3 ms·

Cold air can hold a lot less water vapor than warm air. If you open the window when it's cold outside, then the air will become very dry as it heats up. So eve

Cold air can hold a lot less water vapor than warm air. If you open the window when it's cold outside, then the air will become very dry as it heats up. So even if you have 80% humidity outside on a cold winters day, once that air gets inside and heats up to room temperature it's going to be like 20% humidity.

nitrogen 5y ago
Indeed, even in San Francisco I measured indoor humidity as low as 18% before I got a humidifier, and dryer+colder places (e.g. Salt Lake City) could be even lower inside. Humidity below 40-50% at indoor temperatures has noticeable negative effects for me. As other comments have mentioned, in the long run for maximum health and maximum energy efficiency we really need to design our buildings to be air tight, with energy-recovering ventilation systems and humidity control.
Izkata 5y ago
That happens even if you don't open your windows, since buildings aren't perfectly sealed.
- dredmorbius 5y ago
  It also depends greatly on how well your structure is insulated and where (or whether) vapour barriers exist. Vapour condenses against cold surfaces. This is why in winter poorly-glazed (e.g., single-glazed) windows "steam" up (really: condense), or frost over. Similarly water will condense against exterior walls, and may lead to degraded sheetrock (newer construction) or plaster (in older buildings), or rot. Where walls are insulated, a vapour barrier _before_ the insulation can lead to condensation inside the wall, again, against the exterior (cold) surface. The result is that even if the interior is humidified, moisture is "sucked out" by the exterior cold. (More accurately: vapour condenses against cold surfaces.) In summer, the problem is reversed, with warm humid exterior air condensing on surfaces cooled by interior air conditioning --- the outside of windows, or again, inside walls. Interiors can see higher relative humidity than the outside, and dehumidifiers may be needed.

nitrogen 5y ago

Indeed, even in San Francisco I measured indoor humidity as low as 18% before I got a humidifier, and dryer+colder places (e.g. Salt Lake City) could be even lower inside. Humidity below 40-50% at indoor temperatures has noticeable negative effects for me. As other comments have mentioned, in the long run for maximum health and maximum energy efficiency we really need to design our buildings to be air tight, with energy-recovering ventilation systems and humidity control.

Izkata 5y ago

That happens even if you don't open your windows, since buildings aren't perfectly sealed.

dredmorbius 5y ago
It also depends greatly on how well your structure is insulated and where (or whether) vapour barriers exist. Vapour condenses against cold surfaces. This is why in winter poorly-glazed (e.g., single-glazed) windows "steam" up (really: condense), or frost over. Similarly water will condense against exterior walls, and may lead to degraded sheetrock (newer construction) or plaster (in older buildings), or rot. Where walls are insulated, a vapour barrier _before_ the insulation can lead to condensation inside the wall, again, against the exterior (cold) surface. The result is that even if the interior is humidified, moisture is "sucked out" by the exterior cold. (More accurately: vapour condenses against cold surfaces.) In summer, the problem is reversed, with warm humid exterior air condensing on surfaces cooled by interior air conditioning --- the outside of windows, or again, inside walls. Interiors can see higher relative humidity than the outside, and dehumidifiers may be needed.