BUILDING DYNAMICS:Moisture, Airflows and Construction TechnologyITEP Level 2 WX Training Joseph T. Ponessa, Ph.D. Professor Emeritus Housing, Indoor Environments and Health
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
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
Overview of Today’s Presentation Building Science • Moisture dynamics; applications in buildings • Airflow mechanisms; applications in buildings
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
Moisture Dynamics Outline (cont’d) Basic moisture science (cont.) • Water movement • Gravity; wind • Capillary action • Moisture sources • Moisture Measurement • Air • Surfaces / solids
Moisture Dynamics Water can exist in three states • Vapor • Liquid • Solid
Moisture DynamicsVapor • Vapor-Temperature relationships (At saturation) The amount of moisture that air can hold increases directly with temperature.
Relative humidity Because air’s moisture-holding capacity increases with temperature… …for a given amount of moisture in air RH as Temp …and vice versa Moisture DynamicsVapor (cont.) RH is the amount of moisture In the air compared to the total Moisture capacity of air At a given temperature. Expressed as %.
Moisture DynamicsVapor • Dew point: Saturation Air that is cooled to the limit of its moisture carrying capacity releases the vapor as droplets: condensation (or rain) This is the Dew point temperature
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
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?
Moisture DynamicsVapor Transport viaAirflow Moisture carried into wall via air flow / leakage through openings. What is the driving force? Source: USDOE
Moisture DynamicsVapor Transport viaDiffusion Moisture transport by diffusion: Molecules penetrate through drywall. What is the driving force? Source: USDOE
Moisture Dynamics Which mechanism is most important? • Diffusion? • Air transport?
Moisture DynamicsVapor Transfer – cold climate Adapted from Lstiburek 2001 p 290
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
Permeability- Bldg Materials Four classes of vapor retarders have been identified P = Perm * Vapor Barrier
Permeability- Bldg. Materials Building America Best Practices Series: Volume 4 –…..Mixed-Humid Climate Version 1, 9/2005 • Design-p13
Vapor Barriers and Retarders • Class I vapor retarder = 0.1 perm or less • Class II vapor retarder = 0.1 perm to less than 1.0 perm • Class III vapor retarder = 1.0 perm to 10 perm --------------------------------------------------- A class I retarder is a vapor barrier. A class II retarder is a vapor retarder per IBC Source: BFG MH p108
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
Questions about vapor transmission/dynamics? While vapor migration and condensation can play an important role in moisture problems, most problems are caused by rainwater
Moisture DynamicsLiquid Liquid water can flow via • Gravity (or wind pressure) • Capillary action (wicking) = against gravity
Moisture DynamicsLiquid Gravity “It flows downhill” Source: Builder Magazine
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
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)
Moisture DynamicsCapillary action Source: USDOE
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.
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
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
“Capillary break” can interrupt capillary flow of moisture Use capillary break wherever one porous component (eg., wood) meets another (eg., masonry) (Examples later) Examples of capillary breaks Space; drip edge Closed cell sill sealer Gravel bed beneath slab Poly, other membranes Sprayed sealants Moisture Dynamics
Moisture Sources Outline for this section • Plumbing leaks • Rainwater • Groundwater • Humid air (Including embodied water) • Mechanical equipment (Including Combustion equipment) • Occupant practices
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
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
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
Moisture Sources: Humid air (Summer) Basement ventilation may add moisture / RH Also consider air conditioned interior
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 under Airflows
Moisture Sources Embodied moisture – New construction; Several hundred pounds of moisture in concrete, lumber, drywall compound, paint, etc. New building may exhibit moisture problems for months after construction Cold weather construction –Salamander(100 k BTU/h) produces about 1 gal combustion water per hour Occupant practices -Add moisture, too
Moisture Measurement Air measurement • Sling psychrometer • Hygrometer (electronic)
Moisture Measurement (cont.) Surface / material measurement • Electronic device (eg Protimeter) Measures moisture content- wood, drywall, masonry
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
Keep Water Out: Drain the Building EEBA WMG
Case study: Things Gone Wrong Photo: Nathan Yost, BSC Photo: Joe Lstiburek
Flashing is Key: e.g., Windows Photo: Mark LaLiberte, Building Knowledge.