1. Ecole Polytechnique, Case postale 6079, succursale Centre‐ville, Montréal, Québec H3C, Canada
2. National Fire Laboratory, Institute for Research in Construction National Research Council, Ottawa, Ontario K1A 0R6, Canada
3. Département de physique, Université de Liège, B5‐Sart Tilman, B‐4000 Liège, Belgium