How do I insulate a garage or outbuilding?

Insulating a garage is straightforward as building work and easy to get wrong as building physics, because a well-insulated but poorly ventilated garage is wetter than an uninsulated one.

Safety

Step by step

1. Decide what the space is for.Occasional workshop, hobby room, home gym, or a habitable room. The last one requires building regulations approval and often planning permission, and the standards are much higher.
2. Check the regulations position.Converting a garage to habitable use is a material change of use requiring building control. Insulating a garage that stays a garage generally does not, but check with your local authority — this is the part people find out about late.
3. Solve the damp before insulating anything.Leaks, rising damp through the slab, and condensation. Insulating over a damp problem hides it and accelerates it, and the insulation itself will be ruined.
4. Keep the ventilation you need.A garage storing a car, fuel or gas cylinders needs its permanent vents. Insulating it into a sealed box is dangerous as well as damp. Ventilation and insulation are not alternatives.
5. Do the roof first.Most heat is lost upwards and a garage roof is usually the coldest surface, so it is where condensation forms. It gives the largest improvement for the effort.
6. Leave a ventilation gap above the insulation.In a pitched roof, at least 50mm between the insulation and the roof covering, with airflow at the eaves. Filling the void completely causes condensation and rot in the timbers.
7. Put the vapour barrier on the warm side.Always facing into the room, between the insulation and the plasterboard, taped at the joints. On the cold side it traps moisture inside the structure — this is the single most important detail.
8. Choose the insulation for the space you have.Rigid PIR board gives the most performance per millimetre and suits a garage where space is tight. Mineral wool is cheaper and needs more depth. Insulated plasterboard combines both in one board.
9. Batten out single-skin walls.A single-skin brick or block garage wall needs a timber frame to hold insulation, with a gap behind it so the wall can still dry. Insulation tight against a single-skin wall gets wet.
10. Insulate the door.A metal up-and-over door is usually the biggest single weak point and the main condensation surface. Insulation board bonded to the inside, plus a brush seal at the bottom, makes a large difference. Check the door still balances afterwards.
11. Consider the floor.A cold slab loses heat and lets moisture up. Insulation over a damp-proof membrane with a floating floor above works, but it raises the floor level and may foul the door.
12. Seal the obvious gaps.Around the door frame, at the wall and roof junction, service penetrations, and under the door. Draught-proofing costs little and is often more effective than more insulation.
13. Sort out the electrics properly.An outbuilding supply is notifiable work under Part P in England and Wales, with equivalent rules under the Building Standards in Scotland and the Building Regulations in Northern Ireland, and needs an electrician. Do not bury cables in insulation without checking derating.
14. Add controlled ventilation for the new use.A workshop or gym generates moisture. A trickle vent, a small extractor or a dehumidifier prevents the condensation an insulated airtight space would otherwise get.
15. Heat it sensibly.Once insulated, a small electric heater on a thermostat works well. Never a paraffin or bottled gas heater in a sealed space — they produce large amounts of water vapour and carbon monoxide.

Tips

• Vapour barrier on the warm side, taped at every joint. Getting this wrong makes the structure wetter than leaving it uninsulated.
• Insulate the door. A metal up-and-over door is usually where most of the heat goes and where the condensation forms.
• Keep a 50mm ventilated gap above roof insulation. Filling the void is how garage roofs rot.

Common mistakes

• Putting the vapour barrier on the cold side — Moisture from the room passes into the insulation and condenses against the barrier, soaking the structure. It must face into the heated space.
• Sealing a garage that stores fuel or a car — Petrol vapour and exhaust products accumulate. The permanent ventilation is a safety requirement, not a draught to be eliminated.

If it doesn't work

Questions people ask