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How to Insulate a Basement and Which Insulation to Use

Mark_from_MA | Posted in General Questions on

Hello,
I am struggling with what is the best or preferred way to insulate the inside of my basement walls. In order of priority, I am interested in air quality first and cost last – while meeting code. Apologies for the essay.

Some details:
– 1920s house with poured concrete basement (dry with no water/dampness issues).
– A few miles west of Boston (zone 6). Looks like the current code for my area is R 15/19 (which I think means continuous R15, R 19 cavity, or R13 cavity + R5 continuous).
– Approx. 43′ x 34′ unfinished basement
– Rim joists have already been insulated with 2″ iso. using local MassSave program.
– To save money, I am planning on doing the insulation and framing myself and will sub out electrical and drywall. I will probably use metal studs but have not decided yet.
– I will be pulling permits. The final finished space will be considered ‘finished storage’ rather than ‘finished living space’ as it is just under the height requirement.

So, it seems like I have several options to meet code:
– 3″ of XPS (R 5/inch) stuck to wall
– 4″ of EPS (R4/inch) stuck to the wall
– Some combination of XPS or EPS stuck to the wall (min. 2 ” as a vapor barrier?) + R15 mineral wool in the cavity (R15 seems to be most common in local box store)
– Sprayfoam a little over 2″ to the wall (R7/inch)
– Ball park costs of each seem to be in the region of $2-3K (using new materials). I would consider them fairly comparable in terms of the overall project cost and it would not sway me one way or another.
(I’m not considering fiberglass batt because of potential mold issues.)

Questions:
– Are my assumptions/options correct and am I missing any?

– Which one should I do and are there any pros/cons to each?

– The spray foam is the easiest option as I write the check and it is ready for framing, but I am concerned about air quality (first) and environmental impact (second). Are there any greener spray foam options available now?

– The XPS is an easy DIY option and there is a local place to buy used/seconds which reduces the environmental impact. But are there long term air quality issues to consider about XPS and any concerns about buying ‘used’?

– How much XPS or EPS would I need to stick to the wall so that it is acceptable as a vapor barrier? I think 2″ but wanted to check.

– I need to look into EPS more as I think this is the most air quality and environmentally friendly option. Are there issues with water wicking with EPS? The need to put 4″ on the wall is also giving me cause as it pushes out the wall the most.

– The EPS/XPS mineral wool option is bottom of my list just because it is (slightly) more work but it pushes out the wall the least. Should I give it more thought?

– I have not looked into poly iso. Maybe that is a better option that I have missed?

Thanks to anyone that read this far!!

Mark

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Replies

  1. Expert Member
    Akos | | #1

    The common way of insulating basements is with enough rigid on the walls for condensation control (usually between 1" to 2") followed by stud walls with batts. Since the rigid insulation seals off the concrete and prevents moisture from getting into your batts, you won't have any issues with moldy batts. Going all rigid is not really worth the cost or the space. It also doesn't save you much extra energy.

    I would avoid metal studs as you loose about 1/2 the R value of the insulation batts between them. There is really no benefit in for steel in this application, stick to wood framing. Quicker to build, easier to drywall and easier to hang shelving or anything heavy down the road on it.

    1. Eric Habegger | | #14

      +1

  2. GBA Editor
    Kiley Jacques | | #2

    Hi Mark,

    In response to your question about the pros and cons of different insulations, you might find these articles helpful: How to Choose Insulation, Insulation Choices, and A Buyer’s Guide to Insulation.

    1. Deleted | | #4

      Deleted

  3. Expert Member
    Zephyr7 | | #3

    Polyiso would need to be 2.5" thick to hit R15 (might be labeled as "R16" actually). Polyiso would be a great option here, but if you have had any issues with water getting into the basement, you'll want to leave a small gap at the bottom to ensure the lower edge of the polyiso sheets are never sitting in any standing water.

    I would avoid XPS here as it's the least green option. EPS is fine, but thicker than polyiso. Try to get Type II EPS too since it's easier to work with (less crumbly) than type I.

    If you go with batts, you could use cheap fiberglass batts in the interior walls since the rigid foam is doing all the hard work, basically. I personally would just go with all rigid foam even if it costs a little more, since it simplifies the project a bit (saves the need to put in batts as a second step). I'd look into reclaimed foam here if you have any suppliers in your area, since it's cheaper, greener (reclaimed, after all :-), and the scuffs the used sheets often have isn't a problem in hidden areas like this.

    I would NOT use spray foam here unless you have an irregular wall surface like stone. Spray foam is usually more costly than the other options, and it doesn't add much benefit. Spray foam also gives you an irregular surface to deal with when you try to frame the wall, where rigid foam sheets will be perfectly flat and easier to frame over.

    Note that MassSave pays for all kinds of insulation and air sealing projects, and might cover at least part of your basement insulating project. I'm not very familiar with it since it's far outside my region, but we have some members on here who are pretty familiar with the program and will know for sure.

