The gradient, and why it is a straight line in R-value rather than in inches
Heat flowing through an assembly at steady state is the same at every layer, because there is nowhere for it to accumulate. That single fact gives the temperature drop across each layer: it is proportional to that layer's share of the total R-value. A layer with a fifth of the assembly R takes a fifth of the temperature difference, whether it is a quarter inch thick or eight inches thick.
So the temperature at any interface is the indoor temperature minus the total temperature difference multiplied by the fraction of the R-value lying between the room and that interface. In a wall with R-23.5 total at 70 inside and 20 outside, the back of the drywall sits near 68 degrees, because only R-1.13 of the assembly is inside it. The back of the sheathing sits near 23 degrees, because R-22.1 of the assembly is inside it and only R-1.4 outside. Nearly all of the drop happens across the insulation, which is the point of the insulation.
Draw that against the dew point of the indoor air, and you have the picture this page produces. The dew point is the temperature at which the air in the room would be saturated, and any surface it touches that is colder than that will collect water. Seventy degrees at 40 percent humidity has a dew point near 45. Seventy degrees at 50 percent has a dew point near 51.
Why the back of the sheathing is the plane everyone talks about
In a conventional framed wall with insulation only in the cavity, the sheathing is the first substantial material on the cold side of the insulation. It is also, in most walls, the plane where the assembly changes from something vapour-open to something less open, and it is a material that will rot if it stays wet. That combination is why building science conversations about walls tend to converge on the temperature of that one surface.
The strategy that raises it is continuous insulation outboard of the sheathing, and the reason is visible in the gradient. Adding R-value outside the sheathing puts more of the assembly resistance on the cold side of that plane, which moves the plane itself warmer. Adding R-value inside the cavity does the reverse: the sheathing gets colder as the wall gets better, which is one of the genuinely counter-intuitive results in the field and the reason blowing insulation into a previously empty wall cavity is a moisture decision as well as an energy one.
The results table on this page shows that trade directly for your assembly. The assembly R-value calculator shows what the same continuous layer does to the whole-wall R at the same time, and the two effects are the argument for exterior insulation taken together.
What a steady-state calculation cannot see
| What is missing | Why it changes the answer |
|---|---|
| Drying | Assemblies wet in winter and dry in spring. A plane that goes below the dew point for a few hours in January is not the same as one that stays there for weeks. |
| Air leakage | Air carrying moisture through a gap delivers orders of magnitude more water than vapour diffusion through intact materials. Air sealing usually matters more than the vapour layer. |
| Moisture storage | Wood, gypsum and cellulose absorb and release water over days. A material can take up condensation and give it back later without ever causing damage. |
| Vapour permeance | This calculation is temperature only. It does not compute how much vapour actually reaches a plane, which depends on the permeance of every layer inside it. |
| Sun and rain | Sun on wet cladding drives moisture inward, sometimes strongly. Rain that gets behind cladding is a bulk water problem no vapour analysis addresses. |
| Real weather | One instant at one temperature is not a season. The useful question is how many hours a year a plane spends wet, which needs hourly modelling. |
None of this makes the gradient useless. It makes it a way to see the shape of the problem and to compare options against each other, which is what an organizer is for. It is not a way to conclude that a wall will be fine.
The two things worth doing regardless
Control the humidity and control the air. Indoor humidity in winter is something you can measure and influence: bathroom and kitchen fans that genuinely vent outdoors, a cover on a crawlspace floor, and not running a humidifier up to a number that feels nice are the ordinary levers. Every point of relative humidity you take out of the room lowers the dew point and moves every plane in the wall further from trouble.
Air sealing does the other half. Vapour moving by diffusion through intact drywall is a slow trickle. Vapour riding on air through a gap around an outlet box, at a top plate or around a window is a flood by comparison, and it deposits water in the exact places the gradient says are cold. If you are choosing between a vapour retarder argument and an air sealing project, the air sealing is the one with the better evidence behind it, and the air sealing payback calculator puts a number on the energy side of it. For the humidity side, the humidifier sizing calculator covers what adding moisture to a house actually does in winter, which is the lever pointing the other way.
What belongs in your specific wall, in your climate, with your cladding and your interior finishes, is not something to settle from a web page. Assemblies fail slowly and invisibly, and the failures are expensive. Take the picture this page gives you to someone qualified in building science before you change what a wall is made of.
Questions people ask
Does this tell me whether my wall will get condensation?
No, and it is built not to. What it tells you is where the temperature in your assembly crosses the dew point of your indoor air, at one instant, under one set of conditions, assuming steady state and ignoring everything else that moves water around a building. Real assemblies wet and dry across a year, store moisture in their own materials, and lose or gain far more water through air leakage than through diffusion. A plane sitting below the dew point in this calculation is a reason to think harder, not a prediction of damage, and a plane sitting above it is not a clean bill of health. If you need an answer you can rely on, that is a building science professional and often an hourly hygrothermal model, not a web calculator.
Where does the vapour retarder go?
That depends on your climate, your assembly, your cladding and what the interior finishes are, and getting it wrong traps water inside the wall. In a cold climate the conventional position is toward the interior, on the warm side of the insulation, so that indoor moisture does not reach the cold sheathing. In a hot humid climate the moisture drive runs the other way for much of the year and an interior vapour barrier can be actively harmful. Many modern assemblies deliberately use a variable-permeance or vapour-retarding layer rather than a barrier, precisely so the wall can still dry inward. What none of them tolerate is a low-permeability layer on both faces, because then the wall cannot dry in either direction. This is the question to take to a professional rather than to settle from a rule of thumb.
Why does adding cavity insulation make the sheathing colder?
Because temperature drops in proportion to R-value, and cavity insulation adds R-value on the warm side of the sheathing. In an uninsulated wall the sheathing sits close to indoor temperature, because heat pours through the cavity and keeps it warm. Fill the cavity and that heat stops arriving, so the sheathing drops toward outdoor temperature. The wall now loses far less heat, which is the point, and the sheathing is now colder in winter, which is a moisture consideration that did not exist before. This is a real and well-documented effect and it is the main reason cavity fill in an old wall is a decision with two sides rather than an obvious improvement.
What indoor humidity should I run in winter?
Lower than feels comfortable is usually the honest answer, and the constraint is set by the coldest surface in the house rather than by comfort. If the windows are streaming, the air in the room has a dew point above the glass temperature, and every point of relative humidity you remove lowers that dew point. Running exhaust fans that genuinely terminate outdoors, covering a bare crawlspace floor, and not chasing a humidifier setpoint are the ordinary levers. Very cold weather can produce condensation on single glazing at humidity levels that would be entirely reasonable in milder conditions, which is why one target number for the whole winter does not work.
I already have mould in the wall. What does this page tell me?
Nothing you should act on. Existing mould, staining, damp framing or a musty smell means water is arriving from somewhere, and it is far more often bulk water — a roof, a flashing, a gutter, a plumbing leak, ground water — than interstitial condensation. A steady-state gradient does not find leaks and will not tell you where the water is coming from. Stop, find the source, and get someone in who can open up enough of the assembly to see what is happening. Insulating or air sealing over an active moisture problem makes it worse and hides it while it gets there.