Three common continuous mineral fibre myths debunked

by carly_midgley | April 5, 2017 10:24 am

By Antoine Habellion, M.Eng., M.A.S.

Misinformation is unfortunately common in the mineral fibre insulation industry.
Photo © BigStockPhoto

The Pseudologoi were the personifications of lies and falsehoods in Greek mythology. Myths are fine—they are stories people know to take as parables, rather than interpret literally. Falsehoods, however, are dangerous. With the explosion of social media and self-proclaimed experts commenting on building industry websites and blogs, personal views and false information (whether intentional or not) are now everywhere. It can be difficult to sort through the falsehoods to find the truth.

‘Pseudologoi’ occur in the building industry when experts specify products on the basis of this prevalent misinformation. The buildings they construct become the tangible embodiments of falsehoods, and the repercussions can be serious, ranging from wasted money to buildings that may not perform as planned. This article discusses three common falsehoods about mineral fibre as continuous thermal insulation, relating to wind-washing, water absorption, and loss of R-value in cold temperatures. The data provided was obtained by third-party testing labs.

Myth #1: Wind-washing impedes thermal performance

Cavity air-speed impact on sheathing thermal performance.
Photo © Randy Van Straaten. Photo courtesy Building Science Laboratories | Building Science Consulting Inc.

Wind-washing is defined as wind-driven air passing through or around insulation. Bulk movement of air can carry or ‘wash’ significant amounts of heating out of a structure. The exposure of insulation to wind determines its impact.

Variables dictating the force of wind acting on a building include:

Therefore, wind loads on a building are not always constant; they vary from low to high concentrated loads. High concentrated wind loads only come from gusts, and typically do not last long enough to have any meaningful effect on the performance of the continuous fibrous insulation. The effects of these gusts are also specific to the type of façade, and do not have the same impact on the windward side as on the leeward side. For instance, cavity airflow is higher behind a loose open-joint siding than it would be behind a brick veneer with weep holes, as it has a higher air permeance. Most of the time, the continuous fibrous insulation is protected by a cladding or wind break, which drastically minimizes the impact of the wind loads acting on the insulation.

Not all fibrous insulation is meant to be installed as continuous. For instance, some stone wool manufacturers have designed specific rigid mineral wool board materials with a higher density to serve this application. This higher density also minimizes the impact of reduced R-value due to airflow over and through the product. Third-party wind-washing testing on typical exterior insulations—including fibrous products—at both low and high levels has shown wind’s impact on thermal performance is negligible (see the image above). With a high-density insulation material on the interior side of the cladding, significantly reducing the impact of the wind gusts occurring periodically and windward forces occurring on façade areas only, designers should not worry about the impact of wind-washing.

Myth #2: Water absorption reduces mineral fibre’s effectiveness

Typically, moisture is the main cause of building failures. Therefore, designers and architects have many concerns to address when designing their enclosures for durability and performance. There is an unsubstantiated perception in the industry that fibrous continuous insulation absorbs water and takes many hours to dry out, but this is not the case.

As it does with wind, the façade or rainscreen effectively blocks approximately 90 to 95 per cent of rain as it falls on the building. The remaining five to 10 per cent tends to drain out the back side of the cladding, but a small amount may fall on the face of the insulation. The s water hitting the face of the insulation repels off the product, as the fibres are treated with water-repellent oil. Moreover, the non-directional fibres create small voids in the insulation board, which minimize the wicking of water into the product, similar to water draining though crushed stone at the foundation wall. Gravity also helps propel water down and to a location where it can be directed outside of the enclosure. This is even the case with open-jointed rainscreen systems.

The image below shows an open-jointed rainscreen with the water absorption of extruded polystyrene (XPS) board insulation compared to mineral fibre board insulation, as well as the elapsed drying period. As shown in the chart, the difference between the two is minimal. The perception of water absorption in mineral fibre was derived from submerging mineral wool in water for two hours, then weighing it and seeing how long it takes to dry. If a wall enclosure sees this amount of bulk water in two hours, mineral fibre is the least of the design team’s worries.

Comparison of drying after a 10-minute wetting test at 10 Lpm (2.6 gpm) with drainage troughs.
Photo © Jonathan Smegal. Photo courtesy Building Science Laboratories | Building Science Consulting Inc.

Myth #3: Mineral fibre loses R-value at cold temperatures

Graph of stone wool guarded heater plate results.
Photo © Ivan Beentjes and John L. Wright, University of Waterloo

Since fibrous insulation does not use blowing agents in the manufacturing process, there is no potential for R-value loss due to temperature. R-value of fibrous insulation actually increases when the temperature decreases, due to the radiation function of heat transfer, which dramatically drops with the temperature (as shown at right). Third-party testing of fibrous insulation supports this as well. This is similar to most other insulations, although certain products, such as polyisocyanurate (polyiso), see a decrease at low temperatures. The reason for the decreased R-value in these insulation products is the fact the blowing agents in the products condense in the cells, which increases the thermal conductivity of the insulation. This does not happen in fibrous insulation products, as the R-value is obtained from trapping still air.


For the most part, there is no such thing as bad insulation—just bad application. Using a higher-density fibrous insulation as a continuous layer is a very good application, and there should be no concerns regarding wind-washing, water absorption, or loss of R-value. It is important to understand fibrous insulation is only a component within a system, so a complete understanding of the enclosure and its performance should always be achieved.

[5]Antoine Habellion, M.Eng., M.A.S., is a building science project manager at ROXUL Inc., where the majority of his work entails managing research projects related to energy efficiency, enclosure performance, summer comfort, hygrothermal analysis, building information modelling (BIM), durability, and resilience. He holds master’s degrees in civil engineering (from Hautes Etudes d’Ingénieur in Lille, France) and green building design (from Arts et Métiers ParisTech in Paris). Habellion can be reached via e-mail at

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