Rab are newcomers to camping mats, but their decades of experience in making sleeping bags suggest they knew what they were getting into. Their current range of inflatable mats covers more or less the full spectrum of uses:

• Ionosphere 5 for lightweight 3-4-season backpacking/mountaineering
• Ionosphere 5.5 for more demanding alpine/winter conditions
• Stratosphere 5.5 for more affordable 4-season use
• Stratosphere 4 for all-round 3-season use
• Exosphere 3.5 for valley camping or backpacking on more of a budget

We're looking here at the Ionosphere 5.5. This is top of the range in terms of warmth, and comes with a respectably low weight and pack size for its performance, making it ideal for mountaineering and alpine trips as well as 4-season backpacking. Although it's able to perform to much colder temperatures than we have yet been able to test in, we still managed to give the Ionosphere 5.5 a good work-out. Bar a few minor niggles, we've been impressed. On the strength of this it looks like Rab have made a great start with their new mats.

Weight, pack size and durability

The warmth-to-weight ratio on offer here is particularly good. Rab have managed to keep the weight way down at just 690g (size regular), which is pretty impressive for such an insulating mattress. This may have come at a slight cost to durability as the 20 denier outer fabric may not hold up as well as some other slightly more rugged mats on the market. So far this has not posed a problem for me, and build quality does feel tough enough for general outdoor and mountain use. If the worst does happen and you manage to tear or puncture it, there is a handy repair kit included, with two self-adhesive patches.

Pack size is small for a warm mat like this, although I do wish the stuff bag that comes with it was slightly more generous to allow for a little bit more play as I have needed to re-roll the mat back down perfectly a couple of times now to ensure that it fits.

Dimensions

The Ionosphere only comes in two sizes: Regular (183cm x 51cm) and Long Wide (196cm x 64cm), at £170 and £190 respectively.

At 182cm, I find I'm close to the limit for size Regular. Anyone much taller than me is going to need to look at a Long, but there'd be a significant catch here, since its overall massive dimensions mean that it comes out at a hefty 950g versus Regular's much more modest 690g. That's going to be a totally different warmth-to-weight/pack size calculation. These days 183cm/6 feet does not seem particularly tall for a man, but on the other hand a lot of people - the majority of women for a start - are going to be a fair bit shorter. It'd be good if Rab could bring out a size Small, with shorter men and most women in mind, and then make Regular a couple of centimetres longer in order to cover most of the rest. There will be folk who need Long Wide, but we'd bet this size is not going to be the big seller in the range.

The rectangular shape of the Ionosphere 5.5 prioritises comfort but comes at a small cost in terms of added weight and bulk in comparison to a lighter mummy shape. In Rab's range, you can have a mummy in the form of the slightly-less-warm but lighter Ionosphere 5, and for 3-season lightweight backpacking that might be a better option.

Width wise the Regular Ionosphere 5.5 is adequate for me to roll over comfortably or lie on my back without my hands lying on the ground (I'm neither particularly wide nor narrow), so if you value a comfy sleep over saving a few tens of grams then you may favour the oblong shape.

Sleeping Comfort

The hefty 8cm thickness of the Ionosphere is one of my favourite aspects of the mat. This adds a great deal to comfort when sleeping on uneven ground as the mat is able to absorb any lumps easily. This thickness also means greater comfort for those who sleep on their side where your weight is not as distributed as it is on your back. This mat also manages to avoid suffering from any noisy rustling when moving about during the night, an issue with some other brands.

The vertical baffles are slightly wider on the edges, which helps keep you centred in the middle of the mat and provides a certain cradling effect. The grip print finish on the top of the mat also helps to hold you in place during the night, ensuring you don't wake up cold and on the ground. Overall the comfort on offer from the Ionosphere has been really welcome on all my camps this spring and summer, and I've had a great night's sleep even after a week-long trip or on uneven ground.

Warmth

Though warmer mats are available, at a price - generally insulated with down - the Ionosphere 5.5's R-Value of 5.5 puts it firmly in four-season category. Although we haven't yet managed to test this on any particularly cold nights, it is clear just by lying on the mat that there is very little heat lost to the ground. This is achieved by using two different technologies. A heat-reflective TILT lining helps reduce radiant heat loss. Also used to good effect in some of Rab's sleeping bags and insulated jackets, TILT boosts performance while adding hardly anything in weight. Combined with this is a synthetic insulation that fills the chambers to reduce the movement of air, reducing convective heat loss.

The net result is a mat that's impressively warm for its weight. Given its performance, you may be able to get away with using a lighter sleeping bag than you otherwise might, which could help save weight overall on your sleep system. For its balance of weight, pack size, and performance it's a thumbs up for the Ionosphere 5.5.

Inflation and Deflation

For inflation and deflation there is a simple and easy to use low profile valve which works like a 1-way valve when configured for inflating. There is a pump sack included which can be attached to the valve and rolled up to easily inflate the mattress in a minute or so without getting any moisture from your breath into the mat.

There are slightly better designed valves out there which make it easier to fine-tune your desired inflation level once inflated, but this is only a minor niggle for me. To pack it away, simply pop both parts of the valve and the mat should deflate in seconds.

Ethics and Environment

The main fabric and internal insulation are made from recycled materials, with a fluorocarbon-free DWR coating. This is great, and Rab could probably have made more of a song and dance about it than they have!

Summary

The Ionosphere 5.5 offers fantastic warmth to weight as well as superior comfort to almost any other mat that I have tried. A small pack size for such a warm and large-dimension mat makes it perfect for mountaineering, alpine outings and backpacking in cold conditions. I have very small issues such as the too-tight stuff sack, and I think the valve could be easier to use (some designs are more convenient), but in all the most important respects the Ionosphere is really difficult to fault. I'm now looking forward to using the mat in cold conditions, where it should really come into its own.