Therm-a-Rest MondoKing 3D reviewed

The Therm-a-Rest MondoKing 3D is an 11cm self-inflating foam mattress, the deepest pad Therm-a-Rest makes and the one it describes as its highest level of comfort and support. It comes in two sizes, Large at 196 x 64cm and XXL at 203 x 76cm, and costs from £216.75 at UK retailers. Car campers who want home-like depth without paying MegaMat money are the people it suits best.

Sizes and prices

Therm-a-Rest has no UK web shop, so you buy it through outdoor retailers. At the time of writing (October 2026) Facewest and Base Camp Food both charged the prices below, under the list price shown on their pages.

The XXL costs only a little more and adds 7cm of length and 12cm of width, the better deal for anyone over six foot or anyone who sleeps with their arms out.

How StrataCore is built

Many self-inflating mats use a single slab of cored foam. The MondoKing’s StrataCore construction stacks a continuous layer of thermal foam between alternating ridges of air and foam, which Therm-a-Rest says gives a better warmth-to-weight ratio and lets the pad compress for the boot. Vertical side walls turn it into a box rather than a wedge, and Facewest points out that they also let two MondoKings sit flush side by side as a double. The top is a 50D polyester knit with some stretch, the base 75D polyester, all listed on Therm-a-Rest’s specification page.

Warmth and the R-value of 7.0

Therm-a-Rest rates its pads to ASTM F3340, the standard test for camping mattresses, which measures heat flow through the mat between a hot plate and a cold plate. The result is the R-value, and the higher it is, the less heat you lose to the ground. At 7.0 the MondoKing handles UK camping in any season. One cold-weather tip from Therm-a-Rest is worth knowing: in freezing conditions, top the pad up with the pump sack rather than your breath, because the moisture in your breath can freeze inside the foam.

Valves and set-up

The TwinLock valve splits the job between two one-way valves, one to let air in and a larger one to let it out. Therm-a-Rest claims it inflates the pad three times faster and deflates it five times faster, without saying faster than what. In practice you open the valve, let the foam breathe in, then top up with the stuff sack, which doubles as a pump sack. For the very first use, Therm-a-Rest recommends inflating it fully and leaving it for two to four hours.

Durability and care

Therm-a-Rest’s European warranty page offers a Limited Lifetime Warranty on manufacturing defects, with claims handled by its office in Ireland. Punctures and wear are not covered, and only the pump sack comes in the box, so pack a patch kit. Store it unrolled and inflated with the valve open; Therm-a-Rest warns that mildew from damp storage is not covered.

MondoKing 3D against its rivals

Against the Exped MegaMat, the Large is 13cm narrower and less warm at 7.0 against 8.1, but it is 1cm deeper and costs well under the MegaMat’s £290. The XXL closes most of that width gap at 76cm and is longer than either rival. The NEMO Roamer packs smaller and has a finer valve for adjusting firmness, but its Single costs more than the MondoKing Large.

Choose the XXL if you are tall or broad. Choose the Large if you are of average build and want the cheapest deep pad of the three. Couples have no MondoKing double, so they need two pads side by side.