Exterior wall psychrometric analysis calculator

Jonathan Ochshorn

Directions: This calculator finds the temperature and dew point temperature gradients within exterior walls and indicates if, and where, condensation might occur. Use the drop-down menus in the yellow cells to define any exterior wall by listing the various layers (components, or materials) of the wall, from the exterior to the interior. A few user-defined materials can be added if those found in the drop-down menu are insufficient. It is common to start with an exterior air film and end with an interior air film.

R-value, perm rating, and temperature can be set for I-P or SI units at the top of the calculator. It is also necessary to input values for inside and outside temperature and relative humidity (RH).

And note that perm values can neither be equal to zero (since the inverse perm used in the calculations would crash the program) nor can they be infinitely large (so, if a material is extremely permeable to vapor, a very large number can be used, and such numbers are displayed as "100,000" in the calculator, no matter what their actual value is, since such subtleties have no practical impact on the results).

The results are displayed below the drop-down menus and show the calculated temperatures, saturation vapor pressure (SVP), vapor pressure (Pv), relative humidity (RH), humidity ratio (ω),and dew point temperatures (DP), within the wall. R-values, perm ratings, and temperatures are displayed in the units selected (i.e., I-P or SI). Condensation occurs, and is noted in the bottom row, wherever the temperature falls below the dew point temperature within the wall (i.e., where the relative humidity reaches 100%).

Commentary: This calculator should be taken with a grain of salt, so to speak, since it abstracts from many important considerations. For one thing, many materials have variable permeability depending on the humidity, which this calculator does not account for. Instead, this calculator assumes a fixed perm rating for each material, and doesn't fully account for materials with a range of perm values. Similarly with R-values, the numbers used are somewhat arbitrarily picked to represent values that may not apply to all instances of a given material. Furthermore, modern walls may be able to accommodate some condensation if allowance is made for the wet materials to dry out, either to the interior, the exterior, or both. So the choice of water-resistive barriers and vapor retarders becomes much more nuanced in modern building codes and in modern practice. And the calculator does not account for the so-called framing factor in framed walls, in which the presence of studs and plates reduces the overall R-value of the assembly.

For those and other reasons, this calculator should be understood as more of a teaching tool than as a practical guide for exterior wall design. For example, the default wall section shown in the calculator (click on the "reset" button to see it) generates interstitial condensation between batt insulation and plywood sheathing; but adding a vapor barrier between the gypsum board and the fiberglass batt insulation — changing the value in the drop-down menu from "not used" (between "interior finish" and "stud-wall insulation") to either an aluminum or polyethylene vapor retarder — eliminates the condensation. This seems straight-forward and is illustrated in figure 1. But life is not that simple …

Fig. 1. Schematic wall section (left) showing interstitial temperature and dew point temperature gradients. Condensation occurs where the temperature falls below the dew point temperature. In this example, taken from the default values in the calculator, the dew point temperature — shown as a red line — crosses over the temperature between the batt insulation and the plywood sheathing, indicating the potential for condensation at this location. When a vapor retarder is added between the gypsum board and batt insulation, the dew point temperature — shown as a green line — is always below the temperature, so condensation does not occur. The psychrometric chart (right) plots the combination of temperature and humidity ratio and shows where the relative humidity rises above 100% (i.e., where condensation occurs — at point "C" in both diagrams). Images by Jonathan Ochshorn.

In spite of what the calculator suggests, the use of an ordinary Class I vapor retarder to prevent condensation is not always a good idea and is actually constrained in modern codes. As described in the 2024 International Building Code, one must sometimes use a so-called responsive vapor retarder — made from materials that increase their permeability as humidity increases — where, e.g., "a Class I or II vapor retarder is used in combination with foam plastic insulating sheathing installed as continuous insulation of the exterior side of frame walls."

