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Community and Q&A

How to Insulate a Basement With Stone Walls

coolbordel | Posted in General Questions on
Hello
I would like to finish a room in my basement but I am not sure of the best way to do it.
Foundations are irregular limestones below grade, then concrete above grade (see the picture). I have never had any water issue in the basement and the foundation seem to be in a good state in my view. I live in MA.
I have been told to keep 1-2 inch air gap between those stone wall and the framing because those type of foundation needs to breath and also to allow moisture to evaporate. But then I do not know how to go after that.
Can I go with 1-2 inch air gap, framing with rock wool between studs, then moisture resistant drywall ? Or would it be missing some vapor retarder ? In that case, should I add some solid foam insulation between the framing and the airgap?
What if I add 1 or 2 inch XPS glued, to the above grade concrete part all around my basement (where we usually lose the most of heat (from what I read)) or would it be stupid ? This part is one inch deeper than the stone parts, so adding XPS would not overlap with any air gap.
I would like to keep things cheap. Doing the work likely without permit because i do not have enough ceiling space, so I will not have much return on investment.
Also, the idea is only to finish half of the basement. The other part will stay unfinished for storage and utilities. I think by leaving air gap, the foundations walls may still be connected to the other part of the basement (I am not sure if i will need to seal it for fire safety purpose maybe)
Thanks a lot for all your suggestions.

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Replies

  1. Expert Member
    Michael Maines | | #1

    Coolbordel,
    1. Read this: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
    2. The foundation doesn't need to breathe. The best, and really the only reasonable way to insulate a rubble foundation is with closed cell spray foam, after you ensure there are no water leaks. HFO-blown foam has a much lower environmental impact than conventional HFC-blown foam, and it performs better, but you will probably need to request it.
    3. Building codes are for protection of current and future occupants. You should get a permit. If you do not have enough headroom to meet code, there are solutions, or you should not finish the basement.

  2. coolbordel | | #2

    Thanks a lot Michael.
    About the permits, I am trying to get them. If I can't, because of this head room, the only way for us would be to dig the basement or raise the house, and we do not want to do that.
    I thought using spray foam would be bad for this kind of foundation because it would trap moisture and also prevent being able to see the state of the foundation and the mortar.
    I will look into this though.
    In the situation we are not going with spray foam, what would be a 2nd alternative even if it is not the best insulation ?

    1. Kevin J | | #5

      You should be able to pull a permit to convert it to finished storage space/playroom as long as you don't have a bedroom. They may want a means of egress. The square footage won't count towards finished space, but you'll at least have covered your bases. Most times you are grandfathered in for your ceiling height. Call your local inspectors office.

      1. coolbordel | | #7

        I will try. I am waiting for a feedback from the building department of my town on my project. I am not very optimistic because they require 6'8 for every habitable space, even storage area. My basement is 6'5/ 6'6.

  3. Expert Member
    Dana Dorsett | | #3

    >"I live in MA."

    In MA code-min for a basement wall is R15 continuous insulation, or other insulation yielding a U-factor of comparable performance. Installing a mere R5 of foam and a 2x4/R13 wood studwall has that performance, and has the bare-minimum foam/fiber ratio for wintertime dew point control at the foam/fiber boundary on the above-grade section in your climate zone for using only standard indoor latex paint on wallboard as the interior side vapor retarder. A bit more than the bare minimum R5 would still be better if going with a foam + fiber solution though- it's hard to install more foam-R after the studs are up if the wall assembly develops mold.

    >"I have been told to keep 1-2 inch air gap between those stone wall and the framing because those type of foundation needs to breath and also to allow moisture to evaporate."

    That's mis-information. Both concrete and limestone (and other stone types) are highly tolerant of high moisture content. Allowing a limestone (or concrete) foundation to "breathe" allows moisture to pass through, dissolving some of the limestone along the way, weakening it slowly (over centuries, not decades.) Blocking it preserves it's mineral content and strength.

    The moisture-suscptible materials

    >"What if I add 1 or 2 inch XPS glued, to the above grade concrete part all around my basement (where we usually lose the most of heat (from what I read)) or would it be stupid ?"

    Yes.

