Garden Room Insulation Guide: Why SIP Panels Are Best in 2025
Garden Room Insulation Guide: Why SIP Panels Are Best in 2025
Planning a garden room? Proper insulation is critical for year-round comfort. Discover why SIP panels outperform traditional insulation methods and how to choose the right specification.
A garden room is only as good as its insulation. Get it wrong, and you'll have a freezing workspace in winter and a sauna in summer. Get it right, and you'll enjoy comfortable year-round use with minimal heating costs.
The insulation method you choose affects everything: comfort, energy bills, build time, and long-term value. This guide examines all the options and explains why SIP panels are the best choice for most garden rooms in the UK.
Whether you're building a home office, art studio, gym, or entertainment space, this guide will help you make the right insulation decision.
Why Garden Room Insulation Matters
Poor insulation ruins garden rooms. Here's what happens:
❄️ Winter Problems
- Freezing temperatures = unusable space
- Condensation on walls and windows
- Mould growth on cold surfaces
- Sky-high heating bills to maintain comfort
- Heat escapes as fast as you add it
☀️ Summer Problems
- Overheating—too hot to work/relax
- Need for air conditioning (expensive to run)
- Uncomfortable even with windows open
- Direct sun turns space into greenhouse
✅ With Good Insulation
- Comfortable 365 days/year
- Low heating costs (£300-500/year typical)
- No condensation or mould issues
- Stable temperatures year-round
- Adds significant property value
Garden Room Insulation Options Compared
There are 4 main insulation methods for garden rooms. Let's compare them honestly:
1. SIP Panels (Structural Insulated Panels)
What they are: Prefabricated sandwich panels with PIR insulation core bonded between OSB facings. Structure and insulation in one component.
✅ Advantages:
- Best thermal performance: U-values 0.209-0.264 (97-122mm)
- Fastest installation: Weathertight in 1-2 days
- No thermal bridging: Continuous insulation
- Exceptional airtightness: ≤3 m³/h/m²
- DIY-friendly: Like flat-pack furniture
- Factory precision: No on-site cutting/fitting
❌ Disadvantages:
- Higher upfront material cost (£89-99/m²)
- Less flexible for unusual shapes
- Requires careful handling (large panels)
Best for: Anyone wanting the best thermal performance, fastest build, and long-term value. Ideal for home offices, studios, and year-round garden rooms.
2. Timber Frame + PIR Insulation
What it is: Traditional timber frame (89mm or 140mm studs) with rigid PIR boards fitted between studs.
✅ Advantages:
- Lower material cost (~£60-80/m²)
- Maximum design flexibility
- Easy to modify on-site
- Familiar to most builders
❌ Disadvantages:
- Thermal bridging at studs (15-20% of wall area)
- Typical U-value: 0.25-0.28 (not as good)
- Slower installation (3-5 days to weathertight)
- More labour intensive
- Harder to achieve good airtightness
Best for: Budget-conscious builds where upfront cost matters more than long-term performance. Works well for occasional-use garden rooms.
3. Timber Frame + Mineral Wool
What it is: Timber frame with glass/rock wool insulation batts fitted between studs.
✅ Advantages:
- Cheapest insulation option (~£50-65/m²)
- Breathable (good for moisture management)
- Fire-resistant
- Widely available
❌ Disadvantages:
- Worst thermal performance: U-value 0.30-0.35
- Thermal bridging at studs
- Can sag over time (gaps = heat loss)
- Absorbs moisture if vapour barrier fails
- Itchy/unpleasant to install
- Takes up more space (thicker for same U-value)
Best for: Very tight budgets or non-habitable spaces (sheds, stores). Not recommended for year-round garden offices.
4. Budget "SIP-Style" Panels
What they are: EPS (polystyrene) core panels or lower-grade alternatives to proper SIPs.
⚠️ Warning: Not All "SIP" Panels Are Equal
EPS (expanded polystyrene) panels are cheaper but have worse thermal performance (λ = 0.035-0.038 vs PIR's 0.022). To achieve the same U-value, you need 60-70% thicker panels. Often marketed as "SIP panels" but performance is significantly lower.
Our advice: If budget is tight, choose timber frame + PIR rather than low-grade panels. You'll get better value and known performance.
Performance Comparison: Real Numbers
Here's how each method performs for a typical 4m × 3m garden room:
| Criterion | SIP Panels (97-122mm) |
Timber + PIR (89-140mm) |
Timber + Mineral Wool (140mm) |
|---|---|---|---|
| U-Value (W/m²K) | 0.209-0.264 | 0.25-0.28 | 0.30-0.35 |
| Annual Heating Cost* | £420-480 | £580-650 | £720-850 |
| Airtightness (m³/h/m²) | ≤3 | 8-10 | 8-12 |
| Installation Time | 1-2 days | 3-5 days | 3-5 days |
| Material Cost (40m²) | £3,560-3,960 | £2,400-3,200 | £2,000-2,600 |
| Total Project Cost** | £10,500-12,000 | £10,000-12,500 | £9,500-11,500 |
| 20-Year Total Cost*** | £18,900-21,600 | £21,600-25,500 | £23,900-28,500 |
*Year-round use, 8hrs/day heating, electric heating @ £0.28/kWh
**Including all materials, labour, windows, doors, finishes
***Initial cost + heating costs over 20 years
💡 The Lifetime Cost Reality: While SIP panels cost £1,000-1,500 more upfront, they save £2,700-6,900 over 20 years in heating costs. They literally pay for themselves—and then keep saving you money for decades.
