This credit is designed to minimize duct leakage. (Duct leakage? Sounds gross.) Just as a home’s envelope should be tight to save energy, so should the air duct system – the system that distributes hot and cold air throughout the house, usually invisible to us homeowners. According to the USGBC, duct leakage may account for 15%-25% of total heating and cooling energy use. There is also a health and comfort issue: leaky ducts can draw moisture, dust, and other contaminants into the house; they also may distribute air unevenly throughout the house, so some rooms are hot and some are cold. While it never occurred to me to ask our HVAC sub-contractor if the duct system they were installing would have minimal leaks, now that I understand this credit, it makes perfect sense.
So, how do we know? Again, a green rater has to come in and test the leakiness of the duct system (just as he came in and tested the leakiness of the building itself through a blower-door test). This test took the longest, because all air supply and returns had to be sealed before our green rater could run the test. He sealed them with a plastic-wrap-type product, and then ran the test through an air return, as shown in the photo.
The requirements for this credit are divided into two types: one for forced-air systems, and one for non-ducted HVAC systems (also called hydronic systems, because they use hot water to run through radiators or in-floor pipes to heat the house). We do have a hydronic system to heat the house through in-floor heating — it is very comfortable heat and quite efficient when used in conjunction with our geothermal heat exchange system. But we decided to have an air duct system as well, for two reasons: it’s the only way to have air conditioning/dehumidification, and well-designed air systems are necessary to keep the indoor air quality clean and healthy.
The prerequisite for this credit is as follows:
a. Limit duct air leakage rate to outside the conditioned envelope. The tested duct leakage rate must be ≤ 4.0 cfm at 25 Pascal per 100 quare feet of conditioned floor area, verified by the energy rater.
b. Do not install ducts in exterior walls unless extra insulation is added to maintain the overall US for an exterior wall without ducts. Ducts may be run inside interior wall cavities but must be fully ducted (i.e., do not use the wall cavity as the duct).
c. Use at least R-6 insulation around ducts in unconditioned spaces.
We meet the prerequisites, and perhaps get some points here. In 5.2 Greatly Reduced Distribution Losses, we can get 2 points if duct leakage is ≤ 3.0 cfm at 25 Pascals per 100 square feet of conditioned floor area. Our green rater, Jimmie Sparks of the Neighborhood Energy Connection, gave our home 108 CFM duct leakage to outside. He also counted our garage, mechanical room and crawl space in the total “conditioned area” for square footage. Using that calculation, we get 1.67 CFM, easily meeting the requirement for 2 points.
OR we can get 3 points in 5.3 Minimal Distribution Losses by meeting just one the following:
a. Limit duct air leakage to ≤ 1.0 cfm at 25 Pascals per 100 square feet of conditioned floor area. (We don’t meet this.)
b. Locate the air-handler unit and all ductwork within the conditioned envelope and minimize the envelope leakage (i.e., meet the requirements of EA 3.3). (Our air handler unit and all ductwork all located within the conditioned envelope. One of the benefits of the geothermal heat exchange system is that there is not air conditioner outside. But we don’t meet EA 3.3 — we meet EA 3.2.)
c . Locate the air-handler unit and all ductwork visibly within conditioned spaces (ie., no ductwork hidden in walls, chases, floors, or ceilings). (This would be a very interesting design element and probably help us all be better connected to how the air is distributed throughout the house. Our builder actually has this in his house; it looks pretty cool but very has a very industrial aesthetic. Most people would not consider this, however, unless they view ductwork as artwork.)
Again, I am surprised that we are not at the 3-point level for this, as our ductwork is not only tightly sealed, but we also have no outdoor air conditioner. In the Performance Path (the HERS rating method), we are at the 5 Star Plus rating, so I assumed each component would be at the highest level. Are our ducts a little leaky? I don’t think so; maybe it’s just because our house is long and thin, and the ducts had to be tightly maneuvered to get everywhere.
I should note here that we considered a high-velocity ductwork system — it requires much smaller ductwork everywhere, but more air supplies and returns. This was an issue between our builder and architect. (There is never enough room designed for ductwork, according to most builders; according to most architects, builders always seem to figure out a way to make it work.) We decided against high-velocity because it can be a little loud when it goes on and off. We also liked the system with a variable-speed fan — meaning the air handler could be set really low, so we would barely feel it or hear it, but it would still be filtering the air. Since our number goal was a healthy home, we liked this benefit.
EA Prescriptive Path points so far: 6