recommend a bag

[old-skool-lite]

Barely got a signal. Typical English damp winter night. 1c in tent. Under silk surface dry. Not under silk surface damp. Temperature benefit unknown but not needed. Dinner time now I've had my nap. Over & out for now.

 

[BogTrotter]

Barely got a signal. Typical English damp winter night. 1c in tent. Under silk surface dry. Not under silk surface damp. Temperature benefit unknown but not needed. Dinner time now I've had my nap. Over & out for now.

View attachment 51664

That’s a great result.

 

[Rog Tallbloke]

That’s a great result.

A cunning design is needed to make the silk overblanket into a wearable for daytime drying as a mid-layer.

 

[Odd Man]

A cunning design is needed to make the silk overblanket into a wearable for daytime drying as a mid-layer.

 

[Winki]

My bag outer is fairly dense (20d ripstop gelanot nylon), but it still doesnt block all the wind when it's really bad.

Do you know the weight of that nylon by any chance? Has it retained it's waterproofness well do you reckon?

 

[echo8876]

Do you know the weight of that nylon by any chance? Has it retained it's waterproofness well do you reckon?

it's not waterproof by any means.
just dwr.

 

[Winki]

it's not waterproof by any means.
just dwr.

Has it retained it's water resistance?

 

[echo8876]

Has it retained it's water resistance?

Yes, i washed it once, it still beads.

 

[old-skool-lite]

Silk. Not a total success. An improvement. After a full night it is damp under the silk but not as damp as the bag not under the silk. Need a better material.

 

[Mole]

From what I have read, moving the dewpoint outwards correlates with the amount of insulation. A single layer of fabric isn't going to do much. If it's breathable enough and water resistant, it'll protect your bag from when it condensates on the outer surface. But if you want better than that you need a certain amount of insulation.

SYNTHETIC TOP LAYER FOR A WINTER MUMMY BAG? - Backpacking Light

I'd like to get a synthetic "topper" for my down mummy bag so it, and not my down mummy, collects body moisture condensation. I'll sew a 3" wide mesh strip...

backpackinglight.com

 

[BogTrotter]

References to overbags always state pushing the freeze point to the outer insulation where normally a synthetic material over a down bag can cope with the moisture/wet/freezing better. It seems after @old-skool-lite test, just an additional outer cover is insufficient.

I’m out tonight trying a synthetic (8C) overbag over a 5C down bag. I think the overbag is inadequate compared to the Cocoon ordered since, by all accounts the double top layer of insulation on the Cocoon is quite substantial.

The trial overbag uses Primaloft too, but only a single layer. I’m considering whether to add a second 1/2 season bag of low insulation or not? Doing so may skew any reasonable resulting data.

In theory, an 8C overbag and 5C down bag should be good for -7. Tonight the forecast is for -6C.

 

[WilliamC]

.

 

[Rog Tallbloke]

References to overbags always state pushing the freeze point to the outer insulation where normally a synthetic material over a down bag can cope with the moisture/wet/freezing better. It seems after @old-skool-lite test, just an additional outer cover is insufficient.

I’m out tonight trying a synthetic (8C) overbag over a 5C down bag. I think the overbag is inadequate compared to the Cocoon ordered since, by all accounts the double top layer of insulation on the Cocoon is quite substantial.

The trial overbag uses Primaloft, but only a single layer. I’m considering whether to add a second 1/2 season bag of low insulation or not? Doing so may skew any reasonable resulting data.

In theory, an 8C overbag and 5C down bag should be good for -7. Tonight the forecast is for -6C.

In my experience, using a snugly sized off-the-face suspended bivy with a layer of agricultural row cover stapled or tacked to it will do the job for around 320g. A heavier overbag will reduce the loft of your down bag. The mesh version is about £18 on Ali-E

 

[BogTrotter]

The solution suggested by Mountain Equipment for dew point condensation on the outside of a sleeping bag was to increase ventilation. However, that might not always be practicable or desired.

 

[Rog Tallbloke]

The solution suggested by Mountain Equipment for dew point condensation on the outside of a sleeping bag was to increase ventilation. However, that might not always be practicable or desired.

Yeah, moving the dew point outside the bivy is the aim of my recommend above. Leaving a section of mesh free above the head end will help reduce water vapour load, while the row cover constrains enough body and breath heat inside to put the dew point on the outside of the bivy/insulation.

 

[echo8876]

that just isnt' true.
if you want to move your dew point away, it's basically warming up the tent so that dew falls on your other equipment, not bag. That means reducing ventilation, not increasing it, thus provoking condensation. And it means scraping down the snow in shifts into neat plastic bags.
Condensation and morning dew are fundamentally same process, but in practise they are very different.
For example overventing may move dew point INSIDE of your bag, and that's big trouble. Membrane outer wont help, because you generate enough moisture yourself, all it needs is a temperature border. There is whole topic on vapor barriers and maintaining sleeping bags in extreme cold.

 

[BogTrotter]

that just isnt' true.
if you want to move your dew point away, it's basically warming up the tent so that dew falls on your other equipment, not bag. That means reducing ventilation, not increasing it, thus provoking condensation. And it means scraping down the snow in shifts into neat plastic bags.
Condensation and morning dew are fundamentally same process, but in practise they are very different.
For example overventing may move dew point INSIDE of your bag, and that's big trouble. Membrane outer wont help, because you generate enough moisture yourself, all it needs is a temperature border. There is whole topic on vapor barriers and maintaining sleeping bags in extreme cold.

I’d be inclined to agree with you, but I’d also expect that the people involved with this type of issue all the time would some idea too?

