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Tyvek and / or fanfold insulation under siding

Erich Riesenberg | Posted in General Questions on

I have a 1950s home and am adding new vinyl siding.

There is an option for Tyvek and / or fanfold insulation under the siding. My understanding is water will get through the siding so the main goal is to prevent water from reaching the sheathing. Tyvek seems to accomplish this. Is fanfold better or would both be better?

I have heavily insulated on the inside. I think the main potential problem with fanfold is to prevent possible moisture on the sheathing or between the studs. This is Iowa, hot humid summers with cold snowy winters.

From the inside of the house to the outside, there is:
1) 1/2 inch plywood
2) 2.5 inches mineral wool
3) 4 inches expanded polystyrene
4) drywall or plaster on the ground floor
5) either loose fill insulation (ground floor) or 3 inches of expanded polystyrene (2nd floor) between studs
6) plywood sheathing
7) wood shingles

The wood shingles are being left in place.

Suggestions on Tyvek and / or fanfold and in which order if both would be appreciated. Thank you.

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  1. Erich Riesenberg | | #1

    Fanfold would be 1/4 inch.

  2. GBA Editor
    Martin Holladay | | #2

    Every wall needs a water-resistive barrier (WRB), and Tyvek is a good choice. It's easier to integrate with flashings than fanfold.

  3. Nick Welch | | #3

    Another problem with fanfold is that it's XPS which is blown with high global warming potential gases.

  4. Erich Riesenberg | | #4

    Thank you Martin and Nick.

    It doesn't sound like there would be a benefit to both, and Tyvek seems more durable (does not shrink) and friendly. Thank you.

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