If you want your garden room to be comfortable year-round, insulation and vapour control are essential. A DIY build is a cost-effective way to extend your living space, but without proper detailing, the room can quickly become too cold in winter and uncomfortably hot in summer.
I’ve insulated countless garden buildings, and there are several effective ways to achieve a warm, usable garden room depending on how the space will be used. In this article, I’ll show you how to insulate a garden room for both thermal efficiency and acoustic performance, and how to install vapour control to prevent condensation, mould and long-term damage.
Starting from the ground up, here’s how to insulate your garden room to a professional standard.
Table of Contents
- Floor Insulation and Vapour Control: Best Practice
- How to Insulate a Timber-Framed (Suspended) Floor Base
- Solid Concrete Slab Base (Insulation Above the Slab)
- Wall Insulation and Vapour Control: Best Practice
- Typical Timber-Framed Wall Build-Up
- Installing Wall Insulation Materials
- Roof Insulation and Vapour Control: Best Practice
- Typical Garden Room Roof Build-Up
- Cold and Warm Roof Construction
- Cold Roof Construction
- Materials
- Step 1. Cutting and Fitting PIR Insulation
- Step 2. Installing a Vapour Barrier
- Step 3. Covering with plasterboard, or the finishing of your choice.
- Warm Roof Construction
- Step 1. Create a Vapour Barrier
- Step 2. Laying the PIR Insulation
- Step 3. Installing a layer of OSB
- Step 3. Install the EPDM Rubber Waterproof Roofing Cover
- Roof Lights and Penetrations
Floor Insulation and Vapour Control: Best Practice
The two most common base types I use are a raised timber-framed base or a solid concrete slab.
Let me start with the timber-framed base first…
How to Insulate a Timber-Framed (Suspended) Floor Base
This is the most commonly used base in our garden room builds, due to the ease of construction and cost efficiency. It’s a raised timber frame suspended on piers, concrete piles or ground screws.
Typical Insulation Method
TOP
- Structural floor deck (e.g. moisture-resistant T&G flooring) – Egger Protect, etc.
- Vapour control layer (warm side)
- Insulation fitted tightly between joists – 100mm PIR ideally
- Floor joists – Timber frame (100x47mm treated construction timber in this example)
- External ground protection or membrane where required – Weed protection (not shown)
- Ventilated air gap beneath the building – Raised piers (see here for options)
BOTTOM
Insulating a Timber Floor
A timber-framed base will always be raised from the floor to prevent rising moisture and allow air ventilation. With the timber frame construction complete, we can focus on the steps required to insulate the floor.
Materials
I always use at least 90mm PIR insulation boards for the floor. Manufacturers such as Kingspan, Celotex, EcoTherm and Recticel all provide roughly the same level of insulation. (A U-Value of 0.022 W/mK).
My Recommendations
- 100mm x 2400mm x 1200mm Celotex PIR Insulation Board GA4000
- 22mm x 2400mm x 600mm Egger Protect Tongue and Groove Chipboard Flooring
- Polythene Vapour Control Layer 500 Gauge – 125MU Green Tint 2.7m x 50m Roll
- Vapour Barrier Tape 25m x 75mm
- Soudal 750ml Champagne Gap Filler Expanding Foam
Step 1. Cutting and Fitting the PIR Insulation Boards
Using a standard panel saw, we can measure the gaps in the timber frame and cut the boards to size. Accuracy is important here. You’re aiming for a tight, snug fit. A bit of 4×2″ timber will help prevent the saw blade from meandering and keep the cut to 90 degrees.
To prevent the insulation from falling out of the bottom, we can install small timber supports.
Step 2. Creating an Air-Tight Seal
Using the cutting technique above should give you a nice, clean cut, but if there are any gaps, you can use expanding foam to fill them before continuing.
Once dry, the excess expanding foam can be cut off, and foil sealing tape can be used to achieve an airtight finish.
Step 3. Vapour Control Over the Insulation and Timber Frame
A vapour control layer (VCL) must be installed on the warm side of the insulation, directly beneath the structural floor deck. This prevents warm, moisture-laden air from entering the floor structure and condensing within the insulation or on cold surfaces.