    As Michael pointed out, XPS is by far the LEAST green insulation material in common use in the US, primarily due to it's extremely potent HFC greenhouse gas blowing agents. It's not even close to the next-worst, which is closed cell spray polyurethane blown with HFC245fa, which is also a potent greenhouse gas (~1000x CO2 @ 100 years.) While there are HFO blown versions of closed cell polyurethane it only cuts the CO2e footprint by about a third- the high polymer weight per R of 2lbs density closed cell foam is still a big chunk of it. By contrast, foil faced polyisocyanurate uses half the polymer of closed cell spray foam, and very low impact hydrocarbon blowing agents with a 100 year greenhouse potential < 10x CO2 @ 100 years, comparable to the HFO blowing agents. Graphically it looks like this:

    https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

    The foil faced sheathing polyiso would be about half that of HFO blown closed cell polyurethane in that graph, putting it in the same level as rock wool or fiberglass.

    EPS is blow with isopentane (~7x CO2), most of which escapes the foam at the factory where it is recaptured, not vented to the atmosphere.

    In MA there are multiple vendors of used fiber faced roofing polyiso, which is also 2lbs density, but since no new polymer or blowing agents are being used, it's effectively CO2 neutral- it's just extending the useful lifecyle of an environmental hit already take decades ago. The R-value of roofing foam is "only" about R5.5-R5.7 per inch when new, and three inches would make it to code-min for a foam-only solution. In most cases it's good for about R5 per inch when used, assuming it isn't waterlogged. (You'll know when you try to lift a waterlogged 3" thick sheet of polyiso- WAY heavier than 2lbs per cubic foot!)

    The biggest vendors of used foam in MA are Nationwide Foam/Insulation Depot on Waverly street in Framingham, and Green Insulation Group in Worcester (google 'em for contact & directions info) but there are several smaller vendors that pop up from time to time. Used foam is typically priced less per R per square foot than fiberglass batts, something like 1/4 the cost of virgin-stock roofing foam. The big foam vendors also have factory reject (usually cosmetic blemishes- dinged corners, dented/cut facers, etc.) at about half price. Green Insulation Group usually has a good selection of heavily discounted foil faced new foil-faced polyiso.

    They also carry used roofing EPS, which runs about R4.15/inch. The performance of EPS is very stable over time- it' shrinks a bit more than XPS or polyiso in the first decade or two, but think of used foam as "pre-shrunk" foam- it doesn't keep shrinking forever.

    One way to deal with your concrete pony-foundation over limestone wall is to snap a line on the slab 1" out from the most prominent foam, and with a plumb-bob or level measure the distance from the face of the concrete to the plane above that line. If it's more than an inch, installing rigid polyiso on the concrete thick enough to reach that level, building a studwall butted right up against that foam, then blowing an inch of closed cell foam on the limestone portion before installing (unfaced) batts and drywall. The DIY foam kits are reasonable to use if only doing it a section at a time- even though it's blown with HFCs , 1-1.5" of HFC blown polyurethane + R13 fiberglass still has a lower environmental impact than 3" of HFO blown foam (no fiberglass) for making code-minimum, and if using zero impact used foam on the upper half it's really not bad at all.

    Note: If using steel studs to be more moisture resistant, the thermal bridging of the steel framing cuts the performance of batts in half- figure on about R7 "whole wall" for the steel & R13s, so to hit the code minimum would require at least 2" of polyiso (or EPS) to hit the necessary U-factor.

    FWIW: A bit over a decade ago I did my basement in Worcester with 3" used roofing polyiso, strapped to the poured concrete foundation with 1x4 furring through screwed to foundation with 1x4 furring and masonry screws (and fender washers), mounting the wallboard to the furring. Since I have a basement with a flooding history, I stopped the polyiso at the historical high-tide mark (6" above the floor), since polyiso can take in a lot of moisture if submerged, giving it up slowly when the tide goes out. (EPS doesn't have this issue.) It was a HUGE improvement in both wintertime comfort, and lowered indoor RH in summer (=less dehumidification required to keep the "musty basement smell" at bay) due to the enhanced air tightness and slightly higher summertime basement temps.

