Temperature Rating: What the Number on Sleep Gear Does and Does Not Cover

A rating is a laboratory result, not a forecast

Temperature ratings come from a test rig, not from a field trial. A heated manikin, dressed and positioned to a defined specification, is placed inside the bag on a mat of defined insulation, inside a chamber at a controlled temperature with still air. The heat the manikin has to supply to hold a steady surface temperature is measured, and the result is converted into the figure printed on the label. Everything about that description matters: the dressing, the mat, the posture and above all the still air.

Because the method fixes those variables, two bags rated to the same figure should behave similarly on the rig. That is the real value of a rating, and it is a genuine one: it makes comparison between products possible. What it does not do is tell you how you will sleep, because you are not the manikin, your mat is probably not the test mat, and the air outside a tent on a Canadian ridge is not still.

Read the rating, then, as a comparison tool rather than a promise. If you want it to behave like a promise, you have to rebuild the missing parts of the test yourself: the insulation under you, the clothing on you, and the shelter around you.

• The test fixes the mat beneath the bag; your mat is a separate decision.
• The test fixes clothing on the manikin; what you sleep in changes the result.
• The test chamber has still air; wind outside does not.
• The test says nothing about damp fill, condensation or a wet night.

Which of the published figures you are reading

Most makers publish more than one figure from the same test, and they mean different things. One figure describes the temperature at which a sleeper is expected to be comfortable in a relaxed posture; another describes the lower boundary at which a sleeper curled up can still sleep without waking cold; a third, where it is published at all, is a survival figure and not a sleeping figure. Retail listings frequently quote only one of these, and not always the same one.

This is where most disappointment originates. A bag advertised with a single number that turns out to be the lower boundary figure will feel colder than a bag advertised with its comfort figure at the same stated temperature, even though both are reported honestly. Before comparing two bags, open each maker's own specification page and find out which figure is being quoted and which test method it cites. If the page does not say, treat the number as unverified rather than conservative.

The same applies to quilts, liners and clothing sold with temperature claims. A liner that advertises added warmth is describing a contribution to a system, and the contribution is measured differently from a bag rating. Add claims together with scepticism, not arithmetic.

The sleeper changes the outcome more than the fill does

A sleeping bag does not produce heat. It slows the loss of heat you produce, which means the person inside is the heat source and therefore the dominant variable. Two people in identical bags on identical mats routinely report different nights, and nothing about the gear explains it. Body size, metabolism, how recently and how well you have eaten, hydration, how hard you worked that day and whether you went to bed already cold all shift the result by more than a fill upgrade would.

This is why experienced campers treat going to bed warm as part of the system. Eating before sleep, changing out of damp layers, and doing something briefly active before getting in all raise the starting point. Conversely, going to bed dehydrated after a long day in the cold lowers it, and no rating accounts for that.

The practical move is to calibrate yourself rather than trusting a number. Use the gear on a night where conditions are known and the escape route is short, note the temperature and how you slept, and carry that offset forward. After 2 or 3 such nights you will know whether you sleep warmer or colder than the figure on your label, which is more useful than any comparison between products.

• Record the overnight low and how you slept, every trip, in the same place.
• Note what you wore in the bag and what you ate before sleeping.
• Note whether you were damp when you got in, and from what.
• Build a personal offset from the label figure, and plan from the offset.

The ground is rated separately, and it is not optional

Insulation works by trapping still air, and compressed insulation traps much less of it. Under your body the fill of a sleeping bag is squashed almost flat, so for the part of the night that matters most the bag is barely insulating at all. That job belongs to the mat, which is why mats carry their own rating rather than being folded into the bag's figure.

Because the bag test is run with a specified mat underneath, using a thinner mat than the test used means you are not getting the tested result, whatever the label says. This is the single commonest reason a bag feels colder than its rating. Check what the maker says about the mat used for the test, check the rating on your own mat, and treat the pair as one purchase decision rather than two.

Ground conditions also change through the season. Bare rock, wet sand and old snow all draw heat faster than dry duff, and a pitch that was fine in summer may need a second layer beneath the mat in the shoulder season. Carrying a thin closed-cell mat to stack under an inflatable one is a common and cheap way to add insulation when the forecast turns.

Wind and damp move the night without changing the label

The laboratory removes air movement, and the outdoors puts it back. Wind chill is a measure of how much faster exposed skin loses heat when air moves, and Environment and Climate Change Canada publishes a wind chill calculator so the combination of temperature and wind speed can be worked out rather than guessed. For planning purposes that calculation matters more than the overnight low on its own, because a modest low with strong wind can present a harsher problem than a lower low in still air.

