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Community and Q&A

Roxul for crawlspace stemwall? And a related detail.

Andy CD Zone 5 - NW Ohio | Posted in Green Building Techniques on

Basic question: Is Roxul Comfortboard a suitable substitute for the usual interior rigid foam insulation in retrofitting a crawlspace? (House in zone 5, crawlspace floor is about a foot below outside grade, walls are CMU. drainage is very good. I do plan to use cut’n’cobble XPS in the rim joist area, to ensure good air seal. Homeowner very opposed to foam if it’s not necessary, which has led me to look into Roxul.)

Related: Martin’s article on conditioning crawlspaces ( recommends running the floor membrane clear up the wall to within 3″ of the top of foundation, and THEN installing insulation over that. Conversely, the drawings in the “Strategies and Details ” section show the membrane ending at the base of the wall. How does this detail affect the facing (if any) on the insulation?

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  1. GBA Editor
    Martin Holladay | | #1

    Mineral wool insulation is air-permeable, so I don't recommend it for this application. The mineral wool can't prevent humid interior air from contacting the cold crawl space walls. The likely result will be moisture accumulation and mold.

    Instead, choose an air-impermeable insulation (one of the rigid foam products with carefully air-sealed seams or closed-cell spray polyurethane foam).

    When it comes to the floor membrane question, don't sweat it. Either way will work just fine.

  2. Charlie Sullivan | | #2

    I agree with Martin that this will lead to moisture accumulation on the walls. In theory, clean cement and mineral wool wouldn't support mold, but if it's a retrofit, there will probably be enough gunk there that you can't clean off that it will still support mold growth.

    If the homeowner's objection is the global warming impact of blowing agents in foam, EPS rather than XPS is a good option, but if the homeowner is philosophically opposed to petrochemicals, that doesn't help.

    So your best option might be foamglas, which is completely impermeable, and tolerates moisture with no problems.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Standard basement wall where I live is;

    poured concrete full basement foundation
    1-2" air space off concrete build 24" O.C. frame wall fit with fiberglass batts
    Cover wall with roll foil faced insulation wrapper.

    No mold issues.

    I have no clue if the wall adds reasonable R value, has anyone seen a study?

  4. GBA Editor
    Martin Holladay | | #4

    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. 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.

  5. Aj Builder, Upstate NY Zone 6a | | #5

    Martin, I do not do what I posted above. But all, thousands around here are done that way or with the fiberglass directly in contact using 4'wide poly backed metal building insulation run in two horizontal strips. Building code here requires insulation and then poly.

    I know it's wrong.

    I also have never ever 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.

    Myself and a very small harmful of one off custom builders do otherwise.

  6. Andy CD Zone 5 - NW Ohio | | #6

    FOAMGLAS: Yes, Charlie (#2), clients' objection is to everything about foam that is toxic, which is everything. Foamglas Board looks like a great alternative. I called the manufacturer--apparently it's only available directly from them, but they will sell and ship in small quantities for residential projects. Comes in 2'x4' boards, of various thicknesses (R-3.4/inch if I understand the charts). Cost is $1.20/bd.ft., so to get enough Foamglas to match the thermal resistance of a 4x8 sheet of 2" XPS, it would cost about $120. I'll let them decide about that.

    I assume faced polyisocyanurate is the more reasonable choice?

    And no, AJ (#3,5), wood and FG ain't gonna happen. Although there are all those floor batts hanging like spanish moss throughout the crawlspace--I could frame up some walls and re-use those ;-).

  7. Charlie Sullivan | | #7

    $1.20 a bd. ft. doesn't sound bad....until you do the multiplication!

  8. Andy CD Zone 5 - NW Ohio | | #8

    Martin, you advise against the use of Roxul. But one manufacturer of a crawlspace membrane recommends installing rigid insulation on the wall first, then running the membrane up over it and securing to the wall above. Wouldn't this completely solve the problem you described of air contacting the cold wall, thus allowing use of Roxul?

  9. GBA Editor
    Martin Holladay | | #9

    I assume that the "membrane" you are talking about is a type of polyethylene or thin synthetic rubber that has basically no R-value.

    If polyethylene is placed against a cold concrete wall, the polyethylene will be cold. So if humid interior air contacts the cold polyethylene, the moisture in the air will condense and will trickle down the membrane and puddle on the floor at the base of the wall.

  10. Andy CD Zone 5 - NW Ohio | | #10

    Martin, yes, it's one of those 16- or 20-mil liners. But I'm asking about putting the liner on the warm side of an R-10 or R-15 insulation on the CMU stem wall. I presume the dew point then will always be inside the insulation, and thus no condensation. And thus can I use Roxul?

    Overall goal: to condition a crawlspace WITHOUT using foam.

  11. GBA Editor
    Martin Holladay | | #11

    There are three problems with your plan.

    The first problem is that the air between the fibers of the Roxul is warm, humid indoor air -- not magic dry air.

    The second problem is that daily changes in temperature create a pumping action that provides air exchange between the air among the fibers of the insulation and the basement air. Eventually the humidity gets in there.

