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Q&A Spotlight

What’s the Best Way to Insulate Crawl Space Walls?

A builder tries to accommodate a customer’s objection to rigid foam insulation. Will mineral wool work just as well?

Polyethylene keeps moisture out. A layer of plastic will keep moisture from migrating into this crawl space and transforms the previously damp area into good storage space. An even more effective solution would be to insulate the foundation walls rather than the crawl space ceiling.
Image Credit: Fine Homebuilding/Harold Shapiro

Andy Chappell-Dick is at work on a house in Climate Zone 5 where the task at hand is to upgrade a crawl space by adding insulation as well as a membrane to block the infiltration of moisture. The catch? The owners want to avoid the use of rigid foam insulation if at all possible.

The floor of the crawl space is about a foot below grade, Chapell-Dick writes in a Q&A post at GreenBuildingAdvisor, and the area seems to be well drained. Foundation walls are made from concrete block (CMUs).

He plans to foam in pieces of rigid extruded polystyrene in the rim joist area. To insulate the crawl space walls, Chappell-Dick wonders whether Roxul Comfortboard would make a good substitute for rigid foam. Roxul, according to the manufacturer, is non-combustible and chemically inert, and it’s made from natural and recycled materials, including rock. Rigid foam is a petrochemical.

A second issue is how the waterproof membrane should be installed: should it be run up most of the crawl space wall, or can it be terminated at the base of the wall? And, Chappell-Dick wonders, does this detail have any bearing on the performance of insulation?

That’s the backdrop for this Q&A Spotlight.

This is not the place for Roxul

The inherent air-permeance of mineral wool insulation makes it inappropriate for this application, writes GBA senior editor Martin Holladay.

“The mineral wool can’t prevent humid interior air from contacting the cold crawl space walls,” he says. “The likely result will be moisture accumulation and mold.”

The best route, Holladay says, is to use an air-impermeable insulation — either rigid foam with seams that have been carefully sealed or closed-cell spray foam.

In theory, mineral wool and a clean cement surface shouldn’t support the growth of mold, adds Charlie Sullivan. “But if it’s a retrofit,” he adds, “there will probably be enough gunk there that you can’t clean off that it will still support mold growth.”

He suggests that if the homeowner’s objection to rigid foam insulation is the global warming impact of the blowing agents used to manufacture it, choosing expanded polystyrene (EPS) over extruded polystyrene (XPS) is a good option. “But if the homeowner is philosophically opposed to petrochemicals,” he adds, “that doesn’t help.”

Another option, Sullivan says, would be to use a product called Foamglas, described by its manufacturer as “cellular glass.”

Sullivan’s tip has Chappell-Dick on the phone with the manufacturer, and at first blush Foamglas looks like a great altnerative. It comes in 2-by-4-foot sheets, has an R-value of 3.4 per inch, and costs $1.20 per board foot.

Fiberglass batts are not really an option

Writing from upstate New York, AJ Builder says the method typical in his area is to frame a wall 1 or 2 inches away from the concrete basement wall, insulate it with fiberglass batts and cover the wall with foil-faced insulation. “No mold issues,” AJ Builder writes.

“I don’t know if you are being deliberately provocative, or whether you honestly think that this is the appropriate way to insulate a basement wall,” replies Holladay. “The technique you describe is about two or three decades out of date, and there are plenty of reports of failures resulting from this technique.”

“I know it’s wrong,” AJ Builder says. “I also have never seen mold or moisture. We build in gravel and glacier moraine, and poured concrete here is quite water-resistant from my experience. Just telling it like it actually is. No Roxul use, no foam, all batts of fiberglass, done. Thousands.”

Be that as it may, Chappell-Dick says, “wood and fiberglass ain’t gonna happen.”

What about adding a waterproof membrane to the assembly?

If the risk of using an air-permeable insulation is that moisture will condense on the cold, inside surface of the foundation wall, what about keeping the moisture out of the wall assembly with some kind of a barrier?

“I’m asking about putting the liner on the warm side of an R-10 or R-15 insulation on the CMU stem wall,” Chappell-Dick says. “I presume the dew point then will always be inside the insulation, and thus no condensation. And thus I can use Roxul?”

He adds: “Overall goal: to condition a crawl space without using foam.”

Holladay finds three flaws with this approach. The first is that air between the fibers of the Roxul insulation is warm, humid indoor air, “not magic dry air.” Second, daily changes in temperature will create a “pumping action” that provides an air exchange between basement air and the air within the insulation, so that eventually humidity finds its way into the wall assembly.