If increasing ventilation was taking the sleeping bag outside to direct wind in a cooler, less moisture filled environment was the premise of their solution, I could see that working. I do struggle with the idea that opening a tent flap would be enough though.

Condensation on the sleeping bag surface is not that much of an issue as it can be just wiped off. Perhaps in this practice of balancing compromises this could be lived with. Freezing dew point inside the insulation would be the next level in this challenge.

 

[echo8876]

I’d be inclined to agree with you, but I’d also expect that the people involved with this type of issue all the time would some idea too?

If increasing ventilation was taking the sleeping bag outside to direct wind in a cooler, less moisture filled environment was the premise of their solution, I could see that working. I do struggle with the idea that opening a tent flap would be enough though.

Condensation on the sleeping bag surface is not that much of an issue as it can be just wiped off. Perhaps in this practice of balancing compromises this could be lived with. Freezing dew point inside the insulation would be the next level in this challenge.

ok, let me try to explain it better, as im not a native speaker.
dew point is the case when changing (falling) of air temperature causes fallout with the given saturation. You CAN'T VENT it. you can only control the temp(to a degree ) . there is already enough water in the air, it just waiting for temperature to drop.
condensation is when saturation increases, while temperature is constant (for simplification), so it breaches dew barrier from another side. It can be vented, since your actions are the source of saturation increase, and it can be depleted.
moving dew point is prime advantage of double-walled tents, since they were invented.
and obviously real life is more complex than that.

 

[WilliamC]

dew point is the case when changing (falling) of air temperature causes fallout with the given saturation. You CAN'T VENT it. you can only control the temp(to a degree ) . there is already enough water in the air, it just waiting for temperature to drop.

What are we talking about when we mention "venting" here? Venting the sleeping bag, or venting the tent?
Venting the sleeping bag isn't going to be practical as it will make you colder, unless the bag is too warm for the conditions.
Venting the tent can work - depending on conditions. The dew point depends on the temperature, the air pressure and the amount of water in the air. If the outside air is not saturated, and there's a breeze, venting can remove moisture from breathing etc., thus lowering the temperature at which the dew point occurs.

 

[echo8876]

If you dont vent, temperature rises and dew point emerges on the skin of you tent. Also known as condensation.
If you have perfect ventilation, say, under the tarp, air inside is same temperature as outside, so dew point emerges on your sleeping bag skin, because insides of your sleeping bag are warm, and prevent moisture from condensing.
That's it. you cant prevent dew if you are using an air gap. It's just a matter of where it forms, on your tent's outer skin, inner skin, outside sleeping bag or sleeping bag cover, or inside of a sleeping bag.

 

[WilliamC]

That's it. you cant prevent dew if you are using an air gap.

You can prevent dew if you stay above the dew point, which you can do if you increase the air pressure (impractical), the temperature (use a double skin, light a candle/stove etc), or decrease the humidity (vent, assuming outside conditions are suitable). Admittedly, if the outside air is saturated enough, or the inside air cool enough, there's nothing to be done but mitigation.

 

[echo8876]

You can prevent dew if you stay above the dew point, which you can do if you increase the air pressure (impractical), the temperature (use a double skin, light a candle/stove etc), or decrease the humidity (vent, assuming outside conditions are suitable). Admittedly, if the outside air is saturated enough, or the inside air cool enough, there's nothing to be done but mitigation.

One day ill manage to reduce humidity in a country where usual temp at night is +5c and has 2m of yearly rainfall.
But it's today is not the day, i guess.

 

[Rog Tallbloke]

You can prevent dew if you stay above the dew point, which you can do if you increase the air pressure (impractical), the temperature (use a double skin, light a candle/stove etc), or decrease the humidity (vent, assuming outside conditions are suitable). Admittedly, if the outside air is saturated enough, or the inside air cool enough, there's nothing to be done but mitigation.

Dew won't form on the outside of the bag if the temperature in the air gap is above the dew point. That's why the method using a smallish suspended and lightly insulated bivy which retains some body heat works without crushing the loft of your down bag. Water vapour rises through the bivy mesh and row cover (agricultural fleece), and dew point is reached as it gets cooled in the larger volume of ventilated colder air beneath the tarp canopy. Condensation then occurs on the underside of the canopy, or at worst on the outside of the row cover, where it can be dealt with in the morning.

 

[Jshdudhwodj]

Really interesting talk about dew point.

So to get this right you need the outer skin of the tent to be warmer than the top of your sleeping bag?

The sierra design nitro turned up


The tr is 650fill with 540g = 351000
The SD is 810fill with 418g =338580 yet has warmer rating


Next to the thermarest Questar 20 bag, both regular sizes

Both good bags the finish and materials are better on the thermarest.

Both bags have clever foot boxes the tr has an angled toe box with a added foot warmer section. The SD has a vent function were you can pull your feet out of the bag.

Zipper is longer on the tr and better quality, both bags zips are dual. The SD siz actually comes apart at the bottom. Which is a weird way of doing it.

The loft on the tr appears more, this is because they have more fill on the top than the bottom. You can however distribute the filling in the SD to the front by pushing it through the 5" baffles.

Hard to say which is better. The SD is tighter fitting, but less tapered than the tr.

Shall try it this weekend and think either will be perfect for me and the lad.

Also made my own pad straps.

 

[Rog Tallbloke]

So to get this right you need the outer skin of the tent to be warmer than the top of your sleeping bag?

No, the inner skin of the inner tent/bivy/sleeping bag cover needs to be warmer than the dew point of the moisture laden air.