GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Thicker walls

ScottNeels | Posted in General Questions on

I live in Minnesota. I’m remodeling an older home with full 2×4 studs. On the outside there is a layer of 1 inch buffalo board and 1 inch foil covered foam.
Is it worth the time and money to add wood strips to the 2×4’s to make them 2×6’s on the inside and go from a R-15 to R-19 or 21 insulation or add 2 inch certifoam to the outside or should I just leave it alone with 2×4 walls and R-15?

Thank you

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    There is no simple answer to your question.

    If you are asking whether you will get a quick payback in energy savings for your investment in insulation improvements, the answer is almost undoubtedly no -- but even that answer depends on the cost of your materials and labor. If the labor is free, you have to decide how much your time is worth to you.

    I'm not sure what "buffalo board" is, but I'm guessing that it is asphalt-impregnated fiberboard sheathing.

    The fact that your house has 1 inch of foil-faced rigid foam on the exterior is unfortunate. (I'm guessing that you are talking about foil-faced polyiso.) In Minnesota, exterior rigid foam needs to be either very vapor-permeable (yours isn't) or thicker.

    Minnesota has two climate zones: Zone 6 and Zone 7. If you live in Zone 6, the minimum R-value for the exterior rigid foam installed on a 2x4 wall is R-7.5. In Zone 7, it's R-10. (For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

    Right now, the R-value of your fiberboard sheathing plus polyiso is probably about R-7 -- or less in cold weather, since polyiso doesn't perform well when it is cold. That's almost enough in Zone 6, but not enough in Zone 7.

    Are you planning to open up your walls from the inside? If so, what type of insulation do you now have between your studs?

    Are you planning to remove and replace your siding?

  2. ScottNeels | | #2

    [Editor's note: Scott Neels accidentally posted his response on the wrong page. Here it is:]

    The interior is gutted to the studs. Was going to replace old insulation with R-15. Will be residing in the future. Vinyl is brittle and we are replacing all windows. Doing all the work myself so it's just what I think I'm worth.
    Thank you.
    --Scott Neels

  3. GBA Editor
    Martin Holladay | | #3

    If you want to replace the insulation between the studs, the best insulation to use is dense-packed cellulose. For more information, see How to Install Cellulose Insulation.

    When it's time to replace your siding, you'll have a golden opportunity to thicken up your exterior rigid foam layer so that it's thick enough to keep your sheathing above the dew point during the winter. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |