What is the warmest sleeping bag?

Buying a warm sleeping bag for a camping or hiking trip can be overwhelming. With so many different shapes, seasons, warmth ratings (and a good quality night’s sleep on the line!), it’s hard to determine what is going to be the most appropriate. With more than 30 years assisting campers and hikers, whether you need a sleeping bag for a music festival or something to keep you comfortable whilst trekking in the Himalaya, Open Air is here to assist you buying your next warm sleeping bag, whatever your adventure.

Sleeping Bags at a Glance

The basic function of the sleeping bag is to keep you warm and cosy at night by capturing body heat released as you sleep. This is achieved through the use of an insulating material, either natural or man-made, to trap this warmth and retain it.

Warm and toasty, what ever the temperature.

The ability for the sleeping bag to provide warmth and comfort depends on a number of different factors; 

  • The time of year (and expected overnight temperatures)
  • The type (and quality) of insulating fill
  • The shape of the sleeping bag 
  • The use of a sleeping pad and its thickness / insulating properties
  • Most importantly, the user themselves

As everyone feels the cold differently, mileage may vary between users of the same sleeping bag under the same conditions. Generally speaking, body temperatures differ between men and women and also vary according to age, size and metabolism. Some sleep warm whilst others will feel the cold, even on mild nights. 

Sleeping Bag Temperature Ratings

Sleeping bag manufacturers provide a number of different ratings to assist you with selecting the warmest sleeping bag for your adventure.


When selecting a sleeping bag, it’s important to consider what time of year you’re likely to be using it. Most sleeping bags will be categorised by a ‘Season’ rating which will provide a guide to the appropriate sleeping bag for the time of year you’re likely to be using it.

  • 1 Season Sleeping Bags (10 degrees or warmer): lightly-filled, compact summer sleeping bags perfect for use in the warmer months (June – August) or travel abroad to hotter locations.
  • 2 Season Sleeping Bags (5 degrees or warmer): ideally suited for use in Late Spring to Early Autumn. Appropriate for UK ‘family campers’ as even in summer, a warm night every night isn’t guaranteed. 
  • 3 Season Sleeping Bags (0 degrees or warmer): If you’re camping or hiking in Spring or Autumn, a 3 season sleeping bag is likely what you will require. Many of these sleeping bags strike a balance between keeping you warm on cold nights, and still being appropriate for milder nights (though expect to sleep with the sleeping bag unzipped). Prepare to have the cold impact your night’s sleep as the temperature approaches zero.  
  • 4 Season Sleeping Bags (-10 degrees or warmer): sleeping bags for use in snow or very cold winter nights. 
  • 4 Season + Sleeping bags (-40 degrees or warmer): sleeping bags for use in extremely low temperatures (ie. high mountain or polar use).
Rab Andes 1000 – Available upon request.

Temperature Range

To complement the season rating, sleeping bags typically carry temperature ratings. Since everyone feels the cold differently, a sleeping bag’s temperature range can be incredibly useful as a guide for the conditions the sleeping bag will be most comfortable in. These ratings are typically expressed as Comfort, Lower Limit and Extreme and form part of the EN13537 European Standards for sleeping bag temperature ratings. Introduced as a way of standardising the testing procedures by sleeping bag manufacturers, these ratings provide consumers with the ability to make relative comparisons between sleeping bag models.

  • Comfort Rating (T-Comfort)
    The comfort rating is the temperature at which an average adult woman can expect to sleep comfortably in a relaxed position. If you’re a cold sleeper, we recommend that you use rating to decide the coldest temperature the sleeping bag is suitable for.
  • Lower Limit (T-Limit or Limit of Comfort)
    The limit of comfort is the temperature at which a standard man can sleep for eight hours in a curled position without waking (wearing thermals and sleeping on a 2.5cm sleeping pad). If you’re a warmer sleeper, we recommend that you use rating to decide the coldest temperature the sleeping bag is suitable for.
  • Extreme (T-Extreme)
    This is a survival rating and is not recommended for general use. Use of the sleeping bag between the limit and extreme ratings exposed the user to the risk of hypothermia.

Pro Tip: If you’ve ever purchased a duvet, you might have come across
tog ratings which typically describe warmth (ie. the higher the tog rating, the warmer the duvet). As tog only considers the material itself (not the shape or construction of the sleeping bag), tog ratings are rarely used to describe the warmth of outdoor sleeping bags.


Sleeping bag insulation fills typically fall into one of two categories; natural (down) or man-made (synthetic).


Synthetic insulation keeps you warm by trapping warm air between the polymer fibers. The best-designed synthetic sleeping bags will often feature overlapping or offset insulation layers to ensure there are no cold spots. Synthetic insulating materials have the capacity to keep you warm, even when the sleeping bag is wet. The hydrophobic (water-hating) nature of the synthetic fibres means they dry relatively quick also making them well suited to damp and wet camping and hiking adventures in the UK. The service life of a sythetic sleeping bag is sadly shorter than that of a down-filled sleeping bag as, with use and storage, the polymer fibres compress and break down resulting in a loss of loft (and therefore a loss of warmth).

Synthetic sleeping bags are often graded according to their weight (often expressed in gsm or ‘grams per square meter). The higher the gsm number (ie. 300gsm), the more insulating fill contained within the sleeping bag, therefore the warmer you will be. This does not take into consideration the quality of the insulation or the shape and design of the sleeping bag, so be sure to consult the sleeping bag’s temperature ratings when comparing models.


