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Getting Ready for “Winter” Building Energy Inspections

John Snell in Winter Mode

As winter makes itself known—undeniably so here in Vermont, with hard frosts, flurries of “hard water” and a couple feet of snow in the mountains—it is useful to review the procedures and techniques for using our imagers to find problems, especially those in buildings.

Industry standards, including the new RESNET Guideline, call for a temperature difference of at least 10oC (18oF) between the interior and exterior walls’ surfaces. Most of the time, this is sufficient to produce good results. With experience, work can be done with a lesser difference but be cautious so you don’t get into trouble—the sun, wind and even differences in nighttime radiational cooling can all result in confusing patterns on the exterior and even on the interior of a wall.

Working on the interior side is usually much more productive because the influence of the sun and wind are minimized, and the conductive path is more direct. Generally you’ll want to keep the span as narrow as possible in order to have sufficient thermal contrast. Use the framing as a guide—if you can see it, you probably will see issues related to the insulation.

Ideally we want to locate any areas with missing insulation. Exactly where these are might depend on the type of insulation used. Voids in blown-in cellulose, for example, tend to be at the top of the wall, while problems in spray foam or fiberglass can be anywhere.

It is essential to also check the performance of the insulation by testing it in conjunction with a blower door. Only a few degrees difference in temperature from inside to outside is needed to detect active air leakage. The blower door quickly subjects the insulation as well as the entire conditioned space to a strong pressure difference (negative is recommended). Any air that can flow will flow—either through cracks/holes or directly through the insulation. This cools the building surfaces and quickly creates thermal patterns that are remarkably diagnostic. Fiberglass batts, in particular, can exhibit a degradation of performance of up to 50%. Spray foam, which often appears to be perfectly monolithic, will quickly reveal any small holes, all of which can result in significant problems. And don’t forget that it is also very common to find air leakage in interior partition walls!

This wall, insulated with fiberglass, is showing classic signs of cold air moving through the insulation otherwise know as “air wash.” This occurs when the building is subjected to a negative pressure.

It is a good idea, once the blower door is set up and running, to work quickly to record any and all changes related to abnormal air flow. Depending on where the air is moving from and how it is moving, the process of finding and understanding all the signatures can easily take 30-60 minutes in an average home. Eventually, however, you will cool the building to the point where the temperature differences are minimized and it is time to wrap up your inspection.

If detection of moisture-related problems is a concern—and these often show up in the higher humidity transition to winter—you’ll need to work more slowly, with a moisture meter in hand, to understand this often complex situation. At its simplest, areas of wet insulation will (from the inside) be starkly colder than almost anything else you will see. But show up under less favorable conditions and the signature may be completely different, or in the worst case, non-existent.

This is a great time to get out and look for energy-related issues in buildings! In warmer areas, the early morning is generally a good time, and in colder areas, as is my experience in Vermont, we can work pretty much any time of day or night for the next five months!

To learn more about home inspections, tune into the upcoming webinar, sponsored by Fluke that I’ll be presenting for the Wisconsin Energy Center on Tuesday, November 9th from 1:00-2:30 CST. See for details. Please note that a Fluke TiR imaging system (a great unit to use for building inspections!) will be given away to a very lucky participant! You can also have as many people as you want listening in for one registration fee so plan to have the whole crew on hand if you can.

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

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