# Relative Humidity, Vapor Retarders, and Mold Risk Calculations

| Posted in General Questions on
Relative humidity can be calculated if you know dewpoint and temperature – by formulae such as the August-Roche-Magnus approximation.

All methods – all formulae – ignore vapor resistance (perms).

Surely scientists/engineers have a way to then, as a subsequent step, bring vapor retarders into the calculation of relative humidity.  If not, that would surely guarantee major mistakes.  How can you calculate relative humidity at a point in a roof/wall assembly if you ignore vapor barriers and vapor retarders?

Is there something I’m missing?

Seeking a formula for how vapor retarders affect mold risk, or at least a way to integrate perms into a mold risk spreadsheet, I ended up converting a printed spreadsheet [edit: calculating condensation in a wall assembly] into a functioning spreadsheet – almost.

Substituting complex thermodynamics formulae into the sheet resulted in the same numbers as in print, except for 2 lines + 2 cells.
Where do the highlighted parts come from?
If you wish, you can edit this file.  Perhaps highlight your additions / changes in a different color so other readers can see what was original and what’s been changed.  I suggest inserting lines instead of overwriting existing lines.
Thanks!
I don’t think you need to read the source article, but if you wish, the article with the source table is “Vapour Pressure and Condensation”, in Canadian Building Digest.

## Join the leading community of building science experts

### Replies

1. | | #1

I'm trying to follow what you're doing. It looks like you're trying to calculate the vapor pressure in the midpoint of an assembly.

Since the vapor flow in is greater than the vapor flow out you're showing condensation. But the vapor pressure is far below saturation, so that's not possible. I think what has to happen instead is you have to calculate the vapor pressure where the vapor flow in is equal to the vapor flow out. If you know the vapor pressure on the inside and outside of the assembly, and the permeability of the entire assembly you can calculate the vapor flow through the entire assembly (assuming no condensation). One of two things is happening. Either:
1. All the vapor passes through the whole assembly, and the vapor flow at every point in the assembly is the same; or
2. Condensation is happening. Vapor is entering the assembly and not leaving. At the point where the condensation is happening the RH is 100%.

2. | | #2

It should show condensation happening - the source table has identical numbers.

I just highlighted blue the cell where the saturation vapor pressure equals the actual vapor pressure.

3. | | #3

You don't show where line 24, actual vapor pressure, is coming from. In line 22 you have the vapor resistance of the plaster as 0.07, the insulation as 0.63 and the concrete at 1.25. With a total vapor resistance of 1.94 that's 3.4% for the plaster, 32% for the insulation and 64% for the concrete. If the vapor diffuses proportional to the vapor resistance you get a vapor pressure of 0.278 at the plaster-insulation boundary; 0.195 at the insulation-concrete boundary 0.03 at the exterior.

Now you can't have a vapor pressure of 0.195 at the insulation-concrete boundary because the temperature there is -14C and the saturation vapor pressure at that temperature is 0.054. So instead of vapor flow you get condensation.

I don't see the point of trying to quantify the amount condensation. That assembly is what's called a wrong-side vapor barrier, there is more vapor resistance on the outside of the insulation than on the inside. In a cold climate that can cause problems (in a hot climate you want more vapor resistance on the interior). The general rule is to calculate the dew point on the interior, and the temperature at the vapor barrier, on the design day. If the wall temperature is below the dew point there is a risk of condensation, rot and mold. The solution is to have more insulation on the exterior.

4. | | #4

Line 23 looks to be a units conversion from line 22. Line 26 are constants depending on the material.

Is WUFI an easier and better proven option?

5. | | #5

I'm surely missing something (definitely not following all of your tables) but what do perm ratings have to do with calculating RH from dewpoint and temp?

Are you trying to calculate RH in an assembly using Dewpoint/Temp outside the assembly? Why? There's plenty of formulae that calculate vapor drive using perm ratings from which you can then calculate RH if you want.

As a side note, vapor can collect in an assembly before reaching dew point temperature due to hygroscopic adsorption and absorption.

• |
• |
• |
• |