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BUILDING DYNAMICS: Moisture, Airflows and Construction Technology

BUILDING DYNAMICS: Moisture, Airflows and Construction Technology. James Biddle, Mansel Nelson Northern Arizona University Presentation Credit to: Joseph T. Ponessa , Ph.D. Professor Emeritus Housing, Indoor Environments and Health. Goals.

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BUILDING DYNAMICS: Moisture, Airflows and Construction Technology

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  1. BUILDING DYNAMICS:Moisture, Airflows and Construction Technology James Biddle, Mansel Nelson Northern Arizona University Presentation Credit to: Joseph T. Ponessa, Ph.D. Professor Emeritus Housing, Indoor Environments and Health

  2. Goals • Review basic dynamics of moisture movement, control in buildings • Review basic dynamics of airflows in buildings • Proper management of moisture and airflows provides better buildings and reduces callbacks ---------------------------------------------------- • Understanding of these mechanisms is essential for diagnostics

  3. Objective: How does this fit with weatherization? Weatherization can improve comfort and save money. Downside is ‘inadequate’ ventilation When is ventilation inadequate? • When it is less than prescribed ventilation • When it is inadequate to take care of building excesses • Too much moisture • Excessive pollution sources

  4. Overview of Today’s Presentation Building Science • Moisture dynamics; applications in buildings • Airflow mechanisms; applications in buildings

  5. Section I: Moisture DynamicsOutline • Basic moisture science: Vapor & Liquid • Air – vapor – temperature relationships • Relative Humidity • Putting it all together – Psychrometric chart • Vapor movement • Diffusion • Bulk transfer – air flows

  6. Moisture Dynamics Outline (cont.) Basic moisture science (cont.) • Water movement • Gravity; wind • Capillary action • Moisture sources • Moisture Measurement • Air • Surfaces / solids

  7. Moisture Dynamics Water can exist in three states • Vapor • Liquid • Solid

  8. Moisture Dynamics (cont.)Vapor • Vapor-Temperature relationships (At saturation) The amount of moisture that air can hold increases directly with temperature.

  9. Moisture DynamicsVapor • Dew point: Saturation Air cooled to the limit of its moisture carrying capacity releases vapor as droplets condensation (or rain) This is the Dew point temperature

  10. Moisture DynamicsVapor • So what does it all mean? Moist air that is cooled down OR… Moist air that meets a cool surface will condense! Wet surfaces that don’t/can’t dry rapidly will produce mold

  11. Moisture DynamicsVapor Transport Vapor transport: how does vapor get from point A to point B? POSSIBLE MECHANISMS • Diffusion • Bulk transport: Airflow Which is more important?

  12. Moisture DynamicsVapor Transport viaAirflow Moisture carried into wall via air flow / leakage through openings. What is the driving force? Source: USDOE

  13. Moisture Dynamics Which mechanism is most important? • Diffusion? • Air transport?

  14. Moisture Dynamics Diffusion vs. air transport While air transport accounts for bulk of moisture migration in most buildings, there are circumstances when diffusion is most important factor

  15. Permeability – Bldg Materials Four classes of vapor retarders have been identified P = Perm * Vapor Barrier

  16. Permeability – Bldg. Materials Building America Best Practices Series: Volume 4 –…..Mixed-Humid Climate Version 1, 9/2005 • Design-p13

  17. PERMABILITY An important note about building materials and water • Permability of many materials changes when they are wet. Ppermeability of wet plywood, for example, changes from 0.75 (dry) to 3.0 when wet

  18. Questions about vapor transmission/dynamics? While vapor migration and condensation can play an important role in moisture problems, most problems are caused by rainwater

  19. Moisture DynamicsLiquid Liquid water can flow via • Gravity (or wind pressure) • Capillary action (wicking) = against gravity

  20. Moisture DynamicsLiquid Gravity “It flows downhill” Source: Builder Magazine

  21. Moisture Dynamics Capillary action Water can flow against gravity when moving in a tight space… …and, by the same process, can wick through porous materials

  22. Moisture Dynamics Practical applications • Water can travel up and behind flashing that is not properly dimensioned • Water can diffuse/wick through masonry, adding humidity to spaces and wetting components (e.g. sill plates)

  23. Moisture DynamicsCapillary action Source: USDOE

  24. Moisture dynamics Some builders, at least, have known about capillary action for a long time… Abbey grange, Great Coxwell, England XXX barn Photo: Barn. Houghton Mifflin, 1992.

