Interstitial condensation Moisture movement within the materials making up a structure leading to local accumulations sufficient to cause problems: Rot Corrosion Frost damage Wetting of insulation Staining of internal surfaces Damage to equipment within the building
Standards for Interstitial Condensation BS5250:1979 - Dewpoint Method BS5250:1989 - Appendix D contains a calculation procedure BS5250:2002 references BS EN ISO 13788:2002 CEN TC89 WI 29.3 Standard for ‘Assessment of moisture transfer by numerical simulation’ in preparation
Building Standards (Scotland) Regulations Regulation 18: A building of purpose group 1 (i.e. housing) shall be so constructed as to protect the building and its users, so far as may be reasonably practicable, from harmful effects caused by condensation.
Building Standards (Scotland) Regulations Standard G4.1 A floor, wall, roof or other building element of a dwelling must minimise the risk of interstitial condensation in any part of a dwelling which it could damage.
Building Standards (Scotland) Regulations Provisions deemed to satisfy the standards: Interstitial condensation (G4.1) The requirements of G4.1 will be met where the walls, roofs and floors are assessed and/or constructed in accordance with Appendix D and Clauses 9.1 to 220.127.116.11 of BS5250:1989
Draft Approved Document C : 2004 Requirements Resistance to moisture C2. The floors, walls and roof of the building shall adequately protect the building and its users from harmful effects caused by - ground moisture; precipitation and wind-driven spray; interstitial and surface condensation; and spillage of water from or associated with sanitary fittings or fixed appliances.
Draft Approved Document C : 2004 External walls (resistance to damage from interstitial condensation) 5.3.4 An external wall will meet the requirement if it is designed and constructed in accordance with Clause 8.3 of BS 5250:2002, and BS EN ISO 13788:2001. 5.3.5 Because of the high internal temperatures and humidities, there is a particular risk of interstitial condensation in the walls of swimming pools and other buildings in which high levels of moisture are generated; specialist advice should be sought when these are being designed. Similar requirements for floors and roofs AD F2 moved into C
Available Models BRECON - BS5250:1989 but includes ventilated cavities ICOND - BS5250:2002 and BS EN ISO 13788 MATCH, WUFI, MOIST …..
Theoretical basis of the BS5250 / EN 13788 method Both use the ‘Glaser’ method Steady state 1D vapour diffusion Constant material properties Materials are dry until condensation occurs at interfaces when RH=100% Ventilation of cavities can be included
Glaser misses out: Materials are hygroscopic, liquid water stored in pores Materials can start wet from built in water or rain ingress during construction Water moves by a combination of vapour and liquid flow Material properties are effected by moisture content 2D and 3D flows can be important Driving forces change on diurnal scales
Glaser method Wall or roof divided into a series of homogenous layers Thermal and vapour resistance of each layer used to calculate the temperature ( SVP) and vapour pressure (VP) profiles If the VP is less than the SVP at all points no condensation VP > SVP at any point condensation Recalculate profile Condensation rate. = Vapour flow in - Vapour flow out
Air Infiltration from building into structure No models and no data Stack effect raises internal air pressure in upper half of the building in winter Wind forces may raise internal pressure intermittently Operating theatres etc. operate at over pressure
Condensation standards BS5250:1989 - Two months of winter weather : if condensation predicted the designer should decide whether it is important BS5250 : 2002 / BS EN ISO 13788:2002 - Twelve months of condensation and evaporation : three pass/fail criteria
EN 13788 Criteria No condensation in any month Pass Condensation in winter, which evaporates in summer Pass or Fail depending on amount and material Condensation in winter, which does not evaporate in summer. Fail because assumed to cause accumulation over successive years
Boundary Conditions BRECON External - January and February mean T & RH Internal - Any T & RH appropriate to the building type ICOND External - 12 monthly means of T & RH Internal - T = 20°C + 12 monthly RHs determined by internal humidity class
Internal Humidity Classes -50510152025 Monthly mean outdoor air temperature, 1080 810 540 270 1 2 3 4 5 0 0,002 0,004 0,006 0,008 e v p Pa kg/m 3 C o
Boundary Conditions MATCH requires Internal – Monthly means or hourly values of T & RH External – Years of hourly values of : Temperature Dewpoint Wind speed Cloud cover Global, diffuse and direct solar radiation EC TRYS for Kew, Aberporth, Eskdalemuir, Lerwick METEONORM??
Central England Temperature 1950 - 2050 Annual Mean January Mean
Material properties BS EN ISO 13788 method requires Thermal conductivity – widely available with corrections for moisture content and information on likely variability Vapour permeability – wet cup and dry cup values available for many materials, but little information on variability
Material properties Match requires Thermal conductivity and Vapour permeability Density and specific heat – generally available Water sorption coefficient – standard test, but data not generally available Sorption Isotherm – standard test data catalogues available. Liquid water diffusivity – no standard test and no data available
Conclusions Boundary conditions should represent ‘extreme’ rather than ‘mean’ years - once in ten years?. We need information on air flows in cavities We need models that can take account of air infiltration from within a building into the structure and we need the data to run them
Conclusions Simple ‘Glaser’ models are adequate for many lightweight structures, with little storage capacity More complex models are needed for ‘heavy’ constructions that store water We need the material properties data and the climate data to run these models.
Questions What is your experience of interstitial condensation problems? Do you use prediction models? What guidance documents are needed Regulations are starting to require more thermal complex calculations – should moisture be going the same way?