Which SIP Panel Thickness for Your Garden Room?
If you've decided on SIP panels (smart choice!), here's which thickness to choose:
72mm SIP Panel (50mm Celotex GA4000) — U-value: 0.396
Price: From £88.90/m²
Best for:
- Storage sheds, workshops (no heating)
- Summer-only use
- Ultra-tight budgets
⚠️ Not recommended for year-round heated garden rooms. U-value too high—you'll spend more on heating than you saved on panels.
97mm SIP Panel (75mm Celotex GA4000) — U-value: 0.264
Price: From £88.90/m²
Best for:
- Garden rooms (occasional heating)
- Studios & hobby rooms
- Budget-conscious year-round use
- Gyms, entertainment spaces
✅ Good balance of cost and performance. Comfortable in winter with modest heating. Popular choice for garden rooms that aren't primary workspaces.
122mm SIP Panel (100mm Celotex GA4000) — U-value: 0.209
Price: From £98.70/m²
Best for:
- Home offices (daily use)
- Art studios, music rooms
- Year-round garden rooms
- Heated spaces requiring comfort
✅ THE GOLDILOCKS CHOICE: Excellent thermal performance, reasonable cost, professional-grade comfort. Stays warm in winter with minimal heating, cool in summer. Perfect for daily-use garden offices.
142mm (U=0.179) & 172mm (U=0.144) SIP Panels — Premium Options
Prices: £115.50/m² (142mm) | £133.40/m² (172mm)
Best for:
- Maximum comfort & energy efficiency
- Passive house standards
- Very cold climates or exposed sites
- When heating costs are a major concern
Worth the upgrade? If you'll use the garden room daily for 10+ years, yes. The heating savings justify the extra £15-35/m². If occasional use, stick with 97mm or 122mm.
Top 5 Installation Tips for Garden Room SIPs
1️⃣ Get the Base Perfect
SIP panels need a level, square base. Use a laser level. Even 5mm out of level causes problems. Install treated sole plate with DPC underneath to prevent damp rising into panels.
2️⃣ Use PIR Splines at All Joints
PIR splines create thermal continuity between panels. Skipping these creates cold spots and thermal bridges. Budget ~£10-15/linear meter for splines—essential for performance.
3️⃣ Seal Everything
Use expanding foam at all panel joints, window/door openings, and penetrations. Airtightness tape around windows. The goal: ≤3 m³/h/m² airtightness. This is where DIYers often go wrong—be thorough!
4️⃣ Get Weathertight Fast
Install roof panels same day as walls if possible. Cover any exposed OSB with tarpaulin overnight. OSB is moisture-resistant but not waterproof—prolonged rain exposure degrades performance.
5️⃣ Install Ventilation
SIP garden rooms are so airtight they NEED mechanical ventilation. Budget £200-500 for continuous ventilators or MVHR system. Without it, you'll get condensation and poor air quality. Building Regs requirement anyway.
5 Common Garden Room Insulation Mistakes (And How to Avoid Them)
❌ Mistake #1: Under-Specifying Insulation
The problem: "I'll just use 72mm panels to save money" → Results in £300-400/year higher heating costs.
The fix: For year-round use, minimum 97mm panels. For daily use (home office), choose 122mm. The savings pay back the difference in 3-5 years.
❌ Mistake #2: Ignoring Thermal Bridging
The problem: Using timber frame without understanding thermal bridges. "100mm insulation" doesn't mean U-value 0.22—timber studs reduce actual performance to 0.25-0.28.
The fix: If using timber frame, add external insulation or use thicker studs (140mm minimum). Or switch to SIPs for continuous insulation.
❌ Mistake #3: Skipping Ventilation
The problem: "It's just a garden room, it doesn't need ventilation" → Condensation, mould, and poor air quality within months.
The fix: Install continuous background ventilation (trickle vents) PLUS extract fan, or MVHR system. Budget £200-800. Non-negotiable for airtight buildings.
❌ Mistake #4: Poor Airtightness
The problem: Gaps around windows, unsealed joints, no expanding foam → All your insulation performance wasted through air leakage.
The fix: Take airtightness seriously. Use expanding foam liberally at all joints. Airtightness tape around all openings. Seal every penetration. Target ≤3 m³/h/m² @50Pa.
❌ Mistake #5: Choosing Cheap "SIP-Like" Panels
The problem: EPS (polystyrene) panels marketed as "SIPs" but with worse thermal performance. Saving £1,000 upfront costs £4,000+ in extra heating over 20 years.
The fix: Only buy proper SIPs with PIR insulation core (Celotex, Kingspan, etc). Check the λ value: should be 0.022-0.023 W/mK. If it's 0.035+, it's not premium insulation.
Ready to Build Your Insulated Garden Room?
UltraSIPs supplies premium SIP panels perfect for garden rooms. Expert advice, competitive pricing, and fast UK delivery.