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Two layers of R4.2 on attic ductwork

user-3101858 | Posted in Mechanicals on

Hello all,

A client just had her ducts insulated. Unfortunately, the HVAC company installed R4.2 instead of R8 and to ‘ease’ the repair just wrapped another layer of R4.2 over the existing layer. Now I’m concerned about condensation or vapor lock here in Chicago (hot summers and cold winters). Should I/we be concerned and should it be fixed or is the risk relatively low? The ducts are rigid and mastic-ed at every junction. Thanks…Ross

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  1. Expert Member
    Dana Dorsett | | #1

    If air can't convect into the insulation, you can't have accumulating condensation between the insulation layers or at the duct surface.

    Outdoor dew points in summer in Chicago average in the mid-60s Fahrenheit. With a 50F duct surface and a 100F attic, the temp in the middle where the two layers meet will split the difference, at about 75F, which is well above the average dew point. In an 80F attic with 50F ducts you might approach condensing at the layer boundary, but the AC probably won't be running significant duty cycles under those conditions.

    If the ducts & insulation are completely inside the pressure boundary of the house and not exposed to exterior air, the risk of condensation is going to be zero anyway, since the dew point of the air inside the conditioned space will be below the surface temperature of the ducts, since the indoor air is being dried out by the even colder air conditioning coils.

    Bottom line- as long as the ducts and the insulation of the inner layer of insulation is well sealed, air leakage through the second layer is not a high risk proposition.

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