How to Factor Vapor Retarders into Relative Humidity & thus Mold Risk

[Jonathon]

TL;DR Summary

Many methods of calculating Relative Humidity exist. All that I've seen ignore vapor retarders, which seems disastrous. Am I missing something?
Attempting to find a way to integrate perms into a mold risk calculation, I converted a printed spreadsheet (from a journal article) into a functioning spreadsheet, almost. Where do the highlighted parts come from?

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 (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 yellow-highlighted parts come from?

The blue cell shows where the actual vapor pressure = the saturation vapor pressure.

If you wish, you can edit the spreadsheet - except that physicsforums.com wants to display the file instead of link to it. You'll have to insert a dot before the "com" in this address: docs.google com/spreadsheets/d/11smIARQELJuGApXnJM_Wd-VyPr9eORoFZ4VKseyk-w0/
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.

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.
https://nrc-publications.canada.ca/eng/view/ft/?id=897fca5e-67e2-46e0-9e83-c2c456faadf1

Thanks!

 

[russ_watters]

Summary:: Many methods of calculating Relative Humidity exist. All that I've seen ignore vapor retarders, which seems disastrous. Am I missing something?...

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?

I feel like may be confusing the calculation of RH from other parameters(a single state) with the calculation of permeation. Relative humidity is just a single parameter of many that describes a single point/state of a parcel of air. The relationship between the various properties has nothing specifically to do with flows or processes. Flows and processes are how you determine what the state is, not what the relationship between RH and vapor pressure (for example) is.

Relative humidity itself doesn't really cause/foment mold growth; condensation - a wet surface - does. The article models a wall by individual components to determine the temperature and humidity gradients to predict where condensation - and therefore mold growth - may occur. Vapor retarders/barriers reduce or block moisture permeation to lower the vapor pressure curve and prevent condensation.

 

[jrmichler]

You seem to be attempting to calculate moisture inside a wall under assumptions of temperature, humidity, and vapor permeability. If so, this is overly simplistic. Building Science Corporation, https://www.buildingscience.com/, has published a lot of information on moisture in walls and roofs. It turns out that moisture in walls has more to do with liquid water getting in (rain), and ability of the structure to dry out, than with vapor permeability letting moisture in. Here's a link to their information on water in walls: https://www.buildingscience.com/document-search?topics[0]=696.

 

[russ_watters]

Building Science Corporation, https://www.buildingscience.com/, has published a lot of information on moisture in walls and roofs. It turns out that moisture in walls has more to do with liquid water getting in (rain), and ability of the structure to dry out, than with vapor permeability letting moisture in.

The details of wall construction have been investigated and developed over the past several thousand years. Today, building codes include the results of modern scientific investigation and all that experience. A properly constructed and maintained building just won't experience excess moisture/mold growth. So yeah, when excess moisture/mold is found in a modern building it is virtually always due to a code violation, design error or disrepair.