    The third problem 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 the moisture, leading to mold.

  12. Richard Beyer | | #12


    If Andy installed a waterproofing membrane over the foundation walls and floor, then installed Roxul Mineral Wool over the membrane (as I understood from the posted comment), how would this contribute to mold if the moisture is trapped behind the membrane?

    To grow mold you need moisture +60% RH and a food source.
    Andy stated there's good drainage. We could assume moisture and mold is not a problem rather the consumer is seeking a warmer conditioned floor above with no added poison to their IAQ. 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.
    Did I misunderstand something here?

    I also agree with AJ Builder... Many homes in New England have fiberglass installed against raw cement walls with no mold issues. There are many homes in New England with fiberglass insulated cement walls. I have to assume these homes also have good drainage. Moldy basements which have the same or similar system installed most likely were DIY projects or wanna be contractor systems who do not understand building dynamics.

    Andy should start with asking a local engineer about the lands topography before he leaps. Does the land have the potential to flood but remain dry most of the year? This would ease the consumers mind over blog advice and give Andy peace of mind knowing he did the right thing.

    Sometimes published building science is not always correct. 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 $$$$.
    Foam is not always the perfect answer either. We both know it brings it's own environmental, fire and health problems with it and millions of $$$$ to silence people who speak out against it.

    If the consumer does NOT want foam in their home why are you pushing foam Martin, as stated here; "Instead, choose an air-impermeable insulation (one of the rigid foam products with carefully air-sealed seams or closed-cell spray polyurethane foam).

    I should also note there are many failures of foam out there too.

    Has anyone opened up any past basement walls sprayed with open cell foam in the Boston area or anywhere else? Remember this was preached about being the right thing to do on This Old House years back. Lets see the numbers from all those "This Old House" episodes aired out of Boston which sprayed open cell foam over rubble stone foundation walls during the late 1980's to early 1990's. What's that verdict?

  13. Andy CD Zone 5 - NW Ohio | | #13

    I can understand Martin's first two objections to my plan (#11), but I'm still curious how he would address Richard's counterargument (first sentence of #12). Would mold grow in Roxul just from the now-conditioned air in the sealed crawlspace? Martin's third point, about moisture behind the impermeable liner on the concrete foundation wall, makes sense but don't all the crawlspace conditioning companies routinely run the liner clear to the top of the wall?

    My modified plan would have the Roxul BEHIND the liner, next to the CMU wall. Martin, what say you on that?

    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. 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.

  14. GBA Editor
    Martin Holladay | | #14

    Richard Beyer,
    Q. "If Andy installed a waterproofing membrane over the foundation walls and floor, then installed Roxul Mineral Wool over the membrane (as I understood from the posted comment), how would this contribute to mold if the moisture is trapped behind the membrane?"

    A. The problem is that moisture can come from either direction. If Andy followed your advice, the waterproofing membrane would be chilled by the cold concrete, and would form a condensing surface for moisture in the interior air.

    You're right that some installations using this method are fine. 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), and it is safer in a house with a very dry basement than a house with a damp basement.

  15. GBA Editor
    Martin Holladay | | #15

    Q. "My modified plan would have the Roxul BEHIND the liner, next to the CMU wall. Martin, what say you on that?"

    A. That plan is not good, for the three reasons I listed in my Comment #11. I can continue to provide building science explanations for my advice, but you and Richard don't seem to be convinced by building science. Here is another answer, based on failures discovered in the field: every imaginable type of sandwich using air-permeable batts (usually fiberglass) and polyethylene has been tried on the interior side of basement walls.

    Some people put the polyethylene against the concrete, and then install fiberglass batts.

    Some people install the fiberglass, and cover it up with polyethylene.

    People have even tried two layers of polyethylene, with fiberglass in the middle.

    What happens? 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.

  16. Richard Beyer | | #16

    Martin why would you make such a statement?... "you and Richard don't seem to be convinced by building science"

    In my opinion, Building Science has specified junk science for years! Building Science has made numerous error's over the years specifying junk science in favor of the manufacturer(s) who line wallets with $$$$. No different than why codes are rewritten over time due to structural and material failures. At one time galvanized hangers were specified on salty coast lines. Today's new standard is stainless steel. Why? Because galvanized steel corroded and failed. At one time code officials would sign off after a deck was nailed up with 12d to 16d nails. Decks failed, people got hurt and lawsuits became profitable to lawyers. Today, decks must be bolted to the house. Remember SPFI language from 2009 thru 2013? ("Spray foam is Inert") and now in 2014 after the failed Federal Class Action filings by Wolf-Provato, the new published statement is ("Spray Foam is Relatively Inert"). Manufacturer's were forced to acknowledge know there are plenty of cases where the foam is not inert and they could foresee liability.
    Back to my question about previous This Old House Episodes which installed open cell spray foam over rubble stone foundations.... Building Science stated this was okay too. Today it's not specified in direct contact with rubble stone. Why is that? Did Building Science recognize another one of their error's?

