My friend and I had an argument about this topic. He says that the cold air in the mattress will stay at the ambient temperature, so if it's -20 outside the air in the mattress will stay at -20 so I'll keep freezing as opposed to sleeping on the ground (both with a sleeping bag). I think that this is false since the air is trapped in the mattress and that air is an insulator. The ground will keep on being cold though. I'm not sure which one is right here.
Effectively, it isn't much better to sleep on an air mattress that isn't specifically made to insulate from the ground than on the ground itself although it does make a slight difference.
The main takeaway is that you should always avoid sleeping directly on the ground so even a poorly insulated air mattress will be better than nothing.
Your friend isn't right in assuming the air in the mattress will stay at ambient temperature as you are constantly heating it and it doesn't mix with the rest of the air outside of the mattress. It should remain a bit warmer than the air around although it would lose heat both to the air and the ground and as such won't be very effective.
Mechanisms of heat loss
There are three mechanisms involved in losing heat to the ground when camping (provided the tent is setup on the ground).
Your body is the heat source and will shed heat away in the form of electromagnetic radiation (between 8 and 15 µm wavelength on the spectrum). That's why emergency blankets work by reflecting these radiations, and why a fire feels hot on your skin even from a distance.
When a hot body is in contact with a cold body, heat will be transferred to equilibrium. Between solids, conduction is the most significant method of transfer as:
conduction is greater in solids because the network of relatively close fixed spatial relationships between atoms helps to transfer energy between them by vibration.
In the case of sleeping on the ground, since you keep producing heat to maintain temperature, this is a constant process. Considering the ground is essentially infinitely large, your body will never be able to heat it up enough so that the system reaches equilibrium and heat loss will remain constant.
This is the process where parts of a fluid (like air) of different temperatures will mix and transfer heat to each other. It is a very efficient transfer method.
If your air mattress doesn't include a mechanical barrier to convection, any air that you heat up will eventually mix with colder air. You will still feel cold.
Here's a very rough sketch of the process. Your body (red) heats the air at the top of the mattress. This air mixes with the colder air from the bottom and any heat that gets to the ground gets transferred to it.
There are a few ways to combat the heat exchange in an air mattress:
- The idea is to restrict air movement with physical barriers. The longer it takes for the air to reach the bottom, the slower it will cool.
- Insulating fibers
- Similarly to baffles, adding an insulating material like down, synthetic or not, will restrict air movement.
- Thermal reflector
- It is possible to add a reflector, like aluminium foil, somewhere in the mattress. Just as with an emergency blanket, heat will be reflected.
- A foam structure
- Self-inflating mattresses like Therm-a-Rest are composed of a loose open cell foam that gets filled with air. This is a relatively effective insulating material as it greatly restricts convection currents.
Some brands combine those solutions (mainly baffles and reflectors) to reduce bulk and weight although you then have the issue of mattress integrity. If not airtight, an air mattress is useless.
How to use R-ratings
The higher the R-rating, the better your mattress will be at keeping you warm. As noted on the MEC website in the How to choose a sleeping pad article:
Sleeping pads with an R-value of less than 3 are best for really warm weather. R-values of 3–4 are best for 3-season use. For winter camping, choose a sleeping pad with an R-value of 4.5 or higher.
As a reference, the 12cm thick Reactor Air Double Mattress from MEC has an R-rating of 1.5. This is even though there are basic structural baffles inside. This is a laboratory measured rating, which is accurate. As you see, the effective insulation is marginal and won't even work in the interseasons, nevermind in the winter.
First hand knowledge/anecdotal evidence
There are many field experiments that have been conducted (intentionally or not) where the end result is that a plain air ballon mattress isn't insulating at all. In my 8 years of outdoors retail, I had plenty of angry customers coming back to the store with a summer mattress saying they hadn't been able to sleep all night when in the intermediate seasons when the ground freezes or sometimes winter when camped on snow, even if they had a sufficient sleeping bag.
The blue foam sleeping pads will maybe fare better than an air mattress but I can't confirm first hand. On frozen ground, open cell foam like the blue stuff is usually not considered enough and closed cell EVA is preferred (like Evazote yellow pads from Zote Foams).