“The third problem,” Holladay adds, “is that the concrete is damp, so that it’s possible for the area between the concrete and the membrane to get damp from that direction, too. The membrane traps moisture, leading to mold.”

But lots of basements are insulated with fiberglass

Richard Beyer is not understanding why a wall assembly that keeps moisture out of the mineral wool insulation with a waterproofing membrane is going to result in mold.

“This proposed system will work providing a back-up dehumidifier and sump pit is added to ward off the unknown here and/or the potential freak storm which could change the drainage dynamics of this property,” Beyer writes. “Did I misunderstand something here?”

Further, Beyer says, AJ Builder is correct: many homes in New England have fiberglass installed against raw cement walls with no mold issues.

“Sometimes published building science is not always correct,” Beyer says. “Hence, why it’s consistently rewritten when failures occur, no different than our building codes. Most writings come from manufacturers who are selling product and who are filling the pocket’s of specifiers with $$$$.”

Beyer wonders why Holladay is suggesting foam insulation when the homeowner doesn’t want the material in the house, adding, “I should also note there are many failures of foam out there, too.”

Chappell-Dick also is curious about why a wall assembly in which the Roxul is isolated from the crawl space wall by a membrane would be a problem.

“The most important thing I have learned on this site is that while pure building science is exact and completely unarguable, applied building science is far more nuanced,” Chappell-Dick adds. “And, frustratingly arguable. It’s not so simple as ‘managing moisture.’ We’re managing risk and clients’ expectations, all at the lowest price possible while somehow extracting an income.”

Also, says Beyer, building science has been in error many times over the years. For example, galvanized steel joist hangers were once specified in coastal locations, but it’s since been replaced by stainless steel. Why? Because galvanized steel corroded and failed.

The membrane will trap moisture

The problem, Holladay replies, is that moisture can come from either direction. “If Andy followed your advice,” he writes to Beyer, “the waterproofing membrane would be chilled by the cold concrete, and would form a condensing surface for moisture in the interior air.”

Holladay concedes that some installations using the method that Chappell-Dick proposes are successful. “The method is safer in warmer climates than in cold climates (because a concrete wall doesn’t get as cold in Alabama as it does in Vermont),” Holladay says, “and it is safer in a house with a very dry basement than a house with a damp basement.”

But the bottom line is that any wall assembly including a waterproof membrane and batt insulation against a foundation wall is risky, Holladay says. This applies to walls with a layer of polyethylene plastic against the concrete, followed by fiberglass batts, as well as walls where the batts come first, followed by poly. Ditto for walls with two layers of poly and fiberglass in between.

“What happens?” Holladay asks. “If you are lucky, and the soil around your house and the air in your basement are dry, these methods can work. In other cases — and plenty of remodelers have seen the failures, again and again — you end up with a moldy mess.

“In other words, these sandwiches of fiberglass and polyethylene are risky. You are rolling the dice. But if you are feeling lucky, go ahead and roll the dice.”

As to the fallibility of building science, Holladay says this: “At the risk of stating the obvious, here’s how science works: scientists write papers presenting data and theories to explain the data. New data that contradict old theories are used to propose new theories that explain the data better. Over time, our scientific understanding evolves and moves closer to the truth.

“Science is a more useful way of separating falsehood from truth than the usual alternatives, which include superstition, religious beliefs, and stories heard while leaning on the counter at the local lumberyard.”

Our expert’s opinion

Here’s how GBA technical director Peter Yost sees it:

A crawl space foundation is just a short basement; you need the same three barriers that you need for any assembly — continuous air, water, and thermal barriers — as well as provisions for directional drying.

Just as you would not insulate a basement before managing moisture, you need to manage moisture in the crawl space first, and then move on to insulation and air sealing. Check out this resource from Building Science Corporation.

And if indeed crawls are just short basements, then check out these other BSC resources.

Air-permeable insulations, including rock wool, need a separate air barrier (and more than one of the BSC foundation details accomplishes this with a sealed rigid insulation layer between the masonry foundation and the air-permeable “cavity” insulation). Above-grade walls can have interior air barriers, like the Airtight Drywall Approach (ADA), but it is hard to consider ADA as appropriate for a crawl space or think of other interior sheathing that you could or would use as an interior crawl space air barrier.

