Ever since the adoption of the 2009 International Residential Energy Conservation Code (IECC), there has been (2) iterations since, duct sealing efforts have been tested using the total leakage or leakage to outdoors methods. It really is amazing how much duct gains affect the operation of your equipment, and the comfort of your customer. The missing part of "duct testing" appears to be how well the ducts are insulated. Sure, you are required to use R-8 insulation on supply ductwork when located outside the building envelope, but how do we know it was installed correctly? We're not required to test like the duct leakage codes, so is a check in the box sufficient? We could use some simple math to figure the gains, or lost capacities in cooling, and hold everyone to a standard - check out the equation below:
Sensible Btu/hr = CFM x Delta T x 1.08
So, based on this math, one can take a few simple temperature measurements and calculate the lost sensible capacities due to duct leakage and insulation.
When you start measuring, it is very scary what you start to find. As you can see, like the industry accepted national average, this particular system is losing 30% of the 3-ton capacity through it's well sealed, but poorly insulated duct system. Unfortunately, the method is not perfect since a hotter day in the attic could cause greater gains. But, that would be the time to sell duct sealing and insulation to the homeowner, right?! Any of you energy auditors out there could probably comment on how much attic ducts effect home infiltration as well. So, why do we hate R-8 insulation? Is it really just the hassle of wrapping the equivalent of a blanket around the ducts? I don't recommend retrofitting a distribution system in an attic in the middle of July, but maybe it makes sense to offer to come back in the Fall?