Step 4. Installing the Moisture Resistant Floor Boards
For this, we tend to use Egger Protect 22mm tongue and groove boards. They’re super strong and add another layer of moisture protection. They are easy to install by inserting 40mm screws into the frame. A quick-drying glue can be used to seal all the boards together between the grooves.
You can now build your timber-framed walls on top of the Egger Protect or OSB floorboards.
Solid Concrete Slab Base (Insulation Above the Slab)
If you’re constructing your garden room with a solid concrete base, the approach to insulation normally comes after the walls and roof have been constructed. The height of your garden room walls will need to take into consideration the depth of the insulation and flooring, which can take up about 135mm. This depth consists of the PIR insulation boards, a structural floor deck and a finishing flooring such as laminate flooring.
In our garden room builds, insulation is always installed above the concrete slab, creating a warm floor structure without needing insulation beneath the slab itself.
Top
- Finished floor covering (e.g. laminate flooring including underlay)
- Structural floor deck or floating floor system
- Rigid floor insulation boards (PIR)
- Damp proof membrane (DPM)
- Concrete slab
Bottom
Materials
Again, I always use at least 90mm PIR insulation boards for the floor.
My Recommendations
- 100mm x 2400mm x 1200mm Celotex PIR Insulation Board GA4000
- 22mm x 2400mm x 600mm Egger Protect Tongue and Groove Chipboard Flooring
- Damp Proof Membrane – All sizes
- Foil Vapour Barrier Tape 25m x 75mm
- Everbuild 30MINPU3 LumberIrwin 30min Polyure Wood Adhesive
- Soudal 750ml Champagne Gap Filler Expanding Foam
Step 1. Vapour Control on a Concrete Base
With the majority of the garden room structure complete, the floor insulation can be installed. This typically occurs when the walls and roof have been erected and the internal wall finishing has been completed, such as the fitting and plastering of plasterboard.
Now, a vapour barrier can be laid on the concrete floor, with enough overlap around the edges to cover the insulation. This does not need to be fixed down, but care should be taken not to rip or tear the membrane.
Moisture management is particularly important with concrete, as slabs can retain and release moisture over time.
Step 2. Installing the Insulation Slabs
The insulation boards should be laid tightly together with staggered joints to minimise thermal bridging. Any gaps should be avoided, as cold spots in the floor can lead to discomfort and condensation issues.
Expanding foam and foil tape should be used to seal any gaps.
Step 3. Creating a Solid Floor
With the insulation installed and sealed, we can now fit moisture-proof 22mm chipboard flooring. Ensure you leave a 5mm expansion gap around all edges and glue all joining boards with a fast-drying wood glue.
This now leaves the floor in a good place to install and finish flooring. Options such as laminate flooring are a good option
Wall Insulation and Vapour Control: Best Practice
The walls of a garden room make up the largest surface area of the building envelope, which means they play a major role in both heat retention and moisture control. Our garden rooms are built using a timber stud wall system, and while there are variations in materials and thickness, the core principles of insulation and vapour control remain the same.
Typical Timber-Framed Wall Build-Up
A best-practice garden room wall build-up (from outside to inside) typically consists of:
- External cladding
- Ventilated cavity (if required by the cladding system)
- Breathable membrane
- Structural sheathing (OSB or plywood)
- Timber stud frame
- Insulation between studs
- Vapour control layer (warm side)
- Internal lining (plasterboard, plywood, or panelled finish)
Each layer has a specific role. Changing the order or omitting components can significantly affect performance.
Materials
Here’s a list of materials we use in our garden rooms.
