The interstitial condensation calculation is performed in line with the Glaser Method (based on ISO 13788) and determines if any condensate will gather in the system throughout the year.
This method is a standard static interstitial moisture calculation, initially developed in 1958. To date, the Glaser methodology has been widely used to assess the amount of water vapour likely to be generated within the building and the resultant increase in internal vapour pressure above that of external air. The simplified calculation used is based on average monthly temperatures, vapour pressure and steady-state conduction of heat to determine if critical condensation points are reached within one year.
The Glaser method identifies vapour diffusion; how easily water vapour can pass through the fabric of the building. This method assumes, however, vapour moves in one direction only, from inside to outside, and omits driving rain completely from its calculations. Therefore, it is only useful in colder climates. Absorption and porosity are not measured, which also means the potential risk attributed to moisture storage is omitted.
- The R-Value and vapour resistivity are collected for each layer.
- External temperature and humidity data for each month are collected based on the project's climate settings.
- The first part of the Glaser Method collates/calculates all necessary data to then determine if any vapour flow occurs within the system.
- The second part of the Glaser Method determines the total condensate at the end of the year, and/or if any detected condensation will evaporate throughout the year.
- After all twelve months have been analysed, a determination is made based on the amount of condensate in the system. There are three possible outcomes:
- No condensation at all
- Condensation occurred but has evaporated throughout the year
- Condensation is still present in the system after a full year
Material Data Collection
The wall system calculation methodology to gather the corrected R-Values for any insulation and cavity layers, including any possible derating of layers. Thermal bridging is not performed this time around as this will make the insulation layer seem a lot lower than expected. Instead, we opt for using just the insulation R-Value as it takes up the majority of that layer.
As AS/NZS 4859.2 is used for data collection, the more conservative (i.e. lower R-value) results are selected based on the outcome for both summer and winter. The vapour resistivities are either included in the material specifications from the supplier or default values which best match the layer is adopted.
Temperature and Humidity Data Collection
The external temperature and humidity data are collected from EPW (Energy Plus Weather) files collected from all over Australia. The coordinates determined in your project settings will be used to find the closest location with an EPW file. Once the correct weather file has been identified, the data within is compiled from all 8760 data points (24 hours in 365 days). Due to the nature of the condensation calculations, the data points have been compiled in two different ways depending on the climate zone of the project. Climate zones 1-5 will have the monthly mean values for temperature and humidity used, whereas climate zones 6-8 will have the monthly mean of the daily minimum compiled.
For internal temperature and humidity, a constant 20 degrees temperature has been chosen, but humidity is slightly different based on climate zone. Climate zones 1-5 will have a constant 65% internal relative humidity, and climate zones 6-8 will have 70% relative humidity.
The above decisions have been made to assure results as close as possible to reality.
Glaser Method (Part One)
The temperature at each interface (the connection between two layers) is calculated starting from the internal side. This temperature is dependent on the specified layer's R-Value proportion of the overall total system R-Value. The external air film’s outside surface will have the same temperature as the outside air temperature.
Using this temperature, the saturation vapour pressure is calculated for each interface; the threshold which will determine if condensate forms. The vapour resistance of each layer is then calculated, dependent on the thickness and vapour resistivity of the material.
These are cumulatively assigned to each interface, starting at the internal side with a value of zero. Using the overall resistance and cumulative resistances at each interface, the vapour pressure for each interface is determined.
If at any interface the vapour pressure is greater than saturation vapour pressure, we have condensation! Speckel has adopted a simplified approach here - only looking at the interface with the highest vapour difference, also known as the condensation interface.
Using all the previously collated properties, the vapour flow can be determined - the quantifiable amount of condensate produced at an interface.
Glaser Method (Part Two)
Using the calculated vapour flows for each month, the first month in which condensation occurs from January is determined. If no months have condensation, no further processing is done. Or if every month has condensation, then January is selected as the starting month. Starting from this month, each month is sequentially assessed for vapour flow.
If vapour flow is found to be positive, it is simply added onto the accumulated vapour flow. If vapour flow is found to be zero, then there is a chance evaporation may occur at this interface. Vapour flow for that month is recalculated, this time flipping a switch at the condensation interface, which may cause the calculated vapour flow to become negative in this iteration. This is then taken from the accumulated total condensate until reaching zero and continues throughout the rest of the months in the year.
- Condensation can only occur on one layer at a time (or more when more than one layer has the exact same lowest Vapour Pressure difference)
- If evaporation occurs at more than one layer, it occurs uniformly across the entire system (each layer is not taken in isolation; one layer can have condensate evaporated but the other not)
- Inside air temperature and humidity are set to 20 degrees and 70% respectively