Wednesday, October 15, 2008
Because the barn was too tight to produce accurate readings when they were here the first time, Minnesota Power returned to complete the air infiltration testing with a more precise blower door assembly- the results were as good as we could've hoped for. The magic number measured in this test is referred to as CFM50, which represent the amount of air leaking through all the gaps in the building when it is under a negative pressure of 50 pascals. The smaller the number, the better. Most houses today, being of older stock built before we knew much about air sealing, are very leaky, so CFM50 values of 1.0 or larger are common. A "Code" house built today should be .50 or less. To meet the minimum Energy Star requirements, we needed to be .25 or better. The more stringent Energy Star Tier III requirement is .15 or better. To satisfy the even stricter PassiveHaus standards (arguably the most stringent energy-efficient building standards on the planet), the barn's CFM50 would need to be under .08. That said, I was thrilled when our building measured .05 in this test- one of the tightest buildings they have ever measured...we did it! Since the CFM50 value represents the air infiltration per square foot of building size (in other words, the barn was .05 cubic feet per minute airflow per square foot at 50 Pascals pressure), it can then be converted to the ACH50 (Air Changes per Hour) number, which takes into account the total size of the building. This number represents how often the air is exchanged through the building due to uncontrolled leakage. Obviously, when you are trying to conserve precious heated air in our brutally-cold winter climate, the lower the number the better. the ACH50 value for the barn was 0.4, so we bettered the PassivHaus requirement of 0.6 ACH by 33%. This is exponentially better than many of the older homes in this area, and a substantial improvement over even the well-built new homes today. So what does all this math mean? Minimal heating system requirements, low heating costs, and a comfortable indoor environment...we'll take it.