Emily: A quick way to make the call would be to look at the outdoor dew point temperature, yes. If it’s below about 15 °C, it’s not a problem to open the windows.

(goal is to decrease moisture inside!)

Thanks for the help – posted from the very wet la niña in eastern Australia!

I’d also add that negative indoor pressure can be undesirable because it can draw crawl space air (with all the attendant damp, spores, smells, radon, etc.) into the occupied space.

I find myself referring to dpcalc.org often! I believe mould risk is also a function of temperature – at very high or very low temperatures it cannot grow, regardless of availability of moisture – but in this case, I agree that it’s the low RH that makes it low risk.

I’d add that indoor dew point can be a useful metric when trying to avoid winter condensation on surfaces such as aluminium window frames with no thermal break (common here in New Zealand). But even if you avoid condensation, the RH near the cold surface can still increase to the point where mould can set up shop. Without ventilation, it can be a challenge to keep the indoor dew point low enough. Of course, if you do have MVHR, the dry outdoor air in winter really works in your favour!

Excellent observation – I’ve certainly found that heating indoor spaces generally raises the dew point, sometimes considerably. I wonder if Joe Lstiburek would call this “accidental humidification”. 🙂

Tom, I tell people all the time that 55° F is a special number for air conditioned spaces in humid climates. That’s the dew point temperature for air at 75° F and 50% relative humidity.

I live in SoCal and in Europe before that, where opening windows was considered quite normal albeit oldschool. This article about dewpoints is excellent for when I want to humidify or dehumidify the house, heat it or cool it, or some combination. Closing the windows when we leave is not much more effort than opening them. In a strange way I feel good about opening windows, just like I do about using my car’s horn, or functional buttons on a suit sleeve: They seem to exist for a reason. To me this feels like a feature of the house.

@Ken, if conditions are appropriate to open windows and operate a fan, I don’t think we have to worry about condensation due to cold sheathing 😉

When talking about year-round ventilation design, supply ventilation (positive pressure) pressure is preferred over exhaust ventilation in warm & mixed humid climates like Allison’s (Georgia) so as to avoid condensation on back of drywall in summer. Conversely, exhaust ventilation is preferred in cold climates so as to avoid condensation on back of exterior sheathing in winter (and also why we want some insulation outboard of sheathing, and why vapor retarder goes on warm side in both warm-humid and cold climates).

However, rather than taking a fan and driving the drier outdoor air into the building, (thus positively pressurizing the wetter indoor air), I would configure a strategy that pumps the wetter indoor air to the outside.

If you positively pressurize the wetter indoor environment… it can be driven into the exterior cavities and in some cases, encounter the cold exterior sheathing and condense. That’s not good.

But under negative pressure… no issues are created.

Good message though!