Rab Ultrasphere 4.5 sleeping mat | Tested and reviewed

The Rab Ultrasphere 4.5 is one of the best sleeping mats if you're looking for 3-season levels of warmth at the lowest weight and most compact size. It delivers a competitive R-value of 4.3 (not 4.5, confusingly), but weighs just 463g (including pump sack, storage bag and repair kit) and packs down incredibly small (16x9cm).

The toasty warmth is achieved through two layers of heat-reflective TILT (Thermo Ionic Lining Technology), which works like an emergency foil blanket to reflect heat back towards your body and prevent radiant heat loss.

You also get “offset air chambers that trap heat within the construction, without the need for high-volume insulation”. The low weight is achieved through the use of an ultralight 20-denier polyester fabric and a tapered, minimalist, mummy-style cut.

This overall design ethos may polarise opinion. The pros? The Ultrasphere is ultralight, super-compact and impressively warm. The cons? It feels quite narrow, it’s not the thickest (only 8cm) and its sleeping surface is far from the roomiest. The thin, somewhat flimsy materials may be vulnerable to durability issues over time too.

However, comfort levels are high, thanks to the inoffensive horizontal baffles, while inflating and deflating via the pump sack and modern valve system is straight-forward enough (although not as fast or efficient as some others).

Comfort

The 8cm-thick Ultrasphere features an unfussy array of horizontal baffles for its sleeping surface. I personally prefer this kind of design to vertical baffles, though there’s admittedly not a lot in it when it comes to getting a decent night’s sleep.

For me, the horizontal layout provides a flatter overall sensation and it feels less ridgy or bobbly than other pads. The polyester outer fabric is also quite nice and soft against the skin. On a scale of 1-10, I’d rate the Ultrasphere at around a seven.

When it comes to comfort, everyone has their own preferences. It’s always worth trying a few different kinds of pad to find the design you prefer, whether that’s horizonal baffles, vertical baffles or more of a textured surface.

If you prefer vertical baffles and you’re not overly fussed about keeping things as light and as packable as possible, Rab’s Stratosphere 5.5 or Ionosphere 5.5 mats are good alternatives to the Ultrasphere.

Materials, shape and size

Rab employs 20 denier, recycled polyester for the main outer of the Ultrasphere. This is bolstered by internal TPU and a fluorocarbon-free DWR (durable water repellent) coating for protection against water ingress. There are more rugged sleeping pads out there, but this should hardly come as a surprise given how light the Ultrasphere is.

Some online reviews of the Ultrasphere suggest air leaks and damage from prolonged use, so durability may be an issue in the long term – though we’re yet to have any qualms ourselves. Time will tell. Rab has seen fit to include a lightweight repair kit for quick fixes on the go.

Rab’s designers had packability front of mind when dreaming up the Ultrasphere (which is great for wild camping), so it features the kind of lightly-tapered, mummy shape that fast and light pads often sport. I tested the regular version, which measures 183x51cm – relatively narrow and not the longest, either.

It’s not a pad you’ll ever find in a Big and Tall shop and if you’re after a large mat profile or even a bit of luxury, this ain’t the pad for you. At 8cm thick, it’s also slightly thinner than some of the competitors, though I'm splitting hairs a bit here.

Also available is the substantially more spacious ‘long wide’ version, which measures 196x64cm. What you gain in footprint, you also gain in packed size (4cm longer sack), weight (135g more) and cost (an extra tenner). However, for larger individuals, this is a no brainer.

Inflation and deflation

The Ultrasphere features a low-profile, two-way valve for inflation and deflation, with a triple-layered closure system similar to Sea to Summit’s Ether and Nemo’s Tensor ranges. It’s an innovative design where a single hole is covered by two caps.

Opening the top (one-way only) cap allows inflation but not deflation, meaning no air is lost during the inflation process. The second cap opens everything up, allowing the pad to quickly deflate.

You also get a micro-adjustment system, so once fully inflated via the pump sack, you can manually press your finger against a little nodule below the top cap, which lets out small bursts of air. This allows you to fine-tune the mat’s pressure to your preference.

For inflation, the top cap pops off and is attached to the inflation sack in order to bring the pad to life. The sack itself has a canny resemblance to a regular Rab dry bag – that is, until you notice its funnel and inflation valve.

I found around four sack-fulls was enough to get the pad ready for slumber time. However, the inflation process was slower than with some of the competition, as there’s only so much air that can move through the narrow funnel at the base of the pump sack at any one time.

The use of a pump sack like this is preferable to directly blowing the pad up by mouth, as it reduces the amount of condensation that enters the interior. This can lead to mould in the long run.

Warmth and R-value

Rab has really pulled the rabbit out of the hat with the warmth-to-weight ratio. Considering the mat alone weighs just 370g, an R-value of 4.3 is very impressive, putting it in the same league as something like Mountain Equipment’s Mirrostat 7.0.

The brand suggests expeditions, fast and light adventures, mountaineering, trekking, bikepacking, thru-hiking and multi-day mountain running are activities the pad is suited to – a nod to its versatility.

With a 4.3 R-value, I'd put a lot of faith in its insulating ability for 3-season use, though I'd opt for something with a higher value for the cold depths of winter.

The Ultrasphere achieves its high R-value thanks to its Thermo Ionic Lining Technology (or TILT), which Rab describes as “radical” on its website.

In actual fact, it’s similar to many thermal films seen in other leading sleeping pads, efficiently reflecting heat back towards the body, providing cosy warmth. Offset air chambers within the pad trap heat too, insulating you from the cold ground beneath.

Weight and packed size

When it comes to achieving a dinky packed size, this is one of the best pads I've tested. However, having deflated and rolled the mat up, it’s a bit of a nightmare to squeeze into its stuff sack, often taking numerous attempts to finally shove it all in – not what you need in the groggy early hours while you’re being feasted on by hungry midges.

So, it’s a bit of a double-edged sword. If you eventually master the unsubtle art of getting the pad into its sack, you’ve got yourself a highly packable, lightweight unit.

While the official measurements of the pack are 15x9cm, I measured it as 16 x 9cm, which is still remarkably compact. In fact, it’s shorter and less girthy than those classic 1-litre Nalgene bottles that brands always whip out for size comparisons.

Rab has taken things even further on its website, using a tennis ball as a comparison! (It’s not as small as a tennis ball.)

At 370g, the Ultrasphere is one of the lightest mats in our latest crop of reviews, which is impressive considering its very decent R-value. It raises questions about durability in the long run – time will tell whether or not its featherweight qualities will negatively impact the pad’s lifespan. The whole package weighs 463g, including the pump sack (66g), storage bag (22g) and repair kit (5g).

Verdict

I'd give the Rab Ultrasphere 4.5 a rating of 80%. An ultralight, super-compact sleeping mat with a superb warmth-to-weight ratio – but how durable will it prove over the long-term?

If you're shopping for a whole new sleep system, you may also be interested in our lists of the best sleeping bags and best camping pillows.

About the author

James Forrest writes regular features and route guides for Trail and LFTO, and has been one of our main gear testers for the last few years. James is based on the edge of the Lake District so when he isn’t off on his latest crazy adventure or challenge, he’s walking in his local fells.