In the past few years, the Finnish building code's demands for energy efficiency has increased the thickness of insulation layers in building envelopes. Natural convection is considered in the building code by not allowing structures whose modified Rayleigh's numbers exceed the limit values. However, it has been unclear how considerably the natural convection can affect the heat loss through envelope in instantaneous or annual level. The aim of this study was to inspect the actual effect of natural convection in relatively thick, air-permeable vertical insulation layers by laboratory measurements and computational modeling.

Several U-factor measurements by calibrated hot box method were carried out to wood frame test walls with mineral wool or cellulose insulation. Attributes presumably affecting the convection process were varied. Poor workmanship's effect was studied with intentionally sloppy installation work and with intentional horizontal air gaps in the insulation layer. Careful installation with the convection barrier was used as a reference where the effect of convection was supposed to be negligible. Measured U-factors were compared to numerically calculated U-factors and to simplified U-factor calculations. Permeabilities and thermal conductivities of the used materials were measured for the calculations' initial data.

Measured and calculated results were in relatively good agreement. Results showed that during cold periods natural convection can slightly increase the actual U-factors of exterior walls. The convection barrier can be used effectively to restrict natural convection as demanded by the building code. However, the quality of workmanship should not be forgotten when pursuing a very low U-factor by increasing insulation thickness.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings XII, December 2013