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Community and Q&A

Cold climate basement insulation and code questions

navigator16171 | Posted in Building Code Questions on

I live in SE Wisconsin (Climate Zone 6) in a 1930s house that has a currently unfinished cinder block basement with a concrete slab floor. The basement is unconditioned but stays at or above 55 degrees in the dead of winter. Basement walls are 95% below grade, and there are some small windows at the top of the walls. The house is heated with hot water radiant heat but there are no radiators or other heat / HVAC sources in basement.

I would like to add a rec room in part of the basement with finished walls and a floor and ceiling, as well as a workshop. I do not plan on adding heating or cooling, other than using an electric fireplace on occasion as a heat source in the living room.

I’m planning on using 2″ XPS (R10) foam board on the perimeter walls and butting 2×4 wall studs up to the insulation. I’ll either drywall over that or use some other type of finish wall paneling. I have read that Code requirements for basement wall insulation in climate zone 6 are R13, but does that apply to this situation? Is this even a finished basement if there is not a heat source and it is not otherwise conditioned? I am not sure I meet other Code requirements for finished basements either, as the max. ceiling height is about 6’6″ or slightly less after I add a floor and ceiling. Is there a risk of condensation issues if I am only using 2″ of R10?

Other question is whether I should insulate the entire perimeter of the basement or just the perimeter walls adjacent to the rec room area. Due to some gas and waste pipes running parallel to a wall about 1.5″ inches from the wall (that is not adjacent to the rec area), I cannot get 2″ of insulation and framed walls there. Any recommendations for how to handle that without rerouting pipes? Can I use foam faced polyiso and leave it exposed? Or does use of that risk condensation issues?

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  1. Expert Member
    Dana Dorsett | | #1

    XPS is one of the least-green options here, due to the high environmental impact of the HFC blowing agents, with a lifecycle global warming potential more than 1000x CO2. As the HFCs diffuse out over a few decades it's performance drops to that of EPS of similar density, about R4.2/inch.

    R10 on it's own would be way-skimpy but fine if coupled with a fiber insulated insulated studwall (with NO INTERIOR VAPOR BARRIER!!!). IRC 2015 code minimum for zone 6 is R15 continuous insulation, or R19 if thermally bridged by studs:

    If it's an insulated 2x4 studwall, you'll need at least R7.5 foam for dew point control at the foam/fiber boundary at the above-grade portion of the wall. Going with 2" of EPS (R7.8-R8.4) or 1.5" of foil faced polyisocyanurate (R9-R10) would be greener than XPS, since they are blown with hydrocarbons, typically pentane (about 7x CO2), most of which leaves the foam at the factory, and is recaptured (to meet local air pollution requirement), and often burned for process heat.

    With R8 continuous and even R11 in the cavities it would exceed code-minimum requirements, but high density fiberglass R15s or R15 rock wool would be preferred.

    If you have a local source for reclaimed roofing foam (any type), it's dramatically cheaper than virgin stock foam, usually < 1/3 the material cost, often

    Going with 3" roofing polyiso (~R17-ish) held to the wall with 1x4 furring through-screwed to the foundation with masonry screws, mounting the wallboard on the furring can be cheaper than a 2" foam + fiber insulated studwall solution, and takes up less interior space to boot. With polyiso the cut edge needs to be kept off the slab, or above the high water mark if the basment has a flooding history, as it can wick up moisture and take forever to dry. Finishing out the lower part with EPS of the same thickness works.

  2. navigator16171 | | #2

    Thank you for the advice. At what point does a basement become "finished" and trigger these code requirements?

  3. Expert Member
    Dana Dorsett | | #3

    There is no such trigger. The IRC doesn't distinguish between finished vs unfinished basements- the requirements are the same in either case.

    If it's going to be an un-CONDITIONED basement, there needs to be R30 between the floor joists on the first floor. See the "FLOOR R-VALUE" column in TABLE N1102.1.2:

    Wisconsin code is currently at IRC 2009:

    But IRC 2009 also calls out R15 continuous insulation for conditioned basements, or R30 under the floor between joists when over unconditioned space:

    Insulate the entire basement wall with R15 continuous foam or R8 + 2x4/R13 and you won't be sorry. Even if it's 55F in the dead of winter it's a HUGE heat loss through that R1- 1.5-ish foundation wall. Insulate the walls to IRC 2009 code-min and it'll be ~65F or so year-round, and it will cut your heating bills by a double-digit percentage, even if you're not actively heating the space.