Insulating any building assembly on the interior makes the assembly colder; it’s just that masonry walls tend to care a lot less than framed walls, particularly ones sheltered below grade. For me, it’s that portion of the “below-grade” wall that is actually not below grade that is worrisome. And does it really matter if that condensation is only occurring in the portion of the wall above grade? It still represents a problem for any materials that can grow unintended biology.

We tend to think of below-grade spaces as damp and cold because they are in contact with the soil and often aren’t moisture-managed. But if a crawl space is moisture-managed, you can air seal and insulate it just like a basement. Also bear in mind that any work to insulate and air seal the crawl space may have impact on levels of radon in the crawlspace and possibly the living spaces above.

Using Foamglas is definitely a premium approach: the product has a good R-value, is inert, and is air-impermeable. With any other insulation approach, establish the three barriers and then check for directional drying potential. And frankly, if you can’t moisture-manage the crawl space, don’t insulate it.


  1. Aj Builder, Upstate NY Zone 6a | | #1

    fiberglass stud walls with
    fiberglass stud walls with poly in NY basements is code required.

    Clarification to all,

    DO NOT INSULATE THIS WAY. All I am saying is that in my county they mandate for us to insulate wrong. And so far I have not found one spec of mold in OTHER builders homes that I have gone in AFTER THE FACT to do additional work.



  2. Roger Anthony | | #2

    Mold is a living thing!
    Mold is alive and it needs food and water to live!
    Mold does not grow on clean concrete - water is not enough, it needs food.
    Why insulate a crawl space? You don't live there. What will it achieve? Why would anyone spend good money on a space that is unused and unseen?
    Insulate the floor, at least this helps keep the room above warm and saves money.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Roger Anthony
    Q. "Why insulate a crawl space?"

    A. Here are four possible reasons:
    1. To keep pipes from freezing.
    2. To make sure that any ducts are located within the conditioned envelope of your house.
    3. To lower your energy bills.
    4. To reduce crawl space humidity levels.

    For more information, see Building an Unvented Crawl Space.

  4. User avater
    Robert Swinburne | | #4

    poly against concrete - new construction
    I have been specifying poly under the slab and wrapped up the foundation walls to a point high on the wall where it get taped to Intello or similar air barrier which then forms the continuous air barrier for the above grade sections of the house. There is also a break at the top of the concrete wall to prevent moisture from reaching framing. This keeps the concrete foundation wall and it's inherent moisture to the outside of the envelope. An interior stud wall is spaced out enough from the foundation wall to hold rigid foam in place behind it. (EPS or otherwise) No interior air or vapor barrier in the basement. The stud wall may be insulated or not. Is this a valid approach? With the poly in this location, could I simply build a stud wall and fill it with cellulose and eliminate the rigid?

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Robert Swinburne
    Your approach sounds fine with me, as long as you keep a layer of EPS (or some other type of rigid foam) between the cold concrete and the stud wall.

    I would never recommend the use of cellulose in a crawl space, because the chance that the cellulose will get damp some day is just too great. I know that some builders will insulate a stud wall like the one you are talking about with fiberglass or mineral wool (again, with the proviso that there must be a layer of rigid foam between the concrete and the studs) -- although I don't particularly like that approach either, because crawl spaces are subject to damp conditions and flooding.

    In short, I'm a "foam only" guy when it comes to crawl spaces. If you want more R-value, just make your foam thicker -- and leave the studs uninsulated.

    Anyway, that's my recommendation. If you decide to insulate the stud bays anyway, in spite of my advice, I would use mineral wool, not cellulose.

  6. User avater
    Robert Swinburne | | #6

    Thanks Martin
    Good answer. I'm actually thinking more along the lines of either meeting the VT energy code for a basement or, more commonly, a house on a sloping site where one side of the basement is open to grade and it is used as living space. I am a big proponent of a stud wall service chase and rock wool if necessary but would rather gain the insulation using EPS. Definitely "foam only" in crawl spaces though.
    Good to meet you in person in Portland earlier this week.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Robert Swinburne
    Yes, it was good to meet you. Glad we agree on crawl spaces.

  8. Anthony Papay | | #8

    In the picture the bubble on the floor is likely created by the pressure of outside air infiltrating, but now stopped by the poly film. I watched amazed as the poly in my crawl filled like a big balloon.

    My assembly is foam board on top of the poly on top of the concrete wall. A tar tape seals the perimeter of the poly before the top of the concrete. There's moisture under the poly where there is concrete and a pebble floor. Should be OK - yes?

