D.1 General
BS EN ISO 13788:2002 contains recommended procedures for the assessment of the risk of: a) surface condensation and mould growth; and
b) interstitial condensation.
D.2 Surface condensation and mould growth
BS EN ISO 13788:2002 contains a method for calculating 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 problems including condensation.
Besides the external climate (air temperature and humidity) the method uses three parameters to determine the risk of surface condensation and mould growth:
a) the “thermal quality” of each building envelope element, represented by thermal resistance, thermal bridges, geometry and internal surface resistance. The thermal quality can be characterized by the temperature factor at the internal surface;
The internal surface temperature at any point will depend on the nature of the structure, especially the presence of any thermal bridges causing multidimensional heat flow, and most importantly, the value of the internal surface resistance Rsi (see Table C.3).
b) the internal moisture supply (see Annex B); c) the internal air temperature.
To avoid mould growth the relative humidity at the surface should not exceed 80 % for several days. The principal steps in the design procedure are to determine for each month of the year:
i) the internal air humidity;
ii) the acceptable saturation vapour pressure psat at the surface, based on the required relative humidity at the surface;
iii) a minimum surface temperature and hence the required fRsi of the building envelope. !D.3 Interstitial condensation
D.3.1 Principle"
BS EN ISO 13788:2002 contains a method for establishing the annual moisture balance and calculating the maximum amount of accumulated moisture due to interstitial condensation within a structural element. The method should be regarded as an assessment tool, suitable for comparing different
constructions and assessing the effects of modifications rather than as an accurate prediction tool. It does not provide an accurate prediction of moisture conditions within the structure under service conditions, and is not suitable for calculation of drying out of built-in moisture.
In building elements such as cold roofs, where there is air flow through or within the element, the calculated results can be very unreliable and great caution should be used when interpreting the results. 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
(D.1) where:
Úi is the internal air temperature in degrees centigrade; Úe is the external air temperature in degrees centigrade;
Úsi is the temperature of the internal surface in degrees centigrade. fRsi Úsi–Úe
Úi–Úe --- =
Moisture transfer is assumed to be pure water vapour diffusion, and the thermal conductivity and the thermal resistance are assumed constant and the specific heat capacity of the materials not relevant. Heat sinks/sources due to phase changes are neglected.
Calculation methods according to this principle are often called “Glaser methods”. More advanced methods, which are not currently standardized, are available and are described more fully in the references quoted in the bibliography.
There are several sources of error caused by these simplifications.
a) 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.
b) The use of constant material properties is an approximation.
c) Capillary suction and liquid moisture transfer occur in many materials and this can change the moisture distribution.
d) Air movements through cracks or within air spaces can change the moisture distribution by moisture convection. Rain or melting snow can also affect the moisture conditions.
e) The real boundary conditions are not constant over a month.
f) Most materials are at least to some extent hygroscopic and can absorb water vapour. g) One-dimensional moisture transfer is assumed.
h) 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 building components in which there is significant storage of water and which can experience large diurnal changes in temperature. Further guidance is given in a BRE Information Paper. In any case, neglecting moisture transfer in the liquid phase normally results in an overestimate of the risk of interstitial condensation.
!D.3.2 External climate data
The calculation method specified in BS EN ISO 13788:2002 requires monthly mean external temperature and vapour pressure data. Monthly mean temperature data can be obtained for many locations relatively easily, however vapour pressure data are much more difficult to obtain. One useful source is the CD ROM “International Station Meteorological Climate Summary, Version 3.0” available from the US National Climate Data Centre, which contains information for 43 UK stations and many more around the world. Table D.1 summarizes the mean temperatures and relative humidities, calculated from the mean temperature and vapour pressure, for London, Manchester and Edinburgh.
