Sizing can be a rather tricky, but critical process. There are a lot of factors to consider. Comfort, efficiency, and performance are all things that really hinge on proper sizing. If you aim for any one of those, without considering all, you can end up severely compromising the other two. If you aim only for comfort, performance goes out the window and you need to compensate, which ruins efficiency. If you aim only for top performance, you can end up with a system that is too uncomfortable to sleep well in. You could aim only for efficiency, which tends to mean snug fitting, but then it ends up compressing loft or not allowing you to spin inside the system, which destroys performance and comfort. One should also consider that there will be inevitable margins for error in production. It’s just about impossible to hit a finish dimensions exactly and everyone should build in a small buffer to accommodate this.
The most typical issue is an over-focusing of comfort. This usually means over-sizing for sprawl sleeping. As outlined above, this degrades performance dramatically and heads into an inefficiency feedback loop. A big cavity promotes sprawling, which increases heat loss from the body. This heat enters a larger cavity, which takes longer to heat up and feel warm to your body. The increased surface area to the outside then loses much more heat out of that cavity. All this then means that you won’t feel the same warmth you would expect from a well sized system. Large-sized systems consistently underperform compared to smaller ones, so you have to bump up the calculated loft to compensate for all the loss. It’s just more and more weight for less and less insulation. Not a good path for the ULer.
The ideal scenario is finding the right balance of performance, efficiency, and comfort. This involves finding the minimum space you need to be comfortable and the minimum amount of space for the system to perform well. On the comfort side it means considering what habits are essential and what habits are luxuries you can train yourself out of. There are many habits we engage in, simply due to living in conditioned spaces, which we can easily just trend away from. All it takes is a little adaptation. For most people I think this only needs to go so far as to eliminate the most detrimental habits that prevent you from carrying a relatively light sleep system. However, this process can go to extremes where people train themselves to be comfortable and efficient in scenarios where most people could not. Think the hardened thru-hikers who’ve been out so long that their basic quilts are pretty clapped out and flat…..but they’ve adapted to it by being out for months and months on end. I’ve seen many a quilt that looked like it was little more than two layers of nylon shell with clumps of down here and there.
On the performance side, it means looking for a balance between over-sized inefficiency and the performance detriment that comes with under-sizing. There is a bell curve for this where a large system will lose a lot of heat. Trimming size off that system will increase performance, until you get to the point where shell tension and point loads start to push performance back down. Shell tension will reduce loft. Point loads create cold spots. Coverage also starts to become an issue, which promotes drafts in quilt or hybrid false bottom systems. This is really the worst scenario. Over-sizing can be pretty bad, but under-sizing is far worse. I recommend trying to dial in that balance, but if you’re unsure, trend slightly to over-size.
Effects of oversizing sleep systems
In most cases, adding internal volume to a down sleep system will have detrimental effects on insulation efficiency. It’s usually an inefficiency feedback loop. Increased size promotes sprawl sleeping, which increases heat loss from the body. Then, that lost heat enters the system cavity, but the increased volume increases the lag time before that space can be heated by body heat. Last but certainly not least, the larger size has increased surface area exposed to the outer cold, which increases that rate of heat loss out of the system. So the body loses more heat, the cavity is slow to warm, and the outer surface loses the heat faster.
That said, it doesn’t always apply. There is an argument that added dimension in a sleep system doesn’t matter because the unused sections would collapse and close off the extra surface area. While there are some scenarios where this applies, it’s mostly short-sighted in the context of UL gear. Of course, this certainly applies to the big, heavy comforter on your bed at home. Something like that has enough weight to essentially press down and seal off the excess, leaving a small and warm cavity around your body. However, this is one of the reasons that UL down comforters don’t work that well. They are so light that they tend to float on top of you and not seal the edges. An open comforter really needs some weight to it to seal the warmth around you, unless you are going to actively manage it by tucking it around you.
The other place this theory could apply is with basic quilts. If a quilt lacks the technology to affectively manage drafts, the blunt method for dealing with them is to make the quilt wider and wider. This is where we run into the issue of quilt weights starting to click back up to the weight of sleeping bags. It’s a pretty thoughtless method instead of engineering intelligent systems to deal with drafts. However, this could be one place where added width doesn’t directly affect insulation efficiency. That extra width can just be tucked further around the user so that the internal cavity can remain small and efficient. Of course, it’s still a very blunt and inefficient method because you’re carrying the weight of all that width and there are much better ways of dealing with drafts. Adding width is simply throwing more weight at the problem. Additionally, this really only applies to the open top portion where the sleeper can use their hands to tuck it around them. The closed footbox is still going to remain cold if it is also sized large. The light weight of down construction is never anywhere near enough to actually collapse the excess volume and it’ll remain exposed surface area.
Dimensions before filling, or after?
