Right, insulation. Not the sexiest topic, is it? But here’s the thing – a combination of wall, loft, and floor insulation measures can cut your home heat loss by a remarkable 67%. And insulating your walls, floor, and loft can save you £630 – £730 on annual energy bills.
That’s proper money. And the difference between a warm, comfortable home and one where you’re wearing three jumpers indoors in January.
But choosing insulation material is genuinely confusing. There’s about twenty different types, all with different names, pros and cons, and purposes. Mineral wool, fibreglass, PIR boards, spray foam, sheep’s wool – what even is half this stuff?
Let me break it down properly so you can make an informed decision without needing a degree in building physics.
Understanding The Basics First
Before we dive into specific materials, you need to understand what we’re actually measuring.
Thermal Conductivity (Lambda Value)
This is the key number. It’s shown as λ (lambda) and measured in W/mK. Lower is better.
A material with λ of 0.022 is more insulating than one with λ of 0.040. The lower the number, the less heat passes through.
U-Values
This measures how much heat passes through your entire wall/roof/floor, not just the insulation. Lower is better here too.
Building regulations set maximum U-values you have to meet. For example, new builds need walls with U-values of 0.18 W/m²K or less.
R-Value
This is thermal resistance – basically the opposite of U-value. Higher is better. More common in American products but you’ll see it occasionally in the UK.
The important thing? Focus on lambda values when comparing insulation materials.
The Main Insulation Types
Right, let’s go through the actual materials you’ll encounter.
1. Mineral Wool (Rock Wool/Glass Wool) – The Reliable Standard
Costing just around £13 – £17.5 per m2, rock or glass mineral wool are some of the most affordable, available, and sustainable options on the insulation market these days.
What It Is
Mineral wool is made from rock or glass products that are spun into sheets and blankets. Looks like candy floss, feels like… well, itchy candy floss.
Lambda Value
0.032-0.044 W/mK (decent)
Where It’s Used
- Loft insulation (the most common use)
- Cavity walls (blown in as loose fill)
- Between timber studs in walls
- Between floor joists
Pros
✅ Cheap and widely available
✅ Fire resistant (doesn’t burn)
✅ Good acoustic insulation (blocks noise)
✅ Easy to cut and fit
✅ Often made from recycled materials
✅ Doesn’t absorb water easily
Cons
❌ Itchy as hell to work with (wear gloves, mask, and long sleeves)
❌ Can compress over time, reducing effectiveness
❌ Not the best thermal performance compared to modern alternatives
❌ Loses effectiveness if it gets wet
Cost
£13-17.50 per m² for materials
Best For
Loft insulation on a budget. Between timber joists. Anyone doing DIY who wants something simple and affordable.
2. Fibreglass – The Budget Option
Very similar to mineral wool but made from recycled glass. Slightly cheaper, slightly less effective.
Lambda Value
0.032-0.044 W/mK
Where It’s Used
- Lofts
- Between studs
- Internal walls
- Floors
Pros
✅ Cheapest option available
✅ Easy DIY installation
✅ Non-flammable
✅ Widely available
Cons
❌ Even more itchy than mineral wool
❌ Absorbs moisture and loses effectiveness
❌ Can compress easily
❌ Lower thermal performance
Cost
£10-15 per m² typically
Best For
Budget loft insulation. DIY projects where cost matters most.
3. PIR (Polyisocyanurate) Boards – The Premium Choice
This is where things get serious. PIR insulation board, short for PolyIsocyanuRate (PIR), is one of the most capable and valuable heat resistant material used in UK constructions.
What It Is
Rigid foam boards with aluminium foil on both sides. Feels solid and lightweight.
Lambda Value
0.021-0.032 W/mK (excellent)
The Lambda value of PIR is situated between 0.021 to 0.032 W/mK. This means that PIR offers superior insulation with the same thickness.
Where It’s Used
- Under concrete floors
- Flat roofs
- Between rafters
- External wall insulation
- Pretty much anywhere you need serious insulation
Pros
✅ Best thermal performance for thickness
✅ Moisture resistant (foil facing repels water)
✅ Strong and rigid (can be walked on)
✅ Doesn’t compress
✅ Achieves required U-values with less thickness
✅ Easy to cut with a handsaw
Cons
❌ More expensive than mineral wool
❌ Not breathable (can trap moisture if installed wrong)
❌ Releases toxic fumes if it catches fire
❌ Made from petrochemicals (not eco-friendly)
Cost
£20-35 per m² depending on thickness
Best For
Anywhere space is limited. Under floors. Flat roofs. When you need maximum performance in minimum thickness.
4. Phenolic Foam Boards – The Top Performer
Phenolic foam is the top performer in most aspects, but it’s pricey.
Lambda Value
0.018-0.023 W/mK (the best available)
What It Is
Similar to PIR but even better thermal performance. Pink/orange rigid boards.
