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Community and Q&A

Foil faced insulation on basement walls?

Bryson | Posted in General Questions on

I am in the process of remodeling my basement. I’m in climate zone 7. 

A previous owner had insulated the concrete basement walls with 1.5″ of foam with thin wood paneling attached directly on top. In some areas there was fiberglass insulation in a 3.5″ cavity directly against the concrete. Totally inadequate and not up to code. The basement is relatively dry and it rarely gets very humid here. I have already removed everything down to the concrete. 

I’m planning on installing 1.5″ of EPS foam followed by a 2×4 advanced framed wall filled with fiberglass batts. I believe the basement was unfinished when the house was built, so the ceiling has R-19 faced fiberglass batts currently. I am planning on removing it since the basement is now heated and reusing it in the 2×4 walls. I understand that the R value will be reduced to around 15 since it will not have the space of the 2×6 ceiling cavity. This will be covered in typical drywall. 

I’ve chosen EPS since I have read studies that show XPS significantly degrades in its thermal properties over time (and costs over 2x as much) and polyiso is a poor insulator in cold temps. 

Three questions:
1: Is it really that much of a problem to use foil faced EPS boards? Everything I have read on GBA blog posts and on white papers specifically shows the use of UNfaced boards since you don’t want a vapor barrier between the concrete and the inside area. However, I have both read that a vapor retarder before the batts is likely okay, and I have also seen that the manufacturer installation documentation (R-Tech and Polyshield) both show the use of foil faced EPS in basement wall applications.

Home Depot and Lowes seem to only stock foil faced EPS. I can try sourcing unfaced EPS from another supplier, but I am in the mountains away from any large city so I imagine the cost would be quite high to get it here. 

2: Should I be fine reusing the R-19 faced batts in the wall, and just slashing the facing to remove the vapor barrier? 

3: Is there anything else about my plan that you would advise against, or anything that I otherwise got wrong?  

New to GBA and thanks in advance!

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  1. DC_Contrarian_ | | #1

    You want to have a vapor barrier between the concrete and the interior. There's no point in trying to let the concrete dry to the interior, you'll never dry it out because it's constantly replenished from the moisture in the soil behind it. You just want to block that moisture and keep it in the concrete, moisture doesn't harm concrete and even makes it stronger. Foil-faced EPS is an excellent choice for that. You may even consider sealing the seams and edges.

    It's a highly controversial subject how fiberglass should be used in basement walls. The question is whether you should try to allow it to dry toward the interior, or whether you should try to keep it out in the first place. The general rule is that vapor follows the heat flow, so you put a vapor barrier on the warm side and allow the wall to dry to the cool side. The problem with basements is the warm side is usually the interior and the walls have no ability to dry to the exterior. My gut is in your situation where the basement wall is going to be kept above the condensation point you want the insulation to be vapor open to the interior. The easiest way to do that is to install it with the facing toward the exterior.

    I work in very humid climates and would never consider fiberglass below grade.

  2. DC_Contrarian_ | | #2

    Oh, and if you're converting from the basement ceiling being insulated to the walls, pay extra attention to the rim joists. You want them thoroughly air sealed and well insulated.

    1. Bryson | | #3

      Yes that will be part of the project. I'll use some extra EPS and expanding foam for those. Right now it is just loose insulation shoved in and not very well sealed.


  3. Expert Member
    BILL WICHERS | | #4

    XPS degrades from the initially rated R5 per inch down to about R4.2 or so per inch over time, which means XPS will end up at exactly the same R per inch as EPS. XPS will never perform worse than EPS in terms of R value. XPS is "less green" compared to EPS though, which is the negative for EPS. XPS is better when used underground in constant contact with high moisture levels though, which is the only real advantage to XPS (aside from being a little more physically durable than EPS).

    Polyiso's "doesn't work in the cold" issue is significantly exaggerated. That said, it doesn't really matter in a basement application because the insulation you use on the INTERIOR of your basement walls will never see temperatures as cold as it would if it was used on the EXTERIOR above grade walls. I used polyiso in my own home. The main advantage of polyiso is higher R per inch over XPS and EPS, and even if derated for the cold, it ends up at about the same R per inch as EPS.

    All that fun stuff aside, there is no issue using EPS in your assembly. Concrete doesn't need to dry, since it doesn't care if it stays wet. The one issue you can have is "rising damp", which means more moisture risk to the sill plate and rim joist area if you seal up the masonry. To address this, you want to be sure you have a capillary break between the top of the masonry wall and the sill plate. That's the only real concern with sealing up the wall here.

    I would also avoid the use of fiberglass here. While it's technically possible to make it work if you're careful and follow the tables for R value ratios of rigid foam to batts, it's much safer to just go all rigid foam. The easiest way to do that is probably to put in code minimum levels of rigid foam directly against the foundation walls, then frame out your interior wall with the 2x4s "on the flat" to minimize wasted space. You can still run wiring in walls built that way -- I recommend using 4" square electrical boxes, which are 1.5" deep just like the studs, then use single or double gang mud rings to bring the devices up to the right level with your finished drywall surface. This is easy to do, and allows the use of standard parts.


  4. Expert Member
    Michael Maines | | #5

    My typical detail is foil-faced polyiso against the concrete walls, and if the space is to be finished, fluffy insulation in a stud wall to the interior of the foam. Foil-faced EPS would work as well. Tape the seams. To gauge how much foam vs. fluffy insulation I follow the same rules as for condensation control in above-grade walls.

    1. Bryson | | #6

      You put the foil facing inside the house, correct?

      Can you direct me where to find information on the condensation control rules?

      1. Expert Member
        Michael Maines | | #8

        The polyiso I use and spec has foil on both sides. If you have insulation with only one foil face, I would put it toward the concrete as that's where the moisture will be coming from.

        Here is Martin's article on insulation ratios:

    2. DC_Contrarian_ | | #7

      Do you use the same exterior temperature as above grade for the calculation, or do you estimate a below-grade temperature?

      1. Expert Member
        Michael Maines | | #9

        I usually use the same rules as for above-grade, but if not much foundation is exposed above grade, sometimes I use the rules for the next-warmer climate zone. It's not a very scientific approach; it's safer to always go with the above-grade rules.

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