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Is Wind Washing Reducing Your Insulation R-Value?

  • 1.  Is Wind Washing Reducing Your Insulation R-Value?

    BPA Staff Member
    Posted 26 days ago
    by Allison Bailes


    What is wind washing?

    Wind washing is the effect of wind causing air to flow through or around air-permeable insulation, thereby reducing its R-value.  One of the most common ways this occurs is when you want outdoor air to come into an attic or a cathedral ceiling for venting. If the pathways for airflow aren't defined precisely, wind washing is often the result.

    The lead photo above and the one just below show fiberglass batt insulation in cathedral ceilings.  Both have baffles installed above the insulation, where the air is supposed to go. But guess what. Air is the honey badger of building science. It don't care what you want it to do. It's gonna do what it wants.

    See those arrows below?  The red arrow is where the architect and the builder told the air to go. And the air says, yeah, I'll take that path. But I'll also go down below the insulation, in the gap where that blue arrow is. In the photo above, the insulation has a huge gap beneath, allowing for easy movement of a lot of air. And because fiberglass insulation is mostly air anyway, it's going right through the middle, too, where the yellow arrow points.

    How well do you think that insulation works with all that air passing beneath it and through it?

    Airflow pathways in a vented cathedral ceiling assembly

    Wind washing and blown insulation

    Perhaps the most well-known example of wind washing is when you have blown insulation in a vented attic. If the pathways for airflow are defined poorly, as above, the wind blowing into the attic can blow that insulation around. The photo below shows one example. But it can be far worse than that, too.  Sometimes you'll see large bare spots near the eaves as all or most of the insulation gets blown inward.

    Wind washing of blown insulation in an attic can leave some areas completely uninsulated [Photo courtesy of Reuben Saltzman of StructureTech]


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    Diane Chojnowski
    BPA Connections Community Manager
    Building Performance Association
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