Mountain Core 250



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TOG (DRY/WET) 2.0/1.4

A lightweight sleep system with great next-to-skin comfort and breathability. With the 250g/m2 weight PrimaLoft® ACTIVE as the liner fabric, the Mountain Core 250 offers an open fleece feel but with the added benefit of an unrivalled warmth-to-weight ratio for any fabric.

Using the thickest 250g/m2 weight PrimaLoft® ACTIVE fabric offers a warmer bag than the lighter Mountain Core 125 but with all the benefits of a comfortable sleep environment.

Externally, the PointZero fabric gives great windproofing with a DWR (Durable Water Repellancy) for when your pack takes a dunking en route to camp. The total package retains the packability you’d expect from OMM.

PrimaLoft® ACTIVE blurs the line between wadding insulation and fabric; PrimaLoft® ACTIVE loosely weaves PrimaLoft®’s industry-leading insulating fibres through a fine polyester mesh. The resultant structure is warmer and less than half the weight of a conventional fleece product – Down like performance when dry whilst also still performing when wet.


Whilst not the warmest bag we offer, the benefit of using PrimaLoft® ACTIVE, instead of a traditional wadding insulation, we achieve a much more comfortable inner climate. With enhanced wicking and breathability, the open weave fleece fabric allows for much better airflow whilst the lofted plume still provides fantastic insulation values.


PointZero face fabric is windproof with good water resistance thanks to the DWR coating whilst PrimaLoft® retains its insulating properties even when wet, perfect for changeable conditions.


At OMM we always choose synthetic insulation over down to be sure that we can get warm even when everything is wet. PrimaLoft® make the best synthetic insulation. PrimaLoft® ACTIVE uses 64% post-consumer recycled material it is Bluesign and Oeke-Tex approved.


Soft textured PrimaLoft® Active
Insulates even if wet
2.0/1.4 tog (dry/wet)
Half YKK zip
Stitched through inner with loose laid outer
DWR (Durable Water Repellancy)
Fast drying easy care
Mummy shape
Length: 195cm
Shoulder Width: 65cm
Pack Size: – 27 x 16cm
Hood drawcord
Hang loops at base


Colour: Navy/Grey
Fabrics: PointZero (Nylon), PrimaLoft® ACTIVE 250g/m2
Weight: 700g

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What's a Sleep System?

 A Sleep System normally just refers to a usually just a standard insulated sleeping bag but there are some other innovative ideas out there such as our own Rotor Sleep System that uses insulated trousers (that unzip to form a half sleeping bag) that then clip into our insulated jackets and the Foot Pod to complete it. So whilst not technically a sleeping *bag* it is an insulated sleep system!

What Temperature Rating Do Our Bags Get?

 Well…that’s a good question. Whilst we do use the EN tests for the bags, we do not believe these are the best measure of a bag’s performance. Instead we use Tog ratings for both dry & wet performance (very relevant when using them in the UK!). You can find a comparison table for our sleep system on each product page. For more information on Togs, check out the OMM Glossary. We do not assign a temperature rating to our bags as we believe the figures gained in a laboratory are not necessarily representative. Each person will have a different experience, depending on their size, what they have eaten, their metabolism, what sleeping mat they use and what they wear in the bag. Temperatures quoted from lab testing also do not take into account compression of the underside insulation when you are in the bag, which has a big impact on the true value.

Our Sleeping Bags are primarily designed as a race-weight option for our event held in the UK every October. This means competitors can experience anywhere from -5C (as they did in 2018) to plus 10C and often in wet conditions.

The emphasis here is race-weight, that is to say, users are willing to put up with a certain amount of discomfort for the space and weight saving.  

Can I Wash My Sleeping Bag

 Yes! And please do. Just like owning a top end sports car, a performance Sleep System is only as good as how well you care for it. In the same way a sports car still needs oil changes and screen wash, you need to regularly maintain your OMM Sleeping Bag to keep it performing at its best. It is important to follow the Product Care Instructions and use the correct wash  product as these can affect your Bag’s performance. The benefit of using a synthetic insulation is it is much more resilient when washing plus dries a lot quicker. 

What Is A Tog?

 The Tog is a measure of thermal insulance of a unit area, also known as thermal resistance. It is commonly used in the textile industry and often seen quoted for duvets and carpet underlay.
Developed at The Shirley Institute in Manchester, as an alternative to the SI unit of m2K/W.
Basic unit of Tog (insulation coefficient) is the RSI, (1m2⋅K/W). 1 tog = 0.1 RSI: Thermal resistance in togs is equal to 10x the temperature difference (in °C) between 2 surfaces of a material when the flow of heat is equal to one watt per m2. Tog is measured from the difference in temperature between a heated metal plate beneath a fabric and the air temperature above the fabric. This is a much more objective measure than Comfort Ratings, as commonly used for sleeping bags and can be applied to a wider variety of sleep systems. It is also possible to provide a Wet & Dry Tog value, useful when describing use in typical UK conditions.

What Insulation Do We Use?

OMM use PrimaLoft® synthetic insulation as it is widely regarded as the best quality available; with insulating properties very close to that of down plus largely manufactured from post-consumer recycled product (PCR). Usually manufactured in a roll or sheet of ‘wadding’, think loft insulation but a lot nicer! From PrimaLoft® we mainly use their Gold Eco with Cross – Core Technology, their highest rated and most sustainable option plus the latest ACTIVE fabric in the Core Ranges, built from a synthetic plume woven through a knitted mesh, CORE offers the warmth of a fleece but with less than half the weight.

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