Measuring Infiltration Rate (air leakage)

MikeSpeed6030

A typical infiltration rate for reasonably tight residences is 3-5 air changes per hour. That's a lot of outside air that must be heated (or air conditioned). The traditional way to measure air infiltration rate is with a blower door device, which pressurizes the house. That is somewhat specialized equipment, not readily available to DIYers or even many HVAC contractors.

I don't know if this is a new idea, but there is an alternate approach, not as rigorous as a blower door, involving indoor humidity measurement. The primary cause of dry indoor air is infiltration. In winter, outside relative humidity might be, for example, 50% at 32 deg F. But when that air is heated to 72 deg, the relative humidity drops to around 20%. This is why many people run humidifiers - a more comfortable indoor humidity is around 40% (although lower humidity may be necessary during very cold weather to avoid condensation).

Let's say, for example, that the outdoor temp is 32 deg at 50% RH. With indoor humidification, stabilize the indoor RH at, say, 40%, and then shut off the humidifiers. Measuring the indoor RH with several digital humidity gauges distributed around the house, measure the elapsed time for the indoor RH to drop to, say, 25%. (Meanwhile, keep the indoor temperature constant and avoid any ventilation into the house. If you have a forced air furnace, keep the fan on.) Using a psychrometric chart and some math, it is easy to calculate the air changes per hour.

Info on infiltration here: https://www.energycodes.gov/sites/default/files/documents/BECP_Buidling Energy Code Resource Guide Air Leakage Guide_Sept2011_v00_lores.pdf
Many downloadable psychrometric charts here: https://www.google.com/search?q=psychrometric+chart&biw=1280&bih=642&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjhrfbz__XKAhUDJCYKHbOtBioQsAQIKg#imgrc=u8tm9jBXnONwiM:

Best to use a chart in inch-pounds units and print out the psychrometric chart in landscape. Google for info on how to use a pschrometric chart.

bmwpowere36m3

Interesting idea... or you can pay a specialist to perform a blower door test. A few hundred and you have your answer in an hour or so, as it's the biggest variable when doing heat loss calcs.

I'm glad I did, turns out my 1950s house is really good 0.35-0.5 ach. Granted I performed a full remodel and did sealing while everything was exposed.

Gordy

I don't think that would come close to a blower door test, and much more time consuming for the lack of accuracy.

I also believe lack of humidity is not all infiltration most of it is, but not all.

Jamie Hall

Good idea -- at least to get a rough guide to infiltration. Of course, it assumes that you can raise the humidity to something reasonable!

A note to @bmwpowere36m3 , though. While I applaud your getting your air change down to that low a level -- 0.5 or less ach -- may I suggest that you keep an eye on indoor air quality? At that low a rate, there is an interesting assortment of organic and inorganic compounds, some of which are really rather nasty, which can build up to levels of concern without the occupant necessarily being aware of them.

Mike Thomas_2

The best part about a blower door test, is it not only gives you good numbers, it will also tell you where your biggest leaks are.
find someone who will not only do the test, but help you with sealing those leaks, so you can tighten up the house as you go.