My Recommendations:
- 50mm x 2400mm x 1200mm Celotex PIR Insulation Board GA4000
- 100mm x 2400mm x 1200mm Celotex PIR Insulation Board GA4000
- Celotex PIR Thermal Laminated Insulation Board 2400mm x 1200mm x 25mm (37.5mm Overall) – Optional
- Rockwool RWA45 50mm Slab 1200mm x 600mm – (6.48m2/Pack) – Optional
- Polythene Vapour Control Layer 500 Gauge – 125MU Green Tint 2.7m x 50m Roll
- Tyvek Housewrap Breather Membrane 1.4m x 100m
- Soudal 750ml Champagne Gap Filler Expanding Foam
Installing Wall Insulation Materials
If you’ve been following my other garden room construction guides, then you’ll have constructed a wall that looks similar to this. A timber-framed wall made with 4×2″ treated timber, backed with 9mm OSB boards to give it rigidity.
This is the starting point for installing the insulation. The timber frame has now been backed with 9mm OSB boards to increase rigidity.
It is best practice to complete the roof construction before insulating the walls. I have not drawn the roof in these diagrams to ensure a clear demonstration of the process.
Step 1. Fitting Insulation Between the Timbers
Rigid insulation boards such as PIR or phenolic insulation are commonly used between wall studs due to their excellent thermal performance. For additional acoustic insulation, I like to add a layer of wool insulation, such as Rockwool RWA45. You have the option to use one or both.
PIR boards should be cut accurately using a panel saw and fitted tightly between studs. Any small gaps should be sealed to prevent air movement, which can dramatically reduce insulation effectiveness. 90mm PIR insulation is ideal for the 4×2″ timbers in this example.
Mineral Wool Insulation
If acoustic insulation is required, reduce the PIR to 50mm and compact 50mm Rockwool RWA45 into the same opening. Rockwool can be cut using a utility knife. It’s available in 1200×600 slabs.
When constructing your timber-framed walls, noggins should be installed at 1200mm heights to allow for easy installation of the insulation.
Step 2. Vapour Control Layer
A vapour control layer (VCL) is essential on the warm side of the wall insulation. Its purpose is to limit the movement of warm, moisture-laden air from inside the garden room into the wall structure, where it could condense on colder surfaces.
The vapour control layer should be installed continuously across the inside face of the wall, with all joints taped and sealed. Particular attention should be paid to electrical boxes, windows, doors and service openings. The layer can be carefully stapled to the timber frame.
Step 3. Breathability
While vapour control is critical on the inside, the outer wall needs to be breathable. A breathable membrane behind the external cladding allows moisture within the wall to escape outward while preventing wind-driven rain from entering the structure.
The breathable membrane can be stapled onto the OSB boards in two sections. Starting with the lower half and then overlapping the top half to ensure good coverage around the whole building, excluding windows and doors.
Step 4. Plasterboard & Thermal Bridging
The final stage of the wall insulation is the internal finishing.
We approach this with a 12.5mm plasterboard, screwed to the studs with plasterboard screws. This can then be finished with a skim of plaster and painted to your desired colour.
Other options may include lining the walls with plywood, OSB or an internal cladding.
Reducing Thermal Bridging
Thermal bridging occurs where heat escapes through structural elements such as timber studs. To reduce this, some of our garden room builds include a continuous insulation layer on the inside of the studs.
Using insulated plasterboard can reduce this and improve the overall wall performance, particularly in high-spec builds.
We recommend using a 37.5mm insulated plasterboard like the Celotex PL4025 Insulation Plasterboard. It includes a 25mm layer of PIR insulation built into the plasterboard for easy installation. You will lose some internal space, and the cost will increase significantly, but it’s a great product. A single board costs around £40 (37.5x2400x1200mm), compared to standard plasterboard, which costs about £12.
This completes the garden room wall insulation guide.
Roof Insulation and Vapour Control: Best Practice
The roof is one of the most critical areas of a garden room when it comes to heat loss and condensation risk. Warm air naturally rises, and without adequate insulation and vapour control, a significant amount of heat can be lost through the roof.
In this section, I’m going to be talking about a flat roof (or a shallow-pitched roof), constructed with timber joists or rafters.
