Why Does a Down-Filled Sleeping Bag Keep You Warm?

As you might have read in my 15 mistakes article, I have chosen my sleeping bag for my hike in Scotland very poorly. That terrible mistake got me shaking during the cold nights of October, and I promised myself to learn from it in future to come campings. That got me digging a bit deeper into sleeping bags, and encountered me with down insulation. So let’s get started and try to understand why does a down-filled sleeping bag keep you warm?

I’ve spent a few days reading about it and figured out what makes a down filled sleeping bags so qualified for cold weather campings. In short, down-sleeping bags keep you warm due to their capability of creating a sufficient thermal barrier between your body and its surroundings – lowering thermal conduction. That would be thanks to the trapped dead air between the feathers featured in down sleepings bags.

So What is Thermal Conduction?

The reason a hot cup of tea gets cold, and for you freezing in cold winter nights, are good examples for thermal conduction. This is merely the process of heat energy transferring from one object to another.

As intuition tells, during that process, energy moves from a hot object to a colder one.

The material which the energy moves through is called the conductor. As you probably know, each element conducts heat differently.

When it comes to sleeping bags, the conductor is merely the cold air, getting heat energy from your warm body.

Sleeping bags trap the air between you and the inner part to prevent your body from getting colder – creating a thermal barrier.

This trapped air creates insulation between your body and the environment, preventing your body temperature from dropping.

What is Down?

Some people believe that down is merely the birds’ feathers; however, that is only partially correct.

Birds have two layers for thermal insulation – the outer layer, which is the tough feather we are all familiar with, and the inner layer – the actual down. 

The second is a fine, thin layer which serves as a thermal insulator in many industries, such as jackets, pillows, sleeping bags and more.

The thin loose structure of the down layer enables air trapping, preventing heat from slipping away.

One interesting fact is that birds which experience annual weather changes tend to grow more down toward winters, adjusting themselves for cold climates.

The amount of air down can trap, depends on the density it features. The more down there is per volume unit, the more air it catches and insulation it provides.

That will be described later on as down fill powers.

What is Down Fill Power Exactly?

If you are interested in down products, you’ve probably heard about down fill power.

So what is that exactly? Well, it’s merely a tool for measuring how well the product can insulate heat.

It is usually described as a three digits number, ranging from approximately 300 to 900. The higher that value – the higher the product quality and insulation ability.

That number represents the volume in cubic inches of a single gram of down when fully fluffed up.

Nevertheless, that number isn’t the only indicator for warmth maintaining capability – you should also take into account the amount (quantity) of down the product features.

For example, a 400 fill power product would be warmer than 600 fill power product, if it has a total of 300 grams of down while the other features only 200.

Nevertheless, the 300 grams would weight more than the 200’s, a factor that should not be underestimated.

Don’t Miss The Down Fill Weight

While down fill powers describe the down regarding volume, the fill weight directly addresses it from the weight angle.

Fill weight takes into account the amount of down invested in the product. You should pay close attention to that because down fill power can be deceiving.

Let’s say you are entering a shop, looking for a down jacket. You go through the current collection, but you have absolutely no idea which one to pick.

Then, you are desperately trying to make a decision based on the only ingredient mentioned on the jacket’s label – down fill powers.

With the seller encouragement you buy the jacket with the highest power, but for your disappointment, that jacket doesn’t do too well in cold weather.

Why is that? Well, that might be because the jacket has a low fill weight.

It could be that the down expands into a significant volume (fill power), although there is not enough of it to keep you warm (fill weight).

When you choose a down filled product, take into account these two ingredients together.

Why Should I Choose Down?

Good Weight-Heat Ratio

As mentioned above, the down is the subtle, thin inner feathers layer that maintains birds body warmth.

For it is so gentle, the products filled with it are incredibly lightweight, like this one from Amazon. Concerning camping or hiking, keeping your gear lightweight has a tremendous impact (On that topic, I recommend reading my article about lightweight hiking boots).

The down’s microscopic structure is perfect for air trapping, ensuring effective warmth insulation for its weight.

Can Be Highly Compressed

I remember those times I was camping when I had absolutely no extra room inside my backpack. That was so frustrating; I couldn’t even storage some souvenirs I’ve bought along the road.

It took me a few hikes to understand that my backpack should be filled for about 70%.

A squeezable sleeping bag would turn your life way more comfortable, especially if you are hiking with a carry on or a 40-liter backpack, which are considered small.

Extremely Durable

We all know that little annoying feeling of spending too much money on a short adventure.

We sometimes buy stuff we end up not using or buying expensive things that get ruined after a single use.

