Can my 21-degree sleeping bag and blankets handle 10-degree weather in a car?

I’ll be car camping, and the temperature is forecasted to drop to around 10°F. I have a 21°F-rated sleeping bag and can bring as many blankets as needed. This is my first time trying this in such cold weather. Will I be warm enough, or is this too risky?

Edit: I’ll be sleeping in my car with the seats folded down. I noticed some comments assuming I’d be on the ground.

If you’re in a car and have plenty of blankets, you should be okay. If your bag is down-filled, consider using it as the top layer. Wear warm base layers to bed for added comfort.

Tobin said:
If you’re in a car and have plenty of blankets, you should be okay. If your bag is down-filled, consider using it as the top layer. Wear warm base layers to bed for added comfort.

So should I avoid putting blankets over the sleeping bag?

@Bennie
If it’s a down bag, use it as the top layer. Compressing it with blankets reduces its insulation effectiveness.

Sloan said:
@Bennie
If it’s a down bag, use it as the top layer. Compressing it with blankets reduces its insulation effectiveness.

Just to clarify, the bag works by trapping warm air in its lofted material. Compressing it eliminates that space, reducing its effectiveness.

@Nyle
Actually, all insulation materials have an R-value, which measures resistance to heat transfer. Compressing a bag reduces its effectiveness regardless of material.

Skyler said:
@Nyle
Actually, all insulation materials have an R-value, which measures resistance to heat transfer. Compressing a bag reduces its effectiveness regardless of material.

Even a concrete block has an R-value, so yes, you’re correct.

Skyler said:
@Nyle
Actually, all insulation materials have an R-value, which measures resistance to heat transfer. Compressing a bag reduces its effectiveness regardless of material.

True, but for sleeping bags, the effectiveness lies more in the loft of the material than in its strict R-value. Still, your point about compression stands.

It depends on the quality of the sleeping bag and blankets. A lot of bags’ temperature ratings aren’t accurate. Do you have a proper winter sleeping pad?

Ziv said:
It depends on the quality of the sleeping bag and blankets. A lot of bags’ temperature ratings aren’t accurate. Do you have a proper winter sleeping pad?

No pad, just the bag and blankets, plus warm clothing like wool socks and fleece pants.

@Bennie
Even in a car, a sleeping pad is critical for insulation. Without it, the cold from the surface will seep through. Layering wool clothing and blankets will help, but don’t over-layer to the point where you sweat.

@Ziv
Thanks for the advice. I’ll try to find a pad before the trip.

Sleeping bag temperature ratings are usually about survivability, not comfort. A 20°F-rated bag might work in milder conditions, but 10°F will be pushing it.

You’ll probably feel cold, but layering thermals, using a hot water bottle, or adding hand warmers inside the bag might help.

Don’t forget to crack a window slightly to reduce condensation buildup in the car. It can get pretty damp and cold otherwise.

If you can, get some foam insulation to place under you. Pair it with blankets and warm layers, and you should manage.