In my cold climate environment, where interior conditions remain consistently warmer and more humid than exteriors during spring, fall, and winter, certain telltale signs appear that directly correlate to the construction defects we've identified.

Biological Indicators

sap can indicate air leaks. When warm interior air leaks out, typically at window corners, it raises exterior temperatures enough for these insects to survive cold conditions. Concentrated populations around window frames signal air leakage. This is very upsetting when you are doing a review of an office tower in barely-above-freezing conditions, and every tenth window is swarming with spiders. 

Interior Ghosting

Always enjoyable to see, thermal bridging failures can reveal themselves through distinctive visual patterns on interior drywall surfaces, in particular for steel-framed commercial buildings. Steel conducts heat far better than insulation, so in steel-framed buildings with friction-fit insulation between studs, dramatic heat loss occurs through steel framing that's warm on the interior (adjacent to drywall) and cold on the exterior (adjacent to sheathing exposed to ambient exterior air). This very cold steel in extreme cold exterior conditions, in contact with the interior drywall, drops the temperature of the interior face of the drywall to just barely below the carrying capacity of the air and collects the smallest amount of moisture on the interior face of the drywall in perfect alignment with the steel stud framing. The minute condensation collects dust over time, creating visible "ghosting" patterns of the steel framing on interior, finished walls.

Exterior Snowmelt and Rain Drying Patterns

While interior ghosting shows thermal bridging effects inside buildings, exterior conditions reveal these same problems from the opposite perspective. Light snow on roofs or stucco walls after rain reveals thermal transmission patterns. Snow melts fastest and walls dry quickest where heat loss occurs—either through conduction (visible at framing locations versus insulated areas) or air leakage (faster drying in fan-like patterns moving from low to high elevations).

Ice Formation

Temperature differentials and air movement create two distinct ice-related phenomena that clearly demonstrate building enclosure failures in action. Icicles form when heat bypasses thermal control layers and escapes the building below pitched roofs, typically through air leakage. This heats the roof underside, melting snow on the roof that runs down until reaching cold eaves exposed to ambient air above and below. Water refreezes into ice, with icicle size determined by roof pitch (speed of water flow) and heat differential (time required to freeze).

Attic rain occurs on the inside of the roof sheathing in unconditioned attic spaces. Moisture-laden air from conditioned spaces leaks into attic areas during winter, condensing and freezing on the sheathing undersides. When exterior roof temperatures rise above freezing, accumulated frost and ice melt, dripping onto insulation below. Severe cases, when concentrated moisture accumulates over winter, cause deterioration to the attic insulation and to the ceiling sheathing and finishes.

These real-world manifestations demonstrate the direct consequences of the construction defects that can be identified during on-site reviews. The biological indicators, ghosting patterns, and ice formation all trace back to fundamental failures in control layers, continuity, thermal bridging, or air leakage control.

Conclusion

These simple observation techniques allow identification of building enclosure failures before they become critical problems on construction sites, and recognition of performance issues in existing buildings. Understanding the fundamental physics of energy movement—from high to low concentration—provides the foundation for spotting most enclosure deficiencies.

By focusing on proper lapping sequences, eliminating holes and gaps, reducing thermal bridging, and ensuring continuity of control layers, we are more likely to catch the majority of the typical building enclosure issues during the construction phase (when remediation remains cost-effective and practical).

Whether you're a building enclosure specialist or simply maintaining awareness on construction sites, keeping these principles in mind will help identify potential problems before they become expensive failures. And go have fun spotting failures in the wild. 

Photos courtesy of Saphron Skinner-Willson