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New Models for Interstitial Condensation Chris Sanders BRE Scotland
New Models for Interstitial Condensation l Importance of Interstitial Condensation l Standards and Regulations l Available models l Boundary conditions l Material properties l Which model should be used?
Interstitial condensation Moisture movement within the materials making up a structure leading to local accumulations sufficient to cause problems: l Rot l Corrosion l Frost damage l Wetting of insulation l Staining of internal surfaces l Damage to equipment within the
Standards for Interstitial Condensation l BS 5250: 1979 - Dewpoint Method l BS 5250: 1989 - Appendix D contains a calculation procedure l BS 5250: 2002 references BS EN ISO 13788: 2002 l CEN TC 89 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 G 4. 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 (G 4. 1) The requirements of G 4. 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 9. 5. 5. 2 of BS 5250: 1989
Draft Approved Document C : 2004 Requirements Resistance to moisture C 2. The floors, walls and roof of the building shall adequately protect the building and its users from harmful effects caused by l ground moisture; l precipitation and wind-driven spray; l interstitial and surface condensation; and l 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. l l Similar requirements for floors and roofs AD F 2 moved into C
Available Models l BRECON - BS 5250: 1989 but includes ventilated cavities l ICOND - BS 5250: 2002 and BS EN ISO 13788 l MATCH, WUFI, MOIST …. .
Theoretical basis of the BS 5250 / EN 13788 method l Both use the ‘Glaser’ method Steady state 1 D vapour diffusion l Constant material properties l Materials are dry until condensation occurs at interfaces when RH=100% l Ventilation of cavities can be included l
Glaser misses out: l l l 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 2 D and 3 D flows can be important Driving forces change on diurnal scales
Three winter days
Three summer days
Glaser method l Wall or roof divided into a series of homogenous layers l Thermal and vapour resistance of each layer used to calculate the temperature ( SVP) and vapour pressure (VP) profiles l If the VP is less than the SVP at all points no condensation l VP > SVP at any point condensation l Recalculate profile l Condensation rate = Vapour flow in - Vapour.
Glaser profile through wall
BRECON Cavity Ventilation But what are the flow rates?
Ventilating cavity - partial cavity fill Unventilated Ventilated
Ventilating cavity on the cold side
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 BS 5250: 1989 - Two months of winter weather : if condensation predicted the designer should decide whether it is important l BS 5250 : 2002 / BS EN ISO 13788: 2002 - Twelve months of condensation and evaporation : three pass/fail criteria l
EN 13788 Criteria condensation in any month Pass l Condensation in winter, which evaporates in summer Pass or Fail depending on amount and material l Condensation in winter, which does not evaporate in summer Fail because assumed to cause accumulation l No .
EN 13788 Criteria
Boundary Conditions BRECON External - January and February mean T & RH Internal - Any T & RH appropriate to the building type l ICOND External - 12 monthly means of T & RH Internal - T = 20°C + 12 monthly RHs determined by internal humidity class l
Internal Humidity Classes Dv 3 kg/m Dp Pa 0, 008 1080 5 0, 006 810 4 3 0, 004 540 2 0, 002 270 1 0 -5 0 5 10 15 20 Monthly mean outdoor air temperature, q e 25 o C
Internal Humidity Classes
Boundary Conditions MATCH requires l Internal – Monthly means or hourly values of T & RH l 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 l Thermal conductivity – widely available with corrections for moisture content and information on likely variability l Vapour permeability – wet cup and dry cup values available for many materials, but little information on variability
Material properties Match requires l l l 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
External Insulation - January profile
External Insulation - ICOND Results
External Insulation - MATCH no liquid
External Insulation - MATCH liquid
Partial Cavity Fill
Partial Cavity Fill - January profile
Partial Cavity Fill - ICOND Results
Partial Cavity Fill - no liquid transport
Partial Cavity Fill - with liquid transport
Full Cavity Fill
Full Cavity Fill - January profile
Full Cavity Fill - ICOND Results
Full Cavity Fill - MATCH no liquid
Full Cavity Fill - MATCH liquid flow
Internal Insulation - January profile
Internal Insulation - ICOND Results
Internal Insulation - MATCH no liquid
Internal Insulation - MATCH with liquid
Timber Framed Wall
Timber Framed Wall - January profile
Timber Framed Wall - ICOND Results
Timber Framed Wall MATCH no liquid
Timber Framed Wall MATCH with liquid
Timber Flat Roof
Timber Flat Roof - January profile
Timber Flat Roof - ICOND Results
Timber Flat Roof MATCH no liquid
Timber Flat Roof MATCH with liquid
Concrete Flat Roof
Concrete Flat Roof - January profile
Concrete Flat Roof - ICOND Results
Concrete Flat Roof MATCH - no liquid transport
Concrete Flat Roof MATCH - with liquid transport
Moisture flows from house to loft
Conclusions l Boundary conditions should represent ‘extreme’ rather than ‘mean’ years - once in ten years? . l We need information on air flows in cavities l 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 l More complex models are needed for ‘heavy’ constructions that store water l We need the material properties data and the climate data to run these models. l
Questions What is your experience of interstitial condensation problems? l Do you use prediction models? l What guidance documents are needed l Regulations are starting to require more thermal complex calculations – should moisture be going the same way? l