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Add External Continuous Insulation

cletus_cassidy | Posted in General Questions on

I have a one-year old home in Climate Zone 5A with “flash and batt” insulation in the exterior walls. My framing is 2×6, specifically (from outside to inside): vinyl siding, vapor barrier (Kimberly Clark), 2 inches of closed cell foam, R-11 fiberglass batts. I’m not a builder so it’s possible I missed something on the outside.

I’d like to add exterior continuous insulation in the future to increase the R-value of my exterior walls and reduce the thermal bridging through the studs, but also keep my existing insulation inside. My goal would be to add as much R-Value as possible (e.g. 1.5-2 inches of foam board, Zip-R, etc.). At the same time, I would likely look to upgrade at least some of my double-paned windows to triple pane.

What are my options with the exterior insulation? What do I need to consider to avoid condensation issues etc. from my existing construction? Thanks in advance for your guidance!

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  1. Expert Member
    Michael Maines | | #1

    Randy Williams recently posted a good two-part series that should answer most of your questions:

  2. cletus_cassidy | | #2

    Thanks. Will review.

  3. Expert Member
    Joshua Salinger | | #3

    You will want to use a vapor permeable insulation material. You don't want to trap moisture between two Class 1/low Class 2 vapor retarders. I would go with a wood fiber, mineral wool or cork.

    1. cletus_cassidy | | #4

      Many thanks for this. Is there no feasible way to use a foam on the outside then? I was hoping for some added air sealing benefits.

      1. Expert Member
        Michael Maines | | #11

        EPS and GPS are somewhat vapor-open so they are the safest of the foams, but not as vapor-open as the others that Josh mentioned. I recommend using a separate material for your air control layer.

  4. walta100 | | #5

    I can’t see any way making the “upgrades” you have suggested will ever save enough energy to recover the very high cost to make them.

    The way I see it the current wall is R25 well beyond the 20 most codes would require and very close to the breakeven point for add insulation at today’s fuel prices. Simply put there is no way upgrading from a R24 wall to R38 can ever recover the very high cost of the siding removal disposal and replacement at any fuel price.

    Replacing existing perfectly fine double windows in zone 5 for fuel saving is total impossible. In fact, the price difference between 2 and 3 pain new windows has no ROI in zone 5.

    Ok so it is not about money as there is no return on this investment.

    If you want the do this to be “green” consider how much stuff your upgrades will send to the land fill the siding and windows are all but new and should be usefull for the next 40 years. How much more stuff are you causing to be mined, manufactured and transported to save a pittance in fuel. If you want to be green avoid all foam products it is all nasty stuff.

    If you are unhappy with the above code home you have and what you real want is a full-on crazy passive house this upgrade will get you closer but leave you unsatisfied and planning more upgrades. I say sell this house and build what you want and be happy.

    I doubt exterior foam will make your house more air tight than the interior foam you already have.

    Be sure to avoid making a foam sandwich anything trapped between 2 layers of foam is likely to get wet and rot.


    1. Expert Member
      DCcontrarian | | #7

      Yeah, this is a 1-year-old house. I'd sell it and build the house I really wanted if I were the OP.

  5. mjhil | | #6

    That isn't a stellar assembly, but it's quite good and, for reference, I'm considering adding 2 inches of interior insulation on my poorly insulated 1970s 2x4 walled house to get roughly where your assembly is already (save for thermal bridging of studs). Here is a great article on the realities of more insulation:
    Chasing down air leaks from ceiling boxes, rim joist, bottom plates, and bringing attic insulation to R-60 might be a good avenue of approach.

  6. cletus_cassidy | | #8

    Thanks for all the feedback. This was a custom built house, so I'm definitely not selling any time soon. Just looking to make some upgrades where possible. The primary goal would be to add more exterior insulation to the outside. If it's possible, I'd love to keep the the current windows. and siding I already have but I'm no contractor and perhaps the siding is "single use" / not able to be taken down and then put back up?

    1. Expert Member
      DCcontrarian | | #9

      Most siding is single use. Small sections can be taken off for repairs, but it's very labor-intensive.
      What kind do you have?

    2. Expert Member
      Michael Maines | | #12

      Vinyl clapb0ard-type siding is easy to remove and reinstall, even easier with a tool like this one:

      1. Expert Member
        MALCOLM TAYLOR | | #15


        The only complication I can think of is that what with an extra 2" of exterior insulation the siding will no longer cover the wall at outside corners. Maybe they sell wider vinyl corner trim?

        1. Expert Member
          Michael Maines | | #16

          They do sell 4" and 6" corner trim, at least in some profiles.

  7. walta100 | | #10

    Why do you want more exterior insulation?

    Windows are the single largest line item in almost every construction budget.
    Assuming it is a good-sized house all new triple pane windows insulation and siding would likely be 100k installed to save $40 a year in fuel. That is crazy thinking.

    The extra labor to reuse the vinyl siding would likely exceed the cost of new materials.

    This ship has sailed you made your choice it is time to stop rethinking and get on with your life. You made reasonably good choices the first time.


    1. Expert Member
      Michael Maines | | #13

      Cletus' current walls perform at about R-20.4, assuming a 25% framing fraction. Adding a modest R-9 continuous insulation to the exterior increases the whole-wall R-value by 44%, to R-29.4.

      Walls are typically responsible for 15-25% of a home's heat loss. I wouldn't assume the windows are triple-glazed in CZ5; they might be but they are still pretty rare from what I've seen.

      If annual costs for heating and cooling are $2500, adding exterior insulation to the walls will cut that by $165 to $275/year. For it to only save $40/yr, annual heating/cooling costs would need to be around $360-600, which is possible but unlikely.

      Even at $165 to $275/yr savings, it would take a long time to pay for upgrading the whole house at once. But vinyl siding is easily removed and reinstalled so it's something an interested DIYer could do one wall at a time on the weekend, if they so desire.

      It takes a minute or two to remove a piece of vinyl that covers 6 sq.ft.; let's say 2 minutes at $80/hr--that's $0.44/sf. New vinyl is at least $1.00/sf.

  8. walta100 | | #14

    Quote from the original question.

    “ At the same time, I would likely look to upgrade at least some of my double-paned windows to triple pane.”

    Reusing the vinyl could be done but getting a contractor interested in bidding a jigsaw puzzle where all the pieces are 4 inches shorter than they need to be and each wall has faded to a different color so you can’t swap anything seem unlikely at best.


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