Typical Garden Room Roof Build-Up
A best-practice roof build-up (from outside to inside) generally consists of:
- Roof covering (EPDM, felt, GRP, or similar)
- Roof deck (OSB or plywood)
- Timber joists or rafters
- Insulation between and/or below joists
- Vapour control layer (warm side)
- Internal ceiling lining (plasterboard, plywood, or panelled finish)
The exact configuration will depend on whether the roof is designed as a cold roof or a warm roof, though warm roof constructions are generally preferred for garden rooms.
To help you decide which roof type is best for your garden room, visit my article: Warm Roof vs. Cold Roof: Which is Best for Your Garden Room?
Cold and Warm Roof Construction
Cold Roof Construction
Materials
Here’s a list of materials we use in our garden rooms.
My Recommendations:
- 100mm x 2400mm x 1200mm Celotex PIR Insulation Board GA4000
- Polythene Vapour Control Layer 500 Gauge – 125MU Green Tint 2.7m x 50m Roll
- Celotex PIR Thermal Laminated Insulation Board 2400mm x 1200mm x 25mm (37.5mm Overall) – Optional
Step 1. Cutting and Fitting PIR Insulation
We’re going to follow a similar process to the floor when fitting the PIR insulation into the roof. With the rafters in place and the roof covering complete, we can continue with cutting the 100mm PIR board to fit into the rafter spaces.
Ensure a tight fit with each piece, and don’t push the insulation all the way into the gap. Leave enough space above the insulation for ventilation. This example features 150x47mm timbers and 100mm PIR insulation, with a 50mm ventilation gap remaining.
PIR boards should be tightly fitted, with joints sealed where possible to minimise air gaps and thermal bridging.
Step 2. Installing a Vapour Barrier
A vapour barrier can now be added to the roof.
The vapour control barrier can be stapled to the rafters and should cover the whole roof.
You’ll need to think about light fittings, but I talk about this in the next section.
Step 3. Covering with plasterboard, or the finishing of your choice.
You can now secure the plasterboard to the rafters to finish the cold roof construction.
Again, you can reduce thermal bridging by installing insulated plasterboards like the Celotex PL4025.
This completes the cold roof insulation process.
Warm Roof Construction
The warm roof construction will be completed before any roof finishing takes place. It’s best to do this as soon as possible after fitting the rigid roof boards for weather protection.
Step 1. Create a Vapour Barrier
Clean the roof of any debris with a broom and lay a vapour-proof membrane over the entire surface. Secure the membrane with staples around the edge and seal any joins with tape.
Step 2. Laying the PIR Insulation
Before laying out the PIR insulation, you will need to fit a timber frame around the perimeter of the roof. The timber needs to be the same height as the insulation. 100mm is recommended. This will house the insulation on the roof and provide a fixing point for a layer of OSB boards, before fixing the EPDM waterproof membrane.
Step 3. Installing a layer of OSB
Using counter-sunk screws, secure the OSB or TG4 roof boards. Aim for the rafter below. A string line can help find the rafter along the length of the roof. Ensure the screws are secured flush to the board to avoid damaging the EPDM rubber roofing cover when installed.
Step 3. Install the EPDM Rubber Waterproof Roofing Cover
The final layer will be installing the EPDM roofing cover. I’ve covered this installation process in another article. For quick reference, here’s a great video that demonstrates the process.
Roof Lights and Penetrations
Electrical wiring, lighting fixtures and electrical sockets all need to be installed within the walls and ceiling, so it’s important to ensure the insulation does not block access behind the plasterboard. Equally, lighting fixtures may get hot, so a suitable gap around the light fixture is important.
If installing down-lights into the ceiling, ensure a good gap around the light by cutting away some of the insulation. We want to remove enough to prevent the light from touching the insulation, without removing it all.
Here’s an example from one of my builds.
Roof lights are a popular feature in garden rooms, but they introduce additional condensation risk if not detailed correctly. Insulation should be continuous up to the frame of the roof light, and the vapour control layer must be carefully sealed around all openings to maintain airtightness.
This concludes the guide on insulating a garden room. Please leave a comment below to ask any questions, leave feedback or share your experiences with the wider community.
Further Reading.
