This is why I don't use pads or reflectix with my UQ. Since I always get condensation with pads or reflectix, regardless of the temps, I worry about the ability of my down UQ to still perform with all those pools of condensation.Originally Posted by Shane
With just an UQ, the moisture my body generates seems to dissipate quickly into the down, then evaporates from body heat. Add reflectix and it might throw the system out of whack.
"A foolish consistency is the hobgoblin of little minds." Ralph Waldo Emerson
Exactly. I feel like my whole system transports vapor pretty well and keeps me warm. It's a system that's been tested in multiple days of subzero with no appreciable condensation. I couldn't sleep well if I was clammy.
Skin>hammock fabric>down>air=warm and dry
I have a 24" wide x 6' long piece of reflectix that's inside a fleece bag. I've used it in 0* temps with a 20* UG & UQP. The fleece will pick up a little moisture and my back side got a bit damp - warm and damp, but I wouldn't go a second night in it without drying it. Don't use it anymore and it wasn't real comfy to sleep on after getting used to the underquilt.
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No offense, but I'm a mechanical engineer. I know how heat transfer works, and you are incorrect in the way you are thinking.
If you're not warm enough with just your UQ, adding a reflectx pad wont make you COLDER. It'll add insulation value.
What your saying is like saying "don't layer you're clothes, you'll be colder than with just your jacket."
Here's a rough explanation:
Insulation retards rate of heat transfer. It doesn't depend on temperature within the insulation. Heat transfer increases as temperature differential increases. Your body is a heat generator. It adds heat to the airspace inside your insulation. As the temp inside the insulation increases vs the outside air temp, heat is transferred out through the insulation at some rate. At steady state your body is generating the same amount of heat that's being drawn out. This determines the temp in the airspace. If you don't have enough insulation the temp equalizes too low and you're cold. If you have too much it equalizes too high and you're hot. The temperature in the insulation itself will always be a gradient between the airspace temp and the outside temp.
When I put on a down jacket that is cold, It is cold at first, but then it warms up and I am toasty.
If I could, I would wear a full down suit au natural underneath.
If you're underquilt is too cold, it's not enough insulation. A reflectix pad may keep you warm(and wet) in a pinch, but it's not the solution to a cold weather sleep system.
my .02
What you're saying doesn't prove anything, and doesn't counter anything I said.
Put on a reflectx jacket, then your down jacket, and tell me what happens. I'd bet my life savings you warm up!
So, contrary to what you said previously, you agree that the UQ will still provide insulation, while using a reflectx pad?
Why? Or I guess I should say: that's fine if the down suit has enough loft to insulate you. What if it doesn't? Get a new suit? Sounds expensive. What if the temp changes and you get too hot?
Last edited by Boston; 02-04-2014 at 17:01.
Boston:
I'm not quite understanding your explanation. You say, "Insulation retards rate of heat transfer." So the reflectix will retard rate of heat transfer to the down? Sounds like it make take a while to warm up.
Also everything you say about heat transfer is also applicable to vapor (i.e., moisture) transfer. Seems the reflectix will inhibit vapor transfer just as it inhibits heat transfer. Every time I've used a vapor/air barrier I've gotten condensation on the medium providing the vapor/air barrier.
Using down alone, the vapor transfer is efficient. Insert a vapor/air barrier in the equation and you localize the vapor transfer to the vapor barrier itself - in other words, you get pools of condensation. Seems like mixing two different insulative systems is a recipe to make both work poorly.
"A foolish consistency is the hobgoblin of little minds." Ralph Waldo Emerson
Hey Boston, I think what happens is that down has a lower specific heat than other insulators, so it feels warm very quickly. This causes people to think that it's "colder" wearing layers under the down. You are absolutely correct, of course, that insulation is additive.
The point is you don't "transfer heat to the down". That's not how insulation works. You don't heat the insulation and it keeps you warm. Your body creates heat and the insulation prevents you from losing it. If your body is directly against the insulation it retains heat in your body. If there is an airspace between you and the insulation you have to heat that up first (hence the "it felt cold then warmed up" feeling).
Keep in mind my explanations aren't exact or perfect. Heat transfer is a complcated subject and it's difficult to explain everything here.
As far as using a vapor barrier with down..no reason it wont work unless you get the down wet due to the vapor barrier. If the down is outside the vb you shouldnt have a problem. But I'll remind you, the OP didnt say he had a down UQ.
Last edited by Boston; 02-04-2014 at 22:41.
Woo I tried a pad made of reflectix. I didn't freeze to death but it was extremely unpleasant. I was sweating and cold all it the same time! Not recommended. Anyways I do still use a piece in my Poncho liner under quilt no problems yet.
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