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.