5 Interstitial condensation Moisture movement within the materials making up a structure leading to local accumulations sufficient to cause problems:RotCorrosionFrost damageWetting of insulationStaining of internal surfacesDamage to equipment within the building
6 Standards for Interstitial Condensation BS5250: Dewpoint MethodBS5250: Appendix D contains a calculation procedureBS5250:2002 references BS EN ISO 13788:2002CEN TC89 WI 29.3 Standard for ‘Assessment of moisture transfer by numerical simulation’ in preparation
7 Building Standards (Scotland) Regulations A building of purpose group 1 (i.e. housing)shall be so constructed as to protect thebuilding and its users, so far as may bereasonably practicable, from harmful effectscaused by condensation.
8 Building Standards (Scotland) Regulations Standard G4.1A floor, wall, roof or other building elementof a dwelling must minimise the risk ofinterstitial condensation in any part of adwelling which it could damage.
9 Building Standards (Scotland) Regulations Provisions deemed to satisfy the standards:Interstitial condensation(G4.1) The requirements of G4.1 will be metwhere the walls, roofs and floors are assessedand/or constructed in accordance withAppendix D and Clauses 9.1 to ofBS5250:1989
10 Draft Approved Document C : 2004 RequirementsResistance to moistureC2. 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; andspillage of water from or associated with sanitary fittings or fixed appliances.
11 Draft Approved Document C : 2004 External walls (resistance to damage from interstitial condensation)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 roofsAD F2 moved into C
12 Available Models BRECON - BS5250:1989 but includes ventilated cavities ICOND - BS5250:2002 and BS EN ISO 13788MATCH, WUFI, MOIST …..
13 Theoretical basis of the BS5250 / EN 13788 method Both use the ‘Glaser’ methodSteady state 1D vapour diffusionConstant material propertiesMaterials are dry until condensation occurs at interfaces when RH=100%Ventilation of cavities can be included
14 Glaser misses out:Materials are hygroscopic, liquid water stored in poresMaterials can start wet from built in water or rain ingress during constructionWater moves by a combination of vapour and liquid flowMaterial properties are effected by moisture content2D and 3D flows can be importantDriving forces change on diurnal scales
17 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) profilesIf the VP is less than the SVP at all points no condensationVP > SVP at any point condensationRecalculate profileCondensation rate = Vapour flow in - Vapour flow out
22 Air Infiltration from building into structure No models and no data Stack effect raises internal air pressure in upper half of the building in winterWind forces may raise internal pressure intermittentlyOperating theatres etc. operate at over pressure
23 Condensation standards BS5250: Two months of winter weather : if condensation predicted the designer should decide whether it is importantBS5250 : 2002 / BS EN ISO 13788: Twelve months of condensation and evaporation : three pass/fail criteria
24 EN CriteriaNo condensation in any month PassCondensation in winter, which evaporates in summer Pass or Fail depending on amount and materialCondensation in winter, which does not evaporate in summer Fail because assumed to cause accumulation over successive years
26 Boundary Conditions BRECON External - January and February mean T & RH Internal - Any T & RH appropriate to the building typeICONDExternal monthly means of T & RHInternal - T = 20°C monthly RHs determined by internal humidity class
27 Internal Humidity Classes vDpkg/m3Pa0,0081080540,00681030,00454020,0022701-5510152025oCMonthly mean outdoor air temperature,qe
29 Boundary Conditions MATCH requires Internal – Monthly means or hourly values of T & RHExternal – Years of hourly values of :TemperatureDewpointWind speedCloud coverGlobal, diffuse and direct solar radiationEC TRYS for Kew, Aberporth, Eskdalemuir, LerwickMETEONORM??
30 Central England Temperature 1950 - 2050 Annual MeanJanuary Mean
31 Material propertiesBS EN ISO method requiresThermal conductivity – widely available with corrections for moisture content and information on likely variabilityVapour permeability – wet cup and dry cup values available for many materials, but little information on variability
32 Material properties Match requires Thermal conductivity and Vapour permeabilityDensity and specific heat – generally availableWater sorption coefficient – standard test, but data not generally availableSorption Isotherm – standard test data catalogues available.Liquid water diffusivity – no standard test and no data available
70 ConclusionsBoundary conditions should represent ‘extreme’ rather than ‘mean’ years - once in ten years?.We need information on air flows in cavitiesWe need models that can take account of air infiltration from within a building into the structure and we need the data to run them
71 ConclusionsSimple ‘Glaser’ models are adequate for many lightweight structures, with little storage capacityMore complex models are needed for ‘heavy’ constructions that store waterWe need the material properties data and the climate data to run these models.
72 QuestionsWhat is your experience of interstitial condensation problems?Do you use prediction models?What guidance documents are neededRegulations are starting to require more thermal complex calculations – should moisture be going the same way?