    Bill

  4. Jason S. | | #5

    Just went through the same cost/benefit exercise for my basement in zone 6. I arrived at two layers of 1" EPS with joints sealed, then a 2x3 furring wall with R8 batts and drywall. 1" EPS was the lowest cost per R of any 'air impermeable' insulation i could find. Bonus points for stable R value over its lifespan and decent vapor permeance.

  5. Mark_from_MA | | #6

    Thanks for all the responses and suggestions. Seems like all EPS on the wall or EPS + batts is the preferred approach. My own paranoia about using fiberglass in a basement means that I would only use mineral wool.

    A few more questions - I don't blame folk for rolling their eyes as I am probably overthinking this but I want to do it once and do it right.

    - What is minimum inches of foam board on the wall to act as a vapor barrier? Is there a different minimum for EPS or XPS? (I will probably exceed this but was wondering what is the minimum)

    - Can EPS be in direct contact with the basement floor like XPS?

    - If I used XPS and buy used/recycled material is there any risk of off gassing of something bad given that I won't know the source? New Foamular from a big box store claims all sorts of 'Indoor Air Quality' standards but I don't know if that is just BS advertising and XPS is XPS.

    - Akos - why would you lose about 1/2 the R value of batts when using metal studs? Is it due to thermal bridging or something else?

    - Jason S - you used 2" of EPS + R8 batt which gives you R16 by my calculation. I thought you needed R 19 cavity or R13 cavity + R5 continuous in Zone 6 like me? Would that pass inspection?

    - Zephyr - if I used Polysio and left a small gap at the bottom would that cause any condensation problems? I though the board had the as air tight as possible?

    1. Expert Member
      Zephyr7 | | #8

      How thick the EPS needs to be to act as a vapor barrier depends a bit on the type of EPS you use. If you use Type II EPS, which is a little denser than Type I EPS. With 2-4" you're in class-II vapor retarder territory. That's not technically a vapor "barrier", but it's close, especially in the thicker end of that range.

      You would want EPS rated for compressive strength on the floor. While it's technically possible to use the lightweight non-rated stuff here, I wouldn't recommend it due to the risk of deformation and a weak floor.

      An advantage to using reclaimed foams is that all of the off gassing has already happened. Off gassing is most severe when the product is new, then rapidly drops off with an exponential decay over time (a fancy way to say that there is a lot in the beginning over a relatively short period of time, with a very small remaining amount over a very long period of time). This is one of the reasons that new product smells like new product, but reclaimed material usually doesn't smell like a "product" at all. You don't really have anything to worry about with typical new product either, but there WILL be some off gassing regardless of manufacturer claims -- it just won't be much.

      Bill

    2. Expert Member
      Zephyr7 | | #9

      Metal studs are MUCH better conductors of heat than wood is, so they cause MUCH more thermal bridging as a result.

      If you were to use 2" of EPS that's about R8 of continuous insulation, with the R8 batts making up the rest. That's better than R16 of cavity insulation alone, and while I haven't run the numbers, it's probably pretty close to equivalent, if not a bit better, than R13 cavity with R5 continuous.

      Bill

    3. Expert Member
      Zephyr7 | | #10

      The "small gap" under the polyiso, if you need a gap, usually isn't much of an issue for thermal or moisture performance since the gap is usually 1 inch or less, a very small area compared to the amount of wall surface that IS getting covered. If you're worried about that, just fill the gap with canned foam. I would use the softer "door and window" variant of foam here.

      Sorry for the multiple posts, GBA's anti-attack system seems to have not liked my original wording for the polyiso paragraph.

      Bill

  6. BlueSolar | | #7

    Mark, how thick are your basement walls? Do you know what's on the other side as far as moisture barriers? These days there will often be HDPE or similar, but I don't know the norms for older houses in Mass.

    The R-value drop with steel studs that Akos mentioned is due to thermal bridging. However, there's a simple fix – turn the studs sideways. This configuration eliminates bridging, since the studs don't go through the wall, though you need more studs since now you have pairs of sideways-oriented studs facing each other. This is what ThermaSteel does with their awesome panels: https://www.thermasteelinc.com/content/thermasteel-diy-panel

    You can also use a variety of SIPs that use steel studs in a similar fashion, or even full coverage with steel sheets. The wall you're describing with rigid insulation is basically a SIP. You can of course use any kind of SIP here, like the MgO kind, OSB, etc.

    Using steel doesn't create any problems with hanging things like shelving, so I don't know what Akos is referring to there. You just use screws instead of nails. Proper structural studs are at least 20 gauge (33 mil), and the screw retention is pretty good with anything 20 gauge or thicker. ThermaSteel uses thicker steel, like 18 and 16 gauge.

    The other solution for thermal bridging with steel studs is to just add a layer of continuous insulation outside of the studs, like an inch of XPS, polyiso, Kingspan Kooltherm, mineral wool, etc.

