Start with the Structure, Not the Heater

Every winter, the same thing happens: someone buys a smart little garden office, adds a fan heater, and then wonders why it costs a fortune to keep warm and still feels cold around the ankles. The heater isn't the problem. Heat is escaping faster than you can replace it. A well-insulated garden room of around 10 square metres can be kept comfortable with a 500W panel heater or a small oil-filled radiator, while the same building with thin walls and draughty doors will swallow 2kW and still leave you in a jumper.

The goal is a continuous layer of insulation that wraps the walls, floor and roof without gaps, plus a sealed outer shell that keeps the weather out. Aim for something in the region of 0.20 to 0.25 W/m²K for the walls if you're building from scratch. In practical terms, that means more thickness than most shed kits allow for, and that's fine — this is a building you'll use all year, not a potting shed.

Choose Materials That Can Breathe

Breathable insulation lets moisture vapour pass through rather than trapping it against cold timber, where it turns into damp and rot. In a UK climate, with our damp autumns and driving rain, that matters. Good choices include:

• Sheep's wool — easy to handle, excellent at absorbing and releasing moisture, and forgiving if your fitting isn't perfect. Around 100-150mm in a stud wall works well.
• Wood fibre board — denser and a little pricier, with useful summer overheating benefits, so the room stays cooler in a heatwave.
• Hemp or cork — good breathability, cork particularly good under floors where it also adds a bit of resilience underfoot.
• Mineral wool — cheap and effective, and perfectly breathable, though it needs careful fitting around services and noggins.

Foil-faced rigid boards give the best insulation value per millimetre, and where space is tight they're a sensible choice. Just be aware they act as a vapour barrier: if you use them, seal the joints properly and treat the whole layer as your airtightness line rather than mixing approaches and leaving the structure unable to dry in either direction.

Walls, Floors and Roofs: Where the Heat Actually Goes

In a typical uninsulated garden building, the roof loses the most heat, then the walls, then the floor — but the floor is the one people feel most, because cold air pools at ankle height and never quite leaves.

• Floor: 50-75mm of rigid insulation between battens, or a breathable quilt between joists with a ventilated void below. Never block the airflow under a suspended floor.
• Walls: 100mm of breathable insulation between studs is a solid target. Add a 25mm layer across the inside of the studs to break the cold bridging where the timber runs through.
• Roof: this is where warm air rises and escapes. A warm roof — insulation above the deck, protected by your covering — is the most reliable approach on a garden room. If you're insulating between rafters instead, leave at least 50mm ventilated airspace above the insulation, with vents at eaves and ridge.
• Glazing: double-glazed units with a low-emissivity coating make a bigger difference than most people expect, especially on north-facing walls.

Sealing the Gaps Around Doors and Windows

You can have perfect insulation and still feel cold. Air leaking through a 2mm gap along the bottom of a door will undo a lot of good work, and it's the cheapest thing on this list to fix.

• Run a continuous bead of sealant around the outside of every frame, where it meets the wall.
• Fill the gap between frame and structure with a flexible expanding foam or compressed tape, then trim flush and cover with an internal bead.
• Fit compression or brush seals to the door leaf and frame — brush seals work well on sliding doors, compression seals on hinged ones.
• Check the threshold. An aluminium or rubber threshold plate with a drip bar stops both water and draughts.
• Look at services: cable entry points, extractor ducting and pipework are classic forgotten holes. Seal them with a proper grommet or silicone, not tape.
• Test with the back of your hand on a windy day — you'll find the culprits quickly.

Ventilation and Moisture

A sealed, insulated room full of a breathing human being, a kettle and a laptop will produce moisture. If it can't escape, you get condensation on the inside of the glazing, then mould in the corners behind furniture. Breathable insulation helps, but it isn't a substitute for air changes.

Trickle vents on windows are the simplest solution, and a small humidity-triggered extractor fan is worth fitting if you'll be cooking or drying washing inside. Open a window for ten minutes in the morning during winter — a quick airing loses far less heat than leaving one ajar all day. And if you're insulating a solid concrete base with no damp-proof membrane, add one. Rising moisture will defeat even the best insulation.

A Few Practical Points Before You Start

Insulation works best when it's fitted snugly, with no gaps at edges and no compression that squashes its performance. Wear gloves and a mask with mineral wool, and cut rigid boards slightly oversized so they hold themselves in place. If you're retrofitting an existing cabin, it's usually easier to strip the internal lining, insulate properly and re-board than to add layers on the inside and lose floor space.

Do the work in spring or summer, and by the time the first cold snap arrives you'll have a room that's genuinely usable in January — quiet, warm and cheap to run. That's the difference between a garden room and a garden room you actually use.