1. New Models for Interstitial Condensation
Chris Sanders
BRE Scotland

2. New Models for Interstitial Condensation
Importance of Interstitial Condensation
Standards and Regulations
Available models
Boundary conditions
Material properties
Which model should be used?

3. Surface Condensation

4. Interstitial condensation?

5. 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

6. Standards for Interstitial Condensation
BS5250: Dewpoint Method
BS5250: 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

7. Building Standards (Scotland) Regulations
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.

8. 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.

9. 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 of
BS5250:1989

10. 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.

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 roofs
AD F2 moved into C

12. Available Models BRECON - BS5250:1989 but includes ventilated cavities
ICOND - BS5250:2002 and BS EN ISO 13788
MATCH, WUFI, MOIST …..

13. 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

14. 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

15. Three winter days

16. Three summer days

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) 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

18. Glaser profile through wall

19. BRECON Cavity Ventilation
But what are the flow rates?

20. Ventilating cavity - partial cavity fill
Unventilated
Ventilated

21. Ventilating cavity on the cold side

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 winter
Wind forces may raise internal pressure intermittently
Operating 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 important
BS5250 : 2002 / BS EN ISO 13788: Twelve months of condensation and evaporation : three pass/fail criteria

24. EN 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

25. EN Criteria

26. Boundary Conditions BRECON External - January and February mean T & RH
Internal - Any T & RH appropriate to the building type
ICOND
External monthly means of T & RH
Internal - T = 20°C monthly RHs determined by internal humidity class

27. Internal Humidity Classesv D p
kg/m 3 Pa
0,008
1080 5 4
0,006
810 3
0,004
540 2
0,002
270 1 -5 5 10 15 20 25 o C
Monthly mean outdoor air temperature, q e

28. Internal Humidity Classes

29. 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??

30. Central England Temperature 1950 - 2050
Annual Mean
January Mean

31. Material properties
BS EN ISO 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

32. 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

33. External Insulation

34. External Insulation - January profile

35. External Insulation - ICOND Results

36. External Insulation

37. External Insulation - MATCH no liquid

38. External Insulation - MATCH liquid

39. Partial Cavity Fill

40. Partial Cavity Fill - January profile

41. Partial Cavity Fill - ICOND Results

42. Partial Cavity Fill - no liquid transport

43. Partial Cavity Fill - with liquid transport

44. Full Cavity Fill

45. Full Cavity Fill - January profile

46. Full Cavity Fill - ICOND Results

47. Full Cavity Fill - MATCH no liquid

48. Full Cavity Fill - MATCH liquid flow

49. Internal Insulation

50. Internal Insulation - January profile

51. Internal Insulation - ICOND Results

52. Internal Insulation - MATCH no liquid

53. Internal Insulation - MATCH with liquid

54. Timber Framed Wall

55. Timber Framed Wall - January profile

56. Timber Framed Wall - ICOND Results

57. Timber Framed Wall - MATCH no liquid

58. Timber Framed Wall - MATCH with liquid

59. Timber Flat Roof

60. Timber Flat Roof - January profile

61. Timber Flat Roof - ICOND Results

62. Timber Flat Roof - MATCH no liquid

63. Timber Flat Roof - MATCH with liquid

64. Concrete Flat Roof

65. Concrete Flat Roof - January profile

66. Concrete Flat Roof - ICOND Results

67. Concrete Flat Roof MATCH - no liquid transport

68. Concrete Flat Roof MATCH - with liquid transport

69. Moisture flows from house to loft

70. 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

71. 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.

72. 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?