By Antoine Habellion, M.Eng., M.A.S.
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
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:
- orientation; and
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.