Windwashing in Exterior Mineral Wool

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Windwashing in Exterior Mineral Wool

When mineral wool is installed on the exterior side of wall sheathing, is the performance of the insulation affected much by windwashing?

Posted on Jan 6 2017 by Martin Holladay

Fibrous insulation materials like mineral wool do not stop air flow. Unlike rigid foam (which is a pretty good air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., as long as the seams between panels are taped), mineral wool can only slow down air flow, not stop it.

So what happens when builders install mineral wool insulation on the exterior side of wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. ? Is the thermal performance of the mineral wool degraded by wind?

This question comes up every now and then on (See, for example, the dialogue between Lucy Foxworth and Lucas Durand in 2013.) While readers have speculated that semi-rigid mineral wool is (because of its relatively high density) fairly immune to the effects of windwashing — especially when compared to fiberglass, which is much less dense — the speculation hasn’t been backed up by measurements or calculations.

Now, however, thanks to a group of researchers from Ontario, including Randy Van Straaten and John Straube, we have enough data to reach a conclusion on this issue. In their recently published paper, “Wind Washing Effects on Mineral Wool Insulated Sheathings,” the researchers came to the following conclusion: for mineral wool insulations with a density of 4.4 pounds per cubic foot or more, the windwashing effect on the thermal performance of the insulation is “small and practically negligible for design considerations.”

Use of continuous insulation on the exterior side of sheathing is increasing

Straube presented his paper at a recent building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. conference in Clearwater Beach, Florida. (The conference had the most inelegant and ungainly name ever devised: the “Thermal Performance of the Exterior Envelopes of Whole Buildings XIII International Conference.”)

Straube began his presentation by defining windwashing: “Windwashing is the phenomenon whereby outdoor air goes through the insulation, and then goes out again.”

Straube noted that it’s becoming increasingly common for builders to install continuous insulation on the exterior of wall sheathing. “The best place to put insulation is on the outside of the structure. We know that. But we also know that industry routinely ignores good advice. By now continuous insulation on the outside is becoming more common. If the air barrier is on the outside of the structural framing, the air barrier will have far fewer penetrations and much better performance.”

Most builders who install continuous exterior insulation use rigid foam, but a few are beginning to use semi-rigid panels of mineral wool. “We can minimize air flow with compartmentalization at corners and edges,” Straube said. “But still, air can get through claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. systems. So if we install fibrous insulation on the outside of the sheathing, is this a problem or isn’t it?”

Straube and his colleagues (Randy Van Straaten, Trevor Trainor, and Antoine Habellion) set out to answer that question.

Literature review and laboratory measurements

The rate of air flow through a fibrous layer of exterior continuous insulation depends on many factors, including:

  • Wind speed;
  • Building exposure;
  • Building height;
  • The type of cladding, which can range from a relatively tight cladding like brick veneer to a relatively leaky cladding like vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding;
  • The depth of the rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap, if any;
  • The size of the ventilation openings that connect outdoor air to the rainscreen gap.

[Image credit: Van Straaten, Straube, Trainor, and Habellion]

Straube explained, “We know how to translate wind speed on a map into pressures on a building façade. This is a quantitative thing.” Once one knows these pressures, a researcher still needs to translate the pressures into air flow rates through the fibrous insulation. “Wind flows around the building and creates pressure distributions,” Straube said. “Wind-driven airflow can go sideways.”

These days, many designers and builders are including ventilated rainscreen gaps behind the siding. “The trend is to increase air flow behind cladding,” said Straube. “So we decided that it was worth looking at well-ventilated cladding — at least as well ventilated as vinyl siding.”

Straube and his colleagues performed a literature review, noting that previous studies have included measurements of the pressures exerted by wind on building façades and measurements of air flow rates through rainscreen cavities.

Straube discovered that it can be hard to make field measurements of air flow rates behind building cladding. “Usually, the air flows are very low,” he said. “It’s not easy to get reliable measurements unless you have very open cladding.”

Straube also noted that it’s possible to get tripped up by bad assumptions. He pointed out, for instance, that not all high wind events correlate with low outdoor temperatures. Moreover, it's important to remember that “most days are not windy.”

Expected air flow rates: 0.3 to 3.0 feet per second

The researchers' paper explains how they used published wind speed data, published test hut data, and calculations to determine typical air flow rates behind a variety of claddings, including brick veneer and vinyl siding.

They wrote, "the flow behind many types of cladding, when used on tall and exposed buildings or on a small suburban house, will fall with the lower end in the range of 0.33 to 3.3 ft/s." In other words, for most type of residential siding, a flow rate of 0.33 or 0.50 feet per second is much more likely than a flow rate of 2.8 or 3.3 feet per second.

