The pad decides the night. The bag only trims it.

The ground is the problem, and conduction is why

Air and ground at the same temperature are not the same problem. Inside a bag, loft traps still air that your body warms and keeps. Underneath you there is no loft: your own weight flattens the bottom of the bag to fabric with nothing left in it. Everything below you is carried by the pad — and carried by conduction, where touching surfaces pass heat straight through, rather than by the air movement the bag was built to resist. Conduction is the efficient one, which is why a pad is not a comfort accessory but the thing standing between you and a heat sink.

What R-value measures, and what it leaves out

R-value is thermal resistance: how much heat crosses a given area for a given temperature difference. On pads it comes from a standardised test — ASTM F3340, which sandwiches the pad between a warm plate and a cold plate and reads resistance at several points rather than one. What matters to a buyer is the consequence: pads from different brands are measured the same way, so R-value is the one number on the shelf comparable across brands.

What it does not tell you matters as much: nothing about how warm you sleep, whether the pad is wide enough to keep a shoulder on it, whether you will be on rock, snow or a hut platform, or what your quilt is doing above you. R-value is a property of the pad, not a promise about your night.

A temperature rating on a pad is not a measurement

Some pads are sold with a comfort temperature instead of an R-value. That figure is the brand's own, does not come out of the standardised test, and cannot be compared with anything else on the shelf. When a listing offers a temperature where an R-value belongs, a measurement has been replaced by marketing. Ask for the R-value; if none is published, treat the pad as unrated.

Three pad types, three jobs

Air pads are the lightest insulation for the pack volume and the most comfortable on uneven ground. The failure mode is the problem: a puncture or a dead valve takes the whole pad at once, at night, at the furthest point of the trip.

Self-inflating pads are open-cell foam plus air — heavier and bulkier for the same insulation, but the foam is the insulation, so a slow leak degrades the pad instead of ending the night. The honest middle option.

Closed-cell foam has no valve and nothing to puncture: the least insulation per unit of bulk, the cheapest by a distance, and the only pad that cannot fail.

The part nobody photographs: what is inside an air pad

A plain air mattress is cold however thick it is. Air in a chamber circulates: you warm the layer against your back, that air rises to the edges, cools, and sinks back down under you. Internal convection moves heat from your body to the ground about as well as having no pad at all. Warmth lives in the internal structure — baffles, foam layers, a reflective layer — not in the thickness. Two pads of identical thickness and price can differ enormously; R-value is the only published number that separates them.

Width and length are warmth decisions

Insulation you are not lying on does nothing. A narrow pad means a cold shoulder, and anyone who turns at night eventually puts a limb over the edge, where there is bare ground and no pad. Width is worth more to a side sleeper than a few extra grams of fill in a quilt. Length is the same argument: a torso-length pad saves weight by leaving the legs cold, coverable with the sit pad from your pack — but only if you planned for it.

The pad sets the floor; the bag only trims

Because the compressed underside of a bag contributes almost nothing, no bag compensates for an inadequate pad. A warmer bag over a cold pad still loses through the floor; a warmer pad under a modest bag is warm from below, and the bag only has to handle the air. Given one increment of budget between the two, the pad buys the better night.

Stacking is the cheapest upgrade

Thermal resistances in series add, so a closed-cell foam sheet under an air pad gives roughly the sum of the two. That is how a summer setup becomes a shoulder-season setup for the price of a foam sheet, with a puncture layer between chamber and ground thrown in. It also settles a purchase: if you already own a summer pad, find out whether you need a second pad or only a second layer.

The order of decisions

1. Name the coldest ground you will actually sleep on — snow, rock, packed earth, a hut platform. Ground, not air, is what the pad answers.
2. Decide whether you sleep warm or cold. No spec sheet carries this, and it moves the answer more than the brand does.
3. Buy the pad to that requirement, in the type that matches how much failure risk you will carry.
4. Only then the bag or quilt, sized to the pad's width and to the gap between the pad and the cold you expect.
5. Sleep one night on it in the garden on the coldest night you can find before relying on it on a trip. It is the only test that includes you.

What this page is, and is not

This is reasoning about how heat leaves a sleeping body, not a test report. No R-values, temperature thresholds or weights are quoted, because a table produced without measuring anything would be invented rather than measured. Where a figure would normally decide the purchase, the criterion for finding your own figure is given instead.

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The same rule — spend on the part that does the work, skip the part that decorates it — is written up for the kitchen in carbon steel vs cast iron, where thermal mass decides the pan.