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Value of external insulation defeated by air?

dpilot83 | Posted in General Questions on

As I understand it, the value of any insulation is largely negated by air leaks. If the insulation is air permeable and the air barrier of the house is compromised, cold or hot air flowing through the insulation into the house will go a long ways towards bypassing the insulation.

This makes me question the value of external insulation. External insulation is always outside the air barrier so there will be outside air flowing through the external insulation to some degree. Does this somewhat negate the value of the external insulation?

In the example that will affect me personally I’m planning a house with 2×6 construction and ZIP sheathing with 3” of Rockwool Comfortboard 80 on the outside and Rockwool batts in the inside.


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  1. Expert Member


    The effect of air movement on insulation is called wind-washing. It doesn't totally defeat it, but can reduce its R-value. The amount that it loses in R-value is mainly dependent on how dense the insulation is - and luckily that means Comfortboard loses very little. You can see the effects in tables 8 & 9 of this link:

  2. MartinHolladay | | #2

    D. Pilot,
    The most important factor is this one: If you are building a new house, you can employ measures to reduce air leakage as much as possible.

    Here's a link to an article that discusses the effects of windwashing in houses with a continuous layer of exterior mineral wool insulation: "Windwashing in Exterior Mineral Wool."

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