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Community and Q&A

Insulation for finishing my basement

abtesnow | Posted in General Questions on

Hello all,

I have been reading through this forum and many others and I am having trouble getting the information that I am looking for. First off, I live in Western North Carolina so our climate is rather mild with a few short cold snaps. We do receive a lot of moisture each year but our summers are typically not super humid. My basement is around 9′ 3″ of head space (from slab to bottom of floor joists). The basement walls are 8″ block with 2 coats of Thoroseal and 2 coats of Black Jack Mammy seal. The basement is 28′ x 62′ with only 1 of the 28′ walls daylit. The entire rest of the basement is below grade. I have around 18″ of block around the top of the basement that is above grade. I had a basement refinishing company come out to display a few different options and they were great. While the guys was there he took a surface moisture meter to my walls and never had a reading above 18% (which he said was great). My basement has never been below 56 degrees or above 66 degrees, so I don’t feel like I need to put R-19 all around the perimeter. I do not want to build a 2×4 wall around the basement for 2 reasons. One, the less wood the better and two, I have a main septic drain that runs around the perimeter of 2 walls of my basement that is from 9′ pitched down to 8′ that is around 3″ off the block.

So now on to my question;
I have 2 different options that I am tossing around. I would like for my insulation, framing, and drywall to tuck up behind the main drain line (so I dont have to box out soffits and can easily put it a drop ceiling a the lowest point of the drain in each room)

A) 2″ XPS (R-10) butted together around the entire basement adhered with PL-300 and taped with tuck tape at every seam. Then come back and put 1″ furring strips every 24″ OC fastened to the block with 3-4 tapcons per stud. That would give a 3/4″ void between the insulation and drywall to run wires and use shallow mount boxes for electrical outlets.

B) 1″ XPS (R-5) butted together around the entire basement adhered with PL-300 taped with tuck tape at every seam. Then come back and use 2×4’s as furring strips layed flat every 24″ OC fastened to the block with 3-4 tapcons per stud. That would give a 1-1/2″ void between the insulation and drywall to run wires and or electrical outlets.

I am leaning more towards option B for 2 different reasons… Cost and I don’t really think I need the 2″ XPS because over 85% of my basement is below grade.

I have started to purchase some of the materials, such as: PL-300, tuck tape, and tapcons. I am only planning to finish one room at the moment (within the next month). I am planning on the room being roughly 1/4 of my basement 12’x32′.

As always, any information would be greatly appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "My basement has never been below 56 degrees or above 66 degrees, so I don't feel like I need to put R-19 all around the perimeter."

    A. The temperature of your basement is no indication of whether or not you need insulation. Your existing heating system may well be keeping your basement warm -- but that fact has nothing to do with whether or not the basement walls would benefit from insulation.

    If you live in Climate Zone 4, codes require basement walls to be insulated to R-10. If you live in Climate Zone 5, codes require basement walls to be insulated to R-15. Your idea of using 1 inch of XPS is less than minimum code requirements for either zone.

    For more information, see How to Insulate a Basement Wall.

    -- Martin Holladay

  2. Expert Member
    Dana Dorsett | | #2

    IRC 2015 code-min for zone 4 foundations/basement is R10 continuous insulation or R13 if thermally bridged by studs. (The northwestern border counties from Yancey county through Alleghany are zone 5, and would need R15 c.i. or R19 if studs.)

    A 2" XPS is labeled R10, but in a few decades it will decay to about R8.4, the same performance as EPS. The higher initial performance is due to the HFC blowing agents used, but at it dissipates (taking it's environmental toll) it eventually settles in at EPS performance levels. EPS is blown with pentane which dissipates quickly (most of it is gone before it leaves the factory), and has stable performance over time.

    Polyisocyanurate is also blown with low environmental-impact pentane, and would be ~R9 @ 1.5". With a foil facer the trapped air gap would deliver the necessary R1+ to meet code, and the 1.5" of wood from the 2x furring is close to R2, so it would meet the code minimum performance with only 1.5" of foam.

    The amount of wall that is below grade doesn't really matter. That insulation requirement is as much about moisture resilience as it is about heat gain/loss. The subsoil temperatures in your area are well below your summertime outdoor dew points, and basement walls that are below the ventilation-air's dew point becomes mold-prone, since the walls then take on moisture from the room air. With continuous R10 wall insulation the wall temperature will track the room temperature in summer, with very little moisture accumulation. With only R5 (R4.2 after the HFCs are gone) that may not be the case.

  3. abtesnow | | #3

    I have been told what the code is in my area multiple times. I am simply asking what you guys would recommend... or if someone has used this rigid foam insulation, what did you use and your review?

  4. GBA Editor
    Martin Holladay | | #4

    To answer your questions directly:

    1. Two inches of rigid foam is preferable to one inch of rigid foam. From an energy-savings perspective, the upcharge for the thicker rigid foam is a cost-effective investment.

    2. From an environmental perspective, either EPS or polyisocyanurate is preferable to XPS (because XPS is manufactured with a blowing agent that has a high global warming potential). For more information, see Choosing Rigid Foam.

    -- Martin Holladay

  5. abtesnow | | #5

    Thank you,

    I have read that EPS is not "waterproof" and doesn't act as a true moisture barrier, do you know if this is true? Shouldn't EPS be a little bit cheaper that XPS anyways?

  6. GBA Editor
    Martin Holladay | | #6

    Q. "I have read that EPS is not waterproof and doesn't act as a true moisture barrier, do you know if this is true?"

    A. Any type of rigid foam will absorb water if it is submerged or in constant contact with liquid water for years. I've heard reports of waterlogged XPS as well as waterlogged EPS -- basically, you don't want to submerge your rigid foam for long.

    EPS is used for floating docks and coffee cups. It is waterproof, within the range of expectations I have described.

    EPS can be ordered in different densities. If you want to invest in high-quality EPS, buy the denser stuff.

    Q. "Shouldn't EPS be a little bit cheaper that XPS anyways?"

    A. The answer depends on the specifications (density, etc.) you select, plus the vagaries of the marketplace. Recycled (reclaimed) rigid foam is always cheaper than new foam.

    -- Martin Holladay

  7. Expert Member
    Dana Dorsett | | #7

    I used relcaimed 3" fiber faced polyisocyanurate to insulate my foundation (getting on to 10 years ago now), and it works just fine. The slab is close to the natural water table (below the water table during the spring thaw sometimes), so I stopped the polyiso about 5-6" off the floor, since polyiso CAN get waterlogged if wet (much more susceptible than EPS or XPS).

    EPS is usually quite a bit cheaper per labeled-R than XPS, less than 10 cents per R per square foot compared to 12-13 cents per R-foot. Low density Type-I EPS can be even cheaper still (but only R3.9/inch), but none of it is as cheap (or green) as reclaimed roofing foam. Even derating the 3" polyiso to R5/inch, at $20 per 4x8' sheet it's down around 4 cents per R-foot. There are many square miles of 1.5" - 3" roofing polyiso and roofing EPS reclaimed every year in the US. Most of the roofing EPS is 1.5lbs density "Type-II", rated R4.2/inch- the same stuff used in most insulated concrete forms, and the most common density used under slabs, and perfect for your application. Most roofing polyiso is 2lbs density, rated R5.5-R6/inch. This reclaimer might be near enough to you to be a useful resource (?):

    There are others...

  8. abtesnow | | #8

    Thank you Dana,

    I have contacted this person about the recycled Polysio.

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