Where It’s Used
Same applications as PIR, but used when you need even thinner insulation or maximum performance.
Pros
✅ Best thermal performance available
✅ Thinnest insulation for same performance
✅ Moisture resistant
✅ Strong and rigid
Cons
❌ Most expensive option
❌ Harder to source
❌ Releases toxic fumes if burned
❌ Not eco-friendly
Cost
£30-50 per m² (ouch)
Best For
Situations where every millimetre of space matters. High-spec builds. When budget isn’t the main concern.
5. Spray Foam (Polyurethane) – The Gap Filler
This is the stuff they spray in that expands to fill every gap. Looks like yellow-ish foam once it’s set.
Lambda Value
0.020-0.030 W/mK (very good)
Where It’s Used
- Tricky spaces with odd shapes
- Sealing gaps around pipes and wires
- Under slates in roofs
- Cavity walls
Pros
✅ Fills every gap and crack
✅ Creates an airtight seal
✅ Excellent thermal performance
✅ Adds structural strength
✅ Moisture resistant
Cons
❌ Expensive installation (needs professional)
❌ Permanent (can’t remove it easily)
❌ Can trap moisture if not done right
❌ Makes future electrical work difficult
❌ Can cause issues with roof ventilation
❌ Mortgage lenders sometimes don’t like it
Cost
£20-50 per m² installed
Best For
Professional installations. Difficult-to-reach areas. When you need absolutely no gaps.
Warning
Spray foam in roofs can cause problems. If not done correctly, it can trap moisture and rot your timber. Some mortgage lenders refuse properties with spray foam lofts. Do your research before committing.
6. EPS/XPS Polystyrene – The Rigid Budget Option
Polystyrene boards. Like fancy packaging material but for insulation.
Lambda Value
EPS: 0.030-0.038 W/mK XPS: 0.028-0.036 W/mK
Where It’s Used
- Under concrete floors
- Below ground (foundations)
- Flat roofs
- External walls
Pros
✅ Cheaper than PIR
✅ Moisture resistant
✅ Doesn’t rot
✅ Lightweight
Cons
❌ Lower thermal performance than PIR/phenolic
❌ Can compress under load
❌ Burns and releases toxic fumes
❌ Not eco-friendly
Cost
£15-25 per m²
Best For
Budget insulation under floors. Below-ground applications. When PIR’s too expensive but you need rigid boards.
7. Wood Fibre Boards – The Eco Option
Wood Fibre Insulation – my personal favourite for a number of reasons. It can be used as loft insulation, internal or external wall insulation or floor insulation and it’s easy and pleasant to work with.
What It Is
Made from wood chips compressed into boards. Natural, sustainable, breathable.
Lambda Value
0.038-0.043 W/mK (decent, not amazing)
Where It’s Used
- External wall insulation
- Between timber studs
- Eco-builds
- Breathable construction
Pros
✅ Natural and sustainable
✅ Breathable (regulates moisture)
✅ Good acoustic insulation
✅ Pleasant to work with
✅ Sequesters carbon
✅ Can be recycled
Cons
❌ Thicker than PIR for same performance
❌ More expensive than mineral wool
❌ Can absorb moisture if exposed
❌ Heavier than foam boards
Cost
£25-40 per m²
Best For
Eco-conscious builds. Breathable wall systems. Period properties. When sustainability matters as much as performance.
8. Sheep’s Wool – The Natural Alternative
Yes, actual sheep’s wool. Treated and processed but basically what it says.
Lambda Value
0.035-0.040 W/mK
Where It’s Used
- Lofts
- Between timber studs
- Anywhere you’d use mineral wool
Pros
✅ Natural and sustainable
✅ Breathable
✅ Regulates humidity naturally
✅ Non-toxic
✅ Pleasant to work with (not itchy)
✅ Fire resistant when treated
Cons
❌ Expensive
❌ Can attract moths (if not treated properly)
❌ Limited availability
❌ Performance not better than synthetics
Cost
£20-35 per m²
Best For
Eco-builds. People with chemical sensitivities. Those who prefer natural materials.
9. Cellulose – The Recycled Paper Option
Made from recycled newspapers treated with fire retardants. Usually blown in as loose fill.
Lambda Value
0.035-0.040 W/mK
Cellulose is by far one of the most sustainable options you could opt for when it comes to any home insulation upgrade. Costing a mere £10 – £12 per m2, it’s one of the cheapest options on the market.
Where It’s Used
- Lofts (blown in)
- Cavity walls
- Between timber studs
Pros
✅ Very cheap
✅ Made from recycled materials
✅ Good for filling awkward spaces
✅ Decent fire resistance
Cons
❌ Needs professional installation (blown in)
❌ Can settle over time
❌ Absorbs moisture
❌ Not widely available in UK
Cost
£10-12 per m² (materials), plus installation
Best For
Budget eco-option. Large lofts. When you want recycled materials but can’t afford wool.
