Cooling a garden room or backyard office

Garden offices, studios, she-sheds and converted outbuildings all share a set of problems that no room inside the house has, and they are worth naming before choosing equipment.

A bedroom has one or two exterior walls. The rest of it is bounded by other rooms at roughly the same temperature, which do most of the insulating for free.

A detached garden room has four walls, a roof and a floor, all facing the weather. Every one of them is a route for heat to arrive, and the roof is the largest and the most exposed because there is no attic above it to absorb and delay the gain.

Two consequences follow:

• It heats up much faster than its floor area suggests. The standard square-footage sizing rule understates what a fully exposed building needs — the adjustments for exposure.
• It also cools down fast once the sun is off it. Low mass works both ways, which makes early-morning and evening ventilation unusually effective.

Garden rooms are bought for the view, which means a large glazed elevation, usually facing the garden rather than facing north.

Solar gain through glass is the single largest heat input in most buildings, and a small room with a large south or west-facing glazed wall is close to a worst case. On a summer afternoon that glass is delivering more heat than a small air conditioner can remove.

The order to address it in:

1. External shading — an awning, a brise soleil, a pergola or a well-placed tree. Stopping the sun before it reaches the glass is several times more effective than stopping it after — shading and film compared.
2. Solar-control film on the glass itself, which helps and does not stop the heat arriving at the glass.
3. Internal blinds, which are the least effective option because the energy is already inside by then — but they are cheap, immediate and better than nothing.

This is the same problem a sunroom has, and the same one a west-facing room has, in a smaller box.

This is the constraint people discover last and should discover first.

Outbuildings are usually fed by a single submain from the house consumer unit, sized for lighting, a few sockets and perhaps a small heater. It is often considerably less capacity than a room inside the house has.

A standard 15 A circuit should not carry a continuous load above about 12 A. A portable air conditioner at 11.6 A effectively takes the whole circuit, before a computer, two monitors, a printer and a kettle.

If you do not know what feeds your building, find out before buying anything — and if the answer is a long run of undersized cable, a low-power approach is not a compromise, it is the only safe option.

A garden office has an advantage over a bedroom: nobody is in it for sixteen hours a day, and you control exactly when it is occupied.

• Open it up first thing. Purging overnight-cool air through the building before the sun gets on it starts the day several degrees lower, for free — the night purge principle.
• Pre-cool before you go out there, using a timer or an app. Fifteen minutes of compressor before you sit down beats an hour of it while you work.
• Close it up once the outside is hotter than the inside. The rule is the same as for a house and it is easier to follow in a building you leave.
• Leave it ventilated when empty in humid weather, or it will smell of damp by Monday. A sealed, unheated, unoccupied timber building is a good place for condensation — a small dehumidifier on a humidistat solves that better than leaving a window ajar.

In a room this small, internal gains stop being negligible. A desktop computer under load, two monitors, a laser printer and a person add up to a continuous load in a volume that has nowhere to put it.

In a house that heat disperses into the rest of the building. In a detached 100-square-foot room it stays where it is. This is the same problem, at a smaller scale, as cooling a server closet, and the same fixes help — get the equipment out of a closed cabinet, and move air across it.