Most collectors own a hygrometer. Far fewer own a thermometer, or at least one they consult. The humidity reading is checked weekly, sometimes daily; the temperature of the room is taken for granted. Yet the two are not independent quantities. Relative humidity is, by definition, a statement about temperature as much as about water, and a humidor that is watched for one and ignored for the other is only half supervised.
What "relative" means
Air can hold a certain amount of water vapour, and that amount depends on how warm the air is. Warm air can carry more; cool air can carry less. Relative humidity expresses how much water is actually present as a percentage of that ceiling. A reading of 69 per cent means the air is holding 69 per cent of the vapour it could hold at its current temperature, and no more than that.
The consequence is easy to state and easy to overlook. Take a sealed box at 69 per cent RH and warm it by a few degrees without adding or removing a single drop of water. The ceiling rises, the same quantity of vapour now represents a smaller fraction of it, and the hygrometer falls. Cool the same box and the reverse happens: the ceiling drops, the vapour present becomes a larger share of it, and the reading climbs. Nothing has entered or left. Only the temperature has moved, and the humidity number has moved with it.
This is why a collector can leave a humidor untouched over a weekend and return to find the reading several points from where it was. The packs have not failed and the seal has not opened. The room has simply changed temperature, and the air inside the box has followed.
The daily cycle inside a room
A living room in a Canadian house is not a constant environment. It warms through the afternoon, especially with a west-facing window, and cools overnight once the heating or the sun withdraws. In winter, forced-air heating cycles on and off through the day. In summer, an air conditioner does the same in the opposite direction. Each cycle is a small temperature swing, and each temperature swing is, inside the humidor, a small humidity swing.
A wooden box slows this down but does not stop it. Cedar has thermal mass and the lid is closed, so the interior lags the room by some hours. But it does follow. Over a day the interior might travel through two or three degrees, and over a season through considerably more, and the hygrometer traces every one of those excursions in its own units.
Seen this way, a steady RH reading in a room whose temperature is unsteady is not evidence of stability. It is evidence that something inside the box is working continuously to compensate, and that work has a cost.
What a two-way pack does about it
This is the situation Boveda's two-way humidity control is built for. Each pack holds a saturated salt solution behind a membrane that passes water vapour in both directions. When the room warms and the relative humidity inside the box falls, the pack releases vapour until the air returns to its rated level. When the room cools and the reading rises, the pack draws vapour back in. It does not need to be told which direction to work; the difference between the air and its own equilibrium is the instruction.
A one-way humidifier, such as a block of foam, can do only the first half of that job. Through a cool spell it has no way of taking moisture back, and the box sits above target until the temperature happens to rise again. Two-way control means that both halves of the daily cycle are answered, and the tobacco is spared the swing that the room is trying to impose on it.
The pack is doing more work in an unsteady room than in a steady one. Every temperature excursion asks it to move water one way and then the other, and each of those movements consumes some of its reserve. A pack in a humidor kept in a room that holds a constant temperature will last towards the longer end of its two-to-four-month life. The same pack in a box that sits in afternoon sun will stiffen sooner. That is not a defect. It is a measurement of how much the room was moving.
Why the temperature itself matters
Beyond its effect on the humidity reading, temperature has a direct consequence of its own, and it is the more serious of the two. The tobacco beetle, Lasioderma serricorne, lays its eggs in tobacco leaf, and those eggs are present in a proportion of cigars as they leave the factory. At cool temperatures they remain dormant indefinitely. Sustained warmth is what wakes them.
The threshold most commonly cited is around 24 °C. Above it, hatching becomes progressively more likely, and a single warm week in July can be enough. Once larvae are active they tunnel through wrappers and filler, and the pinholes they leave are usually the first warning. By then the affected cigars are lost and their neighbours are at risk.
No humidity pack can address this. Boveda holds the moisture level where it should be; it has no influence on the temperature of the room, and it would be misleading to suggest otherwise. Humidity control and temperature control are separate disciplines, and a well-kept humidor practises both.
What to aim for
The comfortable band for cigar storage is a room between roughly 18 and 21 °C. Within it, the daily swings are small enough that the packs are not overworked, the humidity reading stays within a point or two of target, and beetle eggs remain asleep. Cooler than 18 °C does little harm. Warmer than 21 °C is tolerable for short periods but should not be the resting state.
In practice this means choosing where the humidor lives with some care. A few principles are worth following.
- Keep the box out of direct sunlight at every hour of the day, not only the hour you happen to check it.
- Avoid exterior walls in winter and the wall shared with a kitchen at any time.
- Do not place a humidor on top of, or beside, any appliance that generates heat, including audio equipment and some lighting.
- Basements are often the most stable rooms in a house, provided they are dry and the humidor is not on the concrete floor.
- If the only available room runs warm in summer, a climate-controlled cabinet solves the temperature problem in a way no passive box can.
Reading both instruments together
Once temperature is understood as half of the equation, the hygrometer becomes easier to interpret. A reading that has drifted two points high on a cool morning is not a signal to remove a pack; it will settle by afternoon. A reading that has drifted two points low in a heatwave is not a reason to add water, and it would be a mistake to do so; the pack will bring it back as the room cools, and the priority is the temperature itself.
The pattern to watch for is a reading that stays away from target once the room has returned to normal. That is the signal that a pack has finished its work and stiffened, or that the seal of the box has begun to fail. A reading that moves with the room and returns with it is simply physics, and the packs are handling it.
A digital hygrometer that also displays temperature, placed inside the box rather than in the lid, gives both numbers at a glance. Checked at the same hour each day, it builds a picture of how the room behaves over a season, and that picture tells you more about the health of the collection than any single reading does.
A practical routine
The routine that follows from all this is short. Choose a location in the coolest, most stable room available and keep the humidor there year-round rather than moving it. Read the temperature alongside the humidity, and treat a sustained reading above 24 °C as the more urgent of any two problems. Use Boveda at the RH your cigars call for, size the packs generously for the volume, and accept that they will need replacing somewhat sooner in a room that moves than in one that does not. And when a reading looks wrong, look at the thermometer before touching anything else.