Installing Mineral Wool Insulation Over Exterior Wall Sheathing

A subset of green builders have always been grumpy about foam. Such builders look at rigid foam panels and spray foam as suspect products: they are made from petroleum, laced with mysterious chemicals, and impermeable to vapor flow.

Not all green builders agree with this analysis, however. Here’s the opposing view: most new homes include many products derived from petroleum, including drain pipes, bathtubs, window frames, and the plastic sheathing on Romex wiring — so why focus only on insulation? Moreover (the argument goes), there’s nothing wrong with using petroleum-derived insulation; in fact, converting oil into insulation makes more sense than burning the stuff to drive our cars.

Regardless of whether you are a “foam is evil” builder or a “foam is a useful product” builder, you probably agree that it’s handy to have a range of available products to solve construction quandaries. That’s why it’s good news for builders that mineral wool insulation can be installed on the exterior side of wall sheathing, just like polyiso.

The thermal performance of a wall is greatly improved when wall sheathing is protected by an exterior layer of insulation. Exterior wall insulation keeps wood warm, keeps wood dry, reduces the condensation potential, reduces thermal bridging, and probably increases the durability of the wall.

However, some builders worry about the use of rigid foam to keep stud bays warm — not only because “foam is evil,” but because foam does not allow the wall assembly to dry to the exterior. (Although this concern is unwarranted, it exists.) Such builders have often wondered whether rigid mineral wool panels (for example, insulation panels made by Roxul or Thermafiber) could be sandwiched between wall sheathing and vertical furring strips — in other words, whether mineral wool could be substituted for XPS, EPS, or polyiso to insulate the exterior of a wall.

Builders who insist that “a wall has to breathe” will probably prefer mineral wool insulation (which is vapor-permeable) to rigid foam insulation (which has a relatively low vapor permeance).

Although the term “mineral wool” is sometimes used (especially in Europe) to refer to ordinary fiberglass insulation, the phrase is typically used in the U.S. to refer to rock wool or slag wool insulation. To be clear, in this article I’m talking about rock wool or slag wool products, not fiberglass.

Rock wool insulation is made from basalt, an igneous rock. Slag wool insulation is made from steel-mill slag. (In the U.S., most mineral-wool insulation is slag wool.) Both types of insulation are valued for their resistance to high temperatures.

Mineral wool insulation can be fluffy like fiberglass, and the R-value per inch of fluffy mineral wool is similar to that of fiberglass. Denser types of mineral wool have a higher R-value per inch than fluffy types; if the mineral wool is dense enough (8 lbs. or more per cubic foot), it can be formed into rectangular panels and can be installed like rigid foam.

The mineral wool products discussed in this article have R-values ranging from R-4.0 to R-4.3 per inch. (Fiberglass insulation can also be formed into high-density panels, but that’s a topic for another blog. Suffice it to say that such rigid fiberglass panels are not generally available at this time.)

Builders have experimented with the use of mineral wool on the outside of their wall sheathing for many years. Unfortunately, there have been several barriers to their efforts:

• The ideal product would be very dense; however, many common mineral wool products haven’t been dense enough.
• The ideal product would have a high R-value; however, most common mineral wool products are rather thin.
• The ideal product would be readily available in small quantities; however, most types of mineral wool are commercial products that can only be ordered in very large quantities.

In spite of these hurdles, experimenters have bravely forged ahead. Many builders who called up Roxul’s technical help line were told that Roxul Drainboard was the densest mineral-wool product available. (Drainboard has a density of 8 pounds per cubic foot.) One downside of using Drainboard: it isn’t available in a high-R version. The thickest Drainboard product is only 2 3/8 inches thick — an oddball dimension chosen because it provides R-10.

