Reducing heat loss by reducing breathability!?!?

 

A new fabric has become available which is claimed"opens and closes like a pine cone depending on ambient temperature". As the temperature rises from 10'C to 20'C breathability improves by 50% to about 20000g/m2/24hrs. As the temperature falls the reverse process is claimed to reuce heat loss.

There seems to me to be some confusion here about heat loss, venting and breathability:

1. Breathability (or moisture vapour permeability) allows moisture to escape through a fabric without allowing cool air back in - i.e. wind resistance.

2. Venting allows moist air to escape and allows cool outside air back in to the garment.

3. Heat loss is improved by increased venting and breathability, but insulation is also improved by increased breathability because trapped air insulates better when it contains less moisture. The reduction in thermal efficiency due to humidity can be seen here.

Therefore the "pine cone fabric" would be of great benefit if it allowed increased venting in warmer conditions (assuming it could also prevent warm rain from getting in)... but the above figures suggest that this isn't the case and that reduced breathability when the fabric is cold will ultimately reduce the effectiveness of any insulation worn beneath.

These arguments also indicate why polar explorers prefer to use the most breathable fabrics available!

Click here for a review from OutdoorsMagic.

Comments

[egor semenov]

hello

i've followed the link to
"Humidity and Insulation"

http://www.typepad.com/services/trackback/6a00d83452157e69e200e54f82ccf08833

but "percentage volume of air replaced by liquid water", as is described in the article, does not mean moist air, but actually WET clothing. the insulation is far more interesting here, than the mambrane, since the clothing is wet allready.

if we were talking about relative humidity, it surely would affect insulation, but not as dramatic as in the table, i suppose?

to evaporate sweat bodyheat is needed. so if the air underneath our clothing is to moist to take more vapour (100% ralative humidity, but not very much volume percentage due to bodytemperature 36°C), the sweat just soaks the first cloathing layer and the other layers can still insulate. this brings the benefit, that no bodyheat is waisted to evaporate sweat. surely not very pleasant, but functional still.

all this is just my personal opnion! i have no experiance with that new membrane. so please correct me, if you find any mistakes, or if i've got something wrong!

thanks and good bye!

[Jane Nielsen]

"Therefore the "pine cone fabric" would be of great benefit if it allowed increased venting in warmer conditions (assuming it could also prevent warm rain from getting in)... but the above figures suggest that this isn't the case and that reduced breathability when the fabric is cold will ultimately reduce the effectiveness of any insulation worn beneath."

I know right. Now I wonder why the manufacturer didn't noticed that!

[Andy]

It's interesting to note that birds fluff up their feathers in the cold (I assume they also avoid sitting in a windy location when they do it) and in very hot conditions, perhaps to benefit from any breeze. In hot conditions they also hold their wings away from their body and pant.