The Ultimate Guide to Sleeping Bag Liners

Insulation is not about having a heater that's actively blasting hot air at you, insulation just slows the rate at which you lose body heat, by trapping it. This is why the same jacket on one person can feel too warm, and on someone else, can feel not warm enough. Every body works differently in generating body heat, no one else can tell you what will be warm enough. So how do you know? Mostly through the trial & error that comes with experience. This is why renting can be helpful!

Insulation counters 3 of 4 ways that we lose body heat

How does a barrier of dead air work?

Without a barrier our bodies lose heat to the environment: there's air flowing all around us (convective heat loss) & we're almost always touching something, even if it's just our feet walking on the ground (conductive heat loss). With a barrier (whether it's clothes, shoes, anything), instead, we lose heat to the barrier, and then the barrier loses heat to the environment. Ideally, the rate at which we lose heat to the barrier is roughly equal to the rate at which the barrier loses heat to the environment; this enables equilibrium, where we feel comfortable. For example: when you put on a jacket, it initially feels cold, until we've lost enough heat to it (warmed it up), then we feel comfortable. If these rates aren't equal, then discomfort can happen. For example:

• When you exercise rigorously, you can feel too hot. What's happening is that your rate of body heat loss is really high, higher than the rate at which clothing can dissipate that heat
• When you put on a really thick jacket, you can feel too hot. What's happening is that the jacket thickness reduces the rate of heat loss to the environment compared to the rate at which we lose heat to the jacket, allowing us to overheat it
• When you put on a really thin jacket, you can feel cold. The opposite is happening, the rate of heat loss to the environment is greater than our ability to heat up the jacket, so equilibrium is never reached & we never feel warm

The most effective barrier isn't any material, it's actually dead air (aka dead space or dead air space). This is air that is trapped & not circulating (because if it were circulating, then that implies a hole in the barrier, in which case you'd lose heat by convection; this is why unzipping helps you cool off & why most outdoor clothing or gear has cinches or velcro at the openings). For example: a thick down jacket is warm not because there's lots of down feathers, but because an inherent property of down is that the feathers are very puffy--they expand & take up lots of volume (this is called lofting*). In the voluminous space that the down feathers take up, there is a lot of dead air that can be trapped! The voluminous space is also why it looks so thick, when in fact, it's not about thickness. The less dense & more voluminous a material, the better it insulates by trapping more dead air. For example, a 'thick' sweater is woven in a way that it traps lots of air pockets, compared to a regular shirt.

By that same rationale, a barrier can lose its insulation power if it's compacted or compressed in a way where it can't trap as much dead air. This is why:

• For sleeping bags you want a sleeping pad below you, since below you, the bag is compressed & doesn't insulate as effectively from the ground (which is a significant source of heat loss through conduction)
• For clothing there are insulation technologies (in mid-layers) that don't depend on lofting, since sometimes clothes need to be tighter fit
• For storage & maintenance it's important to only compress insulation when you need to (e.g., you're in-transit) & not for long periods of time (at home in storage). Specifically if a lofting material has been too compressed, you may proactively beat it out to get it to work

*Fill power is a measure of how much a material can loft. The higher the number, the more volume it expands to take up, and the better insulation