By the Numbers: The Energy Cost of Drying Your Base Layer

It also validates the old wisdom of wringing out your clothes when you fall into a stream/lake.

Very interesting article, thanks for posting. I’m curious how cotton would’ve compared – did you collect any data on cotton?

Hugh,

Your process and resulting conclusion are valid but missing one added step after wringing; tightly wrap it in a pack towel or sacrificial dry garment and then stomp on it to remove about 50% of the remaining water before putting it on. For most of the people, the following details are of little or no value (smile).

Below is a back-of-the-envelope comparison that shows how much extra heat a person would have to supply to dry an Alpha-Direct hoody (light polyester) and a mid-weight 100 % wool hoody after full immersion.

Key assumptions (you can scale the numbers up or down if your garments differ):

1.   Dry mass
• Alpha-Direct hoody ≈ 0.25 kg
• Wool hoody ≈ 0.40 kg

2.   Water remaining in the fabric
(values are typical of field tests; they differ because polyester doesn’t absorb inside the fibre, wool does)

• After simple immersion/drip:
– Alpha ≈ 2 × dry mass → 0.50 kg water
– Wool ≈ 4 × dry mass → 1.60 kg water

• After a strong hand-wring: about 60 % of that water is expelled.
– Alpha 0.20 kg water
– Wool 0.64 kg water

• After “roll-in-towel & stomp”: another 50 % of the remaining water is removed.
– Alpha 0.10 kg water
– Wool 0.32 kg water

3.   Latent heat of vaporization of water (at garment temps around 20 °C):
L ≈ 2.45 MJ kg⁻¹ (= 2 450 kJ kg⁻¹).

4.   Typical time needed to dry while worn (ventilated but not windy):
• Alpha – 3 h (soaked) → 2 h (wrung) → 1.5 h (towel)
• Wool – 6 h → 4 h → 3 h

These times are only to turn the garment “comfortably dry to the touch”; in reality they vary with air flow, humidity, body heat production, etc.

────────────────────────────────────────
How much energy (MJ) must be supplied?

Energy = water_mass × L

Alpha-Direct

1.   Wear soaked: 0.50 kg × 2.45 MJ = 1.23 MJ

2.   Wring, then wear: 0.20 kg × 2.45 MJ = 0.49 MJ

3.   Wring + towel, then wear: 0.10 kg × 2.45 MJ = 0.25 MJ

Wool

1.   Wear soaked: 1.60 kg × 2.45 MJ = 3.92 MJ

2.   Wring, then wear: 0.64 kg × 2.45 MJ = 1.57 MJ

3.   Wring + towel, then wear: 0.32 kg × 2.45 MJ = 0.78 MJ

────────────────────────────────────────
Convert that to average extra power (watts) over the assumed drying time:

power = energy / time

Alpha-Direct

1.   1.23 MJ / 3 h (10 800 s) ≈ 114 W

2.   0.49 MJ / 2 h (7 200 s) ≈ 68 W

3.   0.25 MJ / 1.5 h (5 400 s) ≈ 45 W

Wool

1.   3.92 MJ / 6 h (21 600 s) ≈ 182 W

2.   1.57 MJ / 4 h (14 400 s) ≈ 109 W

3.   0.78 MJ / 3 h (10 800 s) ≈ 73 W

────────────────────────────────────────
What those numbers mean to a wearer

• Resting humans give off ~80–100 W of heat. Light walking/hiking raises metabolic production to ~200–300 W (about half of which is useful heat, half mechanical work).
• Therefore:

– Alpha-Direct, even when dripping wet, can usually be dried just by sitting inside a shelter or strolling around; the 114 W is within resting output.

– A soaked wool hoody needs roughly twice that. You would have to be walking briskly, skiing, or otherwise active to supply an extra 180 W of heat without getting chilled.

– Removing water first (wring, towel) drops the requirement to a comfortable range for both fabrics. A quick towel roll is therefore worth the effort, especially with wool.

────────────────────────────────────────
Caveats

1.   Real drying times hinge on air movement and humidity; with wind the power needed from your body falls because the environment does more of the work.

2.   If the garment is under a shell, moisture must first migrate through that shell, slowing the process.

3.   The numbers above are additional watts needed for evaporation alone. They do not include the heat your body expends to stay at core temperature in cold air.

4.   Latent heat drops slightly at higher garment temperatures; the ±5 % difference is negligible here.

Use these calculations as an order-of-magnitude guide rather than an exact prediction, and adjust the water masses or drying times to match your particular gear and conditions.

Jerry,

You said in part, “My only question is the last step.  If it takes 114 W to be drying off your alpha direct, that exceeds the 100W that your body produces at rest.”~114W drying energy drawn from your body at rest reflects the case in which the AD is still saturated due to something like a recent rain storm or a swim. If  you will be static, have no other insulation layer, and you are not warm, wringing it out and putting it back on drops that to ~68W; the additional step of wrapping wrapping it in a pack towel and stepping on it repeatedly drops that to ~45W.

As I understand the second part of your post, we are in general agreement about the situation in which you must be static but have additional insulation that you can put on.