HVACR NewsMagazine December 2017

Code, Regulation, & Legislation

R-6 Insulation on a

Supply Plenum

following is typical up flow 3 ton air handler. a

How do you insulate a plenum? Most use a custom made metal box (square or rectangle) with 1” internal insulation. The problem is that the most common “R” factor on 1” internal insulation is 4.2. Code in Arkansas requires R-6 per clarifications published by the Arkansas Energy Office relative to 403.2.1 of the 2009 IECC. In fact, many distributors do not carry internal insulation thicker than the 1” and “R” factor of 4.2. So, how is one to comply with code? The easiest and least expensive way is to externally wrap with the same product you are using on round pipe. There are advantages in that there is less resistance in the duct without the internal insulation and there is less likelihood of mold if the system is producing a mist. On the other hand, if you prefer internal insulation, DuctMate makes a polyester product at 1”, 1¼”, 1½”, and 2” thickness with “R” factors of 4.2, 5, 6, and 8 respectively. Internal insulation that complies with the R6 has other problems as well. The biggest may be the effective ID, inside width, of the plenum. The

The cabinet is only 17½ inches wide and the opening for the duct is 16” wide / side to side. Even at only R4 1” internal insulation, you can see the problem. One can offset the connection so the clear internal width dimension is 16” but frequently the plenum is not offset from the connection so the internal dimension is 14 or 16” minus 1” for the insulation on both sides. Increase the insulation to 1 ½ inch to get the full R6 and now the effective opening is only 13”. Even though it is only insulation, the effective width dimension is still reduced by 12.5% to 18.75%. Without an offset, the effective ID reduces from 16 x 10.31 to 14 x 8.31 for 1” R4 internal liner and to 13 x 7.31 for 1½” R6 internal liner. The effective area of the opening as designed by the manufacturer is 165 square inches. With R4.2 1” liner the area drops to 116.375 sq.in. and 95.06 sq.in. with R6 1 ½ “ liner. At a friction rate of 1.12 the system as designed will deliver 1200 cfm. With 1” internal