Envelope · Moisture
Interstitial condensation calculator — Glaser method
A wall gets wet inside when the vapour arriving at a point is more than the air at that temperature can hold. Two lines run through the construction: how cold it gets, which sets the maximum vapour pressure the air can carry, and how much vapour actually gets there, which depends on how easily each layer lets it through. Wherever the second line rises above the first, water forms — usually on the cold face of the insulation, and usually where nobody will see it for ten years.
Saturation vapour pressure and dew point tables
The two numbers a Glaser calculation by hand needs at every interface: the saturation vapour pressure at a temperature, and the dew point of the air. Worked out with the same Magnus formulas the calculator uses, written out in the method below: over water from 0 °C, over ice below it.
Saturation vapour pressure
| θ, °C | psat, Pa | θ, °C | psat, Pa | θ, °C | psat, Pa |
|---|---|---|---|---|---|
| −10 | 259 | 4 | 813 | 18 | 2063 |
| −8 | 309 | 6 | 935 | 20 | 2337 |
| −6 | 368 | 8 | 1072 | 22 | 2642 |
| −4 | 437 | 10 | 1227 | 24 | 2982 |
| −2 | 517 | 12 | 1402 | 26 | 3359 |
| 0 | 611 | 14 | 1598 | 28 | 3778 |
| 2 | 705 | 16 | 1817 | 30 | 4241 |
Dew point of the air, °C
| Air | 30 % RH | 40 % RH | 50 % RH | 60 % RH | 70 % RH | 80 % RH | 90 % RH |
|---|---|---|---|---|---|---|---|
| 10 °C | −6.8 | −3.0 | 0.1 | 2.6 | 4.8 | 6.7 | 8.4 |
| 12 °C | −5.0 | −1.2 | 1.9 | 4.5 | 6.7 | 8.7 | 10.4 |
| 14 °C | −3.3 | 0.6 | 3.8 | 6.4 | 8.6 | 10.6 | 12.4 |
| 16 °C | −1.5 | 2.4 | 5.6 | 8.2 | 10.5 | 12.6 | 14.4 |
| 18 °C | 0.2 | 4.2 | 7.4 | 10.1 | 12.5 | 14.5 | 16.3 |
| 20 °C | 1.9 | 6.0 | 9.3 | 12.0 | 14.4 | 16.4 | 18.3 |
| 22 °C | 3.6 | 7.8 | 11.1 | 13.9 | 16.3 | 18.4 | 20.3 |
| 24 °C | 5.4 | 9.6 | 12.9 | 15.8 | 18.2 | 20.3 | 22.3 |
| 26 °C | 7.1 | 11.4 | 14.8 | 17.6 | 20.1 | 22.3 | 24.2 |
Reading it for a wall: room air at 20 °C and 50 % has its dew point at 9.3 °C. An interface colder than that, if the vapour reaches it at nearly the room's full pressure, is where the wall gets wet. Whether it does reach it depends on the vapour resistance of the layers in front, which is what the calculator above works out layer by layer.
How the Glaser method works
Three passes through the construction, and each one is simple on its own.
The insight the arithmetic gives you, and it is not obvious: temperature is shared out by one property and vapour by a completely different one. Insulation has enormous thermal resistance and almost no vapour resistance — mineral wool has µ = 1, the same as thin air. So the temperature collapses across the insulation while the vapour walks straight through it, arrives on the cold side still carrying most of its pressure, and meets a saturation limit that has fallen through the floor. That is why the classic failure point is the outer face of the insulation, and why the fix is a vapour control layer on the warm side, not the cold one.
Typical values
| Material | λ W/(m·K) | µ |
|---|---|---|
| Mineral wool | 0.037 | 1 |
| Wood fibre board | 0.040 | 4 |
| EPS | 0.038 | 40 |
| PIR board | 0.022 | 60 |
| XPS | 0.034 | 150 |
| Plasterboard | 0.21 | 8 |
| Aerated concrete block | 0.18 | 8 |
| Fired clay brick | 0.77 | 12 |
| Cement render | 0.90 | 25 |
| Softwood | 0.13 | 40 |
| Dense concrete | 1.75 | 100 |
| Vapour control layer | — | sd 2 – 100 m |
| Breather membrane | — | sd 0.02 – 0.3 m |
Membranes are declared by sd directly rather than by µ, so this page takes that number as it comes on the datasheet.
Worked example you can check by hand
The wall this page opens with, at 20 °C / 60 % inside and 0 °C / 90 % outside.
Then put a vapour control layer with sd = 20 m on the warm side of the insulation and run it again: the vapour pressure at that same point drops to about 611 Pa, under the 661 it needs to stay below, and the wall is dry. One sheet of plastic, in the right place.
What this page does not do
- One set of conditions, not twelve. The full EN ISO 13788 runs every month of the year and checks that whatever condenses in winter dries out again in summer. A construction can condense in January and still be perfectly sound. This page is the screening step, not the verdict.
- Steady state, and vapour only by diffusion. Glaser ignores air leakage, which in practice carries far more moisture into a wall than diffusion ever does, and ignores rain, construction moisture and capillary action.
- No moisture storage. Real materials absorb and release water; Glaser pretends they do not. Hygroscopic build-ups — timber, wood fibre, lime — perform much better in reality than this method suggests.
- No thermal bridges. Condensation shows up first at junctions and around fixings, which is a surface problem and a different calculation.
- Plain build-ups only. No ventilated cavities, no studs, no penetrations.
Questions people ask
- What is the saturation vapour pressure at 20 °C?
- 2337 Pa, by the Magnus formula over water used in the Glaser method: p_sat = 610.5 · exp(17.269·θ / (237.3 + θ)). At 0 °C it is 610.5 Pa, and at −10 °C, over ice, 259 Pa.
- What is the dew point at 20 °C and 50 % relative humidity?
- 9.3 °C. Any surface or layer interface colder than that, where the vapour arrives at the room's pressure, will collect water. At 20 °C and 60 % the dew point is 12.0 °C; at 40 %, 6.0 °C.
- How do you calculate interstitial condensation?
- With the Glaser method of EN ISO 13788, in three passes through the construction: share out the temperature by thermal resistance, find the saturation vapour pressure at each interface, and share out the actual vapour pressure by vapour resistance, s_d = µ · d. Wherever the actual vapour pressure is above the saturation pressure, water forms. The wall this page opens with, at 20 °C / 60 % inside and 0 °C / 90 % outside, condenses at 2 interfaces, worst between the fired clay brick and the mineral wool, where the vapour pressure is 416 Pa above what the air can hold at 1.1 °C.