Moisture Control in Buildings

Transcription

1 Moisture Control in Buildings

2 Moisture Control in Buildings The importance of accounting for moisture in building design. Controlling the ingress of liquid moisture into buildings is simple to achieve. HOWEVER (R. Pender in EH, 2010) As insulation and airtightness levels increase controlling water vapour is proving increasingly difficult to achieve.

3 Moisture Control in Buildings Moisture Related Building Damage

4 Moisture Control in Buildings Understanding moisture movement in buildings. Moisture States and Sources Vapour & Water transfer in building fabric Properties of building materials relevant to hygrothermal performance Condensation surface and interstitial Design steps for avoiding moisture risk Condensation risk assessment Design and Material choice Fabric Strategy Case studies

5 Moisture Control in Buildings Moisture States and Sources. Solid Liquid Gas

6 HUMIDITY RATIO (kg/kg Dry air) Moisture Control in Buildings Vapour & Water transfer in building fabric. Vapour pressure is the basic metric : kpa Absolute humidity : g/m 3 Saturation Vapour Pressure : kpa temperature dependent maximum vapour pressure. Relative humidity : % RH = p v p s Dew point o C - temperature at which SVP of air is reached (at constant VP) at defined temperature Increasing moisture content (proportion of air as vapour) = increasing vapour pressure, constant regardless of temp.

7 Moisture Control in Buildings Relative Humidity: Moisture & 20 0 C = 0 0 C = 4.8g Vapour pressure of water in given volume of air as percentage of Saturation Vapour Pressure at the same temperature

8 Moisture Control in Buildings Internal Moisture Sources Activity litres moisture per day A family asleep Typical daytime activities Cooking Washing and bathing Washing clothes Drying clothes Approx. Total 9-15 ltrs. A typical three-bedroom family house: (Adapted from BS5250;2011)

9 Moisture Control Moisture Control in Buildings External Moisture Sources Internal vapour pressure (vapour production, low ventilation..) 2. Wind driven rain 3. Reverse vapour pressure gradient (solar radiation..) Construction moisture & ground moisture

10 Moisture Control Moisture Control in Buildings Moisture transfer MOISTURE TRANSFER (Kuenzel, 1995)

11 Moisture Control Moisture Control in Buildings Moisture Storage Hygroscopic Materials Adsorption at surface of hydrophilic material without SVP according to van der Waals forces Kelvin s Equation Governs rate of capillary condensation according to vapour pressure and meniscus dimensions Young-Laplace Law describes capillary pressure as function of pore radii MOISTURE STORAGE (Image - Martys & Ferraris, 1997)

12 Moisture Control Moisture Control in Buildings Moisture Storage : Sorption Curve free saturation Capillary water region Hygroscopic region (Kuenzel, 1995)

13 Moisture Control Condensation Two types occur in Buildings Surface Visible, treatable & simple to assess risk Incidence governed by internal RH and surface temperature for all building components Interstitial Hidden, hard to remedy & more complicated risk assessment - Incidence governed by wide variety of inter-related conditions & dependent on building component See BS5250:2011, CIBSE,2007 and STBA Moisture Guidance

14 Moisture Control Moisture Control in Buildings Surface Condensation Dew point conditions not necessary for mould growth (Trotman,2004)

15 Surface Condensation Assessment Temperature factor: or Psi value calculation + humidity data

16 Detailing to avoid Surface Condensation Vulnerable area Thermal bridges ADEQUATE VENTILATION! and local increase of heat loss 2D! Surface mould growth

17 Moisture Control Moisture Control in Buildings Interstitial Condensation Actual condensation accumulation relatively rare only possible at interface of non hygroscopic materials or in extreme circumstances Excessive humidity more typical Structural implications of rot & corrosion + health concerns

18 Moisture Control Moisture Control in Buildings Avoiding Interstitial Condensation 1. Add Capacity Use materials which avoid excessive interstitial moisture build up and promote drying.

