My next sewing project is a quilt, they are really simple to make, and could probably be made without sewing, Some kind of tape. for the edge seam. But have machine will give it a go. I already made the insulated bomber hat, and pile hat that function instead of a hood.

Sounds interesting. I can't imagine needing a VBL at anything approaching 60 F. Then again I have slept in my bivy with no insulation in that range, so I guess that is kind of like using a VBL.

I always figured that I'd do that as a last resort, but have never needed to.

How do these quilts differ from sleeping bags? ... Oops, realized my question was answered further down in the thread. Nevermind.

Both for me. Try sleeping next to a lake at that temperature, and I'm sure it'll happen to your bag too. It's not guaranteed that you'll notice though. It's almost impossible to tell if it's happening unless the shell is translucent enough to see the down sticking to the underside.

This is where a VBL can really help. A couple pack liner bags can do the trick. Rip holes into one to make a sleeveless shirt and pull the other one over your legs. I've used this to thoroughly dry out my bag with great success.

?? At 40 below, lakes are frozen. One can drive automobiles on them.

Climashield works well for homemade quilts that are lightweight and insulate well. For my quilt I bought Climashield from this site:

https://thru-hiker.com/materials/insulation.php

I took "sub 40F temps" to mean temps below +40 F. Was I wrong?

FWIW, I have been intrigued by the notion of a quilt but have found a bag more versatile at only a small if any weight penalty. I use my down bag as a quilt sticking out and arm or leg or two to regulate when it is fairly warm (above 45 or so), completely over all but my head when it is chilly (down to maybe the upper 30's), or zipped up when it is really cold adding some clothing and pulling the hood tight as needed. Extra socks are a must when it is really cold. Also using a bivy conserves heat a good bit.

Given that my bag is only 17 ounces there really isn't enough weight to be saved to tempt even a gram counter like me to give it up for a quilt.

I should emphasize that this is only what I find works for me and it may or may not work for others. Also I put out heat like a furnace so my beloved Mountain Hardware Phantom 45 also may be insufficient for most folks in the conditions mentioned.

Imo you are still in the theory world when you say you might not notice. I spent 20-30 days camped next to lakes and creeks at least every other night in the Sierras two straight years. I also used the same down sleeping bag for over 100 straight days (minus the occasional hotel). I'm certain that my sleeping bag absorbed a little moisture but I didn't notice a loss of insulation. If this isn't going cause problems then I think you are talking from a purely theoretical standpoint. I think your argument applies more to winter camping where the sleeping bag absorb moisture at night and it freezes in the bag and it continues to happen night after night.

I am inclined to agree and also to add that most of us do not tour in conditions where the bag doesn't thaw out during the day most if not all days, allowing for some airing/drying out. I know that while I ride at times with overnight lows well below freezing at times, I would be inclined to go somewhere/sometime else if daytime highs were not expected to be well above freezing most days.

Buy the best bag you can afford......synthetic if you travel in the rain alot or are not used to keeping a down bag dry......down if you are.

Synthetics are not as good as down yet for packing small,but they are better if you think it may get wet.