Regarding foam pads, I had the ability to do a field test with a friend. We both had -20°C sleeping bags, a 1" self-inflating Therm-a-Rest, and EVA foams. Mine was 15mm, his was 10mm. The first night's air temps was around -15°C but the next morning, he complained of having chills in his back all night. Since we had a few other pads with us, I proposed he use a 15mm that night and even though the air temp went as low as -25°C, the additional 5mm was enough to cut the cold from his back. There was no other changed variable.
There is an old adage
One blanket underneath is worth two on top.
but I can find little support for this idea except a mention on mumsnet.
The reasoning is, that the floor or the ground underneath is solid, and has a much higher thermal capacity than the air above, and so will draw a lot of heat away from your body. It's not just the temperature of the surroundings which matters, but its ability to draw heat from you too.
If you have ever worn boots or shoes with air-cushion soles, you'll know that your feet stay warmer than in those with solid soles, which conduct heat better.
So any kind of insulation between you and the ground is worth having.
Of great benefit too, is to sleep on a waterproof ground sheet, which prevents moisture travelling up to your bedding and body from the ground. Dry bedding is a better thermal insulator than damp bedding.
Air circulation in the mattress causes some heat loss by convection, but will be much less than by direct conduction to the ground. If taking an air mattress with you is an option, then you will be more comfortable, but warmer than sleeping on the ground.
Some more factors here:
1) Even if the air in the air mattress was the same temperature as the outside air it has much less thermal capacity and thus will take your heat away slower.
2) An air mattress will spread the pressure more evenly--your sleeping bag won't be as compressed at pressure points and thus heat will escape from them slower.
Your friend isn't thinking clearly. Imagine they had asked the question:
On a cold night, is sleeping naked and unprotected better than sleeping in a sleeping bag?
Everyone intuitively knows that you will be far warmer inside a nice, lofted sleeping bag. The insulation retains your body-heat and prevents the cold air outside from reaching your skin to cool it.
Exactly the same argument applies to sleeping mats - the more insulated the mat the better it keeps your body heat in and the cold of the ground out by reducing radiation, conduction and convection. Your friend is right in the limited sense that an uninsulated mat is sub-optimal, but a well insulated mat will make a dramatic difference.
Anyone who spends a night or two in cold conditions will discover from direct and potentially harrowing experience that an insulated pad is vital for comfort and safety. Your sleeping bag alone is little help, because the insulation will be crushed under your body and provide little protection from the ground. The best bag won't help if your pad is under-specified for the conditions, so all experienced winter campers give high priority to a well insulated mat. (In a survival situation, you should improvise ground insulation from something you are carrying, like a rope, or from a natural resource like heather or forest duff).
In response to customer demand the industry is at last rolling out proper standard (ASTM FF3340) for measuring the insulating value of mats in a rigorous and consistent way. So from 2020 it will be possible to purchase mats with a known insulation value that you can confidently match to the conditions and your personal physiology (because some people sleep much colder than others).
TL/DNR: a well insulated sleeping mat is vital for comfort and safety when camping in cold weather. This is beyond any reasonable dispute.
The OP has asked in the comments about whether an uninsulated air mattress would be worse than nothing because the convection inside the baffles might draw heat from the body.
So let's extend our thought experiment.
Imagine a block of ice, with an air mattress on top. You know intuitively that even after a few hours touching the ice is still going to be much, much colder than touching the mattress.
Air insulates well. That's why wool and down are so effective - because they trap air. The convection inside the mattress won't be nearly enough to entirely eliminate the insulating properties of the air.
I began winter camping before modern insulated mats came onto the market. We would pile up heather under our groundsheets, and use an air mattress on top. We all much preferred using a mattress to sleeping directly on the ground. Not as effective as a modern insulated mat, but much, much better than nothing!
Happened to see this and had to register just to reply because I have first hand info. I was fixing up an old mobile home and had removed the skirting and slept on a air mattress in a room with a wood stove. I thought it would be ok but since the floor was the outside temp the air mattress became ice cold. The air in the air mattress just gets colder and colder since the ground sucks any warmth and it never diffuses or has anywhere to go. It was better after I put a piece of carpet between as someone else has mentioned. Unfortunately your friend is right.