A heater on its own in an uninsulated shed mostly warms the garden. Insulation is what lets a bar stay comfortable into autumn and early spring, and it does more for winter evenings than a bigger heater.
Where the heat goes
In a typical timber garden building, heat escapes through the roof, the floor, the walls and every gap around doors and windows. Large glazed areas lose heat fast. Fix the biggest losses first.
Draughts first
Draught-proofing is the cheapest win. Seal gaps around door and window frames, fit brush or rubber seals to doors and make sure the floor meets the walls without daylight showing through.
Roof
Heat rises, so the roof is often the best place to start. Insulation can sit between the roof rafters with a lining board underneath. Leave the ventilation the roof needs and make sure the roof covering itself is sound before you hide it.
Floor
A cold floor makes a room feel cold whatever the air temperature. If the building sits on bearers with a gap underneath, insulation can go between the joists before a floor is laid, or under a new floor layer. Rugs help in the meantime.
Walls
Walls can be insulated between the framing studs and then lined with boards. This also gives you a better surface for fixing shelves and a pub-style finish such as panelling.
Manage moisture
Insulation changes how moisture behaves. Warm, moist air from people and drinks can pass into the wall and condense on the cold cladding behind, where you will not see it. The usual approach is:
- A vapour control layer on the warm, inside face, installed according to the insulation maker's guidance.
- A ventilated gap or breathable membrane on the outer side so any moisture can escape.
- Adequate room ventilation.
Getting this wrong can rot the timber from inside. Follow the insulation maker's instructions, and see damp and ventilation.
Electrics and insulation
Plan cable routes before you close up the walls. Cables surrounded by insulation can need different sizing, which is a decision for the electrician. See power to a garden bar.
Fire and materials
Choose insulation products suitable for the location and follow the maker's guidance on fire performance. Keep heaters clear of linings as their instructions require. See heating.
Is it worth it?
For a bar used mainly in summer, draught-proofing and rugs may be enough. For a bar you want to use in winter, insulating the roof and floor makes the biggest difference for the effort.
Pick a cold evening, stand in the bar with the door shut and feel for draughts with the back of your hand. Seal those first, then decide how far to go.