Moisture and Buildings Joseph Lstiburek, Ph.D., P.Eng John Straube, Ph.D., P.Eng Air-Vapor Moisture Physics Moisture is involved in almost all building envelope performance problems In-service... Durability Examples: rot, corrosion, mould (IAQ) termites, (!), staining etc. presented by www.buildingscience.com No.2/78 Moisture Damage Damage caused by Very high humidity for a long time Wet (100%RH) for a shorter time Time required depends on material Temperature Temperature Accelerates slows or stops process Warm and over 80%RH surface (20% MC) No.3/78 No.4/78 Lstiburek/Straube buildingscience.com 1 of 16
Warm & over 28%MC Below 20 F & near saturation No.5/78 No.6/78 Moisture Control Moisture-related Problems 1. Moisture must be available 2. There must be a route or path 3. There must be a force to cause movement 4. The material must be susceptible to damage Theory: eliminate any one for complete control Practice: control as many as possible Warm & over 80-95%RH No.7/78 No.8/78 Lstiburek/Straube buildingscience.com 2 of 16
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Design Solutions The Water Molecule Balance wetting, drying, and storage Practical Rules Provide a continuous plane of rain control including each enclosure detail Provide continuous air barriers and insulation to control condensation problems Asymmetrical = polar Small: one billion = one foot + 0.28 n! 3 Å Allow drying of built-in and accidental moisture beware drying retarders No.13/78 - Heat Air and Moisture No.14/78 The Polar Molecule Surface Tension: Wettable Hydrogen end is more positive Oxygen end is more negative Water attracted to surface more than self Water attracted to self more than surface H + + H + O 2- H + O 2- H + + No.15/78 No.16/78 Lstiburek/Straube buildingscience.com 4 of 16
Capillary Pressures Surface Tension Result of surface tension = attraction to surfaces pressure varies with pore size e.g., height rise in a glass tube Surface tension up Gravity Down No.17/78 No.18/78 Capillary rise between glass sheets Water: Liquid vs Vapor Elastic band Paper clip Vapor is a single molecule Liquid is molecular clumps, 60 or more Tyvek vs asphalt Vapor Liquid No.19/78 No.20/78 Lstiburek/Straube buildingscience.com 5 of 16
Vapor Pressure: water as a gas com No.21/78 No.22/78 Water vapor in Air 35 C 95 F Water vapor in Air 35 C 0 C 32 F 10 C 50 F 22 C 72 F 0 C 32 F 10 C 50 F 22 C 50%RH 72 F 95 F 1 2 4 8 No.23/78 No.24/78 Lstiburek/Straube buildingscience.com 6 of 16
Water vapor in Air 10 C 50 F 0 C 32 F 22 C 35 C 72 F 95 F 50%RH 25%RH Dewpoint Temperature 50 F 72 F 100%RH 50%RH 22 C 0 C 32 F 10 C 95 F 25%RH = = 1/2 = = = No.25/78 No.26/78 Cold weather condensation 50 F 72 F 100%RH 50%RH 22 C 0 C 32 F 10 C 95 F 25%RH Powerful means of controlling condensation in cold climate buildings Control interior RH 0 C 32 F 100%RH 10 C 21 C 50 F 70 F 95 F 50%RH 25%RH 12%RH 1/2 = = = = = = No.27/78 No.28/78 Lstiburek/Straube buildingscience.com 7 of 16
Warm weather condensation Dehumidification 0 C 32 F 72 F 10 C 22 C 50 F 95 F 35 C 0 C 32 F 50 F 10 C 72 F 50%RH 22 C 95 F 35 C 100% 75 % = = 75 % No.29/78 No.30/78 Psych Chart: Air Vapour Content vs Temperature Air Moisture Content = vapour pressure (Pa, in Hg), humidity ratio (g/kg, grains/pd) Temperature Saturation 100%RH 75%RH 50%RH 25%RH 4000 3500 3000 2500 2000 1500 1000 Psych Chart: Air Vapour Content vs Temperature Air Moisture Content = vapour pressure (Pa, in Hg), humidity ratio (g/kg, grains/pd) Temperature Saturation 100%RH 75%RH 50%RH 25%RH 4000 3500 3000 2500 2000 1500 1000 100% RH 75% RH 50% RH 25% RH -10 ºC 14 º F 100%RH 0 ºC 32 º F 10 ºC 50 º F 20 ºC 68 º F 30 ºC 86 º F -10.0-5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Temperature ( C) No.31/78 500 0 40 ºC 104º F 100% RH 75% RH 50% RH 25% RH -10 ºC 14 º F 100%RH 0 ºC 32 º F 10 ºC 50 º F 20 ºC 68 º F 30 ºC 86 º F -10.0-5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Temperature ( C) No.32/78 500 0 40 ºC 104º F Lstiburek/Straube buildingscience.com 8 of 16
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Air leakage Much more vapor can be carried on back of air flow than diffusion Condensation only happens if air flows towards cold surface Cold-weather air leakage condensation No.37/78 No.38/78.com No.39/78.com No.40/78 Lstiburek/Straube buildingscience.com 10 of 16
.com No.41/78 Enclosures No. 42 / Enclosures No. 43 /.com Enclosures No. 44 / Lstiburek/Straube buildingscience.com 11 of 16
Valid for Diffusion of Air Leakage (Convection).com No.45/78 No.46/78 T back of sheathing = T interior (T interior -T exterior ) R batt /R total Warm Sheathing Approach No.47/78 No.48/78 Lstiburek/Straube buildingscience.com 12 of 16
No exterior insulation Keep it real dry... No.49/78.com No.50/78 R7.5 Exterior Insulation Water Vapour Transport Vapour Diffusion (like heat conduction) more to less vapor No air flow Flow through tiny pores Air Convection (like heat convection) more to less air pressure flow through visible cracks and holes vapour is just along for the ride.com No.51/78.com No.52/78 Lstiburek/Straube buildingscience.com 13 of 16
` No.53/78.com No.54/78 Flow Through Design No.55/78.com No.56/78 Lstiburek/Straube buildingscience.com 14 of 16
Flow Through Colder.com No.57/78 No.58/78 Low Perm Exterior No.59/78 No.60/78 Lstiburek/Straube buildingscience.com 15 of 16
Low Perm Interior No.61/78 No.62/78 Conclusions Air can store much more water vapor as temperature increases Water vapor moves in two modes Diffusion (vapor control) Air Leakage (air control) Vapor control is less important Air control requires all holes sealed No.63/78 Lstiburek/Straube buildingscience.com 16 of 16