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I've been looking at UQ protectors and the debate surrounding breathable vs sil. What would be wrong with having a sil outer and a thin breathable downproof inner? Moisture can't really pass through down anyway and the sil outer would protect against water splash, rain splash etc.
Any thoughts?
Water does pass through the down, destroying it's ability to insulate in the process. The water you can count on being present, is the water vapor coming off your body at night. With a breathable inside cover, you would also need a breathable outside cover t let the vapor pass through.
Alternatively, you could have the inside non-breathable and the down would be fine, but you may have condensation issues next to your skin. People's tolerance for this varies.
Thanks Mad. This is the bit I don't understand. If vapour passes into the quilt through its upper layer, its going to get trapped by the down anyway. it gets heavier and starts to lose its insulation properties. Surely it cannot pass all the way through the down and out the other side (in the event of both sides being breathable)?
The vapour is warm, so presumably it will not sink as such?
The vapor will condense when it reaches a layer that is at dew point temps. That may be at your skin if you use a vb between you and the down or will be in the down if you put the vb on the outside as the vb traps the moisture. As said above, either trap it above the insulation or allow the insulation to vent it to the outside. You may not see condensation issues if you don't sweat much or if the humidity is low.Quote:
Originally Posted by NickJ
cheers mean. I still dont get how vapour could pass all the way through and out the other side of standard UQ with breathable ripstop. Surely it must condense inside or the down just traps it. I cant get my head around how it could move through as when the down gets more moisture in it, even more will then settle. What I mean is that I dont get how a sil protector on the outside of the UQ would lead to more moisture in the down of the UQ.
In the UK we have a lot of condensation issues with tents, and with tarps in the mornings, but in the hammock I've never had condensation on me, or in the hammock (clark with the weathershield) or on the inside of the wethershield. Yes, plenty of times on the inside of the tarp depending on how high its pitched and if there is wind etc but never elsewhere. My primarn concern is heavy rain and windblown rain wetting the UQ in the night and I want to do something to protect that. If vapour can actually go all the way through the down, then I guess a breathable but waterproof UQP would be better, but I cant see how it could go all the way through and the sil UQP cause condensation inside it.
You WANT the vapor to pass all the way through. It will. It has to do with your body heat. And vapor passing through down does NOT destroy the ability of the down to loft - that comes when you actually wet out the down, which under normal use will not happen. I've used my quilts often enough to tell you that from experience.Quote:
Originally Posted by NickJ
I woke one morning to find ice on the outer shell of my underquilt. Given it was a clear night at 9,000 feet elevation without dew or mist or fog or rain, the moisture was no doubt from me, collecting and freezing where the vapor reached the outside radius of warmth emanating from me.
A sil overcover is going to stop the vapor and collect it, and saturate your underquilt if you have not allowed a gap to allow air movement and keep the puddle off the quilt. Vapor doesn't' move through sil the way it will breathable fabric. I have an undercover - it's Momentum, which has a tight weave and better water resistance than DWR nylon, but is breathable and not going to cause as much condensation as sil.
If you are sleeping in the arctic the vapor from your body will in fact collect in the down and cause major problems for you. Given a thick enough down bag and cold enough temps, the dew point will move into the down somewhere between the inner and outer shell of your bag. But those are temps that most of us will never experience - 50 degrees below zero F is not something I am likely to find in my normal three season backpacking here in California. I do experience 20-30F lows, however, and all year long.
Water vapor does pass all the way through all the layers of your breathable insulation, whether it is down, wool, cotton or breathable ripstop nylon.
As I explained in this thread, not only do you have a temperature differential that is the driving force of heat transfer through your insulation, you also have a humidity differential that is the driving force of water vapor transfer through your breathable insulation.
The way you avoid condensation is to choose breathable and windproof materials that improve the chances of water passing all the way through your layers in the vapor phase, so that it does not condense before reaching the ambient air.
It works!
- MacEntyre
P.S. I wouldn't put a vapor barrier like sil-nylon on the outside of a down UQ without putting another vapor barrier between the hammock and the insulation.