A sleeping bag is not a cheap investment, so it is essential to make the most of it. Regarding durability, down-filled sleeping bags feature better performance than synthetic

However, you should still properly treat your bag to ensure that. Make sure it doesn’t get wet and clean it up correctly according to the manufacturer recommendations.

What Are The Downsides?

Bad Performance When Wet

While being extremely warming in a dry condition, a down sleeping bag loses it’s touch when it gets wet.

Avoiding that could be difficult in the field of camping. For that reason, you should stick to a water resistance backpack and a tent which provides proper ground isolation.


If you get a little snizzy around poultry, a down filled sleeping bag may not be the right choice.

It is very likely that the allergenic particles are in the town itself, turning your sleeping experience into a nightmare. If that is the case, you better consider buying a synthetic one.


Down-filled products usually cost more than synthetic ones. That is due to their long and costly manufacturing process.

In my opinion, the price is justified, for their high quality and thermal perseverance capability.

However, if a sleeping bag is not such a significant investment for you, a synthetic one might be a better choice.

What About a Synthetic Sleeping Bag?

Synthetic sleeping bags are made of a manufactured polyester which provides excellent insulation and water resistance.

They are usually cheaper than the down-filled sleeping bags so that you can afford one even with a low budget, like this one from Amazon.

In contrast to down, they perform well even when wet, so you can be less worried about keeping them dry (yet, it is still recommended).

Being made of polyester, this type of bags are hypoallergenic and might be a better choice if you are sensitive to poultry.

Nevertheless, they do have their downsides comparing to the down filling. They feature a lower warmth-weight ratio – they are a bit heavier and insulate warmth less than down.

In addition to that, they are bulkier and not as compressive as down-filled sleeping bags, so you might find it hard to place them inside your backpack.

How To Manage a Wet Down-Filled Sleeping Bag?

As mentioned in its cons, a down filled sleeping bag doesn’t do too well while being wet, as opposed to a synthetic one.

Although, some tips might help you manage a case like that, even when far away from home.

Place Your Sleeping Bag Under The Sun

It is evident that wet stuff dries faster when being warmed out, however, not everyone takes that into account when dealing with a damp sleeping bag.

When you take a break, make sure to spread your bag entirely in a spot exposed to sunlight.

Hang It Up to Dry

Just like laundry, the best way a wet sleeping bag would dry is by hanging it in a windy spot. That might be difficult in the wild, but trees branches might play in your favor for that.

Manipulate The Bag

When down gets wet, it tends to get stuck up together in some spots. When it is in the shape of a big mass, it will take much longer for it to dry up.

Try manipulating the stuffing, and make sure it spreads equally along the entire bag.

How to Wash a Down Sleeping Bag?

Down filling is exceptionally gentle and can be easily mismanaged.

The best way to clean it up is with the washing machine, yet, you should take specific steps to do so.

First of all, remove dirt from the outer part with a rinse of water or a soft towel. After that, zip up the entire bag and close all its velcros, so it doesn’t get snagged while the machine is working.

Add a technical cleaner to your wash, do not use softeners or regular detergent – they could damage your bag.

Then, let the machine wash it under 30 celsius on a delicate cycle.

You might then put it into a dryer machine, yet – make sure to use a low temperature for that, to prevent down clumping (you may also use a few clean tennis balls for that purpose).

How to Store a Down Sleeping Bag?

After you’ve cleaned and dried up your sleeping bag, it is time to store it appropriately until your next trip.

The one thing you have to do is extract it from its currently stuffed sack and put it into a loosely meshed bag.

That is important because when it is stuffed tightly for an extended period, your bag loses its filling resilience.

With low resilience, the down would have a hard time to expand once you use it again. That could end up with some parts of your sleeping bag which do not provide proper insulation.

After that, you should store your bag in a place with low humidity and without temperature extremes (such as too cold or too hot spots).

Avoid storing it in damp basements or unheated places like attics or garages, instead, use a dry and balanced temperature place.

For example, a corner inside your closet would be an excellent place to start.


Down-filled sleeping bags are efficient with creating a thermal barrier which would maintain your body warmth. Their thin, gentle structure provides good insulation while keeping your product lightweight.

When you choose a down-filled sleeping bag (or any other product with down), it is essential to take into account its fill power and fill weight.

While a bit more expensive, down-filled bags are highly durable and would compress efficiently inside your backpack.

If you are looking for a cheaper solution, you might consider getting a synthetic one. These are a bit bulkier and provide less thermal insulation; however, the synthetics are hypoallergenic and would keep you warm even in wet conditions.

When it comes to cleaning your sleeping bag, I recommend you use a washing machine on low temperature and gentle cycle. After drying out, make sure you store it in a balanced temperature spot, with as less moisture as possible.

Recent Posts