A thin bivy shell is not insulation. It is a wind layer and a dew shield, and whether the sleep system inside it is warm enough comes down almost entirely to the pad underneath you: the pad's R-value sets the floor of the whole system, a bag's EN or ISO rating was measured on a pad you may not own, and the shell itself only moves the edges of that calculation, mostly by trapping moisture. Get the pad right, understand what the rating on the bag actually assumes, and the bivy question answers itself before you leave the trailhead.
I learned the arithmetic on a gravel bar in November, on my side, at about two in the morning. The wind had died and the shell above me was quiet, which felt like victory. What I remember is a stripe of cold under my hip, sharp as a ruler, and another under my shoulder. Not a draft. Not a leak. The ground. I was on a three-quarter-length inflatable I had bought for summer backpacking, and I had assumed the bivy would cover the difference. It doesn't. Nothing above you can warm what is below you.
The shell does one job well
A bivy's real contribution is convective. It stops moving air from stripping heat off the top of your bag, and it keeps splash, dew, and spindrift off the fabric. On a windy night that is worth a lot. On a dry, still night it is worth almost nothing, and on a wet night it can cost you.
What it does not do is insulate the underside. A thin shell has no meaningful R-value under your body, and the ground doesn't care how much fabric sits between you and it. Heat moves from you into the ground by conduction, and fabric a few thousandths of an inch thick is not a barrier to that. Only trapped air is.
The number that decides the night
R-value measures resistance to heat flow. Higher is better, and the scale is linear enough that two pads stacked roughly add: a 2.0 under a 4.0 gets you near 6.0, minus whatever the seams and gaps do to the average. Most published pad R-values now come from ASTM F3340, which is a lab test. A warm plate, a cold plate, a pad between them, a steady-state measurement on a dry sample under a defined load.
That is a good number. It is also narrower than most people think. F3340 tells you how a pad behaves in a condition that never occurs on a trail: bone dry, evenly loaded, unchanged from the first minute to the eighth hour. It doesn't tell you what happens when your hip puts a point load on the baffles, when the pad has been inflated and deflated a few hundred times, or when a little moisture has worked into the foam. Those aren't reasons to distrust the number. They're reasons to treat it as a starting point instead of a verdict.
I do two things with that number. First, I read it as a floor, not a comfort rating. Second, I put a thin closed-cell foam pad under the inflatable. Part of that is puncture insurance on gravel and thorns. The other part is honest insulation: closed-cell foam doesn't care about a point load or a slow leak, and it adds its R-value to the stack instead of depending on air that might not be there in the morning.
Where the bag rating stops transferring
This is the part that catches people. An EN or ISO sleeping bag rating, comfort or limit, is measured with the bag on a specified pad and the tester in a specified base layer. Those test conditions are published and specific. Your bivy is not part of them.
So when a bag says it is good to a certain temperature, that claim is a claim about a system, not about a bag. Change the pad and you have changed the system. Put a summer pad under a three-season bag and you have a three-season top half sitting on a summer bottom half, and heat loss through the bottom of a bag is the dominant loss, because the underside insulation is compressed flat under your body weight and stops lofting. Every bit of that loss has to pass through the pad.
That is the reframe I keep coming back to. The bivy is the outermost layer of a sleep system, and it is the least thermally important of the three. The order of importance is pad, then bag, then shell. Most people shopping for cold-weather sleep gear invert that order, because the shell is the visible thing.
The compressed underside
Lying on your back in a bag, the fill under your spine and shoulder blades is crushed to near zero loft. It isn't insulating; it's a spacer. The pad is the insulation. That's why a bag rated for cold with a thin pad under it behaves like a much warmer bag over your chest and a much colder bag under your back, and why you wake up cold in a specific, localized way: hips, shoulders, sometimes the backs of your heels where they press through.
I notice this most clearly in a bivy because there is nothing else to focus on. No tent walls, no floor, just you and the fabric and the ground, and the ground wins every argument it is allowed to have.
Moisture is where the math ends
Here is the thing the spec sheets don't have a field for. A non-breathable bivy shell traps your vapor. You put out a surprising amount of moisture overnight, and in a sealed shell it has nowhere to go but into your bag.
Night one, the system is dry and performs like the numbers say. Night two, the bag has picked up some moisture and the loft is less lively. Night three, in cold conditions, you can feel the difference even though nothing about your gear has changed on paper. Damp insulation conducts heat better than dry insulation, and a shell that keeps weather out also keeps water in. That is the trade, and it is a real one.
So the system you own on night one is not the system you own on night three. If your pad is marginal, moisture is what pushes it from fine to a long night. If your pad is generous, moisture is an annoyance instead of an event.
What I actually do about it
Each morning I pull the bag out of the bivy and drape it over the top of the shell while I eat and pack up. Sun if there is any, breeze if there is any, and twenty minutes is usually enough to move the night's vapor out of the loft. I wipe the inside of the shell with a bandana before I roll it. I don't roll a damp bag into a stuff sack and let it sit all day, because that is how you start night two already behind.
On multi-night trips I check the pad too. Inflatable pads pick up moisture inside from your breath, and that moisture degrades the foam and can freeze. I don't inflate a pad with my lungs if I can avoid it, and I store it open at home.
The honest limits
I am not going to hand you a formula, because there isn't one. Metabolism, what you ate, how tired you are, wind, humidity, snow versus dry duff, how much you roll around, whether you run warm or cold, all of it moves the answer. Two people in identical systems on the same night will sleep differently.
Where I do hold a line is at the manufacturer's stated limits. If a pad maker publishes an R-value or a temperature range, I don't stack past what they say the pad is for, and I don't use a bag beyond its stated rating and pretend the bivy bought me degrees it didn't. The shell earns its place by blocking wind and weather. It doesn't earn a temperature.
So: check the pad first. Read the bag rating as a claim about a system that includes a pad. Then decide whether the bivy is the layer you need or just the layer you can see. On the nights that matter, the difference shows up as a stripe of cold under your hip at two in the morning, and by then the decision is already made.
