How to Use Dow's Steel Stud Wall Condensation Analysis - Cold Climates Calculator
The Dow Building Solutions Steel Stud Wall Condensation Analysis — Cold Climates Calculator will help analyze the condensation potential of a steel stud wall. In addition, the tool helps:
- Determine the effective thermal resistance of steel stud wall assemblies using the ASHRAE parallel path method
- Determine the thermal regime of the interface of exterior sheathing over the heating system
To use it, you’ll need to enter detailed information on the components in the system you want to analyze.
The tool calculates the dew point and the condensation potential of components in the wall. Results are displayed graphically, in real time. Condensation potential exists in situations where the Base Wall or Dow Wall curves fall below the red dew point line. To enlarge the graphic display, select the magnifying glass.
Follow the illustrated instructions in the pages below.
Construct two separate walls for comparison, a Base Wall and a Dow Wall.
From the interior outward, the walls are constructed by selecting desired components for the 8 elements of the wall assembly. Continue making selections for each of the 8 elements of the wall for both the Dow and Base walls.
To select the desired component from each of the 8 elements for each wall, click on the desired wall element dropdown button.
Clicking of the dropdown button shows the various components available for that element of the assembly eg : 1/2 inch gypsum …… click on that component and it is automatically included as part of your wall assembly…….Continue selecting the appropriate selections until both walls are “constructed”.
Here is a final screen with the Base and Dow walls “constructed” ……..
The ASHRAE parallel heat path method is then used to automatically calculate the Effective Thermal resistance of each wall assembly.
Type in the interior temperature and relative humidity for condensation potential evaluation.
Select a City by clicking on the dropdown selection box and clicking on the City for evaluation of condensation potential.
Let’s look at the graphical output for the City, temperature/humidity conditions and walls selected. The graph shows the dewpoint temperature for the interior air conditions we selected….and the CAVITY/EXTERIOR SHEATHING interface temperatures of both walls from August through to May.
While the interior temperature stays constant the exterior temperature drops as we progress to the coldest part of winter and then rises during springtime. As a result the sheathing / cavity temperature undergoes a seasonal swing which is dependent on the severity of the climate (City chosen) and the degree of thermal protection given to the interface by any exterior insulation.
The length of time the cavity/sheathing interface remains below the dewpoint temperature of the selected interior conditions is an indication of the relative potential for condensation IF uncontrolled exfiltration occurs through the wall assembly. In the example we have chosen, the DOW wall, with insulated sheathing, keeps the interface warmer than the non-insulating sheathing of the BASE wall. As a result the interface of the DOW wall is below the dewpoint temperature only 4.5 months for Minneapolis, while the BASE wall’s interface remains below the dewpoint temperature for about 6 months of the year. Thus the benefit of insulating sheathing for cold climates.
For any specific geographical location, completely eliminating the potential for condensation in the cavity demands that the cavity / sheathing interface be above the dewpoint temperature of the interior air for the entire year. This can be achieved by :
- reducing the interior relative humidity (this reduces the actual dewpoint temperature)
- increasing the portion of the wall insulation located outboard of the cavity (by increasing thermal resistance of the sheathing or reducing/eliminating the batt insulation).
The example below (for Minneapolis again) shows the effect of reducing the relative humidity to 30%, keeping the Base wall the same, but changing the Dow wall to have no cavity insulation and R-10 exterior sheathing ….
The effect is the Dow wall cavity is always above the dewpoint and the Base wall condensation potential time is reduced to 5 months. Also note that the Effective Thermal Resistance of the Dow wall is still superior to the base wall.
The user is encouraged to “construct” assorted wall assemblies in order to select a final assembly with the best combination of economics and potential condensation control.