    [edited to add]

    The moisture content of the concrete WILL rise after there is foam board on the interior, due to both the colder concrete temperature and the reduced drying toward the interior. If there is less than a foot of above-grade exposure to the exterior and/or very little roof overhang or poor surface grading/drainage it potentially puts a wooden foundation sill resting on the concrete at risk.

    There are several ways to deal with that depending on the particulars, but a solution that works in nearly all circumstances is to install a sheet of EPDM roofing /flashing between the wood & concrete as a capillary break. This involves jacking up the house 1/4" a section at a time to get the EPDM in there, which sounds radical, but it's easier and safer than you might think. ( At my house I don't have the capillary break, but there are 2' roof overhangs, more than 15" of exterior exposure everywhere, and French drains where the roof valleys dump copious volumes of water during summertime thunderstorm squalls.)

  4. coolbordel | | #4

    Thanks a lot Dana for your information. It is technical but very precise !

    So If I understand, you would advice to:
    - Spray foam 1 inch the limestone wall. If DYI, use HFC kit. Otherwise, HFO.
    - Use fiber faced roofing polyiso on the concrete part of the wall (which is more than 2 feet above grade btw), enough to reach the same plane as the spray foam on the lower part of the wall
    - Stud framing with unface batt and then drywall

    I have a few more questions. I am only planning to finish half of my basement. I should still spray foam and insulate the whole area right ? Since they will not be any drywall in the unfinished area, do I need to cover the foam spray with some kind of fire resistant coating ?
    Also, I do not have any water issue. And I think the limestone / mortar are in a good state. Should I have the wall inspected before spraying anything on it ?

    Thanks a lot

  5. Expert Member
    Dana Dorsett | | #6

    >"- Spray foam 1 inch the limestone wall. If DYI, use HFC kit. Otherwise, HFO."

    An inch would be the minimum for dew point control on R13 fiber insulation using latex paint as the interior side vapor retarder, and is sufficiently waterproof & vapor retardent to keep the rate of ground moisture entry into the basement well (as either liquid or vapor) well controlled.

    >" I am only planning to finish half of my basement. I should still spray foam and insulate the whole area right ?"

    It's "worth it" on a house lifecycle point of view to insulate the entire foundation to at least the current code-min paying a contractor to do the work using virgin-stock materials. It's even more so, with much shorter "payback" horizons on energy savings if using reclaimed/discounted goods and doing it as a DIY.

    >" Since they will not be any drywall in the unfinished area, do I need to cover the foam spray with some kind of fire resistant coating ?"

    Requirements vary a bit from town to town, but yes, at the very least an intumescent paint should be applied to any large expanses of exposed foam board.

    Without the latex-painted interior side gypsum board there isn't sufficient dew point control at the foam/fiber boundary to use fiber insulation in combination with a mere inch of foam. It takes 2.5" of closed cell spray foam (or 3" of used roofing polyiso) to hit code-minimum in those areas where there won't be an interior side studwall. A 600 board-foot DIY kit only covers 240 square feet of wall area R 2.5'" depth at a cost of about $3 per square foot. At 1" depth it covers 600 square feet at about $1.20 per square foot. It's usually cheaper in material cost to just go ahead with an insulated studwall & wallboard + paint, though it's more time consuming than just blasting away with the foam gun.

    Many/most HFC blown foams have a 2" limit per lift, with a cooling period between lifts (both a quality and fire hazard during curing issue), so you might limit it to 2" depth for about 300 square feet if not building an insulated studwall.

    If you have much area to spray-foam at all it's worth getting the 600 board foot kits- it's significantly more expensive per square foot to go with smaller kits. An inch of foam to air seal the band joists and foundation sill to the top of your board-foam is worth it for the whole basement, but you may be able to get the band joists & foundation sills foamed for next to nothing after subsidies through MassSave (less than 50 cents per running foot of band joist after subsidy), assuming you are in the service area of the major utilities that pay into the MassSave. MassSave does NOT subsidize insulating the foundation though- just the band joist & foundation sill. (See: https://www.masssave.com/en ) To improve the continuity of the air barrier it's best to spray foam the band joists AFTER putting up the board foam.

