Applied Structural Drying cheat sheet

Specific humidity (humidity ratio) is the mass of water vapor per mass of dry air, expressed as grains per pound of dry air (7,000 grains = 1 lb); it does not change when air is heated or cooled without condensation.

Relative humidity is the ratio of actual vapor pressure to saturation vapor pressure at the same dry-bulb temperature; heating air lowers RH even though the moisture content stays constant.

Dew point is the temperature to which air must be cooled at constant pressure and humidity ratio to reach saturation; cooling a surface below dew point produces condensation.

Vapor pressure deficit (VPD) is saturation vapor pressure minus actual vapor pressure; higher VPD drives faster evaporation from wet materials.

Equilibrium moisture content (EMC) is the moisture content a material reaches when its vapor pressure matches the surrounding air; drying stops when material EMC equals ambient conditions.

Warm air holds more moisture: saturation vapor pressure roughly doubles for every 20 degrees F (11 degrees C) rise in dry-bulb temperature in the normal drying range.

Evaporation cools the wet surface (evaporative cooling), so a wet bulb reading below dry bulb indicates active evaporation; wet bulb equal to dry bulb means no evaporative drying is occurring.

Drying rate is governed by the difference in vapor pressure between the wet material and the surrounding air, plus airflow across the surface and temperature; all three must be managed together.

Secondary damage from high humidity includes condensation on cool surfaces, swelling of wood and drywall, and microbial growth; dehumidification must control vapor pressure, not just temperature.

Pin-type meters measure electrical resistance between probes; resistance falls as moisture rises, so readings are relative and material-specific, requiring species and material calibration.

Pinless (capacitance/impedance) meters scan a field without penetrating; readings are influenced by density, surface moisture, and thickness, so they are best for mapping, not absolute values.

Thermal imaging identifies temperature anomalies that may indicate evaporative cooling from wet areas; always confirm with a moisture meter before declaring moisture present.

A non-invasive scan followed by pin-meter confirmation at suspected locations is the standard two-step inspection sequence.

Reference (dry standard) readings should be taken in an unaffected area of the same material and orientation to establish a baseline for comparison.

Moisture content percentages differ by material: wood readings are percent moisture content by weight, while drywall and concrete readings are relative or comparative scales.

Penetrating moisture in wall cavities is detected with deep-wall probes or by removing baseboard and testing the bottom plate, which wicks water first.

Document the meter make, model, and mode used, since readings from different instruments or modes are not directly comparable.

Record daily dry-bulb, wet-bulb (or RH), and dew point in both affected and unaffected (reference) areas to prove drying progress.

Document moisture readings at labeled, repeatable points (map with numbered locations) so each day's readings can be compared to the same spots.

A drying log should include date, time, equipment placement, equipment run status, temperature, RH, GPP, and material moisture readings.

Photograph affected materials before, during, and after drying, and photograph meter displays at documented locations.

Equilibrium is demonstrated when material readings return to within a defined tolerance of the dry standard and stay stable across consecutive readings.

Track psychrometric conditions inside the containment versus outside; a negative or flat trend in GPP with rising material dryness indicates the system is working.

Daily documentation supports scope justification, insurance billing, and defensible drying decisions if the project is disputed.

Record equipment runtime hours and any downtime, since dehumidifier performance depends on continuous operation and defrost cycles.

The drying system combines three functions: remove water (extraction), evaporate water (airflow and heat), and remove vapor from the air (dehumidification).

Containment (poly barriers, zippers, sealed doorways) isolates the drying chamber so the dehumidifier treats a smaller, controllable volume.

Calculate dehumidifier capacity from the water load: estimate gallons of water to be removed and required pints per day, then select equipment with margin.

Open drying uses outdoor air exchange when outside conditions have lower vapor pressure than the affected space; closed drying uses dehumidification when outside air is humid.

Air movers should be positioned to sweep the wet surface at an angle, creating a vortex that accelerates evaporation; avoid directing air into corners where it stalls.

Heating the drying chamber raises vapor pressure and speeds evaporation, but it must be paired with dehumidification or moisture redistributes into the structure.

Lay out equipment so air movers do not fight each other and so dehumidifier intake and exhaust are not blocked; airflow should be directional and continuous.

Design to the dry standard: the target is to bring materials to their normal EMC for the local climate, not to zero moisture.

Conventional refrigerant dehumidifiers are most efficient at higher humidity and warmer temperatures; performance (pints/day) is rated at specific temperature and RH conditions, often 80 degrees F and 60 percent RH.

Desiccant dehumidifiers use a rotating wheel and work well at low temperatures and low humidity where refrigerant units lose efficiency.

LGR (low grain refrigerant) dehumidifiers are designed to continue extracting water at lower humidity ratios than standard refrigerant units.

Air mover airflow should be directed along wall and floor surfaces; the goal is turbulent, high-velocity air across wet material, not gentle circulation.

Air filtration (HEPA) is used when the scope includes microbial contamination or when containment requires particle control.

Negative air machines exhaust air from containment to control migration of dust and spores and maintain pressure differential.

Equipment must be sized to the cubic footage of the drying chamber; undersized dehumidification allows humidity to rise and drying to stall.

Defrost cycles and low-temperature operation reduce effective daily capacity; monitor actual water removed rather than relying on nameplate ratings.

Define the scope of drying versus demolition early: materials that cannot be dried to standard (e.g., saturated carpet padding, contaminated materials) are removed.

Set a drying goal and timeline, and communicate daily progress against it; adjust equipment if readings plateau.

Document the moisture map, equipment placement, and daily readings to support billing and to justify equipment days.

Coordinate with the customer, adjuster, and other trades so drying equipment is not shut off or removed prematurely.

Track equipment inventory and days in use; dehumidifier and air mover rental days are a major cost driver.

A plateau in material readings with stable low GPP indicates the drying goal is met; a plateau with high GPP indicates a system problem.

Change orders are needed when hidden moisture, additional affected areas, or extended drying times appear after the initial scope.

Assume asbestos and lead in older buildings until tested; do not disturb suspect materials without assessment.

Use PPE appropriate to the hazard: gloves, eye protection, respirators for microbial or chemical exposure, and slip-resistant footwear on wet floors.

Control electrical safety: use GFCI protection, keep cords out of standing water, and route cords so they are not trip hazards.

Communicate the drying plan to occupants: equipment noise, power use, expected duration, and areas to avoid.

Advise occupants with respiratory conditions, immune compromise, or allergies about mold exposure risk and recommend they consult a physician.

Secure the work area and post signage; keep children and pets out of containment and away from equipment.

Document pre-existing conditions and any customer concerns in writing before work begins to protect both parties.