Good move with the scuttle! How did you air seal the ceiling/attic from the heated area below?

Inside the building envelope? I used code compliant plastic vapour barrier stapled to the trusses and run down the walls a foot, all edges and seams were first sealed with acoustical sealant then taped with tuck tape, (can't be to careful) I installed Resilient Channel below that to fasten the drywall to.  Bob, I see you are a Certified Passive House Consultant, seeing I met the blower door part of a Passive house is there any point trying to get my place certified?

Stuart

Of course; you are in Canada where poly is in the codes & required. Without the poly, sealing is more difficult, although doable.

One can certainly get an estimate this way. I did this: In the winter, I established an indoor RH using a logging thermostat with no humidifier running, then turned on a 3 gal/day humidifier in a part of the house with good air mixing, and measured the steady-state increase in RH after a day or two. Repeat a few times to get a wintertime RH increase % per gal/day of water added. This can be converted to ACH_nat in the winter.

I did this before having a blower door done, and tried to predict the ACH50 I would get by upscaling by 25x, and I was close. My 'guess' was a lot closer than his seat of the pants guess. My auditer thought I was nuts.

My biggest doubt would be the moisture that gets adsorbed by stuff in the house--I think that the moisture content of books, furniture, etc., would take some time to stabilize at higher humidity, even a few weeks. But that's no worse than the doubts about guessing ACH-nat from ACH50.

SF6 is the standard tracer gas to use, but given that it's got a global warming potential of a gazillion, it should be phased out. It appears that in Europe, CO2 is now the tracer gas of choice. The difficulty is that humans emit CO2, so it's best to set up logging equipment and leave the house during the test. For $100 you can get a CO2 monitor that will log data every half our for 24 hours:

http://www.amazon.com/Autopilot-APCEM-Desktop-CO2-Monitor/dp/B008NPA18A

Then you just need something like 0.5 to 1 kg of CO2 to pump the concentration up to one to two thousand ppm which should be easily measurable but still safe. You could buy a CO2 cartridge for a sodastream soda maker for $30 to $50, or you could rent a tank from a gas supply place. Or just mix lots of baking soda and vinegar.

Or buy some dry ice. So using CO2 is easy and inexpensive - someone needs to collect numbers comparing this to a blower door test. For example, how much does outdoor temp (changes stack effect) and wind effect the values and can it still be a more accurate predictor of ACH-normal than using ACH50?

The problem I would see with CO2 is it's weight compared to Nitrogen/Oxygen. I think it's weigth is going to cause it to settle out unless the air is really agitated/mixed. It might also adversely effect outflow from a structure downward as it seeks the lowest parts of the room. Carbon monoxide would be the opposite being slightly lighter than air. Probably not as bad as squirting helium into the room, but it will trend upward. the fact that these gasses displace should effect where the sensors are placed.

I wonder if O2 or Nitrogen might be better. Near the weight of air and will stay mixed, easy to monitor O2 or N content, and readilly available gases...

Interesting discussion.

Once mixed, gases in a house don't "settle out". So release the gas, run some fans for a few minutes, then start taking readings. Evidently the results can be similar:

http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1636645

It might be possible to watch the decay of elevated CO2 levels caused by people to get a daily ACH reading. Ie, if the CO2 level is 1000 and then everyone goes to work, it should decay towards outdoor levels and the rate indicates ACH. OK, plants, pets and a HRV (if left on) might be problems.

jonr, that's a really cool idea to try to extract the ACH from CO2 decay after people leave. Then you'd get a dataset over a whole year to be able to see how the temperature and wind affect it. Pets and plants are pretty hard to control for, but there are certainly households without them. HRVs add a variable, but one that is easier to account for, either by factoring it out or turning it off (or down). You'd probably need sensors in many rooms to figure out what's going on--when occupants leave in the morning the bedroom CO2 is probably quite high but not so much in the rest of the house.

I believe that over a one hour period, any gas in a house with room doors open will be well distributed. Experiments with burning things in the kitchen seems to support this. But I should check with my CO2 meter.