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Sleep systems

R-value

What is 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.

R-value at a glance

Summer
R 1 to 2
Three-season
R 3 to 4.5
Winter
R 5 and up
Standard
ASTM F3340, adopted 2020

Why the pad matters more than the bag

Your sleeping bag insulates you from the air. The ground is a different problem: it's in direct contact with you, it's usually colder than the air, and it conducts heat away far faster than air does. Underneath you, the bag's insulation is crushed flat and doing almost nothing. The pad is what stands between you and that, and R-value is how it's measured.

How R-values add up

The scale is roughly linear, so a pad rated 4.0 insulates about twice as well as one rated 2.0. Values also add up when you stack pads, which is the standard cold-weather trick: a closed-cell foam pad under an inflatable gets you the sum of the two, plus insurance if the inflatable punctures.

Why the numbers only became comparable in 2020

The important part of the story is the date. Before 2020 every brand tested however it liked, and the numbers were marketing. ASTM F3340 fixed that with a single method, and the major brands adopted it, so a 4.5 from one maker now means the same as a 4.5 from another. Ratings printed before then are not comparable to current ones.

How much R-value do you actually need?

The usual bands are R 1 to 2 for summer, R 3 to 4.5 for three-season use, and R 5 or more once you expect frozen ground. Those are starting points rather than rules, and the variable that moves them most is you: cold sleepers, people carrying less body fat, and anyone sleeping after a long calorie deficit all want more than the band suggests.

Ground temperature matters more than air temperature, and it lags behind it. Early in a cold snap the ground is still holding autumn warmth; late in a long freeze it is colder than the air even on a mild afternoon. A pad chosen against the overnight low on a forecast is being chosen against the wrong number.

Does the type of pad change the R-value?

Not the meaning of the number, but very much the weight it costs you. Modern insulated air pads reach R 4 to 7 with internal reflective films at well under a pound, which is why they dominate. Self-inflating pads use open-cell foam and sit around R 2 to 5, heavier but far more robust. Closed-cell foam is roughly R 2, weighs a few ounces, and cannot fail.

That last property is why foam keeps turning up in winter kits despite the poor number. Stacked under an inflatable it adds its R-value to the total and it is also the thing you sleep on when the inflatable punctures at 2am.

What R-value does not tell you

It is a thermal number and nothing else. It says nothing about how wide the pad is, whether your shoulders hang off it, how tall it inflates, how much it crinkles when you move, or how well it survives a night on pine duff. A pad with a great R-value that leaves your arms on the ground is not warm in practice.

It also assumes the pad is doing its job, which means inflated properly. Over-inflate an air pad and you compress against a drum rather than sinking in; under-inflate one and your hips bottom out onto ground that is not insulated at all.

What this means when you're choosing

  • A number tested to ASTM F3340, which any pad sold since 2020 should be
  • R 3 to 4.5 for three-season, and higher if you sleep cold
  • Whether stacking a cheap foam pad underneath is a better buy than a warmer pad

Work it out

Stack two pads

R-values add. Putting a closed-cell foam pad under an inflatable is the cheapest warmth you can buy, and this is what the pair is worth together.

Combined R-value

6.5

Enough for winter and frozen ground.

The common mistake

Buying a warm bag and a thin pad

Most cold nights people blame on their sleeping bag are actually the pad. The bag's insulation is compressed to nothing underneath you, so the pad is doing that whole side of the job on its own.

Common questions

What R-value do I need for three-season backpacking?

Between 3 and 4.5 covers most spring through autumn conditions. If you sleep cold, or expect frost, go to the top of that band rather than the bottom.

Can I add two pads' R-values together?

Yes, that's how the standard works and it's the usual way people extend a three-season pad into winter. A cheap closed-cell foam pad underneath adds roughly R 2 and doubles as puncture insurance.

Where this is defined

Where this comes up

Guides on Tom's Hiking where r-value changes which product is right for you.

Going deeper

More in sleep systems

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.

Baffle

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.

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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