Insulated garden rooms and log cabins

People ask how thick the walls should be. The question underneath it is different: how much of the year do you want to use the building, and how cold are you prepared to be at the edges of it? Answer that and the specification follows.

What you are actually deciding

• Storage and summer use — 28 mm walls, no insulation. A store, a bike shed, a summerhouse you shut up in October.
• A room you use most of the year — 44 mm walls with an insulated floor and roof. The usual garden office or studio.
• Year-round, every day — 70 to 88 mm solid, or twin skin with an insulated cavity, and insulation in the floor, walls and roof.
• Somewhere a person lives — twin skin or glulam. At this point Building Regulations are part of the conversation, not an afterthought.

Each step up costs more to build and less to heat: an insulated log cabin is cheaper to run for as long as it stands. The one that catches people out is the middle: a 44 mm building with no floor insulation is a summer room with a heater in it, not a winter room.

Walls: solid log or twin skin

Solid log walls

Solid timber insulates reasonably well on its own — better than masonry of the same thickness — which is why a 44 mm cabin is comfortable for most of the year without anything added. What it cannot do is hold heat overnight in February.

Twin skin, and when it is worth it

Twin skin is two leaves of timber with an insulated cavity between them. The cavity depth varies with the wall, and the effect does not: it is the difference between heating a room and heating a room that stays warm. If the building is an annexe or a home, this is the answer.

The floor is where the heat actually goes

Almost every complaint about a cold garden building is a floor complaint. The building sits on a base with cold air or cold concrete under it, and heat leaves downwards while everyone stares at the walls.

Insulation between the floor joists, a vapour control layer and a dry base are what fix it, and all three have to be decided before the floor is built. See flooring and insulation for the build-up.

The roof

Warm air rises into it, so an uninsulated roof undoes an insulated wall. It is also the cheapest place to add insulation while the building is being made, and one of the more disruptive to open up afterwards. Insulate it at the start even if the budget is tight elsewhere.

Glazing

Double glazing is standard on everything we make. The variable is how much of the wall is glass: a full-width sliding door is the best thing about a garden room and the weakest part of its envelope. Have the glass — just insulate the rest properly and put the heater where the cold air falls.

Where insulation changes the rules

Insulation does not decide whether you need planning permission — size, position and use do, and those are set out nation by nation on the planning page. What it does affect is Building Regulations, which are a separate question and apply once a building is habitable. We tell you which of the two you are in before anything is ordered.

What it costs

Insulation is one of the cost drivers on the prices page, and it is the one that is hardest to change your mind about later. Walls, floor and roof are all cheaper to specify now than to improve in three years, and the running cost difference lasts as long as the building does.

Tell us how you will use it and how much of the year you want it. We will specify the walls, floor and roof to match and price it with the cabin — or start with the layout if you have not settled one yet.