Down is a natural material and comes from the plumage of a bird. Plumage is the soft, fluffy undercoat that lies beneath the tougher feathers on the underbelly of birds like ducks and geese. Down is lightweight, compressible and an incredibly efficient insulator. During production, down gets combined into clusters which create air pockets between the tiny criss-crossed fibers. In a sleeping bag, down is contained within stitched chambers called baffles. These baffles are created big enough to allow the down to ‘loft’ (puff out) but also hold the down in place so it doesn’t shift around during the night (leading to cold spots.) Down quality varies greatly (species of bird, origin and whether the down has been ethically-sourced or not) but in general, down sleeping bags are more expensive than synthetic sleeping bags for the same temperature rating and tend to last longer due to their resilience to repeated compression.

Some regions of the body require more insulation than others so don’t be surprised to see complex arrangement 3-dimensional baffles and seams around the neck, head and feet to hold the down in place and trap warmth within the sleeping bag.

Fill Power vs Fill Weight

Down quality is determined by its fill power (or loft) rating. Fill power is calculated by measuring how many cubic inches a single ounce of down occupies at its maximum loft. Fill weight on the other hand is how much down is physically contained within the sleeping bag. Both are balanced by the sleeping bag manufacturer to alter the overall warmth, packed size and cost of the sleeping bag.

High Fill Power = More Puff = More Warmth Trapped for Less Weight
A visual representation of the physical size of the various grades of down.
The higher the loft rating, the more ‘puff’ a single cluster of down has. Using higher grades of down allows sleeping bag manufactures to use less down to achieve the same warmth as a sleeping bag filled with a lower grade of down.

What is better? A down sleeping bag or a synthetic sleeping bag?

Despite advances in modern insulation technologies, synthetic insulation fills simply can’t match the exceptional warmth for minimal weight offered by down insulation. Down’s compressibility also endears it for applications where space is at a premium.
However down sleeping bags come with a higher price tag and loose next to all of their insulating qualities of they get wet.

Size matters. Both these sleeping bags have the same warmth rating but the synthetic-filled sleeping bag (left) is significantly larger and heavier than it’s down-filled counter part (right).

If you’re heading out on an adventure and want a good night sleep, you have two options; spend a little more on a down-filled sleeping bag (perhaps considering a model filled with a water-resistant shell or hydrophobic down) or spend a little less on a synthetic sleeping bag, accepting its weight, bulk and longevity penalties.

With water-resistant treatments to both the shell of the sleeping bag and the down fill itself, modern lightweight down sleeping bags endure damp conditions and condensation from hike tents much better than they once did.

Shapes and Sizes

Sleeping bags are typically available in a variety of shapes and sizes to suit the user and their needs.

Like people, sleeping bags come in all manner of size and shapes.
  • Mummy – typically the warmest and most efficient shape sleeping bag design. Insulation is held close to the body with less opportunity for cold spots within the sleeping bag.
  • Semi-rectangular – users that feel restricted in a mummy sleeping bag should consider a semi-rectangular sleeping bag. Whilst these are not as warm as the mummy shape, they are often roomer and can be completely unzipped making them suitable for use in warmer weather. 
  • Rectangular – the most relaxed shape. Perfect for use in the warmer months, these sleeping bags can typically be unzipped and used as a duvet.
  • To address the differing insulation needs between genders, many sleeping bag manufacturers offer Women’s-Specific models which are shorter in length and offer additional insulation at the feet and hips. Additionally, taller users are best to look at Long / XL sleeping bag options.

Tips for additional warmth

The following can increase the warmth and efficiency of your sleeping bag. 

Use a sleeping bag liner. Not only can a sleeping bag liner add valuable warmth to a sleeping bag, but they have the added benefit of keeping your sleeping bag clean from an accumulation of dirt and sweat. Sleeping bag linerscome in various shapes and materials including cotton, silk and micro fleece to suit the sleeping bag and the user’s preference.

The Sea to Summit Thermolite Reactor – warm and stretchy
(perfect for those who wriggle when they sleep!)

Use an insulated sleeping mat. Like sleeping bags, many sleeping mats carry their own rating for resisting heat loss due to contact with the cold ground. This is called the R-value. The higher the R-value, the greater insulation they’re able to provide. Popular ‘winter’ models include the Thermarest NeoAir XTherm as well as the DownMat and SynMat insulated sleeping mats from Exped.

Thermarest NeoAir XTherm – the perfect companion for winter adventures.

Use an additional foam pad. A basic ‘closed-cell’ foam pad under your sleeping mat will provide an extra barrier against the cold ground. Whilst rather bulky, foam pads are lightweight and inexpensive.

A simple closed-cell foam mat like the Backpacker 9mm from Multi Mat provides an inexpensive boost to both warmth and comfort.

Use a reflective emergency blanket under your sleeping mat. A very lightweight and inexpensive way of adding valuable warmth and comfort to your night’s sleep. A reflective emergency blanket will help reflect radiant heat back at you amplifying warmth. 

If you have any questions regarding warm sleeping bags for an upcoming adventure, we’d be only too happy to provide assistance. You can visit us in person at our store on Green Street, Cambridge, or contact us by calling, emailing or using LiveChat.

Thanks for reading!

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