    If they are already tying up the truck & crew for the afternoon they may be able to give you a price break on 1-2" of unsubsidized foam for the stone foundation part. They may have to bill you separately for the band joist and stone foundation parts to stay within the program guidelines, but it's a lot cheaper for them to make only one trip, with only one set-up/break-down cycle to do both parts.

    1. coolbordel | | #8

      Thanks for all this data ! I had forgotten about MassSave. They came 3 years ago but they did not look at my basement at all.
      Tonight, I noticed while walking around my basement that the pony wall on top of the stone wall, seems to be made out of brick surrounded by concrete. The inside layer of concrete is very thin, maybe half a inch, while the outside layer is maybe a foot thick.
      => Is it common ? Would it suffer from the same moisture problem as pure concrete wall, and would you recommend the same method ?

      1. Deleted | | #9

        Deleted

    2. coolbordel | | #15

      Hello

      I am sorry to bring back this thread !
      So I discovered recently that the concrete part of my foundation (which is above the stone wall) is actually brick surrounded by concrete (or mortar), and not a layer of pure concrete.
      I havethe project of insulate all the basement with HFO spray foam sometimes in the incoming months. Do you still think it is a good idea ?

      Thanks !
      Tom

  6. Expert Member
    Dana Dorsett | | #10

    >"I think I will hire a company to spray foam 2.5 inch of HFO on every walls. For the area I plan to finish, I will frame the studs around 1 inch from the wall before having the spray company come."

    With the studs 1" away from the wall the thermal bridging of the 1.5" stud penetration into the foam brings the performance to below code, unless the remainder of the studwall cavity is filled with fluff. There are better ways to go with zero thermal bridging at about the same or even less total depth.

    eg: A co-worker of mine did his place in Arlington with nominally 3" of HFO blown foam using exterior grade 7/8" steel hat furring mounted 3" away from the wall, for zero penetration of the framing into the foam. The top/bottom plates to a furring framed wall can be 1 x 1" angle furring or channels screwed to the joists/slab. One advantage to that approach is that the hat furring "studs" can all be cut ahead of time to a very sloppy fit, saving time.

    >"Should I actually heat this finished space ? I remember the basement not being too cold in winter, and I am thinking that once it is sealed with the foam, it should be much better."

    If the ceiling isn't insulated and there's a boiler or furnace in the basement, the standby & distribution losses to the basement should keep it well into the 60sF once there's an air-tight R15 on the walls. My R17-ish basement in Worcester stays around 65F in winter without actively heating it, as long as most of the rooms above are in the 70-72F range. It tends to be cooler in the spring when the (mostly hydronic) heating system duty cycles are low (for lower distribution losses) and the ground temperatures are still cold. Clearly YMMV.

    1. coolbordel | | #12

      Thanks a lot Dana ! I did not think about the thermal bridging.
      Btw, do you know any company in MA which sprays HFO ? The ones I contacted offer traditional Closed cell (sealtite Pro closed cell)

      1. Patrick OSullivan | | #14

        I contacted Demilec via their online form to get some contractor contacts. I had to ping them a few times before I got an answer, but eventually I did.

        1. coolbordel | | #16

          Thanks Patrick !

      2. DCContrarian | | #17

        I just had a basement sprayed by Ritchie Insulation in Westport. They are an Icynene dealer. I was very pleased with the work and the value.

  7. Tom Woodson | | #11

    Interesting to hear how extreme the effect of thermal bridging is. I had a 12 inch thick 3 wythe brick home framed with 1.5 inch steel studs with a 1 inch gap to the brick and spray foamed to 2 inches. I have a similar basement situation as the OP and was considering doing something similar, now I'm thinking otherwise.

    Dana, how are you calculating total wall R value including thermal bridging?

    I'm curious what my current living space wall performance is now. I was guessing R4 on the brick (I've tried some in-situ measurements, confirmed with real energy consumption data pre-SPF), R13 for the Foam and around R2-3 loss with bridging for around R14 total wall. Think I'm far off?

    1. Patrick OSullivan | | #13

      I came back to this thread to ask the same exact question as Tom, not realizing he'd already asked it! I was considering doing the same thing on a to-be-finished part of my basement. 1 5/8" studs held 1" off the masonry, with the void behind filled with ccSPF, total depth of 2".