Public guidance gives usable thresholds. The Canadian Centre for Occupational Health and Safety, summarising ACGIH guidance, notes that for continuous work in the cold a work-warming regimen with heated shelters nearby is suggested when the wind chill temperature is at or below -7 degrees C, given as 19.4 degrees F, and that at or below -12 degrees C, given as 10.4 degrees F, work should include constant observation through a supervisor or buddy system, a pace adjusted to avoid heavy sweating and wet clothing, and instruction in re-warming procedures and the signs of hypothermia and frostbite. Those thresholds were written for workplaces, but the physiology does not change at a trailhead.

Moisture is the second missing variable. Damp insulation loses loft and therefore loses the still air it depends on, and the same guidance stresses staying dry, including taking steps to prevent excess sweating, and dressing in layers with a wind-resistant outer layer. On a trip that means venting before you sweat, changing before you sleep, and treating a wet bag as a serious problem rather than an inconvenience.

Verifying a rating before the trip, and again afterwards

Verification is a short routine. Open the maker's own specification page for the bag and for the mat, write down each figure, write down the test method named beside it, and write down the mat specified in the bag test. If any of those four items is missing, the comparison you are about to make is weaker than it looks, and the honest response is to plan with more margin rather than to assume the gap is small.

Then check the conditions against public sources rather than memory. Run the forecast temperature and wind speed through the wind chill calculator, read the land manager's own safety page for the area, and note any seasonal hazards that change the plan. Parks Canada, for instance, states that it does not monitor natural ice surfaces for safety or mark potential hazards, and gives a recommended ice thickness of 15 cm for walking or skating alone and 20 cm for skating parties or games. Figures like those are published, checkable and specific to a place, which is exactly what a temperature rating is not.

Afterwards, close the loop. Write the overnight low, the wind, what was under the bag, what you wore and how you slept into the same notebook every time. Within a season that record will tell you your own offset from the label, which is the only rating that describes you.

• Maker's specification page read for both the bag and the mat.
• Published figure and its test method written down together.
• Mat specified in the bag test compared with the mat you own.
• Forecast temperature and wind run through the wind chill calculator.
• Land manager's safety page checked for the specific area and season.
• Overnight low, wind, layers and sleep quality logged after the trip.

Before you buy

• Which published figure the maker quotes, written down for the bag
• Test method named beside that figure, written down too
• Mat specified in the bag test compared against the mat you own
• Mat's own rating checked for the season and the pitch surface
• Extra closed-cell layer packed if the pitch may be rock, sand or snow
• Sleep clothing confirmed dry and set aside before the trip
• Forecast temperature and wind run through the wind chill calculator
• Land manager's safety page read for the area and the season
• Overnight low, wind and sleep quality logged afterwards

Questions readers ask

Does a temperature rating tell me the coldest night I can use the bag in?

No. It reports a laboratory result for a defined manikin on a defined mat in still air. Use it to compare products, then adjust for your own metabolism, your mat, your clothing and the wind and damp the test excludes.

Why do two bags with the same rating feel different?

Often because the two makers are quoting different figures from the same kind of test. One may publish a comfort figure and the other a lower boundary figure. Check each maker's own specification page to see which figure, and which method, is being cited.

Can I add a liner's claimed warmth to my bag's rating?

Not reliably. A liner's contribution is measured differently from a bag rating, so the two numbers are not additive. Treat a liner as a useful margin and a way to keep the bag clean, not as a numerical upgrade.

How much does the sleeping mat matter?

A great deal, because the fill under your body is compressed and barely insulating. The bag test is run with a specified mat underneath, so using a thinner mat means you are not getting the tested result. Check the mat's own rating and treat bag and mat as one decision.

How should I account for wind?

Calculate it rather than estimating. Environment and Climate Change Canada publishes a wind chill calculator for combining temperature and wind speed. Public guidance summarised by CCOHS treats a wind chill temperature at or below -7 degrees C as the point for warming shelters and at or below -12 degrees C as the point for constant observation and re-warming procedures.

Sources

1. Canadian Centre for Occupational Health and Safety — Temperature Conditions: Cold — checked 2026-11-06
2. Environment and Climate Change Canada — Wind Chill and Humidex Calculators — checked 2026-11-06
3. Parks Canada — Banff National Park winter safety — checked 2026-11-06
4. Canadian Centre for Occupational Health and Safety — Cold Environments: General — checked 2026-11-06