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Community and Q&A

Questioning Code Insulation Requirements

Masb333 | Posted in Building Code Questions on

We’re finishing our basement and our northern NJ town says the state building code is we have to insulate our basement to R13 even if we’re not conditioning or heating the air down there. The temperature in our basement is always perfect in winter and summer, so it doesn’t make sense to us to add so much insulation. It’s insulated by the earth. We are using R5 1-inch Rigid Board (Formular  NGX 150) against the cinderblock walls and metal studs and thought that should be enough. Why do I have insulate my basement interior walls to R13?

Does the NJ code make sense to insulate so much? If so, I guess we’ll have Fiberglass insulation.

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  1. Expert Member
    NICK KEENAN | | #1

    Unless you have insulation and air sealing between the first floor and basement, yes you are conditioning and heating the air down there.

    It's impossible to tell how much heat a building is losing by how comfortable it feels. The fact that it feels comfortable just means the amount of heat being pumped into it balances what is lost.

  2. Expert Member


    I'm not quite sure what you are asking? Whether we think the levels of insulation required by New Jersey's building code make sense doesn't affect whether you need to do the work or not.

  3. Expert Member
    BILL WICHERS | | #3

    You really don't want to use fiberglass insulation againt a basement wall. Your choices would be either regular batts, which essentially get moldy and fail right away, or the "bagged" batts, which fail later, usually when you least expect it. Rigid foam is really the "right" way to insulate a basement wall.

    As DC mentioned, if you don't insulate the basement ceiling, and air seal it, you're conditioning the basement by way of leaks. It's usually much easier to just insulate the walls and air seal the rim joist area. As far as I know, all recent codes require some level of basement insulation for this purpose. It's required, you have to do it. The only thing that changes is how much you need based on your climate zone.

    If your question is "does this much insulation make sense?", that's a bit more interesting. In many ways, R10 would make the most sense (you start getting into diminishing returns at that level) in the Northern climate zones, but code is usually R15. I have some ideas as to how the code got bumped up to R15, but that's what it is now in most areas. I'm not sure where your R13 comes from, but it might be a local code.

    Regardless of how much we on this site may think you need R value wise, if your building code requires R13, that is to be read as "thou shalt insulate thy basement walls to R13". It's not something worth fighting the building department people over.

    BTW, reclaimed polyiso is a really great option for basement walls. You can save some money that way.


    1. Masb333 | | #4

      Thanks all for your quick responses. Really appreciate it! We are using Rigid Foam (R5) directly against the basement cinderblock walls. Sorry if i didn't state that clearly. Then, we will probably add the batt insulation in the stud cavities to get it to R13. But, I'll definitely check out reclaimed polyiso as an option. Thanks again!

  4. walta100 | | #5

    You are really asking the energy nerds if you need to insulate? There is only one answer here.

    I am sure you understand that fighting city hall is like yelling at the clouds you might feel better but you changed nothing.

    Remember if you use foam in your basement it must be covered with a fire proof layer.

    Also understand that the metal studs conduct heat so well insulation in between is close to useless. A smart inspector should reject that wall plan.


    1. Expert Member
      BILL WICHERS | | #6

      >"I am sure you understand that fighting city hall is like yelling at the clouds you might feel better but you changed nothing."

      My dad, who was an attorney, liked to say "the problem with fighting city hall is that they fight back using your money"


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