  25. Moisture dynamics Barn interior, showing posts set on stone piers This barn, built in mid- 13th century, in use until 1966, when deeded to National trust Photo: Barn. Houghton Mifflin, 1992

  26. Moisture dynamics Note detail on top of pier A sacrificial wood slab has been placed here. Moisture migrating through pier will enter slab instead of end grain of post; slab is easily replaced Photo: Barn. Houghton Mifflin, 1992

  27. Moisture Sources Outline for this section • Plumbing leaks • Rainwater • Groundwater • Humid air (Including embodied water) • Mechanical equipment (Including Combustion equipment) • Occupant practices

  28. Moisture Sources:Plumbing leaks • Plumbing leaks should be obvious but can be in concealed spaces, and may involve supply or drain lines • Sweating may sometimes be significant

  29. Moisture SourcesRainwater Gutters & downspouts • Water discharged next to foundation and /or against building is almost certain to enter • Most basement moisture problems are due to rainwater

  30. Moisture SourcesRainwater Discharge against building may also penetrate- masonry is not waterproof (Consider masonry as a “Hard sponge”) ------------------ Low spots, backslope next to building also cause problems

  31. Moisture Sources:Rainwater and grading

  32. Moisture Sources: Humid air (Summer) Basement ventilation may add moisture / RH Also consider air conditioned interior

  33. Moisture sources:Mechanical equipment Combustion produces a LOT of moisture 2O2 + CH4 = CO2 + 2H2O 1 lb of nat gas 2.25 lb (1.125 Q) water! Gas furnace, blocked flu, condensation soaks brick in & out Further discussion underAirflows

  34. Moisture Measurement Air measurement • Sling psychrometer • Hygrometer (electronic)

  35. Moisture Measurement (cont.) Surface / material measurement • Electronic device (eg Protimeter) Measures moisture content- wood, drywall, masonry

  36. Moisture Content in Building Materials Mold growth can begin… • In lumber @16% moisture content (this represents equilibrium @80%RH) • In gypsum sheathing @1% moisture content Source: Lstiburek, ASHRAE Journal, 2/02

  37. Keep Water Out: Drain the Building

  38. Case study: Things Gone Wrong Photo: Nathan Yost, BSC Photo: Joe Lstiburek

  39. Anatomy of a Disaster

  40. Flashing is Key: e.g., Windows Photo: Mark LaLiberte, Building Knowledge.

  41. Photo: Mark LaLiberte, Building Knowledge.

  42. Reverse Flashing: A Common Mistake Photo: Mark LaLiberte, Building Knowledge

  43. Building Moisture: Take-away Messages • Building components that get wet must be able to dry out quickly. Assemblies must be able to dry! • If they don’t, mold and other organisms will grow, creating health hazard for occupants and ultimately destroying the building A moisture problem is like a fire: it will not get better with time. It cannot be ignored. Respond promptly!

  44. Section II: AIRFLOWS OUTLINE • Air moves according to pressure differences; These can be created by • Temperature differences • Wind • Mechanical equipment

  45. Airflow • For air to move (leak) into or out of building, you need a hole and a pressure difference • Air in must equal air out–same for moisture, but on a different time frame • Pathways can be direct or indirect, natural or mechanical EEBA BFG

  46. Airflows: Driving forces:Temperature Differences Heating Season: Aka THERMOSIPHONING Source: USDOE

  47. Airflows: Driving Forces:Wind Source: USDOE

  48. Airflows: Driving forces:Mechanical Equipment Airflows can also be influenced by • Ventilation fans • Furnaces / boilers • Ductwork (leaking) • Major Appliances (dryer, water heater) All of the above can remove air from the conditioned space. What about makeup air?

  49. Providing Fresh Air Best practice to control moisture, pollutants and to save energy • Build a tight building • Tightly seal ducts (anything but duct tape) • Ventilate by design!

  50. Building Ventilation by Accident Typical ducts can lose 25% of airflow through joint leakage Daylight!

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