  4. navigator16171 | | #4

    Thanks for all the helpful information. At what point do I become subject to these code requirements? Clearly, I am not up to code now with the original basement setup being completely uninsulated. Do I become subject to these requirements as soon as I put up walls or a finished floor? Wouldn't I never meet the minimum ceiling height requirements, or do municipalities usually waive this for retrofits? I guess I'm not too worried about energy loss since there are no heat sources in the basement, and I'd be comfortable with the ambient temperatures that would come from insulated walls. Or are you saying that it will actually reduce the cost of heating the main living floors? With heat rising, is there much heat loss from the main floors into the basement?

    Is there anyway I could leave the basement unconditioned and insulate the walls rather than the ceiling, relying on the insulation to keep the basement warm enough to be comfortable?

  5. Bob Irving | | #5

    You're losing a lot of heat through those basement walls and through the floor, and the heat to repace that has to come from your boiler. Basements are in fact, within the same environment as the rest of your living space, although many builders and homeowners try to deny this. I've been in hundreds of homes and have never seen one exterior door leading directly to a basement; instead they're all standard interior doors - uninsulated and unweather sealed. So forget the idea that the basement isn't part of your envelope. It would be interesting to compare the SF of your exteior R-19 (?) walls to the Sf of your R-1 walls for a visual comparison. (a single glazed window is also R-1 - same as 8" of concrete.) So when you insulate it to the same as the rest of the house, your boiler will have a lot less work to do. With far less cold wall area to deal with, your house will be far more comfortable.

  6. navigator16171 | | #6

    Thank you. Aside from code compliance, I'd just like to warm the space up somewhat and also gain some of the moisture intrusion protection that rigid foam insulation provides. And I want to do everything possible to make it a mold and mildew resistant setup. I'm still just curious at which point the code requirements become applicable going from an original 1930s setup to having some sort of finished walls. Is there any way to comply with code in an unconditioned basement without insulating the ceiling?

  7. GBA Editor
    Martin Holladay | | #7

    If you decide to remodel your basement, you should contact your local code authority. Only your local code authority can tell you what local code applies.

    In many areas, you can improve the insulation in an older home without the need to bring the insulation up to code requirements. But local codes vary, so you should check.

  8. navigator16171 | | #8

    Martin, if I would do the R8 + 2x4/R13 technique you recommend, what type of insulation would you use between the studs? I thought fiberglass was to be avoided in basements? I'd like to stick to an all-foam setup but don't want to lose much floorspace either by doing R15 continuous.

  9. GBA Editor
    Martin Holladay | | #9

    The all-rigid-foam approach is a great way to go. But if you don't want to do it, don't do it. It's your choice.

    If you are going to install 2x4 studs for wiring, you can combine continuous rigid foam with some type of batt between the studs -- either mineral wool or fiberglass. Or, if you are pressed for space, you can install 1x3 horizontal strapping over the rigid foam, and install cable between the strapping (with shallow electrical boxes).

  10. Expert Member
    Michael Maines | | #10

    Navigator, the building code kicks in when you are converting a utility space to a habitable space. The IRC building code defines habitable space as "a space in a building used for living, sleeping, eating or cooking. Bathrooms, toilet rooms, closets, halls, storage or utility spaces and similar areas are not considered habitable spaces." (Many of them have their own specific requirements, though.)

    Unless you live in a place not subject to the building code, a rec room would be considered habitable space. A workshop really should be as well, though depending on how you use it, you may argue that it's a utility space.

    There are some exceptions in the existing building code ( but they are finicky to parse out without the help of a building inspector or design professional. By the regular IRC building code, basements used as habitable space must have a clear ceiling height of 7', but beams can project down to 6'-4" (R305: If you can't make those headroom requirements, you can't legally use that as habitable space.

    Other requirements for habitable spaces include at least one means of egress that leads directly to the outdoors, via a code-approved stair if not a walkout basement or properly sized window well.

    There are good reasons for these laws (building codes, where they exist, are law), largely relating to fire safety--neither occupants or firefighters should be put in danger of getting trapped or knocked unconscious in the event emergency escape is necessary.

    There are also code minimum window areas (R303) but there are workarounds in many cases.

    I'll reiterate what others have mentioned--for the sake of the environment, please consider using a type of foam other than XPS, and you are losing a lot of heat through your basement walls, whether or not you feel like you are.

  11. navigator16171 | | #11

    For the part of the basement that will not have finished walls but that has pipes running adjacent to the walls, can I use aluminum faced polyiso? Are there any condensation risks with that? I thought that having faced insulation in a basement was generally inadvisable.

  12. T Carlson | | #12

    Navigator, a lot of ill advised code advice given to you, would be helpful if our fine state of Wisconsin used the IRC. It DOES NOT. We use the Uniform Dwelling Code (UDC), one of a very few group of states that went rogue.

    Call the guy that will issue your permit, he/she does not bite. If you want to read the rules it is WI SPS chapters 320-325, standards are 321.

