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Rigid Foam Basement Insulation in Unfinished Basement – Best Practices

josher668 | Posted in Energy Efficiency and Durability on

Hi Everyone,

I read the article “How to Insulate a Basement Wall” and “The High Cost of Deep-Energy Retrofits” and just wanted to run my plan across GBA members to make sure I’m not missing anything.

I am in climate Zone 5A and have 3 rooms in my basement.  There are two unfinished rooms in the basement with bare concrete walls –  a utility room with the furnace and water heater and a large storage room with only 68″ of headroom, but you can stand better between the joists.  There is also a finished room in the basement.  All 3 rooms are separated by doors.

For my two unfinished rooms, I am planning on:

1) Install 2″ XPS on the concrete with insulation glue and tape the seams and then 2″ Dupont/Dow Thermax polysio with staggered seams on top of that.  This should give R-23 and the XPS should do better with my 5A climate zone according to your articles.

2) I’ll keep the insulation 1/2″ off the concrete floor and I’ll fill the gap with Great Stuff foam insulation.

My questions:

1)  Do I need to insulate the interior concrete walls that connect to the exterior walls at a right angle?  Or maybe insulate the first 5 feet?

2)  Is it okay to sandwich electrical conduit between the insulation and the concrete wall?  Or in one case where I have an outlet, should I re-mount the outlet on 4″ of wood that is attached to the concrete so I can insulate all around the wood?

3)  Is it worth it to insulate the floor in these unfinished rooms?  There are no heat vents in these areas, just heat in the utility room from the actual furnace and the vents running in these rooms.  I have minimal headroom in both the unfinished rooms, so I don’t think the traditional rigid foam and sleepers will work.  I used Dricore R+ in the finished room, and although it wasn’t cheap, the floors are noticeably warmer with the R-3 on the floor.  Any other cheaper options or just skip insulating the floor?

Thank you for your time and help.

GBA Prime

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  1. Expert Member
    BILL WICHERS | | #1

    For your first two statements:
    1- I'd reconsider the use of XPS and just go all-polyiso here. Polyiso is both greener and more R per inch. In a basement application, the "R value drops in the cold" issue isn't really something to be concerned with -- interior foundation walls will always be a bit warmer than the worst cold above grade walls will see. The earth itself provides some warmth to below grade walls.

    2- That's a good plan. The softer "door and window" version of great stuff will work best here.

    For your questions:
    1- It doesn't hurt, and will help a little, to insulate any interior concrete walls that tie into the exterior wall. I wouldn't bother going past 4 feet though, since that's the width of a sheet of insulation. The first 2 feet are going to get you most of the benefit anyway.

    2- Yep, no problems sandwiching conduit in insulation -- it's done all the time. The issue you'll have is thermal bridging from the steel electrical boxes and also possible moisture issues inside those boxes. I would try bringing the boxes out from the wall a bit. You could do this with some wood, or you could just use some 1/2" foam between the boxes and the wall. What I did with my own electrical stuff is to put 1/2" pressure treated plywood directly against the concrete wall, then 3/4" noncom (non-combustible treated plywood) on top of that. This gets about 1-1/4" of wood in there, pressure treated against the block wall where it's needed, and "regular" plywood for the box to mount to to avoid any corrosion concerns. I have my electric panels all mounted this way too. The plywood is mounted to the wall with tapcons.

    3- Insulating the floor will help too, but you'll get most of your benefit from insulating the walls. If you want to save some money, insulate all the walls and then see if you get enough improvement from that alone. You can always insulate the floor later if you decide you want to do that.


    1. josher668 | | #2

      Thank you for your responses Bill.

      I was leaning toward XPS and Polyiso as I can get a 4' by 8' by 2" sheet of XPS at my local big box store for $23 per sheet while Dow/Dupont Thermax costs $54 for the same 4' by 8' by 2" sheet. That's over double the price for Polyiso and I want to use the Polyiso for the thermal barrier so that I don't need to drywall. I still need to check with my local building codes on this.

      You confirmed my thought that it doesn't pay to insulate the interior concrete walls for the whole run and I'll use your approach with the conduit. I'll look at the floor once I see how insulating the walls helps.

      Thanks again.

      1. Expert Member
        BILL WICHERS | | #3

        If you're pricing Thermax polyiso, which has a facer rated to be left exposed, that IS expensive polyiso! It's a good product, but not cheap.

        You can use cheaper polyiso behind that layer of Thermax though. You only have to have the Thermax layer on the inside. Note that there has been some discussion on here in the past as to whether or not such an assembly will be accepted in all municipalities though due to potential code interpretation differences in regards to the thermal/ignition barrier requirements. You can make a call to your local building department to find out for sure. This would apply to either the "regular" (not rated to be exposed) polyiso or XPS in this situation.


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