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This article describes How to Recognize & Diagnose Air Bypass Leaks Marks on Insulation and discusses other interior wall and ceiling stains and explains how to recognize thermal tracking, thermal bridging stains, building air leaks, and building insulation defects. Often these stains are mistaken for toxic indoor mold. When investigating a building for a mold problem, you can save mold test costs by learning how to recognize Stuff that is Not Mold or is only Harmless Mold but may be mistaken for more serious contamination - save your money.

Our page top photograph shows black air-bypass leaks on building insulation in a wall. Readers should see HEAT LOSS in BUILDINGS. Also see AIR SEALING STRATEGIES and AIR LEAK DETECTION TOOLS as well as AIR LEAK MINIMIZATION. For photos of snow melt on rooftops that also show points of building heat transfer and loss, see STAIN DIAGNOSIS on Building Exteriors.

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How to Recognize Air Bypass Leaks and Thermal Bridging Marks on Fiberglass Insulation

We have frequently observed evidence of air bypass leaks and thermal tracking as dust deposition on attic insulation, a condition some owners have mistaken for mold (and wasted money on mold testing and remediation). In this photo we've removed a "nearly new" insulation batt that was installed in a cantilevered floor which was leaking air up through the overhanging floor and into the building wall cavity. In just a month of exposure the insulation is already showing dust tracking at its lower right corner, marking the air leak path in this building. On or in building insulation, particularly fiberglass insulation, we see thermal tracking as grayish deposits (actually house dust) left where air is leaking from the conditioned space into the unconditioned space (say from a top floor bedroom into an attic floor and attic above). The warm air passing by the insulation leaves house dust which sticks by moisture condensing out of the warm air and onto the fibers, or the debris may adhere simply by static to the insulation fibers. On building insulation, debris stains left behind by thermal tracking will show us where we have air bypass leaks in a building, such as around ceiling light fixtures, electrical box openings, or where the insulation blanket was not uniformly and thoroughly installed.

We welcome more thermal tracking, soot tracking, air bypass leaks, and similar photos of indoor stains as well as text suggestions to expand this detail and would be glad to credit contributors.

Usually soot marks, thermal bridging, or thermal tracking stains appear, if at all, in the building interior locations discussed in the remaining sections of this article.

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• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.
• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.

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