Therm-a-rest MondoKing 3D Sleeping Mattress Review

© UKC Gear

The MondoKing 3D is Therm-a-Rest's warmest, most supportive and most comfortable camping mat. No lightweight, to put it mildly, this chunky inflatable mattress is strictly for car camping where a small pack size and low weight offer no real benefit, and a bit more luxury is no bad thing.

Thick and luxurious - a car camping bivi with maximum comfort  © UKC Gear
Thick and luxurious - a car camping bivi with maximum comfort

Comfort and Warmth

With a huge 11cm thickness of cushioned foam, the ridged inner distributes weight excellently and provides a comfortable level of support in any sleeping position, which I have never had previously with a camping mattress. It really is leagues apart from a smaller packable camping mat in terms of comfort. The only comfier mat I have used is Exped's Megamat 15 which is all in all a heftier mattress where the difference in price and space within my tent didn't seem worth the difference to me. The obvious comparison for many will be a cheap air bed style mattress, but whilst the price difference is hard to ignore, the MondoKing's comfort is noticeably superior.

With 10cm thick foam inner, it's comfier than my old mattress  © UKC Gear
With 10cm thick foam inner, it's comfier than my old mattress

Bear in mind too that a bog standard inflatable sleeping mattress will generally provide little in the way of insulation. The warmth of the MondoKing is down to its inner foam construction - both in terms of the thickness used and its "StrataCore" design which optimises the warmth to weight ratio. "StrataCore" is essentially a continuous layer of thermally insulating open-celled polyurethane foam between ridges of air and foam. A similar take on this continuous foam design can be seen in Sea-to-Summit's Delta Core which their Comfort SI Mat range utilises.

Internal foam "StataCore" construction - raised baffles run width-wise across mattress.  © UKC Gear
Internal foam "StataCore" construction - raised baffles run width-wise across mattress.

When fully inflated the MondoKing gives an R-value of 7 in accordance with ASTM 3340-18 (R-value is the measure of the mat's thermal resistance - the higher the R-value, the more thermally resistant the mat is). This rating is interestingly down from a whopping 11 in the previous iteration, which I assume would have been overkill for most users. The new version will provide you with insulation from the cold all year round and into more extreme cold nights if you're using it somewhere with that sort of weather. I've really appreciated the insulation on Scottish car camping nights down to 0C and imagine I would need to push this much further to feel any significant loss of heat, which generally I will not be doing with a car camping mattress in the UK.

Square sided walls give a large amount of sleeping suface  © UKC Gear
Square sided walls give a large amount of sleeping suface

Shape and Dimensions

The MondoKing 3D comes in two sizes, Large (W: 63cm, L: 196cm) which we tested, and XX-Large (W: 76cm, L: 203cm). 

It's leagues apart from a smaller lightweight camping mat in terms of comfort

A wide square shape has allowed me to sleep in many different positions without the worry of the bottom of my sleeping bag or my leg falling off and getting cold. Being square-sided also means the mattress maximises its sleep-able surface when compared to its footprint. I am 180cm tall which made the large size more than long enough, and the width should be good for most users who don't need to be completely sprawled out.

Given it is reasonably wide, it is worth considering the dimensions if sleeping side by side with someone in a narrow tent or car. In a large estate car, I have only been able to fit myself in with this mattress whereas in the same car I can usually fit two backpacking-oriented sleeping mats in with a slight overlap between the mats. Similarly, this mattress was not able to fit in my friend's small van.

Just able to fit into the back of a large estate car. No room for a second sleeper.  © UKC Gear
Just able to fit into the back of a large estate car. No room for a second sleeper.

Weight and Pack Size

Anyone buying this sleeping mattress will no doubt have weight at the bottom of their priority list. It may only be useful to know how heavy it actually is if you're packing an airline check-in bag. At 2.05kg on the home scales, including the bag, it's not winning any awards for weight. But then that's not the idea here at all.

Similarly, pack size should also be low on the priority list, although may actually be useful to consider if filling the car with multiple people's stuff. At 70cm in width and 25cm in diameter when packed into the stuff sack, it is bigger than my 2-man tent and can quickly eat up space in the car.

Left to Right: MondoKing, one man tent, standard self inflating mat, water bottle.  © UKC Gear
Left to Right: MondoKing, one man tent, standard self inflating mat, water bottle.

Fabric and Materials

A 50D polyester stretch knit is fabric used on the top surface to provide toughness as well as a soft plush feel, which is nice if sleeping with a quilt on the mattress. On the bottom and side walls is a 75D polyester which feels tough and able to take a hefty amount of abuse - comforting given the price tag. The fabric is also far quieter than any other pad that I have used, which has added to my overall comfort at night. Nobody likes to be sleeping on a crinkly crisp packet.

