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.
12Would you mind doing a simple experiment? Depending on where you live, the floor in the basement or outside might be colder than the surrounding air. Lay down on the floor directly and compare it to any matress.– JasperSep 30, 2019 at 18:04
5Regardless of temperature, I can say from experience that sleeping on the ground is pretty terrible, and I would only do it again if I had no better options. An air mattress is definitely a better option.– Todd WilcoxOct 2, 2019 at 6:49
3Related only: I once spent the night in a very cold airline terminal wearing only long trousers and a business shirt - all my other clothes were "somewhere else in the world". Newspaper was in short supply as others also valued it :-). I constructed a newspaper singlet, which certainly helped. But an immense difference was made by a single layer newspaper blanket over me. This reduced immediate contact with the cold air and built an insulating layer, of sorts, and warmer air pockets. The difference was extremely noticeable. || When travelling I now always carry a lightweight jacket.– Russell McMahonOct 2, 2019 at 23:22
1When camping or hiking with this friend, be sure he sleeps on the ground while you sleep on an air mattress. That’ll fix his wagon.– M.MatOct 18, 2019 at 11:26
2My prediction is that neither on the uninsulated ground nor on a normal air matress you'll get any sleep at -20 (regardless of °C or °F).– cbeleites unhappy with SXOct 19, 2019 at 17:34
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.
9I somewhat doubt that a "simple" air mattress provides only little insulation. I assume that the main contribution of heat loss is due to convection and since the upper air layers will be the warmest and "warm air rises up", there will be little mixing of cold and warm air.– JasperSep 30, 2019 at 18:02
1@Conrad You could combine both. Personally, as soon as the ground is frozen, I carry both an insulating air mattress (Exped Downmat) and a 15mm EVA pad. That way even if my mattress is punctured, I have a backup foam pad that will at least let me sleep, if not perfectly, better than on the ground. Sep 30, 2019 at 20:54
2@Jasper there is always mixing. For example the movement from the sleeper on top deforming the mattress will induce convective currents inside. In any case, why would complicated baffle designs have appeared if convection wasn't an issue? That in itself is the best proof that convection is a major problem. Sep 30, 2019 at 23:04
1@IronEagle Take a look at diagrams of the Therm-a-Rest NeoAir series. Those baffles aren't all structural. Oct 1, 2019 at 15:32
1Your answer is almost complete, but it's missing details of the relative efficiency of conduction, convection and radiation. Direct conduction is by far the most efficient way to transfer heat, which is why you can hold your hands close to a fire without getting burned instantly. For the OP's question of whether you should lie directly on the ground or not, it isn't even up for discussion - conduction drains heat fast. Improving insulation against convection and radiation is good, but if you haven't eliminated direct conduction to the cold ground then it's game over before you start.– GrahamOct 2, 2019 at 12:01
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.
1"One blanket underneath is worth two on top." Glossing over the point of then having one blanket underneath and one on top ;)– FlaterOct 1, 2019 at 8:30
3@Flater the question isn't about blankets; the point is that you need more insulation underneath than you need above. Obviously if all you have is two blankets you would do that. But if you have three blankets it is better to have two under and one over. Oct 1, 2019 at 13:56
1@WeatherVane: I'd phrase that differently: you need isolation below as well as on top (I'm willing to bet that two blankets below and none above is also just awful) Oct 19, 2019 at 17:02
@cbeleites it's a saying, not what to do when you have two blankets. It has nothing to do with the number of blankets you have, only about their distribution. Oct 23, 2019 at 23:09
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.
+1 for Point 2 about spreading pressure evenly. When using hammocks recently I have noticed that cold spots from sleeping bag compression are the coldest of all. Having experienced that, I have noticed that an over-inflated mattress can feel colder than one which is less inflated, and so conforms to your body area and spreads the pressure.– rolingerOct 2, 2019 at 11:07
2With a hammock the most efficient way to keep from getting cold from sleeping bag compression is to have the isolation layer below the hammock. (Best a double layer hammock where the isolation layer is sandwiched between the layers.)– Willeke ♦Oct 2, 2019 at 17:30
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!
Depending on the baffles and the design of the air mattress, it could act more like a heat sink than an insulator. In that case it would be better to sleep directly on the ground than on an air mattress that would spread your heat over a larger area. So then your question becomes "Is sleeping naked and unprotected better than strapping on a heat skink?" The real question is whether or not a particular air mattress' insulation exceeds its capacity to spread your heat over a larger area due to convection, which is not a simple question to answer.– RickOct 17, 2019 at 12:26
Rick - it is a simple question to answer - you simply measure the mat's performance. Most manufacturers have been providing R values for years - this is a ranking of the insulating value of the mat. The only problem has been that the measurements haven't been standardised, which is now being addressed. A non insulated mat provides comfort at a light weight, but not effective insulation. An insulated mat with any kind of significant R value will provide valuable insulation. You are postulating a problem which simply doesn't exist in practice when a user is selecting a mat. Oct 17, 2019 at 13:04
I agree that any mat that has an R-value rating (that isn't terrible) has likely been designed to be insulating, and thus would be fine. My comment was referring to low quality/ indoor air mattresses where there are not R-value ratings. Additionally, an R-value might not really be helpful for these mattresses as an R-value approximates everything as an infinite uniform surface with a constant temperature on each side. The convective heat transfer that would make mattresses worse than nothing would only happen when there's hot and cold areas on one side, so the R-value wouldn't indicate this.– RickOct 17, 2019 at 13:30
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.