Drying clothes: windy but cold outside vs warm(ish) inside?

Question

I know there are a few questions which are similar to this, but I'm asking for facts, about which is definitely better.

We have only one clothes rack but we have a lot of laundry to get through.

Each load needs washing and drying separately because only load fits on the rack (obviously heavy duty items go straight on the radiator).

My question is this: When the weather is cold which is better, putting the clothes rack out on the balcony or in the front room?

Some assumptions:

• It's around 13 degrees Celsius outside
• It's around 19/20 degrees Celsius inside
• There is a light breeze outside, but it's not sunny
• I don't have a dehumidifier
• By "better" I mean quicker

There is a partial answer on this question: How can I get damp clothes to hang-dry faster in the home? but I am looking for hard facts.

Answer 1

The best hard facts are the ones you establish for yourself under your specific conditions.

One experiment is worth a thousand good opinions. — Bill Nye, the science guy.

Determine which location will be more efficient (easier, better, faster, etc.) by taking two identical cake pans filled with the same amount of water and placing each in the locations you want to compare.

After you wait a bit, compare the two water levels by weight or by refilling to a known level (volume).

You will now have the hard facts you need regarding which location will serve your purposes "better."

Have fun. Good luck.

Answer 2

Assuming that it’s dry outside, windy and cool is always better than stagnant air.

The rate of evaporation depends on two factors:

1. The absolute amount of water the air can hold and
2. The difference in humidity between the laundry and the air around it.

While cooler air can hold less water than warmer, the relative humidity inside is very often way higher than outside, typically because we humans produce moisture in our daily activities, from simple exhalation to cooking and bathing. And hanging laundry will add more water, which can bring the air to quite high levels quickly.

Outside, the absolute amount of air is so much, that combined with the movement due to the wind, the laundry will dry quicker - if drying indoors, you’ll find yourself opening the windows regularly, to bring the humidity down again, so you can as well put the rack outside.

Answer 3

The criterion I would use is the outside relative humidity. If it's very humid, then clothes will probably dry just as well indoors, and there may even be a risk of rain. If the humidity is below about 75%, outside drying is probably more effective even if it's somewhat cooler.

The exception I would make to this is if the outside temperature is near or below freezing. The maximum density of fresh water is near 4°C, which I think is a reasonable cutoff point. Frozen moisture is much harder to get out of clothes than when it's liquid, so in this case it's always better to keep them indoors. (Sublimation, mentioned in another answer, only occurs at useful rates at pressures far below normal atmospheric.)

Answer 4

Simple solution: compare the relative humidity

The relative humidity is the major determining factor here! To be more precise, the lower the RH, the more quickly water will evaporate into the air, so you want the environment with the lower RH (as a percentage).

Complicating factors

There are several factors that can affect your decision, however:

Interestingly, RH is temperature dependent-- hence the term relative humidity. RH is a measurement of how much moisture is in the air vs. how much it could have. The warmer it is, the greater the absolute moisture content can be in the air. Therefore, if you start off with a house where the temperature and humidity are the same throughout, you could reduce the RH in one room by raising the temperature with a space heater, stove, or fire. Outdoors, barring weather fronts, you are more likely to encounter changes in temperature and RH that follow the time of day. (Make sure you don't leave clothes out overnight or water can condense back on them if it gets colder than the dewpoint, which is when the RH hits 100%.)

Air movement also makes a big difference. If there is no air current the air containing the water vapor will remain close to its source, with the result that the wet clothes will be surrounded by a zone of high RH and therefore cannot continue to dry at the same rate. Usually there is much more air movement and therefore air exchange outdoors.

Air conditioners prevent spikes in RH inside your house by collecting condensed water from the air as it is cooled down, which means that it is constantly drying out the air in your house-- but it usually just keeps the RH constant because the temperature is dropping.

Clothes spread out for maximum surface area and air flow should dry eventually in either environment, but not so well, e.g., in an indoor swimming area or locker room with tons of showers in constant use-- the evaporation of those water sources will keep the RH elevated. A minor consideration: mold growth happens at high RH, but things need to be constantly damp for about 48 hours before it becomes a problem. If it's really humid indoors and out, consider setting up a heat source to dry out (lower the RH of) the air.

If you want to dry your clothes inside, you'll have the most success if you hang clothes in bigger, warmer rooms, leave doors open between rooms, and set up fans (or dehumidifiers, if you want to invest).

Experiment: which is best for your situation?

This changes with the weather, so you'll have to make a new assessment each time.

If you want to figure out precisely which is better for you via quantitative experiment, you can buy a cheap thermohygrometer to measure the RH in your house. (Here's one for $5 on Amazon, though I encourage shopping locally if you can!)

You can place it in the space where you would dry your clothes and compare that reading to the published weather stats for the general outdoor RH that day and time-- or you can take your handy new hygrometer outside and get a new reading for comparison (give it a minute to adjust). If they're not wildly different, you'll probably get faster drying (not to mention a little bit of natural fabric softening) where there is more of a breeze.

You can also try a practical experiment of drying clothes inside and checking the RH next to them as they start to dry-- the space they are in will probably increase in RH a bit unless you have good air movement; if your indoor drying space gets much too humid you might set up fans for circulation, or just plan to dry clothes outdoors.

Answer 5

A technique used in some parts of Europe to prevent wintertime development of indoor mould is to ventilate the dwelling once or twice daily and to keep all windows and doors closed at other times. For example, Germans call this Stoßlüften or "shock-ventilation". This also replenishes the home with oxygenated air owing to the windows normally being closed in winter.

The process relies on periodically displacing as much as possible of the interior air -- which is comparatively warm and moisture-bearing -- with fresh air from outside. The new air is allowed to then warm up and absorb water vapour from indoor activities like cooking and laundry. And warm air can hold more moisture, and thus appears drier to clothing, than the same air when cold.

Recommendations are to open all windows and doors on opposite sides of the dwelling for 5 minutes or more so as to cause a draught that carries the interior air away, but windy conditions could reduce this time. It won't work well in misty conditions.

I have used this technique to dry laundry, however, if the temperature and RH differences between inside and outdoors are not significant then the effort can be in vain, and outdoor drying might be more effective. Your local conditions could be borderline. To reduce the burden, perhaps place the laundry in a room with a radiator and shut all doors and windows to the room, then experiment with briefly ventilating the room every 2 hours.

If the technique works for one rack of clothes then it will scale to multiple racks.

Advice from a German government department about ventilation (in German):

Translations of the text:

Proper ventilation - like so:

• Shock-ventilation several times a day with windows wide open.
• Best by opening opposite windows ("draught").
• Ventilate with the radiator turned off. (I ignore this step for brief occurrences.)

Source: https://www.bmu.de/themen/gesundheit-chemikalien/gesundheit-und-umwelt/innenraumluft/richtiges-lueften-und-heizen/