I don't know if you've said where you're going, or how cold it's liable to be. If the sleeping bag you're talking about using underneath doesn't compress much, and it's not too cold, you could be fine.. but, if it were me, I'd head right over to a local surplus store and buy a lightweight closed-cell foam pad for $10-15 max, and be sure of sleeping warm. Not necessarily soft, but toasty and a little hard is MUCH better than soft but cold. I've bought them as cheaply as $6. <br><br>I gather you're car camping (or you wouldn't be talking about using two bags), so you might want the full length pad- many people opt for shorter pads (just cut it) for backpacking, and use clothing or packs for pillows and to keep head, legs and heels off the ground. I used to carry one smallish cotton stuff sack to use as a pillow (with clothing inside)- much more comfortable than nylon, and you generally aren't using the bag hood (which would be between you and a pillow) unless it gets REALLY cold- I save the bag hood for the last step. These days, I might try to find a fleece sack instead. I use a balaclava- you might try that, or any sort of knit or fleece cap before using the bag hood if you start to get cold- just put it out, within reach, and reach for it like another blanket at home. It makes a huge difference in apparent warmth.. not as effective as the bag hood, but a lot less restrictive (you can turn your head), and I sleep better than I do with the hood. It's also handy for keeping warm when you stop during the day after hiking or work, and cool off quickly. It can make more difference than a jacket, and it's a whole lot easier to keep handy, and to put on and take off.<br><br>Everybody's different. I have a couple of inexpensive cotton mummy bag liners that I don't use in the field.. for a week or more before a trip, I'll sleep in one of the liners, on the pad, on the floor next to my bed. It takes me a few nights to get used to sleeping on a hard, flat surface again (I actually sleep better once I'm used to it- I have to relax much more), and the constriction of a mummy-style bag (that's what the cotton liner is for). It's much easier to acclimate to this in the comfort of home, and I find that if I do, I sleep much better my first nights on the trail, where it's critical. The real bag is generally too hot to use indoors. YMMV, of course.<br><br>If you have the discipline to do it, most people sleep much warmer if they change into dry clothes or underclothes (whatever you're going to wear) before bedding down- the ones you wore all day contain a LOT of moisture from your body. You can change in the evening, then wear that set the next day. If it's cold enough to worry about it, though, it can be very hard to convince yourself to strip down and change instead of just climbing into the bag.<br><br>I've had two Therm-O-Rest pads, and they are very comfortable for their bulk, and temptingly small when packed.. but they weigh more than closed-cell foam, they're not quite as effective as insulation, they're expensive, and eventually they do fail.. like all inflatables, at the worst possible time... and when they do, you can spend a very cold, miserable night. Two AM in a dark tent is no time to be trying to diagnose leaks and/or valve problems. I went back to foam.<br>