Orest writes from Lloydminster, Alberta: "We have a metal building insulated with aluminum foil backing. The insulation is squeezed between the perlins and the metal roof. We would like add more insulation either R12 or R 20 and then cover this with metal sheeting. Would we need to have vapor barrier between the metal that we are installing and the pink insulation batts? Also as we live in Alberta Canada would we have to cut open the original aluminum foil to permit proper air flow and aeration?"

Answer: Adding insulation to the actual foil-faced insulation is feasible. First rule to follow is a vapor barrier ALWAYS goes on the warm-in-winter side of the insulation in Canada (the only exceptions to this rule in North America are found in a few very humid areas in the U.S.). Aluminum foil is a vapor retarder so if we add insulation in front of it with another vapor barrier this will not be correct and you will run the risk of trapping moisture between the two vapor retarders.

For your project, you can leave the foil as is if the amount of thermal resistance you add is not one-third of the TOTAL ROOF assembly thermal resistance. If the R-value for the new layer of insulation is two-thirds or more of the total R-value of the roof, you will need to cut open the foil on the older layer prior to installing the new insulation and vapor barrier. If the R-value of the new layer if one-third or less of the total roof R-value, you will not need a vapor retarder and will not need to cut the foil open.

On a final note, make sure there are no gaps, or as little as possible, between the layers of insulation.