On the GBA website, several builders have posted suggestions about the best techniques for installing vertical furring strips over mineral wool insulation. For example, here are two comments posted by GBA reader Thomas Jefferson:

• Post #1: “None of the rockwool samples I’ve handled would be dense enough to install strapping on top as you would with rigid foam.”
• Post #2: “I don’t think it would work to install vertical strapping over any thickness of rockwool without some type of squash block. For a very
• short span (like 1"-2" thickness) it might be sufficient to put a sleeve of plastic pipe around the screw, which would be simple to do.”

Another GBA reader, Robert Manninen, echoed Jefferson’s recommendations on the use of squash blocks: “If the ‘springy-ness’ of the rockwool board is really objectionable, cut holes in the rockwool board with a hole saw and either use a plug of wood or high load XPS to attach the nailers…”

Although many builders assume that mineral wool products are too springy, some experts have long suspected that complicated squash blocks are unnecessary. According to building scientist John Straube, “We installed vinyl siding over 3-pound-per-cubic-foot Roxul on a number of walls, including Eric Burnett’s house, about 10 years ago with no issues.”

This year, Straube was able to provide convincing data showing that it is indeed possible to install vertical furring strips over Roxul without any squash blocks. The data came from a Building Science Corporation (BSC) test facility, where Straube and Jonathan Smegal conducted a series of furring-strip tests at the behest of Roxul.

“We tested a range of higher density Roxul products,” Straube wrote to me in an e-mail. “The 6-pound-per-cubic-foot (pcf) and even the 4 pcf products did amazingly well, but the 8 pcf product was easier to handle for people used to the stiffness of foam and it also worked the best.”

Most of the BSC tests were performed on two thicknesses (1 1/4 inch and 3 inches) of Roxul ComfortBoard IS. “ComfortBoard IS is a new product that hasn’t been launched yet,” Straube told me. “It is essentially Roxul 80 — also sold as RockBoard 80. It has a density of 8 pounds per cubic foot.”

According to the BSC report on the testing, “All of the insulations tested showed very little deflection (less than 0.01 inch or 0.25 mm) at the loads imposed by lap siding (of wood, vinyl, or fiber cement). … Testing with various fastener embedment (in framing, in OSB, or a combination) showed no significant differences at loads less than approximately 20 pounds per square foot cladding weight. … Note that these tests were conducted to simulate some of the worst-case realistic scenarios for deflection (i.e., 24-inch-o.c. strapping, and 16 inch vertical spacing between screws).

This is equivalent to only 4 fasteners per square meter. Also, the screws used were the lowest quality, length and thickness that would be reasonable for this application. Using more screws, at a closer spacing would likely decrease deflection, but more testing is required to determine the amount that the deflection could be decreased.”

The researchers unambiguously endorse the use of vertical furring over 8 pcf Roxul — even thick layers of Roxul — for most types of residential siding. The report notes, “A range of target R-values can be easily reached as similar details can be used for the design of walls that have 2, 3, 4 or even 6 inches of insulation.”

At first blush, it would appear that even Portland-cement-based stucco — a relatively heavy type of siding — could be installed on furring strips screwed through Roxul. The BSC report noted, “None of the walls tested in this study exceeded 0.01 inch of deflection at 12 psf (384 lbs total), approximately equal to the typical weight of ¾-inch stucco cladding.”

However, the BSC researchers have decided that further testing is necessary before they recommend the use of stucco over Roxul: “To confirm the very favorable results achieved, it is recommended that field testing, in a test facility or on a jobsite, should be conducted to assess the potential for stucco or adhered veneer cracking over a 1-2 year test period before proceeding with wider deployment. Long-term deflection testing in a laboratory setting may give a better indication of performance with sustained loading that simulates cladding, but field testing is preferred.”

Straube elaborated, “The Building Science Corporation won’t recommend stucco over Roxul — we don’t know enough yet. We need to install some and see if it cracks. If you are talking about synthetic stucco, that’s no problem. We have been recommending and installing synthetic stucco over Roxul for 20-some years. Every major EIFS manufacturer sells a Roxul system for buildings that need the improved fire performance.”

