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Can I leave a layer of tyvek over the 1st course of 1 1/2 foam when adding another course?

kenorakq | Posted in General Questions on

I got a surprise when i stripped the vinyl siding off my walls..I expected XERO exterior foam but found 1 1/2″  covered by a layer of tyvek.

Q- Is there any harm in leaving that layer there and adding another when I complete the addition of a second course of 1 1/2″ foam (total of 3″). 
I will be using 3/4″ furring strips at each stud and furring around the windows…

a follow-up question will deal with how to detail the foam around the windows with 6″ wide furring at the edge of the Dudley box  (furring to permit 4″ wide exterior window trim and room for J channel).

I’m working now and would really appreciate a GO/NO GO on leaving the orig tyvek soon…


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  1. Expert Member
    Michael Maines | | #1

    Tim, as long as you take care with your water-management details, there is no harm in leaving the existing Tyvek in place.

    1. kenorakq | | #2


  2. MattJF | | #3

    Curious what your current wall insulation is and if it is really worth the effort and expense to add more foam. What zone are you in?

    Wall 1 2x4 R11 wall with R6 foam: Total Wall R = 18.4
    Wall 2 2x4 R11 wall with R12 foam: Total Wall R= 24.5

    For my location with 6700HDD, $15/mmbtu, 2500SF surface areas (roughly 2100SF house) that works out to:

    Wall 1 annual heating cost: $326.50
    Wall 2 annual heating cost: $246.10

    Now that difference will vary depending on your location, heating costs, and actual wall.

    Total wall R values

    Heating cost = Area * HDD * (1/total R value) * 24/1,000,000 * $mmbtu

    Heat Pump = 1,000,000/(HSPF*1000)*$kwh
    Gas Furnace = 1,000,000/(Efficiency*1000)*$therm

  3. kenorakq | | #4

    Kenora NW Ontario Canada

    Zone 7

    Electrical heat ay 26c kw delivered

    I've got the foam... at $3 a 2 x 8 sheet of 1 1/2 xps and same price for a 4 x 4 sheet of eps.... so foams not an issue.

    It gets cold up here for a long time.. I'm also adding another 1000 sq/ft so every penny I can save heating is important. The addition will have 6 inches of foam under the slab and foam wings surrounding..

  4. MattJF | | #5

    So you are at 13,000 HDD, higher electricity costs, and dirt cheap foam.

    I didn't run any new numbers, but it likely makes sense to forge ahead if you have the time.

    I am happy to run the numbers if you share your wall assembly details and heating source. Is that reclaimed or seconds that you get for that price? That's $2.26 USD if you are citing CAD.

  5. kenorakq | | #6

    Got a crazy deal on 300 sheets of NEW XPS and about 200 sheets of reclaimed EPS... $1200 CDN for all of it....was such a deal I bought an enclosed trailer to move it and portable garages (two of them) to store it... trying to use it wisely

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