More than an outdoor structure, a garden room is an extension of your home. Whether you’re turning it into a home office, gym, studio or just a place to have some peace and quiet, insulation is what makes it comfortable in any season. One question we get asked a lot is, “What thickness of insulation should I use in my garden room?”
The response will vary depending on how you intend to use the space, your climate, and what type of insulation material you select. But there are some rules of thumb to steer you in the right direction.
Why Insulation Thickness Matters
Insulation controls heat transfer. Inside it holds warm air during the winter; in the summer, it keeps excess heat out. The thicker the insulation, the better its thermal performance (referred to as its U-value – a low U-value means good insulation).
An insulated garden room will mean that:
- Become too cold in winter and overheat in summer
- Cost more to heat and cool
- Develop condensation or damp issues
- Be uncomfortable for daily use
If you’re funding a permanent garden room, insulation needs to match standards commonly associated with that of solidly formed house extensions.
What insulation thickness should I use on a garden room?
Floor Insulation
If you are using your garden room all year round, a typical floor insulation thickness for this is:
- 75mm to 100mm
You might get away with 75mm if you’re using rigid insulation boards like PIR (polyisocyanurate) for example. For mineral wool and your looking at getting more towards 100mm to get the same thermal performance.
Good floor insulation is important as cold air comes up from the ground, particularly in raised or timber-framed garden rooms.
Wall Insulation
Walls typically applications where recommended thickness is:
- 75mm to 100mm
The following is fairly common for high-power applications:
- 75mm PIR board within timber stud work
- Alternatively 100mm mineral wool for enhanced acoustic protection
For home office or living area use (as well as other types of use), you will want to look for insulation with a U-value in the vicinity of 0.18–0.28 W/m²K.
Added insulation may be necessary for colder climates.
Roof Insulation
Heat rises naturally, so it’s particularly important to insulate your roof. The recommended thickness is:
- 100mm to 150mm
For best results:
- 100mm PIR board can be used at flat roofs
- Pitched Roofs: 150mm mineral wool can be used in pitched roofs
An inadequately insulated roof is the number one cause of heat loss in garden rooms so this is not an area to skimp.
Does Usage Affect Insulation Thickness?
Yes, significantly.
Occasional or Summer-Only Use
If the garden room will largely be used in summer, thinner insulation (about 50mm) may suffice. But it’s not good enough for chillier times of year.
Year-Round Use
For a garden room you’ll be using daily — as an office, say, or guest room — use domestic-level specifications. That means:
- 75–100mm walls
- 100mm+ roof
- 75–100mm floor
That way you can be comfortable without breaking the bank on heating bills.
Choosing the Right Insulation Material
Thickness isn’t enough — the kind of insulation makes a difference, too.
PIR Insulation Boards
These provide less thickness with greater heat resistance. For instance, 75mm thick PIR can provide approximately the same thermal performance as 100mm mineral wool.
Advantages:
- Excellent thermal resistance
- Moisture resistant
- Space-saving
Mineral Wool
Commonly used in timber-framed builds.
Advantages:
- Good thermal performance
- Excellent sound insulation
- Fire resistant
Though, it usually takesa little more thickness to get the same U-value as PIR.
Building Regulations and Garden Rooms
For instances, garden rooms under 15m² or standalone buildings under 30m² may not need full building regulations approval. But if the space will be used as a bedroom or a space you’re in daily, insulation should meet those standards.
Even if laws do not apply, building up to higher insulation standards allows better energy efficiency and long-term comfort.
Climate Considerations
If you’re in a colder climate, you may even want thicker insulation. In hot summer climates insulation can reduce unwanted heat gain, so there is a benefit to having insulation in the floor.
Specific attention is necessary to avoid condensation problems in high humidity areas, keeping the insulation thickness and adding a vapour barrier.
Common Mistakes to Avoid
One of the most common mistakes is concentrating solely on wall insulation and neglecting the roof and floor. Heat is lost through all surfaces and hence insulation will need to be relatively consistent throughout the building.
Another error is cramming insulation so it fits into a cavity. This reduces its effectiveness. The depth of the cavity should be equal to the insulation thickness.
Lastly, air leaks around windows, doors and connections can diminish overall efficiency. Air sealing controls; even thick insulation will underperform if it’s not there.
What is the Best Insulation Thickness in General?
For a year-round, quality garden room you should generally:
- Floor: 75–100mm
- Walls: 75–100mm
- Roof: 100–150mm
However, with the use of high performance materials such as PIR insulation boards slightly slimmer builds may be acceptable whilst maintaining a high level of thermal efficiency.
If you want your garden room to feel like a natural extension of your house (so warm in winter, cool in summer, and energy-efficient), then it’s worth investing in proper insulation from the off.
Final Thoughts
There is no one-size-fits-all answer for the thickness of insulation you should have in a garden room – it will depend what it’s used for. Thinner insulation could be sufficient for occasional use. For year round use, it is highly recommended to have residential level insulation thickness.
A well-insulated garden room is not just a nicety — it’s good value in the long term, saving energy and keeping your space durable. Choose the right thickness, and your garden room will be a working, liveable space in all 4 seasons.