We have a condition at where the contractor has asked to use blue duct for underground locations in lieu of Armaflex insulation and concrete encasement. We specified a perlite type product as backfill around the blue duct with an R value of 0.6 per inch.

The contractor is saying that perlite will not work structurally and wants to use sand. A field rep has said they typically do not insulate because the duct is equivalent to R10.

Since ASHRAE 90.1 requires insulation around the duct and meeting 90.1 is also a requirement for LEED, what type of options does the contractor have? Would armaflex without concrete encasement as the insulating material be acceptable? We have discussed with the project mechanical engineer who says insulation is really not necessary in this condition but is required to meet 90.1.