Do Sleeping Bags Lose Their Warmth?

Discussions about what sleeping bag to choose often center around how warm they are going to keep you. But you need to know how long are they expected to keep you warm and what factors will affect the quality of your sleep.

Yes, sleeping bags do lose their warmth, and much of it comes down to their age, how you care for them, the material they are made of, their shape, plus a few more factors.

If you are in the market for a sleeping bag then you will need to know a bit about warmth rating, loft, fillings and how to store, clean and use your sleeping bag to ensure it provides you with many nights of comfortable warm sleep.


The loft in a sleeping bag is what gives it its warmth. The higher the loft the more air is trapped between the particles from which the filling is made and the warmer it is.

Sleeping bags with a down filling, plucked from the soft under feathers of eider ducks, various others ducks and geese, have long been considered the Rolls Royce’s of sleeping bags. However, there are new synthetic fillings on the market which promise to be better than down.

The loft of a filling for a sleeping bag is given in numbers representing how well the filling, whether down or a synthetic fibre, puffs up again after being compressed.

The higher the loft rating the warmer your sleeping bag will be so if you aim to be toasty warm inside your sleeping bag in the mountains in winter then go for the highest loft rating.

Loft rating is based on testing under laboratory conditions to see what one ounce of compressed fill will expand to – the higher the figure the better the insulation.

One ounce of fill that expands to fill 550 cubic inches will have a loft rating of 550 and one that has a loft rating of 900 will expand to fill 900 cubic inches. It’s the spaces between the fibers or down that trap the heat and keep you warm.


The shape of the bag affects the warmth. The bigger the bag the longer it takes for your body heat to warm it. All that spare space takes time and energy to warm.

The rectangular sleeping bags are convenient in that they can be zipped together and two people can sleep inside keeping each other warm.

The semi-rectangular sleeping bag gives less leg room but will heat up faster and the best shape is the mummy shape that fits to your body and warms very efficiently.

What shape you choose though depends on your sleep style – some people don’t like to have their legs confined in the mummy shape.

We lose heat through our heads – that’s why in medieval times when there was no central heating people wore woolly nightcaps, so if camping in a very cold place the sleeping bag with a hood that just allows the face to stick out may be worthwhile for warmth.

Understanding Warmth Rating

In earlier years warmth ratings were a bit hit and miss – who says you’ll definitely be warm in a sleeping bag advertised to be warm at 20 degrees F?

It depends on whether you are a man or woman, your metabolism, what you chose to sleep in, and how you experience temperature.

The result is that a European rating system has been adopted in the US called the EN 13537, which provides a guideline on what you can expect in terms of warmth. There are three ratings:

  1. Comfort – read this as the temperature at which a woman would be comfortable. I guess to avoid being sexist they don’t say this is for women, but we all know many women feel the cold more than men and need more insulation.
  2. Lower limit – this denotes the temperature at which the average man would be comfortably warm providing he is wearing long pyjamas and the bag is on a 1” sleeping pad, not in direct contact with the ground.
  3. Extreme – the coldest temperature where you should survive without freezing. This is definitely not a rating you should aim for as you will probably be a bit uncomfortable and doing your best to stop your teeth chattering in your sleeping bag.

Each of the three ratings will have a temperature in Fahrenheit marked on the sleeping bag indicating down to which temperature you would probably be comfortable.

Down or Synthetic Fill?

Down is lighter than ordinary feathers, and has a high capacity for insulation.

Check when you buy whether your sleeping bag is down only, or a mix of feathers and down. If it is a mix it will be less warm and a good deal bulkier than a pure down sleeping bag.

The advantages of down is that it compresses better than synthetic fill and lasts longer, some people claim their sleeping bags of pure down have lasted up to 30 years.

The average is around 10 years, making them worthwhile for the initial outlay as quality down is very expensive.

The color of the down does not matter – white, grey or black all works the same. The disadvantage is if your down sleeping bag gets wet – then it’s going to lose its insulating ability.

There have been advancements in nanotechnology where a fine hydrophobic coating is applied to the down fibers, binding with them to make the down able to repel water – but this does affect its loft ability – natural untreated down has higher loft.

There are a number of synthetic fillings available like PolarGuard and Quallofil, but these have always been a bit on the heavy side compared to down sleeping bags. Now HeiQ the Swiss textile innovator has launched HeiQ XReflex which gives the same level of warmth with 50% less bulk.

In collaboration with Xefco, an Australian company, they are working on a reduced carbon footprint and their radiant barrier technology cleverly reduces heat loss through reflecting the heat from the body back through the insulating layer to keep the person warm.

The material is made from recycled sources and is also recyclable.

Perspiration Can Make a Bag Lose Its Warmth

The average person perspires out 700ml to 1 liter of water per night while they sleep. This may sound excessive but the skin is the largest organ of excretion and the average adult’s skin measures around 19 square feet.