Developing a laboratory apparatus to make heat flow measurements

Straube and his colleagues decided to look at mineral wool insulation products with various densities and thicknesses. One question that would require measurements in a laboratory was this one: “Does the air flow behind cladding translate into heat loss?”

The researchers wrote, “An apparatus was developed to measure the impact of cavity airflow on heat flow for a number of mineral wool insulation products. ... Multiple surface temperatures and the electrical heating system voltage and current were recorded at 5 minute intervals with a Campbell Scientific CR1000 data acquisition system. ... Airflow velocity was measured by setting a Dwyer 641RM-12-LEDLight-emitting diode. Illumination technology that produces light by running electrical current through a semiconductor diode. LED lamps are much longer lasting and much more energy efficient than incandescent lamps; unlike fluorescent lamps, LED lamps do not contain mercury and can be readily dimmed. air velocity transmitter with an accuracy of ±0.15 m/s (±0.5 ft/s) in the middle air cavity depth over the center of the meter plate.”

Further details on the researchers' laboratory measurements can be found in their published paper.

Effects are very small

At his conference presentation, Straube explained, “We tested different thicknesses and densities of fibrous insulation, under low wind conditions and high wind conditions. If the insulation is thin — like 1 inch — then high wind can reduce the thermal performance by 9%. But with thicker — like 3-inch — insulation, there is only a 2% hit at higher wind speeds. So as insulation gets thicker, the effect is less. If you use low-density batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. [fiberglass insulation] for exterior insulation, you really have to be careful. But if the insulation has a density over 4 pounds per cubic foot, the effect of windwashing is slim to none.” Many builders use denser types of mineral wool, such as Comfortboard IS, on the exterior side of wall sheathing. Comfortboard IS has a density of 8 pounds per cubic foot.

The researchers’ paper noted that “wind washing impacts are expected to be small for well-installed mineral wool board continuous insulation.” Elsewhere, the researchers wrote, “Heat flow measurements were taken for a number of mineral wool board products. The 25 and 50 mm (1 and 2 in.) thick 70 kg/m3 (4.4 pcf) mineral wool board samples tested showed wind washing impacts to reduce thermal performance as much as 0.03 RSI (R-0.2). These values are small and practically negligible for design considerations.”

A summary of the researchers' findings can be found in the table below. The last two columns of the table show the loss of R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. attributable to windwashing effects at to two different air flow rates: 0.3 feet per second (Column 7) and 3 feet per second (Column 8).

[Image credit: Van Straaten, Straube, Trainor, and Habellion]

Research funding

Financial support for the research conducted by Van Straaten, Straube, Trainor, and Habellion was provided by Roxul, a manufacturer of mineral wool insulation. While there is no reason to doubt the researchers' published findings, I hope that their measurements and calculations are eventually replicated by scientists without any financial ties to a mineral wool manufacturer.

Martin Holladay’s previous blog: “R-Value Scammers Sued By the FTC.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Image #1: Patrick Walshe

Jan 6, 2017 11:30 AM ET

Great info
by Dan Kolbert

I've avoided exterior mineral wool mostly because it is such a huge pain, but wind washing was also a concern. Ah well, perhaps I'll get stuck doing it after all! Thanks a lot, pal.

Jan 7, 2017 12:34 PM ET

Edited Jan 7, 2017 2:18 PM ET.

Mineral Wool
by Malcolm Taylor

Price and availability are all that are holding mineral wool back from general use. It's a great material.

Jan 8, 2017 2:26 AM ET

Cost Comparison?
by Kevin Camfield

Is there any published data comparing the installed cost of rigid foam versus the installed cost of Comfortboard IS?

Jan 8, 2017 6:50 AM ET

Response to Kevin Camfield
by Martin Holladay

Construction costs are intensely local. Mineral wool costs are more variable than other construction materials, because distribution is spotty, and some distributors require minimum orders that are quite large. So you have to do your own research on that issue.

On average, rigid foam is cheaper than mineral wool. If you are able to find a local source for used (reclaimed) rigid foam, then the rigid foam option will be less expensive by a significant factor.

Jan 8, 2017 6:26 PM ET

Pain to work with
by Kye Ford

I can't imagine trying to keep a flat plane for siding using furring strips over mineral wool, seems like more of a hassle than its worth. I know some builders are trying to construct vapor open wall assemblies but I can buy 2" thick sheets of EPS foil faced from Lowes for $25, much easier to work with and not itchy.