    You cant use regular foil faced polyiso unless it is listed as a thermal barrier, needs to meet the 250F rating, if you used reg foamboard you would have to cover it with drywall or some other thermal barrier.

    Im a licensed contractor in WI, code officials are not going to make you raise your house to meet height requirements. You wont have to make your stairs compliant if they arent, they would like to see you upgrade your wall insulation like you want to do and have an electrical permit. You may need to provide heat to the room, depends on the inspector. If you make structural changes they look at that, correct smoke detector, handrails, ect. Think safety and structure and common sense.

  13. Expert Member
    Michael Maines | | #13

    These sections from the WI SPS looks a whole lot like the IRC:

    "SPS 321.06 Ceiling height. All habitable rooms, kitchens, hallways, bathrooms and corridors shall have a ceiling height of at least 7 feet, except as follows:
    (a) Rooms may have ceiling heights of less than 7 feet provided at least 50% of the room's floor area has a ceiling height of at least 7 feet. Any area with a ceiling height of less than 5 feet may be ignored in this calculation.
    (b) The 50% limit in par. (a) does not apply to subs. (3) to (6).
    (2) Beams and girders or other projections may project to no more than 8 inches below the required ceiling height."


    "(5) Exits from basements and ground floors.
    (a) General. Except as provided in par. (b), all basements and ground floors shall be provided with at least one exit of the following types:
    1. A door to the exterior of the dwelling.
    2. A stairway or ramp that leads to the floor above."

    Inspectors everywhere may or may not choose to look the other way, but if you are converting a utility space into habitable space, the honest thing to do--for owners, inspectors and licensed contractors--is, at bare minimum, meet the building code. The alternative? Don't convert non-conforming space into habitable space.

    Dow's Thermax is the only foam board tested to be an equivalent thermal barrier to 1/2" drywall. Most jurisdictions allow it to remain exposed. It's available with white facing for a brighter look.

  14. navigator16171 | | #14

    The Wisconsin SPS also appears to require R10 insulation on an unheated slab? What is the thinnest way to achieve that?

    There must be some variances granted locally because most nearby homes have similar basement heights of less than 7' but many have recently finished basements.

    Unfortunately the inspectors happen to be very aggressive where I live so I would like to be as well informed as possible before getting them involved.

  15. T Carlson | | #15

    @Michael, this is existing. You are trying to act smart quoting new dwelling requirements or alterations to a structure (presumably) built after code went into effect. For an existing structure that was probably built prior to the code and therefore exempt unless municipality adopts it, which they usually do, but they apply the code within reason. You dont have to rip out your staircase if you have a 8 1/4 rise with a 8" tread.

    If it isnt a major hinderence they want as close to full compliance as reasonably possible, if it isnt reasonable they arent going to say you cant go in your basement, and believe me they want those permits so the assessor can stop by.

    Again, Navigator, you will not have to add R10 to your uninsulated floor. This site is great for ideas and some practical knowledge, but the code stuff has no place here, Im just trying to correct the misinformed.

  16. GBA Editor
    Martin Holladay | | #16

    As I advised you in Comment #7, talk to your local code official. That's where you will get the information you need -- not here.

    Q. "Can I use aluminum faced polyiso?"

    A. Yes.

    Q. "Are there any condensation risks with that?"

    A. The condensation risk, if any, comes from choosing rigid foam with an R-value that is too low. The condensation risk has nothing to do with the presence or absence of foil facing.

    Q. "I thought that having faced insulation in a basement was generally inadvisable."

    A. You are wrong. For more information, see How to Insulate a Basement Wall.

  17. navigator16171 | | #17

    Martin, what is the minimum thickness polyiso I would need in climate zone 6 to avoid condensation issues? Is there facing on both sides? If so, is it possible to use adhesive to attach it to the wall?

  18. Expert Member
    Michael Maines | | #18

    T Carlson, I know how the game is played--I've designed and/or built hundreds of renovations. I agree with Martin that Navigator should talk with his code official. I simply quoted the code sections that you cited.

  19. GBA Editor
    Martin Holladay | | #19

    All of your most recent questions about insulating basement walls are answered in the article I linked to in my last response.

    I urge you, once again, to read the article. Here, once again, is the link: How to Insulate a Basement Wall.

  20. T Carlson | | #20

    Michael, I didnt cite any sections, only told the guy how to find his state code, maybe read more carefully.
    WI UDC applies to structures built after June 1, 1980. Built before and you are exempt from the building code unless your municipality has adopted it. You still would have to follow electric and plumbing codes. You cannot have uncovered foam in your basement if it is getting inspected unless it is a product like thermax. You could have standard off the shelf polyiso or eps or xps and cover it with approved barrier. A studwall in front of the foamboard with fiberglass insulation and netting holding the insulation is one method if you are not finishing right away.

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