Insulation is the single most important factor in whether your garden room is usable year-round. Here's what the different options mean in practice.
Garden Room Insulation: What You Need to Know
Insulation is arguably the most important specification decision you'll make for your garden room. A poorly insulated room will be cold in winter, hot in summer, and expensive to heat — making it unusable for much of the year.
Why Insulation Matters
A garden room without adequate insulation will:
- Reach temperatures below 10°C in winter, making it uncomfortable to work in
- Overheat in summer, reaching 30°C+ on warm days
- Cost significantly more to heat, as heat escapes rapidly through walls, roof, and floor
- Potentially suffer from condensation and damp issues
A well-insulated room, by contrast, will maintain comfortable temperatures year-round with minimal heating.
Understanding U-Values
U-values measure how quickly heat passes through a material. Lower is better — a lower U-value means less heat loss.
For context:
- A typical uninsulated wall: U-value ~2.0 W/m²K
- Building Regulations standard (new homes): ~0.3 W/m²K
- A well-insulated garden room: 0.15–0.25 W/m²K
When comparing garden rooms, ask suppliers for the U-values of their wall, roof, and floor constructions.
Types of Insulation Used in Garden Rooms
PIR (Polyisocyanurate) Rigid Boards
The most common insulation in garden rooms. PIR boards (brands include Kingspan and Celotex) offer excellent thermal performance in a relatively thin profile.
- Typical thickness: 50mm–150mm
- U-value achievable: 0.15–0.25 W/m²K with 100mm+
- Pros: High performance, moisture resistant, widely available
- Cons: More expensive than mineral wool; requires careful installation to avoid cold bridges
SIPs (Structural Insulated Panels)
SIPs are factory-made panels consisting of an insulating foam core (usually EPS or PIR) sandwiched between two structural boards (usually OSB). They're used as the structural walls and roof of the garden room.
- Pros: Excellent thermal performance, fast construction, airtight
- Cons: Higher upfront cost; harder to modify once built; requires specialist installation
Mineral Wool (Rockwool / Glass Wool)
Traditional insulation used in stud walls. Less thermally efficient than PIR per mm, but cheaper and easier to install.
- Typical thickness: 100mm–200mm
- U-value achievable: 0.2–0.35 W/m²K
- Pros: Cheaper, good acoustic performance, breathable
- Cons: Lower thermal performance per mm; can absorb moisture if not protected
Spray Foam
Occasionally used in garden rooms, particularly for roofs. Provides good air sealing but can cause issues with future maintenance and mortgage lenders.
What to Ask Suppliers
When comparing garden room companies, ask:
- What insulation type and thickness do you use in walls, roof, and floor?
- What are the U-values for walls, roof, and floor?
- Is the construction thermally broken (i.e., are there any cold bridges)?
- What is the expected internal temperature range in winter and summer?
Red Flags
- Suppliers who can't or won't provide U-values
- Very thin insulation (under 50mm) described as "fully insulated"
- No floor insulation (a common cost-cutting measure)
- No mention of thermal bridging or cold bridges
The Year-Round Test
A genuinely year-round usable garden room should be comfortable to work in at 0°C outside with minimal heating. Ask suppliers if they can provide evidence of this — some reputable companies will share thermal performance data or customer testimonials from winter use.
This guide provides general information. Specific performance will depend on your build quality, heating system, and local climate.