(warning – some of this entry is R rated)
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, this is a fairly straightforward question, but the answer can be complex. Here’s why:
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 88g/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: your own metabolism, of course (you may be a ‘warm’ or ‘colder’ sleeper), and your condition (you may be well-fed and rested on one occasion and tired and hungrier on another).
But – it also depends on the design of your sleeping bag and its actual temperature rating.
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.
Temperature ratings are an even bigger issue: 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 is becoming the standard in theUS – this uses heated mannequins dressed in a standard base layer in a sleeping bag, tested in a climate-controlled chamber. The ratings (‘Lower’ for a male sleeper and ‘Comfort’ for a female sleeper) are therefore objective and consistent across brands.
So: 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.
Given this it’s very difficult to guarantee a particular performance plus from one of our liners.
Another factor is that in situations below 32°F, 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 25°F, the ground may have been closer to 15°F for days.
In these circumstances, your sleeping pad is an extremely important component in your sleeping system. You can determine the insulative value of a pad before you buy it by checking its ‘R’ Rating.
Hopefully, the above information gives a sense of how many factors need to be considered in answering the question ‘how many degrees will this liner add to my bag’?
Any further questions, please just ask.