Some campers have suggested weighing your sleeping bag before going on a camping trip and again after the trip to see just how much water is trapped in that lining.

The point of this is that perspiration, if it does not escape from the sleeping bag during the night, remains in the lining and can affect the warmth.

Although manufacturers advertise breathability of their sleeping bags, waterproofing a sleeping bag can affect how much perspiration can escape. Sounds gross – well that’s why sleeping bag liners are advised, with their moisture wicking properties, to keep the inside of your bag clean, dry and warm.

Storing a Sleeping Bag Properly to Retain Its Warmth

It’s tempting to roll the bag is tightly as possible before storing so it doesn’t take up too much space, but this compresses the filling resulting in less loft once it is unrolled months later.

The ideal way to store a sleeping bag is the way you see them displayed in the shop – hung up hanging completely free. However, not many people have the luxury of storing their sleeping bags in this way, so the next best thing is to make a loose cotton bag for it so it is not squished up.

The bag the you use when you take the sleeping bag on a weekend is just for that – squishing it into a small space when you travel. Once home, put the sleeping bag and its travel sack together into a looser cotton bag, or simply fold or roll loosely and store in a linen cupboard.

This will prevent the loft getting compressed and being unable to trap layers of air that keep you warm.

Cleaning a Sleeping Bag to So It Keeps Its Warmth

Machine-Washing a Sleeping Bag So It Keeps Its Warmth

When you machine wash a sleeping bag there are some tips to follow to make sure it keeps its warmth. First make sure all the zips are done up and Velcro fastened – that’s just to avoid them getting caught in the machine and nothing to do with warmth. If the bag has a waterproof coating turn it inside out.

Machine washing in a top loader with an agitator may result in the filling bunching and loosing loft, and therefore warmth – so avoid this type of machine.

Use a heavy-duty front-loading machine on a gentle cycle and make sure the water is cool or warm, not hot. Use a good product specifically for the type of sleeping bag. If it’s a down one try Nikwax Down Wash.

This soap-based cleaner is free from perfluorocarbons (PFCs) and will maintain the loft and insulating properties of the down filling.

A good sleeping bag maintenance combination to buy is the Granger’s Performance Repel and Down wash – two separate products bought together to firstly wash a down bag and secondly spray on a coating that is water-based, fluorocarbon, and volatile organic compound free (VOC-free) to maintain the water-resistance.

If your sleeping bag has a synthetic filling use a pure mild soap, or Granger’s Performance wash.

Once the sleeping bag has finished washing and there is no soap residue – you may need to put it through an extra rinse cycle if there is, then move it carefully to the tumble dryer, set it on a low heat and add some clean tennis balls to the dryer to help stop the down clumping.

Be patient – drying will take some time. Remember not to use fabric softener as the residue left behind can cause damage to your down or synthetic filling.

Synthetic sleeping bags can be put in the machine without too much trouble – just remember to check the wash care instructions on the label.

Handwashing a Down or Synthetic Sleeping Bag

Add sufficient warm water to the bath and soak the inside out bag for an hour or so with a good down wash. Wash by treading on the sleeping bag to squeeze water out rather than trying to wring it – the whole idea is to keep the filling from bunching.

Drain the dirty water, add more water and repeat until there is no soapy residue left. After emptying out the water again stand on the bag to squeeze out water and leave in the bath for 30 minutes to drain.

Get some old towels and wrap up the bag to carry to dry outside, or to the dryer. The towels will absorb the excess moisture.

If drying it outside it will take time to get fully dry, so pick a time when a few warm windy days are expected, you have a secure place to support the bag, and you turn it regularly for even drying.

Dry Clean Using Gentle Chemicals for Down

When harsh dry-cleaning chemicals are used it removes the natural oil from the down filling and this will affect the ability to retain warmth – again resulting in a sleeping bag that underperforms in cold weather.

Down sleeping bags are unique in that the thin layer of natural oil on the duck feathers helps traps warm air and once this is stripped by de-greasing chemicals then they do not puff up as nicely and trap air to keep you warm.

Synthetic sleeping bags should not be dry cleaned as the chemicals could cause damage. If you need to dry clean make sure the company you use is aware of what chemicals can be safely used for sleeping bags.

Don’t Be Too Keen to Wash

If you use a sleeping bag liner, and clean clothes for sleeping, the inside of your bag shouldn’t get too dirty. Simply turning the sleeping bag inside out and giving it a good airing should help ensure it smells fresh.

If the waterproof outside shell has some dirty marks, spot clean in places and reapply the waterproofing with Nikwax Hardshell and Waterproofer.

Age Means Less Warmth

We have all used a sleeping bag at some stage and wondered why it seemed so thin and cold – if it’s old and been machine washed numerous times, the fibers will lose their bounce and become flat resulting in less air being trapped between the layers to keep you warm.

Buying quality sleeping bags means they will retain their warmth for many years but if you buy cheap material it will soon compress and you’ll be left out in the cold.

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