Using 2 sleeping bags, how can I get the effective tempature?

[Starchild]

That is if they match together well, not with one constricting the other.

I am thinking of combining a 40F synthetic semi-rect bag with a 35F down 'alpine blanket', basically what one may consider a semi-rect 'quilt'. In the configuration of sleeping mat, me in the semi-rect bag and the quilt on top with the quilt footbox over the bag. What type of temps can I expect from that?

(Yes I left my 18F bag at another person's house)

[Mobius]

I've done it with a 35 deg down quilt and a 45 down quilt (lightest one on top so as not to compress the lower one). I'd guess it was good for 20-25 deg (based on it getting down to about 25-30 that night and I slept very well.) I was on an XTherm pad at the time so had a pretty good base. Given that all these temp ratings are so subjective it's a tough call.

[Starchild]

Thanks that puts my mine a bit to ease, will report back how well it works.

[Old Hiker]

I'm sure your friend appreciates the late (early?) Christmas present. Feel free to drop by MY house and leave gear behind !!

I've only done this at home, in bed, in North Dakota, with a sleeping bag and blankets, but like the previous poster the heavy one went next to us (wife and me) with the lighter ones on top. Seemed to work well, down to about 25-30* inside the house. Heat was off in base housing for a night.

[slbirdnerd]

I recently bought probably the same 35 degree down blanket for the same reason--but to put over my 23 degree down bag in winter. I froze my butt off in January with just the bag when another hiker who was along had a down quilt over her bag and was okay. People do it, I assume it works, I won't know till next winter though.

[Franco]

At Backpackinglight someone posted a "scientific" way of working that out but I would just add the loft of the two and compare that to the given rating from a reputable manufacturer.For example a 5" loft gives you a 20f rating


http://www.westernmountaineering.com/index.cfm?section=products&page=Sleeping%20Bags&Co ntentId=27

by loft manufacturers mean topp and bottom (total loft)

[Don H]

Single Layer Down Loft and Estimated Temperature Rating
Temp (°F) Loft (in)
50 1.2
40 1.5
30 1.8
20 2.2
10 2.6
0.0 3.0
-10 3.5
-20 4.0

I've used a 15* Montbell and a 32* bag, the lowest rated bag as a sleeping bag with the other bag as a quilt.

[Meriadoc]

Thanks for that chart Don H. So, like I thought and felt, my setup is pretty warm! 20 degree quilt on top of a 15 degree bag.

Regarding the 'total loft' idea of adding the top and bottom - I wouldn't. The bottom is compressed and next to useless when lying flat. Don's chart would be the way to go.

[Starchild]

Well I'm here. So far with oneof
The two but temps are dropping fast and getting the other ready now. I can't see this not working but fingers are getting very cold for smartphone use.

[Franco]

"Regarding the 'total loft' idea of adding the top and bottom - I wouldn't"
I was just pointing out that manufacturers use that way so that there is no confusion a about the two ways...

Do also keep in mind that some use a 40/60 mix, so a 10" loft on a manufacturer's site could mean 5 or 6" on top depending on the bag.

[Franco]

Just in case it isn't all that clear still. the WM 5" (2.5 above and 2.5 below) loft for a 20f rating is close to the 2.2" loft (above) in the Don chart...

[Venchka]

Thanks for the chart. I have been pondering a 40 degree quilt for summer use. Now I wonder if a 1.5" quilt over my 3.5" might not make a better combination than a big down parka inside a mummy bag. Hmmmmm.

Wayne


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[Venchka]

Should be: 3.5" top layer of my


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[Venchka]

Sleeping bag. Dumb fingers.


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[shakey_snake]

I've heard that a synthetic over bag/quilt can help push the dew point out of the down underbag in below freezing temperatures. So, something to think about.

[bigcranky]

Take the two ratings and add them together. In your case, the 45 plus the 30 gives the combo a rating of 75 degrees F.


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[Venchka]

I've heard that a synthetic over bag/quilt can help push the dew point out of the down underbag in below freezing temperatures. So, something to think about.

Are you saying that condensation may wet the sleeping bag under the quilt? That calls for backyard testing. Thanks.

Wayne


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[Meriadoc]

Are you saying that condensation may wet the sleeping bag under the quilt? That calls for backyard testing. Thanks.

Wayne


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The dew point depends on the humidity (absolute). What he is talking about is where the dew point is reached. If the dew point is outside the sleeping bag/quilt then escaping vapor will escape to the atmosphere. But as the temperature changes from ambient to skin, if the dew point temperature should occur within the sleeping bag/quilt then dew will form inside the insulation. The vapor coming off of the human body will condense inside the insulation.

This concept really becomes a big issue in cold weather where water vapor coming off of the body actually freezes inside the insulation.

So placing a synthetic insulation outside where it is cooler may place the dew point temperature inside the synthetic insulation instead of inside the inner down insulation.

I tend to think the biggest issue is compression of the insulation. My quilt can't have anything layered on top, so it becomes the de facto outside insulation.

[Venchka]

I guess there is no free lunch. Two layers of insulation may not equal a third bag. It would be interesting to do some experimenting with a vapor barrier. Close to a warm bed incase none of works.

Wayne


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[leaftye]

I tend to think the biggest issue is compression of the insulation. My quilt can't have anything layered on top, so it becomes the de facto outside insulation.

Backpackinglight did some testing that indicated compression of down wasn't that big of a deal.