LEED-H and the Quest for Less Than 4% Duct Leakage Rate

We just finished our sprayfoam insulation which means we also just went through another LEED-H test. As part of our LEED performance path we needed to install insulation to meet the Grade II specifications set by the National Home Energy Rating Standards (HERS index) and the installation must be verified by an energy rater or Green Rater conducting a pre-drywall thermal bypass inspection. We also had to have our air leakage in our duct work tested.

According the USGBC, duct leakage may account for 15 to 25 percent of HVAC energy wasted per square foot of conditioned floor area. This means in a typical house with no special attention to air sealing, air leakage accounts for about one-third of the heating and cooling costs. Today, most new homes have an average air leakage rate of about 0.5 ach or “air changes per hour” (ach), which refers to the number of times in an hour that a volume of air equal to the volume of the house will pass through the building. Sealing major openings in the building shell, such as around windows and doors, plumbing and wiring penetrations—can reduce the average leakage to around 0.3 to 0.4 ach. This could be considered “standard air sealing.” Installing a “continuous air barrier” can cut the leakage rate to 0.1 ach. This would be “advanced air sealing.”

The quest to achieve these type of numbers seemed simple considering everything we have done in preparing and planning for this system installation. We held more than dozen preparatory meetings between the HVAC team including our builder, architect, mechanical engineer, geothermal system installer and duct work installer. We reviewed system requirements, clearly understood all of the testing, we even had prep meetings with our Green Rater to make sure we were doing everything right and going above and beyond in our effort to achieve a phenomenal score.

So a month ago we ran our duct work leakage test and four of the five zones. The two second floor systems passed with flying colors and achieved less than 4% on their first test. We were confident, cocky even, but on the first floor we failed! How could that be? We went through everything; we did it all right…so we thought. We had to prepare to test again. This time we started with the Wine Cellar which had not been tested the last time, we passed, easily surpassing the less than 4% cfm leakage rate. But we failed for a second time on the other first floor zones which include the guest suite and the main floor of the house.

We think what happened was that when we tested it the 1st time and the mastic was still wet, we blew gaps into the seams that didn’t reseal themselves. LIVE & LEARN – don’t test till the mastic is fully dry! Our duct team went back, removed insulation, and reapplied mastic in a number of areas, but didn’t get the register boots in the joist space. So when our Green Rater (actually we had a substitute, Rob from CSG was on vacation) came out this time, he didn’t realize that the register openings were only covered to prevent debris from getting in, not sealed for the air test. We were under the impression from conversations with Rob that they would seal the openings for testing.

Jeanne, our HVAC mechanical engineer,  got there she  sealed all the register openings in the guest zone and we achieved a +14 cfm number. We got to +1 cfm in the remaining first floor zone. Jonathan (the substitute green rater for Rob) couldn’t stay any longer and said that since we were under the 6%, which you need for LEED, he was ok with it. We told him that we were going for 4%, so he said if we had it all set to go he would just pop back in to test sometime within the next 2 weeks.

We have since sealed the inside of the lavette register boot which should get us that ONE additional cfm, and reopened all of the openings in the guest zone and resealed and recovered those; that should get us the 14 cfm we’re looking for there.  

We are under the 6% on both 1st floor systems which was the minimum required by LEED. But don’t have our final leakage rate. With all of the adjustments, we are still expecting a leakage of less than 4% for the entire system.

Our ultimate objective is when we get to our final blower test is a HERS Grade rating of 50 which is half the HERS rating of today’s average home.

With LEED, credit values increase with every decline in the HERS Index. Essentially, for every 2.5 point decrease in HERS, you gain 1 point in your E&A performance scoring. For a given HERS Index, the credit allocation varies depending on whether the project is in Climate Zone 1-5, or 6-8.

But as you can see, this has not an easy task. Forced air heating and cooling systems are a major source of air leakage, even with our GeoExchange water to air system and the best mechanical engineer on the entire planet Jeanne Reddy of Reddy Piping), we still faced the shortfalls of duct work that came with mastic that wouldn’t dry in the wettest RI spring and summer I have ever known. Duct work affects air leakage rates in two ways: through leaks in the system components and by creating a pressure difference between parts of the home and the outside.

Recent studies indicate how severe the problem is:

• In one study, homes with forced air distribution systems used 16 percent more energy than homes with zonal electric heat. All the homes had similar insulation levels.

• Ducted air distribution losses cut heating and cooling efficiency by 25 to 40 percent.

• In one study, the cracks and openings in duct work represented 13 percent of the house leakage area. But when the furnace blower operated, ducts accounted for 70 percent of the air leakage.

• Duct leakage commonly reaches 350 cubic feet per minute during blower operation.

• During operation, air pressure inside ducts reaches 50 pascals (0.2 in. w.g.). That pressure can create 25 times more air leakage through a hole in the duct than the same size hole in the building shell. So, a one square inch hole in a duct is equivalent to a 25 square inch hole in a wall.

If the duct work or the air handler is outside the conditioned space, air will leak through the joints, seams, filter slots, plenum connections and maintenance openings, unless they are properly sealed. This is why we went the extra step of insulation the full roof deck to an R-51 value and covering each stud with sprayfoam. This allowed our roof to have a continuous thermal barrier and place two of our HVAC units and their duct work in a conditioned environment.

Ultimately however, it is Jeanne’s persistence and our determination to get to that less than 4% leakage that will make this happen, but it was not easy. I can see why most builders do not go this extra step.

Posted by KDL | follow me on Twitter: newscaster