(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 four thermal liners; the Thermolite® Reactor, the Thermolite® Reactor Compact Plus the Thermolite® Reactor Extreme and the Thermolite® Fleece Liner. All 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® Fleece Liner is a brushed fleece fabric made of hollow-core fibers.
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.
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).
Even though the European Union EN 13537 rating system has become the de facto standard for sleeping bag temperature rating in many countries (an improvement over the random numbers brands used to claim), it’s important to understand some limitations of the test. The EN 13537 test protocol uses heated manikins dressed in a standard base layer in a sleeping bag, tested in a climate-controlled chamber. This 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 manikins 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’. An older bag may not have been tested at all to determine that ‘rating’. Or it may have been EN tested, but the static manikin did not reveal points where a human sleeper would lose warmth from the bag.
So – be wary of relying too heavily on ‘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
- whether or not the bag has a draft collar to prevent cold air being drawn into the bag
- whether the overall shape of the bag and footbox allow a natural sleeping position without the insulation being compressed by shoulders, knees, hips and feet
- whether or not the insulation is prevented from shifting (and whether it has deteriorated into clumps and gaps)
All of these things cause heat to 'leak out' of a sleeping bag.
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 well be colder than the air temperature, and may have frozen (here in Colorado, the ground in the backcountry freezes in early October). The temperature you need to know is the ground temperature, not the air temperature (you may want to select a campsite based on how much sun the ground receives during daylight hours - the more the better).
In cooler or cold weather, 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. It's also vital to know that if your sleeping bag has been EN rated, the EN 13537 test mentioned above was conducted with a sleeping pad with an R-Value of 4.
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.
So - back to the question: how much temperature will a thermal liner add to a sleeping bag? Let's take that hypothetical '20° Bag' referred to earlier.
If the following conditions are met:
- the sleeping bag is realistically rated
- the sleeping bag has technical construction features to retain warmth and has a size/shape that prevents unnecessary compression of insulation
- a sleeping pad with a tested R-Value of 4 is used
- the sleeper has a normal metabolism and is well fed
Clearly, there are a lot of factors that 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.