Some friends were vising from the US recently and Jack suggested I check out a blog called Do The Math. In the blog, Astrophysisits Tom Murphy shares his views on “societal issues relating to energy production, climate change, and economic growth. [His] approach is often playfully quantitative, with the aim of arriving at a fresh perspective on our world.”
I’m not sure if Jack knew there was a nugget in there for me, but sure enough, a fairly technical blog post was written up about the air leakage properties in buildings, and demonstrated techniques of using a blower door for apportioning leaks throughout the house! I was stunned as Dr. Murphy delves into some meaty stuff for building scientist in this post titled Let’s Blow this Joint. Once you get past his use of the word ventilation (we building scientist know ventilation means mechanical ventilation – blow/suck/balanced), which he equates with natural air infiltration, it’s a must read for any building scientist.
In a nut shell, this is what I got from it:
- In the blog, he gives a great link to a Lawrence Berkley Lab document that helps you convert an Air Change per Hour at 50 Pascals (ACH50) of depressurisation to Air Change per Hour Natural (ACHNat) or at standard atmospheric pressure.
- He delves into the fact that air leakage isn’t linear with pressure be it induced by stack or wind effect. Though counter intuitive air leakage is more accelerated at low pressure differences than at high. Translation; it doesn’t take much pressure to move lots of air so even. “The littler gaps matter” as he says of his window experiment.
- Room by Room: I had read about doing room by room testing in Home Energy magazine, but Dr. Murphy takes it one step further and makes me think I need the DG700 WiFi attachment so I can walk around the house closing doors get get my data without having to go back to the fan! I love the fact that he even managed to break down and attribute the air leakage to specific item like a pot light.
- Conductance: I’ve never heard of dividing the Cubic Feet per Minute (CFM) by the pressure (Pascals) in this case at 50 Pa. I think this way of normalizing the data might be very useful for comparing leakage from one room to another.
Overall, it was a fresh read and I really appreciated his explanations plus he reminded me of the power of a blower-door as a diagnostic tool when used by the right people!