Sleeping Bag Temperature Ratings: What Comfort, Limit and Extreme Actually Mean
A bag sold as a 32F bag will not keep most people comfortable at 32F. That is not marketing dishonesty, it is a rating standard that almost nobody explains at the point of sale. Here is what the three numbers mean and how to pick from them.
The three numbers
Comfort is the temperature at which a person sleeps through the night relaxed and not curled up. This is the number you should actually shop on.
Limit is the temperature at which a person sleeping curled up can get through the night without waking from cold. You will sleep badly. Most bags are named after this number.
Extreme is a survival figure. It means six hours without hypothermia, curled up, probably shivering the whole time. It is not a camping rating and it should never influence a purchase.
So a bag listed at comfort 42.8F, limit 33.8F and extreme 8.6F is a bag for nights down to about 43F. Take it to 20F and you are using the extreme number as a plan, which is how people end up sitting in the truck with the heater running at 2 a.m.
Why men and women get different numbers
Some manufacturers publish separate comfort figures, typically a few degrees apart. That is not a courtesy, it reflects measured differences in how sleepers lose heat. If a bag lists both, use the higher figure as your planning number unless you know from experience that you sleep warm.
Also worth knowing: ratings assume base layers, a hat, a sleeping pad and a tent. Not a tarp, not cotton pyjamas, and not a bare tent floor.
The pad under you decides about half of it
This is the part that ruins more nights than bag choice. Insulation only works when it is lofted, and the fill under your body is crushed flat by your own weight. Below you, the pad is doing the work.
Below roughly R-2, you feel the ground on a fall night no matter what bag you are in. R-4 and up handles genuine cold. A camp cot solves damp ground and bugs but makes the cold worse, because air moves freely underneath you, so a cot wants a pad on top once temperatures drop.
See the cots, pads and air mattresses before you spend more on a warmer bag.
Doubles, and the cold seam
Zipping two single bags together is the traditional answer and it has a known failure: a seam of thin, uninsulated fabric runs down the middle exactly where two people touch. On a mild night nobody notices. At 40F it is the coldest strip in the tent.
A purpose-built double is cut as one piece, so there is no seam. The trade is that you cannot split it into two bags for solo trips, and it is heavier and bulkier to pack.
One detail worth paying for on a double: two-way separating zippers on both sides. Whoever gets up first leaves without climbing over the other person, which matters more on night six than any number on the label.
Flannel lining, and what it does not do
A cotton flannel lining does not add a meaningful amount of warmth. What it does is feel like bedding instead of a nylon sack, which is the difference between sleeping and lying there. It also handles skin contact better after a dusty day.
The cost is weight and drying time. Flannel-lined bags are heavier and slower to dry than a plain synthetic lining, which is a fair trade in a truck and a bad one in a pack.
Heated bags and liners
Integrated heating runs off a USB or 9V power bank and takes the edge off a night that dropped further than the forecast said. Two honest caveats. First, it is a comfort feature, not a substitute for the right bag: a power bank will flatten before morning at most settings. Second, manufacturers rarely publish how many degrees of extension the heating gives, so treat it as unquantified.
A heated liner is the cheaper route. It drops into the bag you already own and targets shoulders, back, hips and feet, which are the four places that actually wake people up.
What we stock and how it rates
The FREESPACE 250D double is ISO rated at 42.8F comfort, 33.8F limit and 8.6F extreme. It runs 240 by 150 cm with a flannel lining, two built-in pillows, and it is machine washable. At 8.6 lbs it is a vehicle bag, not a backpacking one.
The 35F three-season double rates 44F female comfort and 35F male comfort, with 300 g of fill and an envelope cut that unzips flat into a quilt. It has the two-way separating zippers on both sides.
The 32F flannel-lined single rates 39F female comfort and 32F male comfort, with 400 g/m2 fill and a 230 cm length cut for tall adults. Two of them zip together into a double.
For cold-weather comfort, the FREESPACE HEATING 250 is an extra-wide single with three heat settings running off a 9V 2A pack, and the 6-zone heated liner runs off 5V or 9V USB and doubles as a heated cot mat. KingCamp does not publish a temperature extension figure for either heating system, so we are not going to invent one.
The full range is in sleeping bags and liners.
The short version
Shop on the comfort rating, not the name of the bag. Add 10F of margin for shoulder-season trips where the forecast can be wrong. Put an R-4 pad under it. If two of you are sleeping, buy a real double rather than zipping two singles. Everything else is preference.
Camping where it gets properly cold? A canvas hot tent with a stove jack changes the whole equation.