Inflation and Deflation

It's important to note that a mat straight from the factory will not immediately self-inflate as it has been stored compressed for an extended period of time where the foam's self-inflation memory has essentially become its compressed state. To fix this, simply store your mat with the valve open in an uncompressed state. Under my bed usually works, but this may be an issue for some if storage is tight, so it's an important factor to consider.

Stuff sack mating between inflation valve  © UKC Gear
Stuff sack mating between inflation valve

Roll stuff sack down to inflate mattress  © UKC Gear
Roll stuff sack down to inflate mattress

Reasonably flimsy stitching hasn't kept up with general use  © UKC Gear
Reasonably flimsy stitching hasn't kept up with general use

Inflation comes extremely easy by opening the inflation valve and waiting. For me this has typically taken 10-15 minutes (you get most of the way there within six minutes). Once the foam is at its peak "self-inflation", five or so puffs usually get me to my desired inflation. The storage bag doubles as a pump sack by attaching the inflation adaptor to the valve, filling the bag with air and rolling or "pumping" the air into the mattress. This has probably been my biggest disappointment with the product as the storage bag stitching has started to come undone and now renders this pump sack useless. This is normally not a problem for me as I am able to store my mat inflated (allowing self-inflation to do most of the work) but if I stored this deflated it would mean a long time spent inflating by mouth. Exped's Megamat range provides a more robust mini-pump which is a good solution, although simply a more robust stuff sack would have worked fine and ensured you never forget the inflation device.

The "Twinlock" valves have also been updated from the previous version of the MondoKing 3D, and are now bigger and more robust than before. These help inflation in comparison to more dinky valves (three times faster than their classic valve according to Therm-a-Rest). This in practice is difficult to measure, but they are easy to use and their wide openings allow for a lot of air to be let in or out. Clearly marked IN and OUT, these are one-way valves so that no air escapes/enters between puffs or when deflating. Therm-a-Rest also guarantee that  inflating with your mouth will not harm the mat with the introduction of moisture, sometimes a worry with inflatable pads. Deflation is slightly harder than inflation since this is quite a large mattress, but nothing to lose sleep over.

Wide, robust and easy to use inflation and deflation one way valves  © UKC Gear
Wide, robust and easy to use inflation and deflation one way valves


The MondoKing 3D's comfortable 11cm thickness and generous insulation will ensure almost every user will catch a good night's sleep year round. If stored correctly, inflation is easy and you can let the mattress do most of the work, although a better designed inflation bag would be welcome. Whilst packability may be an issue in a fully laden small car, it is perhaps the price tag that will be the biggest concern for many. However, compared to a bog standard inflatable mattress the comfort provided by the MondoKing 3D is nothing short of luxury. How much are you willing to pay for the best night's sleep possible?!

Therm-a-Rest say:

The MondoKing 3D provides our highest level of comfort and support. Boasting 4.25 inches (11cm) of premium foam, the ultra-cozy pad combines a plush next-to-skin feel with an 7.0 R-value to keep you warm and comfy in any condition or season. StrataCore construction provides lofty three-dimensional shape, and compresses easily to save room in your trunk or closet. The extra-comfy self-inflating pad features the easy-to-use TwinLock valve, allowing the pad to inflate three times faster and deflate five times faster. Like all Therm-a-Rest pads, the MondoKing 3D is made in market to ensure quality and minimize environmental impact. The pad includes a go-anywhere stuff sack with a built-in pump sack.

  • Weight: 1.99kg (L) 2.5kg (XXL) 
  • Length: 196cm (L) 203cm (XXL)
  • Width: 64cm (L) 76cm (XXL)
  • Thickness: 11cm
  • R-value: 7.0
  • Top fabric: 50D polyester knit
  • Bottom fabric: 75D polyester
  • Urethane foam

For more information

As an owner of a truly massive sleeping mat myself I never thought I'd experience FOMO from anyone else's...until I saw this...

Words cannot express the bitterness I felt towards you on that Pembroke trip John ;-)

24 May, 2022

So it's an extremely expensive airbed, basically? :)

I suppose some will pay for something that doesn't involve admitting they're getting old and so want an airbed :)

24 May, 2022

Haha, I know! Especially on a longer trip like Pembroke was, I reckon you get quite a bit of benefit in the later days purely just by being that bit more rested. Genuinely at the time it was comfier than my mattress (without my mattress topper). That may be more due to the fact that my mattress was utterly awful though haha!

24 May, 2022

Like I say in the concluding remarks, I think many who would consider something of the size and comfort of an inflatable airbed will end up being put off by the MondoKing's price tag and opt for the airbed instead. After just spending a week on my aunt's inflatable airbed, there is no doubt that the MondoKing is quite a bit comfier - mainly down to the foam inner. How much that difference in comfort matters to you is obviously down to personal opinion.

24 May, 2022

I Was thinking of getting one of these but am yet to find anywhere that sells them and has them in stock!

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