Jan 9, 2017 11:15 AM ET

Mineral wool board compressibility
by Jeffrey Stark

Not having even seen a sheet of Roxul mineral fiber board, I can only speculate on its properties. I imagine it has the density of the old Fir-Tex fiberboard, one of the early generation of insulating boards, Fir-Tex was not difficult to work with and, while not hard surfaced or rigid, was also not highly compressible and being lightweight. was easy to throw around. My take is that Roxul Comfortboard is similar and that furring, especially 1x4, would not overly compress the board. Of course, there is always laser planing as a back-up strategy.

Living in the Marine 4 zone and within walking distance of the Pacific Coast trail, there is one word for not using foam in any form or thickness: ants. Once carpenter ants make a tunnel in foam insulation it is all over. Resistance is futile. For one thing, the appearance of ants will not be immediately obvious and by the time they are noticed they have established a colony and have moved to the attic. Controlling ants is not easy and eradicating a colony can take months, if not years. And that is with professional help. I can testify to that having lived through a fire ant home invasion.

That is why I am going with mineral wool board for my whole house renovation. My hope is that ants don't tunnel through reclaimed rock like they would through foam or Fir-Tex, and the board is dense enough to discourage an inquisitive ant.

I know that Straube,, are fans of Roxul, but the mineral wool boards do have some properties that make them the right choice in the right place. The only problem seems to be getting your hands on it in the first place.

Jan 9, 2017 12:07 PM ET

Jan 11, 2017 11:51 PM ET

Edited Jan 11, 2017 11:53 PM ET.

by Burke Stoller

Winroc in Nanaimo stocks it regularly, and they are going to start keeping a much larger stock of it on hand. At this point, it is still good to place your order at least 5 weeks in advance if you are going to need a full house worth (2000+ sq. ft.). I know the Winroc in Victoria often steals our supply from Nanaimo, so you should be able to get it from them too, which sounds like it might be closer.

Using 1x4 cedar strapping as rainscreen, you will find that you can compress the strapping up to about a half an inch at the ends of your rain screen pieces, or at the edges of a sheet of the Comfortboard. Continually checking your walls with a long straight edge across their width and height is a must, fine-tuning your screws in and out to ensure that everything stays co-planar. Not that fun, but not the end of he world. Believe me, I have tried to think of every possible way to incorporate spacers, or compression blocks or other methods to allow one to suck the strapping tight against some kind of spacer, but have had, or heard of, no success in coming up with anything economical or quick. I've resigned to the nuisance of checking with a straight edge. In dozens of full house wraps with the stuff, we have never had a single issue with cladding ending up being "wavy" looking, so it's not as big of a problem as it seems. Good luck with a great product!

Jan 24, 2017 5:33 PM ET

Edited Jan 24, 2017 5:35 PM ET.

Is exterior mineral wool worthwhile in zone 4B?
by Aaron Beckworth

My wife and I are working with a design/builder on a new construction project in Silver City, NM. Rather than adobe or pumic-crete (both of which are realistic options in our area) we have decided on 2*6 framing, blown-in cellulose, an inch or so of continuous insulation exterior of the sheathing, and fiber cement siding. I have read several GBA articles discussing mineral wool insulation, and am considering the additional expense as compared to rigid foam. Unfortunately, zone 4B just isn't mentioned in any of the articles I've found.

Our design/builder has not worked with semi-rigid mineral wool, but has not dismissed the idea. His initial concern is the additional expense of the vertical strapping (rain screen gap) as opposed to attaching the siding directly to foam.

Is semi-rigid mineral wool (Roxul Comfortboard 80) well suited for a mixed/dry climate, and is the additional cost worthwhile? Whether or not we choose mineral wool over foam, would it benefit us to have the rain screen gap?
We have seen several projects recently completed using cement fiber board installed without primer or paint. I am not sure what the siding was installed over or if a rain screen gap was included. The siding is visibly saturated after heavy rain, but seems to dry out without issue. Could this possibly hold up over time?

Jan 25, 2017 5:52 AM ET

Response to Aaron Beckworth
by Martin Holladay

I think that it's always better to install fiber-cement siding over a rainscreen gap rather than to fasten fiber-cement siding directly to rigid foam. For more information on this issue, see All About Rainscreens.

Most builders find it easier to install rigid foam rather than mineral wool on the exterior side of wall sheathing. The biggest challenge is keeping the furring strips co-planar; that can be tricky in light of the squishiness of mineral wool. But if you have a compelling reason to prefer mineral wool to rigid foam, go ahead with your plan. Plenty of builders have had success with exterior mineral wool.

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