Uploaded October 2025 | Updated September 2026, 3 weeks ago
In this video we look at three separate homes in greater Fairbanks, Alaska, that exhibit variations of a common thermal bypass problem. There are a lot of ways to define a thermal bypass, but in this video we use the term to refer to anywhere in our building thermal envelope where air leakage-driven heat losses flow through or around insulation, rendering it less effective. Air leakage-driven thermal bypasses tend to be especially prevalent in cases where one heated structure is attached to another, as builders often overlook the critical aspect of transitioning air sealing through the common walls of structures.
In all three cases presented here, the thermal bypasses also contribute to another significant problem: potential moisture damage. During the cold season, warm indoor air will migrate through inadequately sealed common walls, cooling and depositing moisture within walls and roof spaces that are part of the common wall assembly. These moisture problems in common wall and roof assemblies are typically difficult and expensive to repair, as they require tearing into the structure to reach air leakage pathways at the location where the vapor retarder transitions through the wall assembly, also known as the pressure boundary. In this video we demonstrate how to seal these transitions through the use of solid, caulked blocking, which is generally the most effective and durable method for addressing these leakage problems.
Our first case study illustrates a thermal bypass that was overlooked during initial construction. A garage was attached directly to the house in a 1970’s-era split-level, but poor air-sealing led to warm air (and also moisture) escaping into the common wall separating the house and garage. The solution is to install properly sealed blocking in the wall, which we demonstrate here, before re-insulating it.
Our second case study is a different view of the same issue. In this pipeline-era split-level home, air is leaking into the common wall from both the garage and the house. As the home is occupied and the common wall is physically inaccessible, we can’t effect a repair, so we instead diagnose the issue through the use of an infrared camera.
Our final and most detailed case study is a two-story home with a two-story addition. The house has had continuous moisture problems in the common wall and the roof between the two attached structures, and two previous attempts to remedy the issue have failed. Here we take a close look at the house using a variety of diagnostic tools including blower door air leakage testing, infrared imaging and smoke testing. We are able to validate that previous attempts to seal the air leakage failed at the common wall transition between the two structures. We then visually identify the air leakage pathways and discuss the situation in detail, including our approach to fixing the problem.
The parting message in this video is that in all cases, whether they be new construction or remodels, the cheapest and most effective way to stop the air leakage is to identify thermal bypass locations at the start of construction (or demolition). Installing blocking at these stages is a straightforward process, whereas retrofitting it can prove expensive, disruptive and time-consuming.
We welcome questions and discussion in the comments and you can also reach us at info@cchrc.org. Please note that we’re putting these videos on hiatus due to lack of available funds. If you’d like to see more of these, please let us know and we can pass along your support.
CCHRC website: cchrc.org
CCHRC YouTube channel: @coldclimatehousing
Music: "Motivational inspiring music" by Alexander Blu from orangefreesounds.com/. The song is permitted for non-commercial use under license “Attribution-NonCommercial 4.0 International (CC BY-NC 4.0).
In this video we look at three separate homes in greater Fairbanks, Alaska, that exhibit variations of a common thermal bypass problem. There are a lot of ways to define a thermal bypass, but in this video we use the term to refer to anywhere in our building thermal envelope where air leakage-driven heat losses flow through or around insulation, rendering it less effective. Air leakage-driven thermal bypasses tend to be especially prevalent in cases where one heated structure is attached to another, as builders often overlook the critical aspect of transitioning air sealing through the common walls of structures.
In all three cases presented here, the thermal bypasses also contribute to another significant problem: potential moisture damage. During the cold season, warm indoor air will migrate through inadequately sealed common walls, cooling and depositing moisture within walls and roof spaces that are part of the common wall assembly. These moisture problems in common wall and roof assemblies are typically difficult and expensive to repair, as they require tearing into the structure to reach air leakage pathways at the location where the vapor retarder transitions through the wall assembly, also known as the pressure boundary. In this video we demonstrate how to seal these transitions through the use of solid, caulked blocking, which is generally the most effective and durable method for addressing these leakage problems.
Our first case study illustrates a thermal bypass that was overlooked during initial construction. A garage was attached directly to the house in a 1970’s-era split-level, but poor air-sealing led to warm air (and also moisture) escaping into the common wall separating the house and garage. The solution is to install properly sealed blocking in the wall, which we demonstrate here, before re-insulating it.
Our second case study is a different view of the same issue. In this pipeline-era split-level home, air is leaking into the common wall from both the garage and the house. As the home is occupied and the common wall is physically inaccessible, we can’t effect a repair, so we instead diagnose the issue through the use of an infrared camera.
Our final and most detailed case study is a two-story home with a two-story addition. The house has had continuous moisture problems in the common wall and the roof between the two attached structures, and two previous attempts to remedy the issue have failed. Here we take a close look at the house using a variety of diagnostic tools including blower door air leakage testing, infrared imaging and smoke testing. We are able to validate that previous attempts to seal the air leakage failed at the common wall transition between the two structures. We then visually identify the air leakage pathways and discuss the situation in detail, including our approach to fixing the problem.
The parting message in this video is that in all cases, whether they be new construction or remodels, the cheapest and most effective way to stop the air leakage is to identify thermal bypass locations at the start of construction (or demolition). Installing blocking at these stages is a straightforward process, whereas retrofitting it can prove expensive, disruptive and time-consuming.
We welcome questions and discussion in the comments and you can also reach us at info@cchrc.org. Please note that we’re putting these videos on hiatus due to lack of available funds. If you’d like to see more of these, please let us know and we can pass along your support.
CCHRC website: cchrc.org
CCHRC YouTube channel: @coldclimatehousing
Music: "Motivational inspiring music" by Alexander Blu from orangefreesounds.com/. The song is permitted for non-commercial use under license “Attribution-NonCommercial 4.0 International (CC BY-NC 4.0).
