The Physics of Relative Humidity
Relative humidity (RH) is the ratio of actual moisture in the air to the maximum moisture the air can hold at a given temperature. The critical relationship for a cigar room: air's capacity to hold moisture increases with temperature. When temperature changes, RH changes even if no moisture is added or removed.
If room warms by 1°F, RH drops approximately 2–3%
Example: Room maintained at 68°F / 68% RH
Temperature drops to 63°F (5°F drop):
RH rises to approximately 78–83% — above mold risk threshold
Temperature rises to 73°F (5°F increase):
RH drops to approximately 58–63% — below optimal
Conclusion: Temperature stability IS humidity stability.
Without stable temperature, humidity cannot be stable.
Humidity Load Calculation: Full Method
The humidity load is the amount of water vapor your humidification system must add per unit time to maintain target RH against all losses. Loss sources include: infiltration (air leaking in through gaps), ventilation (intentional air exchange), moisture absorption by materials, and door openings.
Step 1: Calculate Infiltration Loss
ACH_inf = Infiltration air change rate estimate:
Well-sealed room (new construction quality): 0.1–0.2 ACH
Average sealed room (good weatherstripping): 0.3–0.5 ACH
Poorly sealed room: 0.5–1.0 ACH
Humidity ratio at 70°F, 68% RH (indoor target): 0.0104 lb/lb
Humidity ratio at 70°F, 35% RH (winter outdoor): 0.0053 lb/lb
Difference: 0.0051 lb water/lb dry air
Air mass per hour = V × ACH_inf × 0.075 lb/cu ft (air density)
Example: 960 cu ft × 0.4 ACH × 0.075 = 28.8 lb dry air/hr
Moisture loss/hr = 28.8 × 0.0051 = 0.147 lb/hr = 0.017 gal/hr
Daily loss from infiltration = 0.017 × 24 = 0.41 gal/day
Step 2: Calculate Ventilation Loss (Cigar Lounge Only)
This step applies when ventilation is running during smoking sessions. A walk-in humidor without active ventilation skips this step.
Session duration: 2 hours
Air exhausted during session = 400 CFM × 120 min = 48,000 cu ft
Air mass = 48,000 × 0.075 = 3,600 lb
Moisture = 3,600 × 0.0051 = 18.4 lb = 2.2 gal per session
Daily equivalent (two sessions): 4.4 gal/day additional demand
This is why active lounge spaces need substantially larger humidification
capacity than storage-only humidors.
Step 3: Calculate Door Opening Loss
Example: 960 cu ft room, 20% exchange per opening
Air exchanged = 960 × 0.20 = 192 cu ft per opening
Air mass = 192 × 0.075 = 14.4 lb dry air
Moisture loss = 14.4 × 0.0051 = 0.073 lb = 0.009 gal per opening
At 5 openings per day:
Daily door loss = 5 × 0.009 = 0.044 gal/day
(relatively minor compared to infiltration losses in a well-sealed room)
Step 4: Total Humidification Demand
Infiltration loss: 0.41 gal/day
Door opening loss: 0.04 gal/day
Cedar absorption: 0.10–0.20 gal/day (first 3–6 months)
Safety margin (20%): add 20%
Total: approx. 0.66 gal/day → Specify: 1.5 gal/day capacity minimum
Active cigar lounge (two 2-hour sessions daily, ventilated):
Infiltration loss: 0.41 gal/day
Ventilation loss: 4.40 gal/day (dominant term)
Door opening loss: 0.10 gal/day
Safety margin (20%): add 20%
Total: approx. 5.89 gal/day → Specify: 7+ gal/day capacity
Humidifier Types and Capacity
| Humidifier Type | Typical Output | Water Source | Best Application |
|---|---|---|---|
| Ultrasonic (small reservoir) | 0.5–2 gal/day | Reservoir (manual fill) | Small humidor, low infiltration rate |
| Evaporative console | 1–4 gal/day | Reservoir or auto-fill | Medium humidor or lounge with light use |
| Large whole-room evaporative | 3–8 gal/day | Direct water line | Active cigar lounge, walk-in humidor in dry climate |
| Ultrasonic industrial | 5–12 gal/day | Direct water line required | High-use active lounge; commercial installation |
Dehumidification: Managing Summer Humidity Spikes
In humid climates — particularly the southeastern US and Midwest in summer — outdoor RH regularly exceeds 80–90%. Door openings, infiltration, and any ventilation running during these periods can drive indoor RH above the 70% threshold where mold risk begins. Mold risk on cigars begins above 72% RH sustained for more than a few days. Tobacco beetles become active above 72% RH AND above 72°F simultaneously.
Indoor target: 68°F / 68% RH
Humidity ratio of outdoor air at 90°F / 75% RH: 0.0227 lb/lb
Humidity ratio of indoor target 68°F / 68% RH: 0.0104 lb/lb
Incoming air carries MORE moisture than the indoor target.
In this condition, the humidifier is off and the dehumidifier is running.
Both conditions occur seasonally — specify equipment that handles both directions.
Control Strategy: Humidistat Placement and Calibration
The humidistat — the control sensor that tells your humidification system when to run — must be placed correctly or the entire control system gives you misleading readings.
- Do not place the humidistat directly above the humidifier output. The localized humidity plume will cause the controller to cut out prematurely before the full room reaches target.
- Do not place the humidistat on an exterior wall where temperature stratification may cause anomalous readings.
- Ideal placement: Center of the room at shelf height (24–36 inches from floor) — the conditions at the level where cigars are actually stored.
- Calibrate against a reference hygrometer verified with a Boveda calibration kit. If the humidistat sensor reads 2% high, the room will be maintained 2% below target — a meaningful variance for long-term storage.
For critical applications, use two independent calibrated hygrometers at different positions in the room and average the readings when assessing condition. A single sensor at one location never tells the full story of conditions throughout a space.