An Insulation Education

Tomorrow morning, we expect a 6:00 a.m. temperature of -31C.

This seems like a great time to talk about how we are insulating our building.  The exterior of the building is completely covered with 1 ½” of rock wool insulation.  The cavity spaces between the wood studs of the exterior walls are filled with approximately 5 ½” of spray insulation.  Together these two thermal barriers provide an effective R-value of R27 for the exterior wall.  There is a big difference between “effective R-value” and the R-values typically discussed in the past.  For example a 2×6 wood stud wall with 5 ½” of fiberglass batt insulation, poly vapour barrier and exterior wood sheathing panels has an effective R-value of about R16, significantly less than the R27 our exterior walls achieve.

This is the result of two major differences between the walls.  First the spray insulation has a higher R-value per inch than batt insulation.  It also provides a more continuous layer of insulation inside the stud cavity.  Secondly, and perhaps more importantly, the rock wool insulation on the exterior face of the building creates a thermal barrier between the exterior face of the wall and the outside environment.  A wall without such a thermal break has only the R-value of the wood studs where they pass from the inside of the building to the outside.  In a typical wood stud wall with studs 16 inches on centre about 15% of the wall would have no insulation.

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