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We are looking at getting new siding here in zone 5 Iowa. Our house was built in 1979 and has 2x4 walls. I have seen that if we add rigid foam insulation we need to use at a minimum of R-5. Do we need to also use something like Tyvek under or over the foam boards?

Thanks!

2 Answers 2

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You already almost have an air barrier. You could simply tape the joints with appropriate tape.

From the Owens-Corning product page for their tape for their foam (no endorsement implied, just as an example of the product class existing for the application):

HomeSealR™ Foam Joint Tape is an effective product to tape the joints of FOAMULAR® extruded polystyrene (XPS) board in residential vertical wall applications when it is used as continuous insulation over steel or wood wall framing. HomeSealR™ Foam Joint Tape provides reliable adhesion to FOAMULAR® XPS insulation. The durable backing is coated with an aggressive acrylic adhesive to offer excellent adhesion coupled with strength and is easy to handle.

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Generally yes you would put the house wrap directly over the sheathing, which means under the exterior foam insulation. The goal of the tyvek is to act as an air barrier between the outside and inside air.

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  • Agreed, except that it might be easier (and cheaper) to tape the foam joints as noted in Ecnerwal's answer. If not taping, then Tyvek would be advised
    – FreeMan
    May 12 at 18:30
  • @FreeMan my concern with the taping foam approach is that the OP hasn’t made it clear what kind of foam it is. I’d it’s low perm, and acts as a vapour barrier as well, it can trap moisture in the sheathing and wall. The tyvek is vapour permeable from the inside out and would allow the sheathing to dry. May 12 at 18:53

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