In general, it's a good idea to consult the IBC or IRC when thinking about the design of exterior walls and the use of vapor barriers. Two key tables from the 2024 IBC are reproduced below:

TABLE 1404.3(1) VAPOR RETARDER MATERIALS AND CLASSES

Vapor retarder class	Acceptable materials
I	Sheet polyethylene, nonperforated aluminum foil, or other approved materials with a perm rating of less than or equal to 0.1
II	Kraft-faced fiberglass batts or vapor retarder paint or other approved materials, applied in accordance with the manufacturer's instructions for a perm rating greater than 0.1 and less than or equal to 1.0
III	Latex paint, enamel paint, or other approved materials, applied in accordance with the manufacturer's instructions for a perm rating of greater than 1.0 and less than or equal to 10

TABLE 1404.3(2) VAPOR RETARDER OPTIONS

Climate zone	Vapor retarder class
Climate zone	I^a	II^a	III
1, 2	Not permitted	Not permitted	Permitted
3	Not permitted	Permitted^c	Permitted
4 (except Marine 4)	Not permitted	Permitted^c	See Table 1404.3(3)
Marine 4, 5, 6, 7, 8	Permitted^{b, c}	Permitted^c	See Table 1404.3(3)

A responsive vapor retarder shall be allowed on the interior side of any frame wall in all climate zones.
In frame walls with a Class I vapor retarder on the exterior side, use of a Class I interior vapor barrier that is not a responsive vapor barrier shall require an approved design.
Where a Class I or II vapor retarder is used in conjunction with foam plastic insulating sheathing installed as continuous insulation on the exterior side of frame walls, the continuous insulation shall comply with Table 1404.3(4) and the Class I or II vapor retarder shall be a responsive vapor retarder.

Underlying psychrometric equations used in the calculator:

To find the temperature gradient within a wall consisting of various material layers:
1. Find the R-values of each material and the total R-value (the sum of the individual R-values).
2. Let the outside and inside temperatures be T_o and T_i respectively; and let the temperature difference between outside and inside = T_o – T_i = ΔT (which will be a negative number in "winter" conditions if the outside temperature is lower than the inside temperature).
3. Starting on the left (exterior side) we find the first material's final temperature by multiplying the total change in temperature from outside to inside by the ratio of the material's R-value to the total R-value of the wall, and then subtracting this value from the temperature on its left. When the final temperature is computed, it should equal the selected interior temperature.
Next, we find SVP, the saturation vapor pressure (Pa units), using the following equations derived by Jianhua Huang and published in the Journal of Applied Meteorology and Climatology in 2018 (the author uses P_s for what I am calling SVP):

As can be seen, there are two equations, depending on whether the temperature, t (°C), is above or below freezing.
We compute P_v, the vapor pressure (Pa), for the inside and outside conditions only, since the relative humidity is currently known only at those two locations:
- P_v = RH × SVP / 100.
We find the inverse perm values for each material, i.e., 1 / (perm value), where perms are in SI units (ng/s × m² × Pa).
Using the two bounding values of vapor pressure found in step "C," we can determine all the other vapor pressures within the wall by making their changes in value proportional to the changes in value of the inverse perms. The method here is analogous to the method used to find the temperature gradient based on R-values: here, we find the vapor pressure gradient based on inverse perm values.
We can now find the relative humidity (RH) at each interstitial material location, taking RH = (P_v / SVP) × 100.
The dew point temperature can be found in these two steps, published by Sensirion:
1. Define a variable H = ((log10(RH) – 2) / 0.4343 + (17.62 × T) / (243.12 + T)), where RH is the relative humidity and T is the temperature (°C);
2. The dew point temperature, DP (°C) = 243.12 × H / (17.62 - H).
Once we know the distribution of temperatures and dew point temperatures within the wall, we can identify potential sites of condensation where the temperature falls below the dew point temperature (or where the relative humidity reaches 100%).
The humidity ratio (absolute humidity) is also computed based on the equation: ω = 0.62198 × P_v / ( P_t – P_v), where P_v is the vapor pressure, defined above, and P_t is the total atmospheric pressure, taken as 101,325 Pa at sea level.

Disclaimer: Building codes (including the International Building Code) are typically structured as a maze of basic statements and qualifying assertions. Often, the qualifiers — sometimes found in other sections, or in footnotes to tables — are more important than the basic statements they modify. For this reason, it is important to scrutinize all relevant sections of the code before drawing any conclusions. This calculator does not necessarily account for all qualifying assertions, and so should not be used for the design of actual structures, but only for schematic (preliminary) understanding of building envelope principles as they apply to simple solid exterior walls. For the design of an actual structure, a competent professional should be consulted.

First uploaded Feb. 16, 2026. Last updated Feb. 17, 2026.