It may not sound like a lot, but if your sleeping bag is rated to about 30 degrees above zero and you are inside a small tent, it can be comfortable to as low as 15 degrees, which is more than adequate. This is not “new math,” but the insulation ability of the dead air space in your zipped up tent. If you don’t know what the rating of your sleeping bag is, ask a sleeping bag expert at a mountaineering outfitter shop such as Sports Chalet or Big 5. They can give you a good estimate of the rating.
So, you’ve got a monster rectangular bag that looks like it sleeps six. It will work! First off, reduce the area of the bag. Tie it or tape it up to reduce the size and volume. Sleeping bags work by your body heat warming the air trapped in the bag and insulation. All of that rectangular space is nice and roomy in the summer, but hard to heat up in the winter. Don’t get it so small that you can’t move nor have too much excess volume. Also, avoid taping or tying so tight that the bag loses it’s loft or insulating space
Next, you need to add more of that loft. If you have two sleeping bags with higher ratings and are able to fit one inside the other and still be comfortable, that will work also. Remember that you can also layer your sleeping bag. Plan on wearing a light sweater, socks and a knit cap to bed, as these count as added layers to your sleeping bag as well.
If you choose to buy a bag, get a synthetic material over “down.” It’s a little heavier, but a lot cheaper. If you select a good synthetic material, it will keep its insulating qualities even when wet. “Down” bags can be dangerous in cold weather when wet. You will need something under you bag to help insulate you from the cold snow or ground. A sleeping pad of closed-cell foam or covered form pad like the “Therm-a-rest” will help. Air mattresses will rob your body heat through the night. Leave them for floating in the pool.
Remember when looking into purchasing a sleeping bag, there are two major shapes. The first being the rectangular bag, usually larger, and with the size comes more dead air space to warm up. Rectangular bags are great for those summer months where staying cooler is the priority over staying warm. The other type of sleeping bag shape is the mummy bag. The mummy bag conforms to the shape of the body, wide at the top and skinnier down low. This allows for less dead air space and thus is a much warmer bag.
How to store sleeping bags
Most good sleeping bags today come with two types of storage bags. One being the compression bag. This bag usually has some sort of strap system, these straps are used to compress the bag, during travel/use, to the smallest size possible. The other bag, included with the sleeping bag, is a storage bag. This bag is about the size of a large pillow case and is use to store you sleeping bag when not in use. Due to it’s over-size it allows your sleeping bag to retain it’s loft and effectiveness to retain it’s insulating properties. A SLEEPING BAG SHOULD NEVER BE STORED COMPRESSED! Also remember to store your sleeping bag in a warm, dry and dust free place. “Store your sleeping bag in a place where you wouldn’t mind being stored.” The worst places for sleeping bags is in garages, in rafters, and anyplace where it will be exposed to water, insects, or dust.