Windwashing in Exterior Mineral Wool
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?
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 GBAGreenBuildingAdvisor.com. (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.”
Insulating Sheathing for Residential Construction by John Straube
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]
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.”
- Image #1: Patrick Walshe
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