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Sleep systems

Baffle

Also called baffle wall, chamber

What is a baffle in a sleeping bag?

A baffle is a fabric wall sewn between the inner and outer shells of a down bag to hold the down in place. Without baffles the down migrates to the low side and leaves cold spots where there's no insulation at all.

Baffle at a glance

Sewn-through
Light, has cold seams
Box baffle
Warmer, heavier, cold-weather standard
Also refers to
Internal chambers in air pads

Why down has to be held in place

Down is loose. Put it between two sheets of fabric and it will slide to wherever gravity takes it, which after a few nights is the bottom and the sides, leaving the top of the bag empty. Baffles are the internal walls that divide the bag into chambers and keep the down where it was put.

Sewn-through against box baffles

The cheap way to make chambers is to stitch the inner and outer shells together directly. That's sewn-through construction, and it works, but every stitch line is a place where there's no insulation at all, so a sewn-through bag has a grid of cold lines through it. Fine for summer bags and light jackets, poor below freezing.

Box baffles solve that by inserting a strip of fabric between the two shells, creating a chamber with depth and no direct contact between inside and outside. That's what every serious cold-weather bag uses. Some bags also add horizontal or diagonal baffle designs so you can shift down towards the top on cold nights, which is a genuinely useful feature on a bag you take across seasons.

Baffles in sleeping pads

Sleeping pads use the word too, for the internal structures that shape an air pad. There it decides whether the pad supports a side sleeper or lets their hip sink through to the ground.

What this means when you're choosing

  • Box baffles rather than sewn-through on anything rated below about 40F
  • Whether down can be shifted around the baffle, which some bags allow
  • Continuous baffles, which let you move down to the top or bottom

The common mistake

Buying a sewn-through bag for cold weather

Stitching the shells directly together creates a cold line at every seam, because there is no insulation where the fabric meets. It is fine in a summer bag or a light jacket and it is the wrong construction below freezing.

Common questions

What's the difference between box baffles and sewn-through?

Sewn-through joins the shells directly, which is lighter and leaves a cold line at every seam. Box baffles add a fabric wall between them so the chamber has depth all the way across. Below freezing, box baffles are what you want.

Can I fix a bag where the down has migrated?

Within a chamber, yes. Lay the bag flat and work the down back towards the top with your hands, which is worth doing before every cold trip. Down that has escaped between chambers means a baffle has torn, and that needs a repair.

Where this comes up

Guides on Tom's Hiking where baffle changes which product is right for you.

More in sleep systems

R-value

R-value measures how well a sleeping pad resists heat escaping into the ground. Higher numbers insulate better, and since 2020 every brand has tested to the same standard, so the numbers finally compare across brands.

Fill power

Fill power measures the loft of down: how many cubic inches one ounce expands to fill. Higher fill power traps more air per unit of weight, so an 850 fill bag is lighter and more compressible than a 650 fill bag of the same warmth.

Fill weight

Fill weight is how much insulation is actually inside a sleeping bag or jacket, given in grams or ounces. Paired with fill power it tells you what a bag's warmth is made of, and it's the number brands are least keen to publish.

Hydrophobic down

Hydrophobic down is down treated with a water-repellent finish so it resists moisture, dries faster and keeps some loft when damp. It delays the failure that untreated down suffers when wet, but it doesn't make down behave like synthetic.

EN/ISO temperature rating

EN/ISO is the standardised lab test that gives sleeping bags their temperature ratings. It produces several figures, and the two that matter are comfort, aimed at a standard cold sleeper, and limit, aimed at a standard warm sleeper.

Mummy bag

A mummy bag is a sleeping bag cut wide at the shoulders and narrow at the feet, with a hood that closes around your head. The taper leaves less air inside for your body to heat, which is why it is the warmest shape for its weight.

Draft collar

A draft collar is an insulated tube around the neck opening of a sleeping bag, cinched to seal the gap at your shoulders. It stops warm air pumping out of the top of the bag every time you move, which is where most heat is lost.

Footbox

The footbox is the shaped end of a sleeping bag or quilt where your feet sit. It's built with extra volume so the insulation isn't crushed flat by your toes, which is the coldest spot in most sleep systems.

Backpacking quilt

A backpacking quilt is a sleeping bag with the back removed and no hood, strapped to your sleeping pad instead. It cuts weight and bulk by deleting insulation that a bag crushes flat underneath you and that does nothing there anyway.

Pad attachment system

A pad attachment system is whatever keeps your bag or quilt joined to your sleeping pad, usually elastic straps that pass under the pad and clip to the quilt. It stops the two separating in the night on any surface that isn't flat.

Underquilt

An underquilt is an insulated quilt that hangs on the outside of a hammock, cradling it from below. Because your body never lies on it, nothing compresses the loft, which is why it works where a sleeping bag underneath you does not.

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