Choosing a Sleeping Bag

Choosing a Sleeping Bag

No matter how you've spent your day outdoors, with the right sleeping bag you can recover at night. The standard sleeping bag will suit people up to 185cm tall, but if that is not enough for you, check for long-sized sleeping bags. So aside from size, what else do you need to look for in a sleeping bag?


Factors to Consider When Choosing a Sleeping Bag:

Temperature Range & Climate – In what kind of temperatures will you use the bag most often? Choose your bag to suit these conditions.

Activity & Weight – If you have to pack and carry the bag over long distances, you want it to be light and compact.

Budget – Synthetic-filled bags are less expensive, but down-filled bags last longer.

Many factors can influence how warm your bag will feel, including your diet throughout the day, the day's activities, your metabolic rate, moisture levels in your bag, ground insulation quality, clothing you're wearing to bed, and distribution of fill in your sleeping bag, to name a few. So which type of sleeping bag to choose? To make it easier, there are two main types of fill for sleeping bags; down fill (natural duck or goose feather) or synthetic fill (polyester). Both have their advantages so consider your activity, climate and price range in your decision-making. The tips below should help you out.

Down vs Synthetic

Pro Down

Better warmth to weight ratio; more compressible; lightweight; lasts longer

Pro Synthetic

Less expensive than down; retains thermal properties when wet; good for asthmatics and allergies


Down Fills & Loft Ratings

Down is the only natural product that still cannot be replaced by synthetic fibres. Percentage of down/feather is one measurement of down quality; the higher the down percentage the greater the insulation. Mountain Designs uses minimum 85% down (maximum 15% down fibre, feathers and residual) in the majority of our down products.

The loft rating, also known as ‘fill power’, is a numerical measurement for the amount that the down expands and traps air within its clusters, for example ‘600 Loft’. It is an indicator of a product’s thermal efficiency. The greater the rating, the better the loft, meaning less down is required for the same warmth, and thus the sleeping bag will be lighter. The loft rating marked on a Mountain Designs sleeping bag is based upon tests completed by the International Down & Feather Laboratory (IDFL) before the down and feathers have been inserted into the finished product. It is the minimum loft value and is the basis for all labels, advertising and marketing claims. After long term storage in a compressed state the loft rating will slightly reduce. The best way to protect your product’s longevity is to store it in a non-compressed state, in a breathable bag and in a dry place.


Synthetic Fills

Synthetic fills have come a long way in recent years. They still don't match down bags, especially in the warmth to weight ratio, as they are heavier and have a shorter life span. Yet synthetic bags are considerably less expensive and can have an advantage in wet conditions. Mountain Designs uses microfibre synthetic fill with weights in various combinations that determine the insulation levels.


Bag Shapes

Because sleeping bags work by ‘trapping’ the air warmed by your body, the less air there is inside your sleeping bag, the less your body has to work to warm it. The three main shapes available are rectangular, tapered and mummy. Rectangular-shaped bags are the most basic and are designed for general use, as they have plenty of room inside for movement and can be unzipped and opened out flat. The down side to this shape is that there is a lot of excess space around your body that reduces the thermal efficiency. The tapered shape is designed to minimise this excess space while still providing space inside to move around. The tapered shape is the most common shape as it is much more versatile and thermally-efficient. They can generally still be unzipped out flat, the same as the rectangular shape. The mummy-shaped sleeping bag is designed to remove as much excess space as possible without making you feel claustrophobic. Mummy-shaped bags are for much more technical use and where thermal efficiency is a priority. A good mummy-shaped bag will ‘fit like a glove’ and since they use less material, will pack smaller and weigh less than other bag shapes.


Bag Construction

Baffles locate fill in the areas where it is required and create a wall between the inner and outer bag (sewing through would create cold spots). Mountain Designs sleeping bags are designed to ensure the correct amount of fill is used in every compartment. Under-filling creates fill movement and cold spots, overfilling is a waste of fill, a problem especially when it is expensive down. Each baffled compartment's optimum fill is therefore carefully measured and filled by hand. Our sleeping bags use a variety of baffle constructions, each with its own benefits, including:

Offset Quilting – This is typically used for synthetic-filled bags, and minimises cold spots between the seams through the offset stitching.

Stitch Through – The shell and lining fabric are sewn directly together to form the baffles. This is a basic construction technique used for warmer weather bags as the stitching can cause cold spots.

Box Wall – Between the shell and the lining, a separate material is sewn in to separate the down. This minimises cold spots and allows the down to loft up more efficiently.

Trapezoidal – Developed by Mountain Designs founder Rick White, the trapezoidal construction uses a separate material to separate the down, similar to the box wall construction. However, the trapezoidal construction uses an offset design to eliminate cold spots between seams. The trapezoidal shape also improves the down dispersion within the baffles.

Curved – Developed to improve down migration, spreading the insulation evenly throughout the baffle and holding it in place when using the sleeping bag.



The best fill and efficient baffling mean nothing if the bag isn't made with fabrics that allow it to perform. Fabrics must allow moisture vapour from perspiration to pass through, prevent fill leakage and feel great against your skin. Some sleeping bag manufacturers save costs by using fabric with lower thread counts, or use 50 denier yarns. The performance costs of this can be significant: aside from the obvious differences of a stiffer feel and rougher touch, the weight penalty is a factor bigger than how much extra the sleeping bag weighs in total. More significantly the extra weight is sitting on the down and restricts the down's ability to loft fully and give you the best insulation.


Temperature Ratings

The temperature rating is a measure of a sleeping bag’s performance under certain temperatures.

All Mountain Designs sleeping bags are tested using EN ISO 23537-1, the most widely recognised international standard for measuring the warmth of sleeping bags. The testing is conducted in a laboratory to standardised user body specifications. The temperature rating outlines three different values – described below – but should only be considered a guide as everybody has different body types and coping mechanisms. 

‘T Comfort’ value refers to the lowest temperature at which a standard user can expect to sleep comfortably in a relaxed body position, such as lying on their back (related to a standard woman and in standard conditions of use). This is a suitable guide for the coldest temperature a ‘cold sleeper’ would want to use the sleeping bag for.

‘T Limit’ value refers to the lowest temperature at which a standard user can expect to sleep in a curled body position and remain asleep (related to a standard man and in standard conditions of use; note that the temperature is lower because the metabolic rate used in the equation for a man is higher than that used in the equations based on a woman’s physiology). This is a suitable guide for the coldest temperature a ‘warm sleeper’ would want to use the sleeping bag for.

‘T Extreme’ value refers to the lowest temperature at which a standard user can survive in when using the sleeping bag (related to a standard woman with a curled body position in a situation of high cold stress with shivering). This is a survival rating only, and at this extreme temperature the user can expect to feel very cold with a real risk of health damage by hypothermia. Use is not advised at this level.

Mountain Designs recommends using the temperature rating as a reference point for comparing other EN ISO 23537-1 tested bags within the Mountain Designs range or across other brands.




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