Too Much Loft; Diminishing Returns?

[Trooper]

I've been thinking about layering UQ's for more warmth. I know it will work, but at what point will there not be any benefit? What I'm thinking is that 5 inches of loft wouldn't be much warmer than 5 feet of loft because the down can only keep so much of your body heat close to your body.

At some point, the down furthest from your body would never be warmed by your body because it escapes from the quilt edges and gaps in the innermost quilts. Imagine it on a grand scale: Fill a large room wall-to-wall, ceiling-to-floor, with down. Then place a human body in the center of the room. Would the down against the walls be any warmer than the down 3 feet from the body? We know the down 3 inches away would be warm.

So, how much down and loft is possible before diminishing returns are encountered?

[Lonely Raven]

You're thinking of it incorrectly. You wouldn't want the down at the end of the room/quilt/whatever to have any of your heat.

If the down is doing it's job, it's *insulating* that heat from moving away from you. e.g. stopping that heat from getting further away from you.

Sure, there is a point that too much down wouldn't do you any better. But the drawbacks there are cost/weight/bulk.

[Trooper]

You're thinking of it incorrectly. You wouldn't want the down at the end of the room/quilt/whatever to have any of your heat.

If the down is doing it's job, it's *insulating* that heat from moving away from you. e.g. stopping that heat from getting further away from you.

No, we are on the same page.

Sure, there is a point that too much down wouldn't do you any better. But the drawbacks there are cost/weight/bulk.

Exactly, but how much is too much?

[Bradley]

Exactly, but how much is too much?

If it is -30°C and your sweating; you've got to much . . .

I would think "To Much" isn't the issue,
but "To Little" could be . . .

I opt for too much.

[tjm]

First, I think you need to determine the ambient temperature you expect to use the quilt at.

Then go to camping goods store at the loft for a bunch of MARMOT or Western Mountaineering sleeping bags vs. their rated temps.

Remember 1/2 of the loft is underneath and 1/2 above.

That will put you in the ballpark for the minimum loft you need.

As for maximum, once you determine the loft vs ambient temp table, the amount of extra left you might want to add should be easier to figure out.

[Wags]

http://www.backpackinglight.com/cgi-...statement.html

that chart indicates the amount of loft needed to be above you to keep you warm to a certain temp. i would be it is fairly similar to the amount needed above and below you in a hammock's case. if you use this info and go out and freeze to death, i cannot be held accountable

[Youngblood]

There is a misconception in thinking that you are trying to warm up your insulation and that you can tell whether the insulation is helping keep you warm by how much you have warmed it up. The insulation closer to you doesn't help more than the insulation that is further away from you because it is warmer.

There is a temperature difference, or differential, between your inner most insulation and your outer most insulation. The inner most insulation is always trying to approach your body temperature. The outer most insulation is always trying to approach the outside temperature. The insulation between those two will be at temperatures in between and proportional to those two temperatures, proportional in terms of distance.

That differential temperature has a time factor to it. You notice that very quickly when you have to initially warm up your insulation. After the insulation has reached some steady state type condition, it is basically how long it takes you to lose your body heat. More insulation holds it longer, less insulation passes it on quicker.

More insulation is not always better because you can overheat. You have to give up some of that body heat at times, even when you are trying to stay warm! You have to maintain a reason balance to stay comfortable and to survive.

It can get complicated because the conditions change and vary so much. Temperatures generally drop during the night. Your body is your furnace and it may initially be generating more heat from exercise and food consumption, and then slow down during the night. And you need more heat initially to get your insulation to that steady state condition-- where you have 'warmed it up'. Sometimes there are a lot of things going on.

[Knotty]

Well said Youngblood. The thermodynamic concepts involved with insulation aren't all that complicated yet misconceptions abound.

Ray Jardine talks about the importance of not overheating in the early stages of sleep because the sweat will cause excessive cooling later on. It's a balancing act.

[kobold]

all other things being equal the coldest time of the night is just before sunrise...

[Sailor]

all other things being equal the coldest time of the night is just before sunrise...

No, no...the coldest time of night is exactly that time after you wake up cold.