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Wall insulation – Are These Options Worth The Cost?

17whippoorwill | Posted in Energy Efficiency and Durability on

Wall insulation – Are These Options Worth The Cost?

Above ground 1,600 SF floor of Modular house in Southern Maine, Climate Zone 6
Wall area is approx. 30% windows
Siding will probably be vinyl

Standard wall cavity insulation is fiberglass batts at R-21, and exterior is ‘standard’ house wrap, but not sure if this value is Center-of-cavity, or Clear-wall

Question: I realize this is limited information, but welcome your thoughts as to whether any of these ‘upgrade’ wall insulation options are worth the cost (“worth” is a relative term, perhaps cost-benefit, or other is more defined)

Zip System R-Sheathing 7/16″                     $4,575
1″ Polystyrene.                                                 $3,850
R-24 Dense Pack in place of R-21 Batt       $2,370


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  1. Jon_R | | #1

    Verify that any choice is compliant with the below moisture related code (even if local code allows otherwise).

    IMO, Zip-R foam is inside the wall, not "over" or "exterior side", which forces a Class I on the interior side.

    1" polystyrene should be FOAMULAR NGX which enables a smart Class II on the interior.

  2. walta100 | | #2

    If you want a real answer and not a guess the only way I know to get one is to build a computer model of you home with your local weather data use interest rate of your mortgage enter your guess at future fuel costs will be and crunch the numbers.

    Your government has created a computer program called BEopt to do just that and is allowing you to use it without any charge. You will need to invest some time entering the data and learning how to use the program. I think it will take most people about 20 to watch the training videos and enter the data.


    1. Tangofm | | #5

      I have been practicing BEopt, with some success, I continue.

      I understand that BEopt is no longer supported by DOE and all that I see at NREL is that they are modeling in Open Studio program.

      Do you have any opinion re which program is better to learn for future use?

      1. walta100 | | #6


        Did you find and watch the training videos on YouTube?

        The program made no sense to me before I watched them.


        1. Tangofm | | #7

          Yes, I agree. the training videos are most helpful.

  3. user-2310254 | | #3

    There are recommended ratios for combining exterior rigid foam with interior air permeable insulation. (See To save on cost while getting thicker foam, buy reclaimed material. It is much cheaper.

  4. Expert Member
    Dana Dorsett | | #4

    IRC 2018 and IRC 2021 code minimums for zone 6 is 2x6/R20 + R5 continuous insulation , so only the 1″ polystyrene (if XPS) option meets the letter of code. The R6 ZIP-R would, but not the R3. The dense pack not at all.

    XPS blown with HFCs is by far the least green insulation in common use today, and at 1" (labeled R5) doesn't provide sufficient dew point control at the sheathing for a zone 6 climate. The IRC calls out R11.25 for 2x6 walls in zone 6 to be able to skip the interior side vapor retarder. But with a polyethylene vapor retarder detailed as an air barrier 1" XPS would be fine from a moisture point of view.

    An alternative to R20 + R5 c.i. is 2x4/R13 + R10 c.i., which is a MUCH safer stackup from a moisture point of view. Chapter 7 of the IRC calls out a minimum of R7.5 exterior insulation for 2x4 framed walls in zone 6, so 2x4/R13 + R10 c.i. meets that with comfortable margins. Going with 2" foil faced polyiso rather than 2" of XPS for the R10 is also an order of magnitude greener from a lifecycle greenhouse gas perspective.

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