Lighter sidings, however, shouldn’t be a problem. “Vinyl siding and fiber-cement are obviously lighter than stucco,” Straube told me. “Small movements are never going to be a problem in those systems. But with stucco, small movements can produce cracks.”

For more information on Straube's recommended details for installing mineral wool insulation on the exterior side of wall sheathing, see Insulating Sheathing for Residential Construction.

Of course, conservative builders are likely to be wary of product they aren’t familiar with. If builders get a chance to handle dense mineral wool, however, they are usually impressed. The Roxul products in question — those with a density of 8 pcf — have a compressive strength of between 4 and 5 pounds per square inch. “They sell it for roof board, and people walk on it,” says Straube.

Roxul is unharmed by water. “It’s coated with an oil-based product,” says Straube. “It’s hydrophobic. Water beads up on it, and it doesn’t absorb water. If you put it in a tub of water and put bricks on top of it to keep it underwater, it will pick up water. But as soon as you take it out of the water, the moisture evaporates in no time flat. It dries out like crazy the moment you have a sunny day.”

What about insects? “Insects don’t eat rocks,” says Straube, “so I’m not too concerned about insects.”

Builders who install rigid foam on the exterior of walls have learned that thin foam is riskier than thick foam. (Exterior rigid foam should always be thick enough to keep the wall sheathing above the dew point in winter; for more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.) Does the same concern arise when using Roxul?

I posed the question to Dr. Straube, who said, “If air is presumed to leak through the inner stud cavity, it can reach the sheathing, where it cools down and dumps its moisture load. But thinner Roxul insulation is not as risky as thin rigid foam, because when the OSB is wet, it can dry much more quickly to the outside through Roxul than through foam. But if you switch the OSB to fiberboard, it would truly change the story, because fiberboard can dry out at a rate that is 10 times faster than OSB.”

Builders excited about building walls that dry in both directions should be cautious, however, because vapor-open walls can perform poorly during the summer if the building is air-conditioned. “If you have a very vapor-open assembly, it’s good and it’s also bad,” Straube said. “In the summer, when rain wets wood siding, these vapor-open walls are the type of walls that taught us about inward solar vapor drive. But knowing that inward vapor drive is a risk, we should make sure there is a gap between the siding and the sheathing, and we should ventilate the gap and back-prime the siding, so water isn’t stored in the siding. Those steps are probably enough to remove the summertime risk from vapor-open assemblies.”

What about housewrap? “I wouldn’t put the housewrap on top of the Roxul — I would put it behind the Roxul,” said Straube. “For one thing, installing the housewrap between the Roxul and the furring strips would be a pain. Anyway, one of the reasons for choosing high-density rockwool is that it is dense enough to resist wind-washing.”

According to Jonathan Ram, technical product specialist at Roxul, “We hope to launch ComfortBoard IS this fall — we’re aiming for early October.” The product has the following specifications:

• Density: 8 pounds per cubic foot
• Available thicknesses: 1 1/2 inch and 3 inches
• Available panel sizes: 3 ft. x 4 ft. and 4 ft. x 6 ft.

Ram said that the price of ComfortBoard IS “will be around the same ballpark as rigid foam.”

Ram is aware that builders looking for Roxul products have often faced problems with availability. “We’re trying to build up our distribution system,” Ram told me. “We’re working on it. We hope you will be able to buy the product in smaller amounts — not just entire truckloads.”

According to Straube, Roxul is serious about improving the availability of their insulation products. “Three years ago Roxul built a new factory,” Straube said. “Until recently, Roxul didn’t have enough capacity to meet the demand. It was embarrassing for us, because we told people about mineral wool, and they would say, ‘That sounds perfect.’ Then when they tried to buy some, they found out it was hard to get a hold of. But a new plant came online about 2 or 3 years ago, allowing Roxul to triple their output.”

If Roxul is able to develop an efficient distribution system for ComfortBoard IS — and especially if Roxul begins distributing a 6-inch-thick version of the product — I predict that many builders will be eager to try it.

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