Fibrous insulation materials like mineral wool do not stop air flow. Unlike rigid foam (which is a pretty good air barrier, 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 sheathing? Is the thermal performance of the mineral wool degraded by wind?
This question comes up every now and then on GBA. (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 envelope 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…