      I figured ~R-7 of thermal break greatly mitigated the effect of using steel studs, but maybe I'm wrong! I did a calc assuming 14.5" of R-14 and 1.5" of R-7, coming in at total wall of R-12.8 (using U-factor and framing fraction).

      I'm not saying I'm right. I want to understand if and why this is wrong.

  8. Karl B (Zone 6A) | | #18

    Despite there being near UNIVERSAL agreement that the way to insulate a rubble-stone foundation is with spray foam (Michael and Dana above, Martin in his basement article, Joe Lstiburek in BSI-041, Akos elsewhere, etc), I share coolbordel's concern about wanting to retain access to the fieldstone/rubble foundation for inspection, repointing, etc. (Yes, I know that ccSPF will reduce moisture migration, and can help physically stabilize the mortar.)

    Thoughts regarding the attached assembly, in which a service chase (of sorts) is created to retain access?

    Thanks!

    1. DCContrarian | | #19

      If there's a gap between the wall and the foundation you need fire-blocking.

    2. Expert Member
      Zephyr7 | | #20

      I’m another one of those agreeing spray foam is the best option here. This and invented cathedral ceilings are the two big niches where spray foam is really the best choice, maybe even more so with the foundation since you can use exterior rigid foam on a roof...

      So you’re basically proposing a sort of “catwalk” around the interior perimeter of the foundation wall? I’m not sure there is a prescriptive code assembly for this, but I’d class it like a crawlspace. Since you’re insulating the interior wall and ceiling of your catwalk pathway, this “crawlspace” would need to be vented to the exterior. I would put some basic lighting in there too, controlled from a switch near the access door, and a few service outlets for convenience.

      If you’ve had bulk water problems at any point in the past, I’d put in a perimeter drain too. If you’ve had only occasional water problems, it might be cheaper and easier to just put in a water monitoring system to alert you to the problem. In the telecom world, we have a “rope sensor” that you can lay on a surface that will detect water over a large area. The better systems can even tell you where along the sensor rope the water is located. These systems aren’t cheap, but they’re cheaper than cutting in a perimeter drain and they’re much less messy to install.

      Bill

  9. Karl B (Zone 6A) | | #21

    DC Contrarian: good call regarding fire-blocking.. I hadn't thought to consider the required blocking every 10' horizontally, which throws a wrench in the works..

    Bill: Yes, an interior perimeter "catwalk" might be a fair description. I'd already figured on adding service lighting, but you're right about it needing to be vented. Coupled with the requirement for fire-blocking (as a "concealed space") that DC Contrarian pointed out, effective ventilation quickly becomes difficult to do. So much for an easily accessible foundation wall?

    Alternatively, since I *mostly* just care about being able to inspect the foundation every 10+ years without clawing out the ccSPF each time, maybe it would be enough to roll a robust vapor/air barrier down the wall, lap it with a similar floor membrane that is turned up at the walls, and seal the two together with a bead of silicone caulk (or grease, even)? EPDM or a heavyweight reinforced poly ought to be able to take the stress of decadal separation. Sketch attached.

  10. Expert Member
    Akos | | #22

    My own home has limestone foundation in the basement. I've been slowly insulating section at a time. What I have done is parge the foundation to get it somewhat even than install the rigid with taped seams over it. Top and bottom of the rigid are sprayfomed in for air seal. The foam is just tapconed in place so it is not flat or vertical by any means. Because of this the stud wall stands a bit away from this rigid. If I ever needed to, I guess I could cut a section of foam out to inspect the foundation behind.

    I can let you know in about a decade or so if this plan was sound.

  11. Chris Bowman | | #23

    This has been such a resourceful thread. I’m looking to insulate and partially finish a limestone basement in Missouri.

    My basement walls were coated in Drylok and have a good amount of flaking and evidence of moisture in the bottom 2-3’ of foundation.

    Two questions:
    1. Would it be of any benefit to install a French drain system and sump pump before insulating and framing?

    2. Should I strip the drylok from the foundation walls and re-point and limewash prior to insulating and framing?

    Thanks!

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