This standard gives calculation methods for:
a) The internal surface temperature of a building component or building element below which mould growth is likely, given the internal temperature and relative humidity – the method can also be used to assess the risk of other surface condensation problems.
b) The assessment of the risk of interstitial condensation due to water vapour diffusion. The method used assumes built-in water has dried out and does not take account of a number of important physical phenomena including:
- the dependence of thermal conductivity on moisture content;
- the release and absorption of latent heat;
- the variation of material properties with moisture content;
- capillary suction and liquid moisture transfer within materials;
- air movement through cracks or within air spaces;
- the hygroscopic moisture capacity of materials.
Consequently the method is applicable only to structures where these effects are negligible (BSI 2002).
Starting with the first month in which any condensation is predicted, the monthly mean external conditions are used to calculate the amount of condensation or evaporation in each of the twelve months of a year. The accumulated mass of condensed water at the end of those months when condensation has occurred is compared with the total evaporation during the rest of the year. One-dimensional, steady-state conditions are assumed. Air movements through or within the building elements are not considered (BSI 2002).
There are several sources of error caused by the simplifications described in 6.2.
- The thermal conductivity depends on the moisture content, and heat is released/absorbed by condensation/evaporation. This will change the temperature distribution and saturation values and affect the amount of condensation/drying.
- The use of constant material properties is an approximation.
- Capillary suction and liquid moisture transfer occur in many materials and this may change the moisture distribution.
- Air movements through cracks or within air spaces may change the moisture distribution by moisture convection. Rain or melting snow may also affect the moisture conditions.
- The real boundary conditions are not constant over a month.
- Most materials are at least to some extent hygroscopic and can absorb water vapour.
- One-dimensional moisture transfer is assumed.
- The effects of solar and long-wave radiation are neglected.
NOTE Due to the many sources of error, this calculation method is less suitable for certain building components and climates. Neglecting moisture transfer in the liquid phase normally results in an overestimate of the risk of interstitial condensation (BSI 2002).
On Numeric Simulation (Hygrothermal Simulation): BS EN 15026:2007
This standard defines the practical application of hygrothermal simulation software used to predict one- dimensional transient heat and moisture transfer in multi-layer building envelope components subjected to non steady climate conditions on either side. In contrast to the steady-state assessment of interstitial condensation by the Glaser method (as described in EN ISO 13788), transient hygrothermal simulation provides more detailed and accurate information on the risk of moisture problems within building components and on the design of remedial treatment. While the Glaser method considers only steady-state conduction of heat and vapour diffusion, the transient models covered in this standard take account of heat and moisture storage, latent heat effects, and liquid and convective transport under realistic boundary and initial conditions. The application of such models has become widely used in building practice in recent years, resulting in a significant improvement in the accuracy and reproducibility of hygrothermal simulation (BSI 2007).
- Assembly, orientation and inclination of building components
- Hygrothermal material parameters and functions
- Boundary conditions, surface transfer for internal and external climate
- Initial condition, calculation period, numerical control parameters (BSI 2007)
- Temperature and heat flux distributions and temporal variations
- Water content, relative humidity and moisture flux distributions and temporal variations (BSI 2007)
- Heat storage in dry building materials and absorbed water
- Heat transport by moisture-dependent thermal conduction
- Latent heat transfer by vapour diffusion; moisture storage by vapour sorption and capillary forces
- Moisture transport by vapour diffusion
- Moisture transport by liquid transport (surface diffusion and capillary flow) (BSI 2007)
BS 5250:2011 Code of practice for control of condensation in buildings.
BS EN ISO 13788:2002 Hygrothermal performance of building components and building elements – Internal surface temperature to avoid critical surface humidity and interstitial condensation – Calculation methods.
BS EN 15026:2007 Hygrothermal performance of building components and building elements – Assessment of moisture transfer by numerical simulation.
BS EN ISO 9346:2007 Hygrothermal performance of buildings and building materials – Physical quantities for mass transfer – Vocabulary (ISO 9346:2007).