Humidity and Insulation

Using calculations for the thermal conductance of air and water, regardless of evaporation, shows that the insulating effectiveness of the airspace in your clothing diminishes dramatically when a small amount of water is introduced (% of Water in Air refers to the percentage volume of air replaced by liquid water):

% of Water in Air	Conductance (Wmֿ¹K ֿ¹)	Increase in Conductance	% of Insulation of Dry Air
Dry air	0.025	1x	100%
10%	0.0825	3.3x	30.3%
20%	0.14	5.6x	17.8%
30%	0.1975	7.9x	12.7%
40%	0.255	10.2x	9.8%
50%	0.3125	12.5x	8%
60%	0.37	14.8x	6.8%
70%	0.4275	17.1x	5.8%
80%	0.485	19.4x	5.1%
90%	0.5425	21.7x	4.6%
All water	0.6	24x	4.2%

This table shows that when just 10% of the dry air in your clothing is replaced by water, its insulation is less than 1/3rd! Introduce 50% water and the insulation value is only 8%!

This is because water conducts heat 24 times more than dry air!

Note: Evaporation has an even bigger effect, as it takes about 550 times more energy to evaporate it than raise its temperature by just 1'C!

Comments

[Christopher Sleight]

Interesting figures, thanks for posting them. I've always found that I need less insulation to stay warm when wearing a soft shell rather than a hard shell. These figures would suggest that's probably because the air trapped underneath my very breathable soft shell is much less humid.

[John Smith]

The wording "X% of the dry air in your clothing is replaced by water" is somewhat ambiguous.

Does the % refer to the relative humidity of the air? (100% being 100% relative humidity, i.e. air at its dew point). Or is it replacing 10% of the air volume with liquid water, so 100% means a soaking wet sleeping bag? Those situations are VERY different, which one do you refer to?

Another way of stating this: Does the 100% situation refer to a foggy night or does 100% mean a sleeping bag soaked in water?

[FurTech]

Just to clarify, what the table refers to is the percentage volume of air replaced by water. In effect the saturated sleeping bag scenario, not humid air.
Of course, the real life impacts of saturated insulation are much greater than even these figures suggest, as evaporation has an enormous effect. The figures also completely ignore the conductance of the insulating materials and fabrics or if they absorb water, as wool does. It also ignores the issue of how water may fill the volume without draining away or clumping up the fibres. For example it's quite difficult to fill the voids in a Blizzard Bag. On the other hand a down bag will lose much of its loft when it gets wet, so the volume actually reduces.