Insulating a Garden Room or Shed So It’s Comfortable All Year

If you are turning a shed into an office, gym, or somewhere you actually want to spend time, insulation is the thing that makes or breaks it. The same goes for a new garden room. Get this right from the start and the space feels solid, warm and dry. Get it wrong and you will be fighting condensation and damp for years.

Insulation is not just about heat. It controls moisture, protects the timber structure, and keeps running costs sensible.

Why foil matters in timber buildings

If you have looked at modern insulation boards or multi-foil products, you will have noticed the shiny foil surface. It is there for a reason.

Heat does not only travel through materials. It also radiates. Standard insulation like mineral wool or rigid boards slows heat passing through the wall, but it does very little to deal with radiant heat.

Foil reflects radiant heat back where it came from.

In winter, heat inside the room naturally moves towards the colder walls and roof. A foil layer reflects much of that heat back into the room instead of letting it escape. In summer, the process works the other way. The foil reflects heat away from the building, helping to stop the room overheating.

That alone makes a noticeable difference in a garden building.

The more important job foil does

The bigger benefit of foil is moisture control.

Warm air carries moisture. Every breath you take adds water vapour to the air. Boiling a kettle does the same. So does drying clothes or exercising. When that warm, moist air reaches a cold surface inside the wall or roof, it turns back into water.

In a timber building, that water has nowhere good to go.

Wet insulation stops working properly. Timber stays damp. Mould starts to appear. Over time, rot follows.

This is why the foil layer is used as a vapour control layer. It sits on the warm side of the insulation and slows moisture before it reaches the cold parts of the structure.

It is one of the most important layers in the whole build.

Condensation is the real enemy

Many garden rooms fail not because they lack insulation, but because moisture was never properly dealt with.

Two things matter more than anything else.

Ventilation

No insulation system can replace fresh air. Moisture has to escape.

Background ventilation such as trickle vents or soffit vents keeps air moving even when everything is shut. If the room will be used as a gym, kitchenette, or anything that produces steam, an extractor fan is essential.

Even something as simple as opening doors and windows for ten minutes a day helps more than people expect.

A sealed vapour layer

A vapour control layer only works if it is continuous.

It must sit on the inside face of the insulation. Every joint needs taping. Every cut edge needs sealing. Any gaps will allow warm air to slip through and condense where you cannot see it.

This is where most DIY installations fall down.

How a well-built wall is layered

From the outside moving inwards, a typical timber garden room wall looks like this:

• External cladding such as cedar or shiplap
• A ventilated air gap created with battens
• Breather membrane to stop rain getting in while allowing moisture out
• Structural sheathing such as OSB or ply
• Timber studs
• Insulation fitted between the studs
• Vapour control layer on the inside face
• Internal lining such as plasterboard or timber panelling

Floors and roofs follow the same logic. Weather protection outside. Insulation in the middle. Vapour control inside.

Choosing the right insulation

There is no single best option. Each material has its place.

PIR boards

These offer excellent thermal performance for their thickness and usually come foil-faced. They cost more and take time to cut neatly, but they are ideal where space is tight.

Mineral wool

Cheaper and easier to work with. It fills awkward spaces well and improves sound insulation. It needs more thickness and must always be paired with a separate vapour control layer.

Multi-foil insulation

Very thin and flexible. Works best when installed with the correct air gaps. Often used in roofs or as an additional layer to reduce cold spots.

Fitting insulation properly

Preparation matters more than speed.

Make sure cavities are dry and clear. Plan cable runs before you start. Do not crush insulation into place.

Floors

Heat loss through the floor is often underestimated. Fit insulation snugly between joists and support it properly. Use the thickest layer you can reasonably install.

Walls

Cut boards or wool slightly oversized so they hold themselves in place. Gaps reduce performance. Compression does too.

Roof

This is where most heat is lost. Insulate between rafters and always leave a ventilation gap above the insulation so moisture can escape. Without that airflow, roof timbers are at risk.

Installing the vapour control layer

This is the stage that deserves patience.

Fix the vapour control layer across the inside face of studs and rafters. Staple it neatly. Then tape every joint. Corners, overlaps, edges and penetrations all need sealing.

If you imagine trying to keep air inside a balloon, you will not be far off.

Many people add a thin batten layer over the vapour barrier before the final lining. This creates space for cables and avoids puncturing the barrier repeatedly.

Common questions

Is insulating a shed different to insulating a garden room?

The principles are the same. The standard is not. A garden office needs deeper insulation and proper vapour control. A basic shed may only need enough insulation to reduce damp and temperature swings.

Can insulation go on the outside instead?

Yes. External insulation is very effective and reduces heat loss through studs. It is more complex and usually means new external cladding or render.

Does bubble wrap insulation work?

Packaging bubble wrap is not suitable for long-term use. It offers very little real insulation compared to proper building products.

How thick should insulation be?

For year-round use, a sensible guide is:

• Floor: 75 to 100mm PIR
• Walls: 50 to 75mm PIR or around 100mm mineral wool
• Roof: 75 to 100mm PIR

Thicker is usually better until space becomes an issue.

What is a thermal bridge?

It is an area where heat escapes more easily than the surrounding insulation. Timber studs are a common example. Adding a continuous insulation layer over the studs helps reduce this.

Final thoughts

A warm garden room is nice. A dry one is essential.

Good insulation works as a system. Heat control, moisture control and ventilation all matter equally. Take your time, fit everything carefully, and be thorough with sealing the vapour layer.

Do that properly and the space will stay comfortable, healthy and usable whatever the weather decides to do next.