10. Multifoil/Reflective Insulation – The Controversial One
Thin layers of foil and bubbles. Looks like space blankets.
Lambda Value
Varies wildly (often overstated by manufacturers)
Where It’s Used
- Marketed for everything
- Actually best as supplement, not main insulation
Pros
✅ Very thin
✅ Easy to install
✅ Reflects radiant heat
Cons
❌ Thermal performance often overstated
❌ Doesn’t work as well as claimed
❌ Building control often won’t accept it alone
❌ Expensive for what you get
Cost
£15-30 per m²
Best For
As a supplement to other insulation. Very specific applications where thickness is critical. Honestly, most people should avoid it as primary insulation.
How To Actually Choose
Right, that’s ten types of insulation. How do you pick?
Step 1: Work Out What You’re Insulating
Different jobs need different materials.
Loft floor – Mineral wool or fibreglass (cheap, easy DIY) Loft roof (warm roof) – PIR boards or spray foam Cavity walls – Blown mineral wool or beads (professional job) Solid walls (external) – PIR boards or wood fibre with render Solid walls (internal) – PIR boards or insulated plasterboard Under floors – PIR boards or EPS Between joists – Mineral wool or fibreglass
Step 2: Check Your Budget
Rough price guide per m²:
- Budget (£10-15): Fibreglass, cellulose
- Mid-range (£15-25): Mineral wool, EPS
- Premium (£25-35): PIR boards, sheep’s wool
- Top-end (£35-50+): Phenolic foam, wood fibre
Step 3: Consider Thickness Available
If you’ve got limited space (like a flat roof or under floors with limited depth), you NEED high-performance boards like PIR or phenolic.
If you’ve got loads of space (like a big loft), cheaper options like mineral wool work fine.
Step 4: Think About Installation
DIY-friendly: Fibreglass, mineral wool, PIR boards (if you can cut straight) Professional only: Spray foam, blown mineral wool, cellulose
Step 5: Eco-Credentials (If You Care)
Most sustainable: Wood fibre, sheep’s wool, cellulose Middle ground: Mineral wool (often contains recycled content) Least sustainable: PIR, phenolic, polystyrene (petrochemical-based)
Step 6: Breathability
For old buildings with solid walls, breathability matters. Use wood fibre or mineral wool, not impermeable boards that trap moisture.
For modern buildings with cavity walls and damp-proof courses, it matters less.
The Common Mistakes
Mistake 1: Choosing Based on Price Alone
The cheapest option isn’t always the best value. If you need 300mm of fibreglass to match 100mm of PIR, you might not have the space. Plus installation costs more for thicker materials.
Mistake 2: Ignoring Building Regulations
You can’t just stick any old insulation up. It needs to meet building regulations. Check the required U-values for your project and make sure your chosen material achieves them.
Mistake 3: Not Considering Moisture
Insulation that gets wet loses effectiveness. If there’s any chance of moisture (like under floors or in walls), use moisture-resistant materials.
Mistake 4: Compressing Insulation
Squashing mineral wool or fibreglass to fit in a gap reduces its effectiveness. If the space is tight, use thinner high-performance boards instead.
Mistake 5: Blocking Ventilation
Lofts need ventilation. Don’t block eaves vents with insulation. Leave gaps or use eaves protectors.
Real-World Recommendations
Let me just tell you what most people actually use:
Loft insulation (DIY): Mineral wool, 270mm thick. Cheap, effective, does the job.
Under suspended timber floor: 100mm PIR boards between joists. Best performance for limited space.
Cavity walls: Get a professional to blow in mineral wool. Don’t try this yourself.
Flat roof: 100-150mm PIR boards. Only realistic option for achieving U-values.
Internal wall insulation: Insulated plasterboard (plasterboard with PIR bonded to it). Easy to install, good performance.
External wall insulation: Wood fibre boards with render for breathability, or PIR boards if you don’t care about eco-credentials.
Final Thoughts
The best insulation material usually depends on the job at hand. There’s no single “best” insulation.
For most DIY loft jobs, mineral wool’s your best bet. Cheap, effective, easy to work with.
For professional installations or where space is tight, PIR boards dominate. Best performance for thickness.
For eco-builds or breathable construction, wood fibre or sheep’s wool make sense despite higher costs.
And whatever you choose, make sure it’s installed properly. The best insulation material in the world is useless if it’s fitted badly with gaps everywhere.
Get multiple quotes from TrustMark-registered installers. Check they understand building regulations. And remember – insulation’s one of those things that pays for itself over time through lower energy bills.
Just get it done. Your future self (and your bank account) will thank you every winter when your heating bills are £500 lower than your neighbours.