Are the advertised dimensions of a sleep system reflecting its finish dimensions or dimensions prior to filling? This is a common question that is often brought up, but one that can’t really be answered in a vacuum. For any of the basic quilts without any differential cut or focused engineering, filling them would decrease the finish dimensions. So, when cutting the fabric you would want to factor this in, so that the finish dimensions end up in the right place. However, for systems with a proper differential cut, there is no loft take-up in the direction of the diff cut. Loft expands into the larger shell and the finish dimension of the system remains the same as the cut dimension of the inner shell. Typically the diff cut is in the width / girth and most systems do not incorporate a length-wise differential cut. Why? First, complication. It gets difficult and laborious to have to manage multiple differentials with all the connections between shells. Second, my view is that, if you have to choose one, the width diff cut is much more effective than a length-wise one. Diff cuts do not manage point loads well, or at all. They can handle general shell tension and avoid having large sections of loft flattened. The tension applied along the length of a system that is sized short tends to concentrate in point loads at the feet and shoulders. A diff cut is entirely ineffective against these issues and they are best dealt with through proper sizing. The only effective way to remedy cold spots from point loads is to increase dimensions to the point where there is no more pressure there. These conclusions reflect the reasoning behind our sizing process. We utilize considerable and variable amounts of diff cut in the width / girth, to handle general tension from different sleep positions with a reasonably snug fit, but we recommend setting the length such that the system is loose.
Performance considerations beyond comfort and efficiency
Long ago, I had a pretty easy time training my body to be comfortable in a handful of efficient sleep positions, and one sorta luxury position. By a huge margin, the most efficient position for retaining warmth is full fetal. Your body’s exposed surface area is very limited and you turn into a warm ball that can take a system way, way below the typical rating. Of course, this didn’t turn out to be all that practical. I tried using a “nest shaped” sleep system and wasn’t really up for the amount of training it would take for me to be able to fully sleep through the night curled up and only able to turn side to side. It’s a long, long ways off fetal, but the next best position, in terms of thermal efficiency, is lying straight with all limbs together, or mummy position. So my sleep position needs started to look like a “nest” with a mummy footbox hanging off of it. This shape also accommodated my luxury position, which is face down, with one knee up and one leg stretched out. It’s relatively efficient, since everything is balled up in fetal, except one leg. That leg is exposing lots of surface area and heat loss, but I tend to only use this position once I’m up to temp and comfy. Anyway, the “nest bag” was super warm and crazy light, but impractical. Next was to experiment with a very narrow footbox coming off the nest shape…..which leads me to my point here. I discovered that I could be comfy with a very narrow footbox, as long as I could transition between fetal, straight, and my luxury position. It was highly efficient and warm……however, it also was not that practical. I really had to carefully manage the orientation of the footbox, because it was so snug that every time I turned, it wanted to turn with me and I’d have to spin it back. The torso area of our systems has a pad strap that can keep things oriented with a set and forget system, but I did not want to add another one to the footbox area. I think it is somewhat important that the footbox remain somewhat mobile. Therefore, the ultimate conclusion was that the footbox should be sized up to the point where I can spin freely, but no more. The footbox is one place where excess size can really harm total warmth.
Variable effects of excess size in different parts of a system
You might be now wondering why we sell the fetal position bags, with generous torso room, if excess room is detrimental to total warmth. It’s a weighing of pros and cons…..There are times and places where large dimensions can add efficiency. Having a space large enough to curl up into is a huge benefit in terms of efficiency and performance. If it is at all practical for someone to use this position, it’s just a massive gain in what you can get out of a system. This is the pro that gets weighed against the inefficiency it brings when you are not curled up in that space. Yes, when you are lying out straight and not curling up in the torso area, the larger space and increased surface area are less efficient than if you were to size this portion more snug to your body. That is why it is important to assess what positions you will actually use. If you will never use fetal position, you’ll be better off with the mummy sizing. That said, the efficiency loss from large torso sizing isn’t as bad as sizing a footbox too large. In the torso, your arms tend to fill out the space and limit the volume. Yes, you have increased surface area for heat loss, but you can typically fill some of that space with clothing and other items you need to bring into your system for the night anyway. With an oversized footbox, it’s just your legs. That inefficiency feedback loop will tend to keep them cold if this space is oversized. Sure, you could stuff some things down there to limit volume and surface area, but the situation is a little different for extremities. The bulk of the body heat is from your core and you typically want your footbox to share this warmth. If you stuff things down there and cut it off from the torso heat, your legs will tend to stay cold, since they are the only heat source down there. Given all this, you can often get away with having a large dimension in the torso area, but you really want to hit that balance in the footbox.
When estimating desired dimensions, one should consider there will be inevitable margins of error in production. Much of the time, finish dimensions can be dead-on, but it’s also rather difficult to land exactly on the mark when loft take-up and seam allowance are factored in. We try to account for these factors, but 1 – 5% margins can still exist. This is pretty normal throughout the industry.