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Garden room insulation: a buyer's guide
Insulation decides whether you use your garden room all year or only in spring. It's also the part of the specification that's hardest to check once the room is built, so it's worth understanding before you buy.
The short answer: insulate all six sides (floor, walls and roof), choose double or triple glazing, and make sure there's a vapour barrier on the warm side and ventilation behind the cladding. Get each company to state the insulation type and thickness for the floor, walls and roof in writing.
Why it matters so much
A garden room has a lot of outside surface for its size. Every wall, the roof and the floor all face the weather, unlike a room in your house that shares walls with warm rooms. That's why a poorly insulated garden room feels cold in winter even with a heater on, and like a greenhouse in summer.
Where to insulate
- Roof. Warm air rises, so the roof loses the most heat. It's also where summer heat comes in.
- Floor. Often skimped. A cold floor makes the whole room feel cold, especially on a base of ground screws with air underneath.
- Walls. The biggest area, so thickness matters.
- Glazing. Big glass doors look great but lose far more heat than walls. Ask for double glazing at least, and consider a smaller glazed area on the side that gets the afternoon sun.
Common insulation materials
| Material | What to know |
|---|---|
| PIR boards (rigid foam) | The most common choice. Thin for how well they insulate, so they suit slim walls. |
| Mineral or glass wool | Cheaper and good for sound, but needs more thickness for the same warmth. |
| SIPs (structural insulated panels) | Insulation and structure in one panel, so fast to build and very airtight. |
Insulation materials themselves are inexpensive. For a typical room they cost a few hundred to around £1,500, and labour is usually included in the builder's price.1 That makes it a poor place for a company to cut corners, and a fair thing to ask about.
Stopping condensation
Condensation is one of the most common complaints about garden rooms. Warm, moist air from breathing, kettles and heaters hits a cold surface and turns to water, which leads to mould and damp. Two things prevent it:
- A vapour barrier on the warm, inside face of the insulation, so moist air can't reach the cold parts of the wall.
- A ventilated gap behind the outside cladding, so any moisture that does get in can dry out.
Ask each company how they handle both. A clear answer is a good sign.
Keeping it cool in summer
Overheating is the other common complaint, especially in south-facing rooms with large glass doors. Roof insulation, an overhanging roof or canopy over the glazing, opening windows on two sides for a through-draught, and blinds all help. Some owners add an air conditioning unit that also heats in winter.
Questions to ask every company
- What insulation goes in the floor, walls and roof, and how thick is each?
- Is the glazing double or triple, and what are the doors made of?
- How do you prevent condensation?
- What heating do you recommend for this size of room?
Questions people ask
Can you use a garden room in winter?
Yes, if it's properly insulated on all six sides and has a heater suited to its size. Rooms that feel cold are usually under-insulated in the floor or roof, or have a lot of glazing.
How do I stop condensation in my garden room?
Make sure the walls have a vapour barrier on the inside face of the insulation and a ventilated gap behind the cladding, keep it gently heated in winter, and ventilate it regularly.
Related guides
Sources
- MyBuilder, Garden room cost price guide, updated 21 October 2026. UK price ranges for garden rooms, foundations, electrics, plumbing and fees.