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Anthony Papay
    I didn't select the photo that accompanies this article -- I don't have any more information to explain it -- and you may be right about the apparent bubble on the floor. I had a different reaction when I saw the photo, however: I assumed that the plastic covered a mound of dirt or a natural outcropping of some kind. (Such features are fairly common in crawl spaces.)

    It's hard to get a full picture of the details in your crawl space from your brief description. But if your question concerns the visible drops of moisture or condensation behind (underneath) the polyethylene, you are correct: these beads of moisture do no harm.

  10. User avater
    Leigha Dickens | | #10

    I Have Run Into This Too
    I have a very green-motivated client building a new home in CZ4 who wants to do the exact same thing. Wants to avoid foam for various environmental and health reasons, wants a sealed crawlspace, found Roxul Comfortboard.

    The Roxul rep stated that Comfortboard was an acceptable product to use in the specific application of a sealed crawlspace...but, if you read their literature on ComfortBoard in basement walls, which he directed me to, they are specifying it here only in combination with a vapor/air barrier on the foundation wall (as you do with a sealed crawlspace), then Comfortboard, then a furred 2x4 wall with Roxul Comfortbatts, a vapor barrier over those if needed in your climate zone, and most importantly, drywall over that as the all important air-barrier. Makes some sense for a basement, but is it worth putting that kind of detail into sealed crawlspace insulation for the sake of avoiding foam plastic? Well, maybe. Or could you find a way to cover the Roxul Comfotboard with some kind of less expensive air and/or vapor barrier as needed, such as Smart Membrain, without going to the extent of 2x4 walls and drywall? Well...maybe.

  11. Gregory La Vardera | | #11

    late to party
    Just saw this linked on FaceBook.

    For those advocating for just using foam in the crawlspace, you really can never just use foam because the IRC forces you to cover the foam. You can cover it in mineral wool though.

    See IRC R316.5.4, I'm looking at 2009.

    I have to disagree that foam is the best material here. The likely hood of getting it good and airtight is slim, and adding the operation of taping to the crawlspace work seems like throwing more time after time poorly spent. If moisture gets behind the foam and you get condensation, then its going to have a tough time drying.

    There are no good options. At least if you use mineral wool, get the crawlspace buttoned up with a slab on poly, the outside of the wall waterproofed, or at least damproofed, get positive air circulation through the crawlspace just like any other room in the house, then at least anything happening against the wall stands a good chance to dry to the inside.

    Other than that, build a slab on grade.

  12. Ross Philip | | #12

    Gypsum board thermal barrier in crawlspace susceptible to mold?
    Thank you for all the information posted in this article. For a crawlspace in Boulder Colorado, we are considering:

    Seal foundation vents.
    Cover dirt floor with impermeable vapor barrier.
    Insulate rim joist with cut blocks of 2" Rmax Thermasheath polyiso foam board and Great stuff sealant.
    Insulate the crawlspace masonry wall with 2" Thermasheath or two layers of 1" Tuff-R polyiso board with taped seams.

    According to fire code, the foam board needs to be covered with a fire protecting layer such as 1/2" gypsum board. I am aware that Dow Thermasheath might not require such a covering, but it is unavailable in our area.

    So, finally to my question: is it good practice to use sheet rock as a thermal barrier in the crawlspace? It is my understanding that the paper backing is an ideal food for mold; also if the crawlspace were to flood in an extreme storm event, the wet sheetrock would all have to be replaced?

    Because of the complexities arising from the thermal barrier issues, many insulation contractors in our area are insulating crawl spaces using perforated vinyl faced fiberglass roll batting, with the vinyl face toward the interior of the crawlspace. Do you think this practice is acceptable? It seems that many articles I've read vehemently oppose the use of fiberglass in crawl spaces.

    Thank you so much for any advice as how best to approach code compliant insulation of crawl spaces in our area.

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Ross Philip
    Q. "Is it good practice to use sheet rock as a thermal barrier in the crawlspace?"

    A. Yes, as long as you have taken the necessary steps to keep the crawl space dry. Properly detailed, a crawl space can be (and should be) as dry as a finished basement.

    Q. "Many insulation contractors in our area are insulating crawl spaces using perforated vinyl faced fiberglass roll batting, with the vinyl face toward the interior of the crawlspace. Do you think this practice is acceptable?"

    A. The practice you describe probably meets code, but it's risky. If the climate is dry, and the crawl space is dry, fiberglass insulation can work. However, there is always the risk with this method that the walls will get moldy.

    This approach works better in Colorado than North Carolina -- but I still wouldn't recommend it.

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