Cooling and airing a guest room

A guest room is not a cooling problem with a seasonal twist. It is a room that has been sealed for eleven months, and everything people complain about in one — the smell, the wet window, the airless feeling at 3 a.m. — comes from that single fact.

Doing these in the wrong order is why people buy an air conditioner for a room whose actual problem is that no air has moved through it since last Christmas.

1. Air exchange, 24 to 48 hours before they arrive. Replace the air in the room several times over. A window fan on exhaust with the door ajar does this properly; an open window on a still day barely does it at all.
2. Measure the humidity, in that room. The EPA target is “below 60 percent (ideally between 30 and 50 percent) ”. A reading from the heated hallway tells you nothing — a closed unheated room routinely reads twenty points higher.
3. Dry it if it is above the band, for a day or three, then remove the machine before the guest arrives.
4. Heat or cool it last, and only to the extent the season demands. In winter the room needs warming, and warming it is also what lifts its surfaces above the dew point.
5. Leave one quiet fan with a published noise figure, for the guest to use or not.

Stale-room smell is produced by things that need moisture to work: mold and bacteria in soft furnishings, in the mattress and in the carpet underlay. The EPA sets the condition plainly — keep relative humidity in the 30 to 50 percent band, and “if wet or damp materials or areas are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow”.

A closed spare room fails that test continuously rather than once. It has no ventilation to carry moisture away, its surfaces are the coldest in the house, and moisture from the occupied rooms migrates in under the door and condenses there. The EPA again: “When warm, humid air contacts a cold surface, condensation may form.”

Which is why airing and drying fix the smell and air freshener does not. One addresses the mechanism; the other addresses your nose.

The whole seasonal routine — a few days of drying and a few nights of a 12-watt fan — costs less than a dollar. The expensive item on that list is the only one most people buy first.

The holiday case has its own shape, and the mistake is predictable: turning the heating on in the spare room the night before people arrive.

Warming cold, damp surfaces quickly drives moisture out of them into the room air, which then condenses on the window as soon as the temperature drops overnight. The room is warm, the glass is streaming and the bed feels damp — and all three happen because the heat arrived without any ventilation or drying beforehand.

The order that works: ventilate, dry, then heat, starting two or three days out. Each step makes the next one cheaper. Whether the room needs a dehumidifier at all turns on the reading rather than the season.

Spare rooms are frequently the worst room in the house in summer, because they are the ones nobody optimized: the loft conversion, the room over the garage, the one facing west.

• Size the cooling for the room, not the guest. 5,000 BTU covers 100 to 150 sq ft on the published table, which is most spare rooms. Oversizing produces a cold clammy room.
• Check which of the awkward-room problems you actually have before buying, because the fix differs: a finished attic, a room over the garage and a west-facing room are three different diagnoses.
• A fan is a better guest-room purchase than cooling for most households, because it works in both seasons and costs cents a night.