Table D.1 — Monthly mean temperature and relative humidity for interstitial condensation calculations (1983–2002)
Heathrow (London) Ringway (Manchester) Turnhouse (Edinburgh)
T RH T RH T RH °C % °C % °C % Jan 4.9 84 4.2 83 3.5 83 Feb 4.7 82 4.1 80 3.7 81 Mar 6.9 77 5.8 76 5.3 78 Apr 8.8 71 7.8 71 7.0 75 May 12.6 69 11.3 68 9.9 75 Jun 15.7 68 16.1 72 14.7 76 Jul 17.9 68 16.1 72 14.7 76 Aug 17.6 70 15.8 74 14.4 78 Sep 14.9 75 13.3 77 12.1 80 Oct 11.2 81 10.3 81 9.2 82
!Most climatic data, including that in Table D.1, is derived from long-term means, consequently the actual climatic conditions will be worse than shown for half the years: constructions which just pass the BS EN ISO 13788 method may fail in those years. To evaluate the risk of such failures analysis may be re-run under more severe climatic conditions, which may be expected to recur within a stated interval. The number of years N should be selected according to the sensitivity of the building to condensation: a ten year period is appropriate for most buildings, whilst a twenty year period is appropriate for particularly sensitive buildings such as computer centres. The environmental conditions are then determined by applying the corrections in Table D.2 to the mean values for the selected location.
Table D.2 — Corrections to monthly mean temperatures and relative humidities from a mean year to achieve condensation risk years with various return periods
NOTE At present there are no standard methods for transforming data measured at airfield locations to buildings in city centres or in distant locations or at different altitudes. The best possible method at present is to define a number of regions similar, to the degree day regions, and specify a condensation risk year for each, with perhaps a correction of altitudes above 100 m.
Where there is air flow through or within a building element, the results of calculations using the methods described in BS EN ISO 13788:2002 can be very unreliable and should be interpreted with great caution. Moisture transport in pitched roofs with a large void above the insulation are dominated by airflows (see 8.4.2.2). When designing roofs of that type the designer is advised to follow the recommendations of this code. Further guidance is given in D.4.
D.4 Calculation of condensation risk in pitched roofs with a large void above the insulation
The method for calculating interstitial condensation risk recommended in D.3 and described in
BS EN ISO 13788:2002 takes account only of water vapour transport by diffusion and ignores the effects of airflow. Also, the method uses monthly mean temperatures, which means it is unable to include the effects of radiation to the night sky or of solar gain. Consequently, the calculation method in D.3 is unsuitable for use for determining condensation risk for the roofs with large voids above the insulation (see 8.4.2.2). Methods for calculating that risk will be described in a future BRE information paper. Most UK buildings are heated in wintertime and the use and occupation of those buildings generates water vapour at a pressure greater than external atmospheric pressure: that pressure difference tends to drive moisture from inside to outside. The transport of water vapour through the building fabric is dominated by air flows between the interior and any voids within the construction (such as the loft and batten spaces of a pitched roof) and between those voids and outside air.
Some water vapour will be removed from the occupied spaces by ventilation and some will pass into the loft by diffusion through the ceiling, but the bulk will be transported by air currents (convection) through gaps and holes in the ceiling. Tests of typical dwellings indicate that transfer of moisture by diffusion accounts for 25 % and by convection 75 % of the total. If a well sealed ceiling can be achieved, transfer by diffusion and by convection will be roughly equal.
Water vapour may in turn be removed from the loft by diffusion through the underlay and/or by the movement of air through unsealed gaps in the construction, laps in the underlay and low level ventilation provided specifically to encourage air flow.
In a pitched roof with an HR underlay and 10 mm low level ventilation (as 8.4.2.2.2) vapour diffusion through the underlay material is negligible compared to vapour transport by air flow through unsealed gaps in the construction, laps in the underlay and ventilation slots.
Pitched roofs with a large void above the insulation are subject to diurnal temperature cycles, particularly in clear, calm weather. On clear nights, radiation to the sky can cause the temperatures of the roof covering and of the underlay to fall to several degrees below those of outside air and of air in the loft. Condensation may then form on the underside of the covering and of the underlay. On clear days solar radiation can raise the temperatures of the roof covering and of the underlay above the temperature of the loft by as much as 20 ºC to 30 ºC."
Risk Temperature Relative humidity
°C %
1 in 5 –1 +2
1 in 10 –1 +4