    Martin, All I was pointing out is that fibrous insulation has been used for years in New England basements successfully. No different than AJ Builders comment.

    If we are to argue over what performs best when wet Mineral Wool will win as studies have shown it's R-value is not depleted like it's counterpart glass fiber and water will drain out of mineral wool material unlike spray foam. Foam also loses r-value as the temperature drops.

    Let's look at this another way...
    On the exterior side of the foundation wall, damp proofing is installed, today liquid applied waterproofing is installed in high end construction. Your theory based on building science this will fail even though a vapor permeable and waterproof membrane is there? So technically you can insulate in direct contact with the cement wall. Grace Vycor's new exterior air sealing membrane is also vapor permeable and so is Tyvek. Are you telling us fibrous insulation will fail when installed in direct contact with the exterior sheathing when craft paper is on the warm side of the room to? Dew point changes and condensation build up is more likely to happen and cause damage on the extreme outside conditions of a wooden building enclosure exposed to the elements in an area that has four seasons than in direct contact with a concrete foundation wall which is protected by damp proofing and or waterproofing membranes, and let's not forget about the curtain drainage.

    In a perfect world the basement foundation and concrete floor should be dry and waterproofed before insulating, period. Insulation will only perform as well as the preparation and water management of the "system", this includes spray foam and rigid foam.

    Martin, Why the biased comments in favor of foam over fiber?

  17. GBA Editor
    Martin Holladay | | #17

    My statement (that it appeared that you were unconvinced by building science arguments) was based on your earlier statement: "Sometimes published building science is not always correct." In any case, your latest comments seem to reinforce this attitude, since you just wrote, "Building science has specified junk science for years."

    Sometimes, people who don't trust building science recommendations are more likely to be convinced by reports of failures in the field. Both sources of information are useful.

    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.

  18. Aj Builder, Upstate NY Zone 6a | | #18

    Richard, I support most of what is posted here at GBA and by Martin. The point of my prior post is that in my area it is mandated by the building code officials to insulate basements counter to best practices which is what Martin should advocate for.

    To this day it does bug me that what I learn here and from going to Joe L. talks for the last 3 decades, well the two don't jive (local mandated poly verses the screams to DON'T DO IT!)

    The wrong way of using poly IS the norm here (except for like 2 or 3 of us.)

    In a wet part of the world like the Carolina humid zones with crawlspaces.... I would expect mold to grow on top of mold on top of mold if anything is done wrong and even if things done right aren't carefully kept an eye on. And who spends time in crawlspaces, home inspectors at sales of properties may be the only folks crawling with the critters...better do it very very right where it be moist.

  19. Andy CD Zone 5 - NW Ohio | | #19

    And it's not that I am "not convinced by building science." Believe me, I salute that flag. I appreciate this forum as much as anybody, because A) building science is complicated, and it takes us newbies a while to catch on; and B) the real-world applications contain so many variables that a single answer or blog post often doesn't speak directly to a situation in front us, and we get the chance to keep asking clarifying questions that may appear inane to the more experienced.

    I thought maybe Roxul (the board) was just enough air-impermeable to work, or that putting it behind the liner might be a solution. I get it--the physics of vapor movement makes this risky. Switching to polyiso. Homeowner will accept that as least-harmful foam.

  20. GBA Editor
    Martin Holladay | | #20

    Sounds like a good approach.

    I didn't mean any offense by switching from building science arguments to reports of failures in the field. As I wrote, both sources of information are useful. In any case, good luck with your project.

  21. Richard Beyer | | #21

    I also appreciate building science when it's not junk science and I enjoy reading Martin's helpful blog. I did not say all Building Science is junk science. You can not disagree that there are many products specified which should not be on the market. Your all smarter than this.

    As for this loosely used modern language, "Building Science". I say modern because it was not spoken about in the middle class residential market place until shortly after This Old House took a back seat to sexy ladies designing painted cardboard furniture for HGTV homes.

    How many insulation companies today employ a "Building Scientist" with a "certificate" versus ones with college degrees like Joe L or Allison Bailes? The answer is few. Every insulation company which sells spray foam seems to have one on staff. They classify themselves as building scientist to consumers ruining the organized trades reputation.
    I hope this gives you all a better perspective of where I'm coming from when I use this term loosely. It's not to insult legitimate college educated building scientist who put their time in a major university. However, I stand behind the examples I provided above involving building science specified and approved material failures. Many are due to theory and those written in favor of manufacturers who have lined wallets with $$$ in order to be specified. After all when building materials fail the contractor takes the heat and is blamed for "Not following Directions" regardless of the cause.

    If you want the correct answer, wouldn't it be best for you to contact each product manufacturer and have them place the correct materials and installation procedures in writing so when and if it fails you have some recourse? This is what a professional designer, architect or building scientist would do when something is in question involving the design of someone else's home. As a specifier you should give the consumer the choice with a non-biased response and let them decide what's best for them. Just my opinion.

    Here's some information on all the options available / fire testing...

    NFPA 286 Fire Test of all insulations

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