(and – why you should think of a liner, a sleeping bag and a sleeping mat as components in a ‘sleep system’)
We get quite a few questions via ‘Ask Baz’ regarding how many degrees a thermal liner will add to a sleeping bag. On the face of it, a fairly straightforward question; but the answer can be complex.
Sea to Summit offers three thermal liners; the Thermolite® Reactor, the Thermolite® Reactor Compact Plus and the Thermolite® Reactor Extreme. All three are made of Thermolite®, which is a hollow-core fiber; this plus the three-dimensional knit structure helps to trap air which insulates you. The Thermolite® Reactor is made of 80g/m² fabric, the Reactor Extreme of 110g/m² fabric, and the Reactor Compact Plus of panels of both fabrics.
The Thermolite® Reactor is rated as adding ‘up to 14°F / 8°C’. So – will it add 14°F / 8°C to your sleeping bag?
This depends on a number of factors including your own metabolism (you may be a ‘warm’ or ‘cold’ sleeper), and your condition (you may be well-fed and rested on one occasion and tired and hungrier on another).
It also depends on the design of your sleeping bag and its actual temperature rating. If you’re skimming through this post, the next four paragraphs are the ones to really pay attention to.
Questions which we receive about adding warmth with a liner frequently begin with (for instance) “I have a 20 Degree sleeping bag” (by which a ‘temperature rating’ of 20°F / -7°C is meant).
It’s important to know that in the US, manufacturers are free to claim whatever ‘rating’ they choose for a sleeping bag. It is not standardized at all (no matter what store staff and bloggers may suggest). The European Union EN 13537 rating system has become more widespread in the US since this post was first written, but there’s still no obligation for a brand to adhere to it. The EN 13537 test protocol uses heated mannequins dressed in a standard base layer in a sleeping bag, tested in a climate-controlled chamber. The test provides a ‘Lower’ rating (essentially a reference value for a male sleeper) and a ‘Comfort’ rating (the same but for a female sleeper). However – the mannequins don’t move inside the sleeping bags, and thus the design weaknesses of some bags do not become apparent. We’ll cover design points in a moment.
Back to the writer who has asked about his or her ’20 Degree bag’. The point here is that the bag may not have been tested at all to determine that ‘rating’. Or it may have been EN tested, but still carry an unrealistic product name: in one instance (to answer a customer service email), we checked on a brand-name bag which was marketed as “Zero °F” – the EN Rating for this bag is 10°F for a man and 23°F for a woman. Women sleep less warm than their male counterparts, as this above example shows (a distinction which is not always readily apparent to the female end-users who write asking about thermal liners).
So – be wary of using ‘ratings’ as a baseline which a thermal liner should enhance.
Design points of sleeping bags which affect performance include how well the draft tube covers the zipper, how effectively the hood seals around your head, and whether or not the insulation is prevented from shifting (and whether it has deteriorated into clumps and gaps). All these things cause heat to ‘leak out’ of a sleeping bag.
Given this it’s very difficult to guarantee a particular performance plus from one of our liners.
Heat loss into the ground. Another factor is that as conditions become colder, you lose more and more heat into the ground. In situations below 32°F / 0°C, the ground may have frozen (here in Colorado, the ground in the backcountry freezes in early October). So – while the air temperature overnight may drop to 30°F / -2°C, the ground may have been closer to 20°F / -7°C for days.
In these circumstances, your sleeping mat is an extremely important component in your sleeping system. You should check the insulation value of a pad – its ‘R’ Value before you buy it (you might care to read up on the physics of sleeping mat design first here).
For summer use, an R value of 1-2 is sufficient. For early Fall an R Value of 1.5-3 will be necessary, and for late Fall you may need a 4. Winter camping requires a 5.
No matter how good a thermal liner may be, it cannot compensate for an inadequate sleeping mat. And it will provide less of a temperature ‘boost’ to a sleeping bag which does not retain warmth effectively, compared to a well-designed sleeping bag.
This is why it is so important to think of each of these ‘individual’ products – a sleeping bag, a sleeping mat and a thermal liner – as components in a system.
Clearly, there are a lot of factors which affect how much extra warmth a liner will add to a sleeping bag. If you have any questions about any of them, please just ask.