    EPS is lame unless you get the good stuff, the higher density, higher R-value versions. Type II is the de facto minimum, with R-value less than 4, and a compressive strength of about 15 psi. You can get much stronger and better insulating EPS, but I'm not sure how easy it is to source in your area. XPS has much better compressive strength and R-values.

  7. Expert Member
    Akos | | #11

    Take a look at the table here for assembly R values with different studs:
    https://www.buildingenclosureonline.com/blogs/14-the-be-blog/post/86806-effective-insulation-r-values-in-steel-vs-wood-framing

    I have done both wood and metal frame walls. Metal walls cost about 50% in material and take about 2x to build. Metal is great for non combustible walls and sound isolation. The rest of the time, there is no benefit to it.

    Part of my home has metal walls, I'm speaking from experience when I refer to issues with mounting things. Doable but never simple.

    In a basement, if your wood stud is molding or rotting away you have much bigger problems than what your walls are made of.

    For the OP's questions.

    Use faced EPS for the walls, any thickness is a vapor barrier. Make sure to tape the seams and seal the top and bottom with canned foam. In zone 5 you need R5 for condensation control with 2x4 walls. You can always add more for a bit of extra buffer and piece of mind.

    Jason's assembly above (post #5) is excellent value and insulation.

  8. Mark_from_MA | | #12

    Thanks

    To answer a few questions
    - The basement walls are approx. 13" thick of poured concrete and nothing else.

    - The only reason that I was leaning towards metal studs was that I thought it would be less hassle/quicker but it sounds like that is not the case, compounded by the thermal bridging issue

    - Sounds like I need to read up on the types of EP some more. These guy are pretty close to me and seem to have reclaimed stock of all sorts.
    https://insulationdepot.com/insulation-types/

    I will give the local inspector a call before I get started to see if he has any preferences/requirements

  9. Mark_from_MA | | #13

    One other question...

    The basement walls were painted by the previous owner - looks like standard latex paint. I don't think foamboard adhesive like PL300 will work with a painted wall and I was hoping to not have to drill and use fastners.

    Would a few spots of sprayfoam (like greatstuff window and door) across the foam board work? - with 2 x 4s to push the foam board against the wall while it dries. I would frame directly against the foam board assuming walls are relatively straight..

    1. Expert Member
      Zephyr7 | | #15

      NO! DON'T GLUE WITH CANNED FOAM!

      I tried that once, it sounded like a good idea to me too. What happens is the canned foam pushes everything apart as it cures no matter what you do to try to clamp the pieces in place, then you end up with a messy assembly that is a LOT of work to clean up to a point where you can redo it right. Been there, done that. Don't be like me! :-)

      My guess is the PL300 will hold to the painted wall OK as long as the paint itself is adhered well to the concrete. If the paint is not adhered well, the PL300 will probably just pull the paint off the wall. The only way to know for sure is to do a test and glue something up temporarily. If you only use a small blob of glue, you can pop the piece off with a prybar, just make sure to leave an edge on your test piece you can get under. I'd probably use a piece of scrap 2x4, and notch out a bit of one end of the face I glued against the wall to make a slot for a prybar to get under.

      If you find you need to use mechanical attachments, tapcons work, but the Hilti insulation anchors are better (but more expensive). With those Hilti anchors, you just drill a hole through the foam and into the concrete to the correct depth, then tap the anchor in with a mallet. The anchors are one piece and made of plastic, and they hold pretty well and install quickly.

      Bill

      1. Mark_from_MA | | #17

        Thank Bill - good to know and will not repeat that mistake. I make enough new ones of my own.

  10. Jason S. | | #16

    Mark, if I'm not mistaken you've listed the above grade wall insulation requirements. Below grade requirement is R15 (or it was when I pulled permit). You could bump it up to 2x4 with R13 or maybe R15 cavity without too much risk but you don't want the foam to get cold enough for condensation to occur. Keep the ratio in check -- air impermeable insulation should be 35% or better of total R in zone 6. R15 batt is pushing it with R8 foam, disregarding the fact that cheap EPS is more like R3.85/in.

    1. Mark_from_MA | | #18

      Hi Jason
      Am I not reading the code correctly for basement?... which is very possible - R15/19 (Which I think means continuous R15, R19 cavity, or R13 cavity + R5 continuous)

      https://www.ase.org/sites/ase.org/files/massachusetts_2015_iecc.pdf

      I am leaning towards 2" of reclaimed XPS on the wall (R10), wood frame butted up to the XPS with mineral wool batts (R15 seems to be all I can find locally) = R25

  11. Jason S. | | #19

    Mark,

    Forgive me I was looking at a different edition. Requirement varies by year and even by jurisdiction, so you're likely correct for your case.

    U factor equivalent of my assembly might just barely get you there (something like U0.06). Better to be conservative to make the inspectors and the green police happy ;)

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