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Wool and Humidity

This page focuses on the physical effect that water vapor (humidity) has on wool.

We have other pages on this site that talk about how water generates heat when it picks up water vapor, and how wool sheds liquid water ...

But this page shows a few questions I posed to some wool people (academics and industry professionals), and the answers I got from Padula.  Nobody else responded, which is kind of what I expected.  But I keep writing to the academics anyway.  Hopefully, it will help somehow.


I have a question, please.   Hope you don’t mind me including several of you.

When wool picks up water, does the fiber get thicker?   If dry wool can ad/absorb water weighing as much as 30% of the wool itself, where does the water go?  Are there empty spaces inside the wool that the water fills?   Or does the wool swell, or????

Padula's answer:

Yes, wool expands as it takes in moisture.

That is why it is tested under standard conditions.

So of course I had more questions:

If one of our jackets is out in humid weather how thick can the fiber get?   And how does that increasing thickness affect the wind resistance?

……. The wool is giving off heat as it draws in the humidity.   And I am guessing that when the fibers get thicker, the coat also resists wind better?  With no wind, would the insulative properties of the coat change?

Padula answers again ...

This is where the academics will and can really confuse reality….

There is a difference between individual fiber measurements and practical application. 

While you can measure individual fibers, they will vary more in diameter along their length, than how much it will “expand” due to moisture gain or release.  Any difference in individual fiber diameter will not be practically detected or noticed, so what if it expands 0.25 or 0.3 micron - you can’t see that difference and it would be within the accepted limits for testing. 

The yarn is not a single fiber - so all the fibers would have to “behave” or react the same way - and moisture gain/release is not consistent, it’s more of a (sorry for this pun) “a fluid or flowing” motion or movement.  And, it will be in constant motion if you are wearing it, because your body is not “standard” conditions.  Think of thermal imaging and how that looks. 

Woolen yarn is different than worsted, and that bulkiness is “air” - which is the “insulating” part.  

Most of the new moisture management discussions are based on next to skin wear and that is generally worsted.   It’s different when the yarn has 40 fibers, versus 140 or more in a woolen yarn. 

So, back to your questions.  In theory,  The fibers will get thicker, filling up that “air space” and air flow or wind resistance would be diminished.  

Not knowing when to quit, I kept going:

So … here’s another thing I’d like to know about … I keep hearing/reading dry wool can pick up as much as 30% of its weight by gathering moisture from the air.  How humid does the air need to get for the wool to add that much water weight?   Also, wool is often traded based on a theoretical weight at 15% humidity.   So … at 15% humidity, how much water is 100 pounds of wool actually holding?   I’m guessing it’s a lot less than 15 pounds!   Here in New Jersey, our humidity is almost always above 50%.   How much water-weight is in the wool stored here at my house?

Padula again:

Ah yes…. This whole regain thing is complex.  And textiles are different than raw wool fiber testing.

Again, this is why testing at standard conditions is so important.   Not only for “research” or scientific knowledge, but commerce is business and dollars.

Standard conditions is both temperature (20 C +/- 2) and relative humidity ( 65% +/- 3) - so if the tests are not done within those parameters, and that includes “pre-conditioning” - the results can be skewed. 

It’s generally agreed that at standard conditions, wool naturally has 14% moisture, for a regain of 16% (water content expressed as a percentage of the oven dried weight). 

In general, let’s just use 16% and so 100 lbs of “wool” would have 16 lbs of water….. and for explanatory purposes and ease use a 10% change in RH will result in about 1% change in weight (SGS Bulletin 4.1B 2014)

100 lbs of scoured wool or clean wool at 65% RH would have 16 lbs of water.  At 75% RH there would be 17 lbs of water.  At 45% RH there would be 14 lbs of water…. At 25% RH - there would be 12 lbs of water. 

So the difference between 75% and 25% RH  is 5 lbs of water… 5 lbs of wool is one price, 5 lbs of water is another….  Think of 100 bales of wool (a container or 40,000 lbs)…. That is 2,000 lbs of “water” vs “wool”. 

Wool is “soaked” and “dried” many different times during the processing from raw wool to fabric state and past/previous experience of the fiber can also have an impact on moisture/regain.  It’s important to know what state or stage of production.

Practical application ….  All things bring equal…. 

Your 5 lb jacket at 65% RH has 0.8 lbs of water (16%) and 4.2 lbs of “other” weight:

  • 75% RH - it will weigh 5.05 lbs
  • 65% - 5 lbs
  • 55% - 4.95 lbs
  • 45% - 4.9 lbs
  • 35% - 4.85 lbs
  • 25% - 4.8 lbs
  • 15% - 4.75 lbs

You might be able to detect a weight difference between 75% and 25% …. But in between are you going to be able to notice say 0.05 lbs … on a 5 lb garment…. While wearing it, dispersed across your body as you “respirate” and change the humidity in your own “micro-climate “ ….

Now you know why I started off with how this is complex.  And this was just the easy stuff, an SGS bulletin…. Not anywhere close to Von Bergen material.


Updated 5 July 2024 --- Ralph