19 2. Assessment of Sources and Sinks Avoid moisture risks at design stage: Can building component dry out? Is capacity for moisture drying out equal or greater than moisture entering the fabric? Material properties Atmospheric moisture loads Moisture risk assessment Internal moisture loads Moisture response Evaluation of the occurrence of moisture accumulation, and/or potential mould growth conditions

20 Assessment : Steady State Calculation Material properties Glaser method BS EN ISO µ W/mK Vulnerable area (critical interface) Atmospheric moisture loads T RH (monthly) Internal moisture loads T RH (monthly) Moisture response condensation

21 Risks of using inadequate assessment Vapour Control Layers as magic bullet?

22 Vapour Control Layers Builder. Building Physics Predicated on incomplete evaluation external moisture ignored Reduced drying potential -Drying to inside prevented Reverse condensation Perfect build quality assumed Difficult to ensure continuous seal, floor voids, partitions, ceilings - DIN Vulnerable over lifetime

23 Dynamic Hygrothermal Modelling BS EN Tools: WUFI, Delphin, MOIST, MATCH.. Additional moisture Sources: Service Conditions Atmospheric moisture loads Dry bulb temperature Relative humidity Wind speed and direction Solar radiation (global and diffuse) Longwave (cloud index) Rainfall Windspeed & direction Total atmospheric pressure (hourly) Material properties Moisture response Density, Porosity, µ, λ, Sorption isotherm, Liquid transfer coefficient sorption, Liquid transfer coefficient redistribution, Moisture dependant λ, short wave absorptivity of surface, Internal moisture loads T + RH (hourly) Defined according to: BS EN BS EN ASHRAE 160 User defined sine curves liquid transport (surface diffusion and capillary flow) moisture storage by vapour sorption and capillary forces Vapour diffusion Moisture content profiles, temp, RH profiles Moisture content/time, temp, RH/time

24

25 Open To Interpretation Failure Criteria? Moisture Control in Buildings Moisture Analysis Comparison GLASER BuildDesk HAM

26 Moisture Control in Buildings The importance of accounting for moisture in building design : Ventilation Relative Humidity Control

27 Moisture Control in Buildings The importance of accounting for moisture in building design : Building Fabric Relative Humidity Control

28 Moisture Control in Buildings Vapour & Thermal properties of building fabric Heat Input Ventilation (Trotman,2004)

29 Moisture Control in Buildings Design Guidance; Controlling moisture sources with drying potential is the only guaranteed means of managing moisture levels in building fabric.

30 Moisture Control in Buildings Design Guidance; It s about regulating moisture not preventing moisture.

31 Moisture Control in Buildings Design Guidance; Strategy Vapour check based system

32 Moisture Control in Buildings Design Guidance; Strategy Material properties: Vapour open slows moisture transfer through the fabric without inclusion of vapour barrier Breathable system

33 Moisture Control in Buildings Design Guidance; Strategy Material properties: Vapour open Hygroscopic Absorbs moisture as it passes through insulation Breathable system

34 Moisture Control in Buildings Design Guidance; Strategy Material properties: Vapour open Hygroscopic Moisture Storage Hygroscopicity combined with relatively high density = capacity for moisture storage that prevents accumulation of liquid condensation Breathable system

35 Moisture Control in Buildings Design Guidance; Strategy Material properties Vapour open Hygroscopic Moisture Storage Capillary Active Moisture redistribution with capillary action, reduces localised excess moisture concentration. Breathable Approach Working with physics = Build Simplicity Breathable system

36 Problem solving with healthy products Woodfibre outperforms synthetic insulation, as no other insulation will breathe, wick moisture and regulate building temperature, both in the summer as well as the winter. Add to this the fact that it is 100% natural and locks up more Carbon than is used to produce or transport it, combined.

37 Moisture Control in Buildings Design Guidance; Case Studies

38

39

40 Suscon Academy

41 Suscon Academy

42 New Court, Trinity College Cambridge

43

44 Moisture Flows through Internal Wall Insulation

45 Calculated 0.88 W/m2K Calculated 0.42 W/m2K Measured 0.58 W/m2K Measured 0.28 W/m2K New Court, Trinity College Cambridge

46 Pavadry at New Court, Trinity College New Court, Trinity College Cambridge

47 New Court, Trinity College Cambridge

48 Steps for a Moisture-Safe Construction Greater the level of insulation, the more attention needs to be paid to moisture Moisture will find a way You cannot change the laws of Physics Bring an understanding of moisture transport to the design stage Identify construction details that are vulnerable to moisture ingress Design in capacity with materials that store moisture at safe levels & promote drying Use buildable solutions, avoid over-optimised approaches which require perfect build quality or complicated interaction with services, it dose not work. Assess risk with appropriate tool: Evaluate likelihood of moisture accumulation and drying potential Balance sources & sinks Include potential faults and failures when assessing risk Maintain & Ventilate Breathable Natural Materials = Drying Potential = Safe Construction

49