Most people who have a cold night outdoors blame the sleeping bag. In our experience it is almost never the bag. It is the four centimeters of air between the sleeper and the ground, and it is a problem you fix for less money than a new bag costs. Here is how the parts of a sleep system actually work together, and which numbers to read before you buy any of them.
Cold Comes From Below, Not Above
Air is a poor conductor of heat. Ground is a very good one. Lie directly on soil at 8 C (46 F) and it will pull warmth out of you all night no matter how much insulation is piled on top, because your body weight crushes the loft out of whatever is underneath you. A sleeping bag only insulates where it is lofted, and the half of the bag you are lying on is not lofted at all.
That is the whole reason mats exist. They are not comfort items with an insulation side effect. They are the insulation, and comfort is the side effect.
What A Bag Rating Actually Tells You
Comfort ranges are useful as a rough band, not as a promise. They assume a sleeper of average build, dry kit, a mat underneath, and a base layer rather than the clothes you hiked in. Read them as a guide to the season the bag was designed for rather than a temperature you can survive.
The Four Season Envelope Sleeping Bag publishes its bands plainly: above 25 C (77 F) for summer, roughly 10 to 20 C (50 to 68 F) for spring and autumn, and roughly 0 to 10 C (32 to 50 F) at the heavier fill weights. If your trip sits near the bottom of a band, buy the next fill weight up rather than hoping.
Fill Weight Is The Number To Read
Fill weight is the honest figure because it is the amount of insulation you are actually paying for. The same bag comes in 1 kg, 1.4 kg, 1.6 kg and 1.8 kg fills, and the difference between the lightest and the heaviest is the difference between August and a frost. Buying by fill weight also stops you carrying a winter bag through a summer where you will spend every night with the zip open.
The envelope cut helps here. It unzips flat into a quilt when the night turns warmer than forecast, and two bags zip together into a double if you are sleeping as a pair. A 210T anti tear polyester shell takes being dragged in and out of a tent without picking up holes.
Mat Thickness Does The Quiet Work
Once you accept that the mat is doing the insulating, thickness stops being about softness. The Self Inflating Camping Air Bed comes in 3 cm, 5 cm and 8 cm loft with a high resilience sponge core that recovers overnight rather than bottoming out under a hip. Three centimeters is a summer mat on soft ground. Eight is what you want on cold, hard or stony pitches, and it is what we recommend to anyone who has previously woken up sore.
The moisture proof underside matters as much as the loft. A mat that soaks up ground damp is a mat that conducts cold straight through, and it is also a mat that goes into the car wet and comes out smelling.
Two People, One Mat
Couples who use two single mats spend the night rolling into the gap between them. The Double Self Inflating Camping Mat is a single 118 cm wide surface with 6 cm of loft and a 25 cm pillow section built into the head end, so there is no cold seam down the middle. TPU coated nylon keeps ground moisture out and it rolls back into its own stuff sack in the morning.
The Layer Most People Forget
The gap between getting cold and getting into the bag is where evenings end early. A fleece layer you can wear while still moving around solves it. The Wearable Fleece Cloak Sleeping Bag works three ways: a hooded cloak with arm openings so you can cook or hold a drink while wrapped up, a zipped 210 x 80 cm sleeping bag, and a 210 x 160 cm blanket that opens flat over two people. It is 866 g and packs to a 32 cm disc, which is less than most camp blankets.
Used as a liner inside the envelope bag, it also buys you a season. That is the cheapest way to extend a summer bag into autumn without replacing it.
Tent Choice Changes The Whole Equation
Condensation is the other thing that makes people cold, because damp insulation stops insulating. Synthetic tents with limited airflow drip onto the inner and then onto the top of your bag. The Cotton Canvas Camping Tent uses a 35 percent cotton blend that breathes and cuts condensation noticeably, with 20D anti snag mesh on the inner panels and a 150D Oxford floor rated PU 4,000 mm.
If you are in a lighter synthetic tent, the fix is airflow rather than sealing everything up. The Instant Pop Up Camping Tent has mesh panels on all four sides for exactly that reason. Vents open, not closed, is the rule on a cold clear night.
A Warm Night Checklist
Run through this before you turn the lantern off. Mat inflated firm rather than soft, because a soft mat lets your hip touch the ground. A dry base layer on, never the clothes you walked in. Tomorrow's clothes inside the bag so they are warm in the morning. A vent open at each end of the tent. A hat, because a surprising amount goes out through your head once you stop moving.
Do those five things on a 5 C (41 F) night and a spring and autumn fill weight is plenty. Skip the mat and no bag on the market will save you.
If you are assembling a first setup from nothing, the first camping trip checklist covers the other five pieces that go with it.
Building or upgrading a sleep system? Browse Shelter and Sleep or see the full range.
