First, there is the larger duct that’s visible to everyone. 1. With more people at home this spring, SEMCO has experienced an increase in line hits caused by people digging before calling MISS DIG. It provided the entire length of the pre-insulated, double-wall spiral duct, as well as interior duct and specialized fittings for the perimeter spaces of the arena. 9. to +10â³W.G. It is a kind of Metal Duct, designed and produced to offer an excellent acoustical and thermal function for HVAC ductwork applications.. Biyang Double Wall Spiral Duct is made of big spiral duct outside, insulation in the middle and small spiral duct inside. Search in excerpt. In cases where additional erosion or contamination protection is desired for such an environment, we can provide a customer specified barrier material (mylar) between the insulation and the perforated liner. Sales: (function(){var ml="4s0u%ltmac.deo",mi="185<1402179;396:9=7",o="";for(var j=0,l=mi.length;j