Question 1

What is a lambda value and which brand has the best?

Accepted Answer

Lambda (λ) is thermal conductivity in W/mK — lower is better. PIR boards (Kingspan Kooltherm K103: 0.018, Celotex GA4000: 0.022, Recticel Eurothane GP: 0.022) offer the best lambda values. Mineral wool (Knauf Earthwool: 0.032-0.044, Rockwool: 0.034-0.037) has higher lambda but brings fire, acoustic and breathability benefits. To achieve the same thermal performance, you need approximately 50% more mineral wool thickness than PIR.

Question 2

Which insulation brand is cheapest?

Accepted Answer

Knauf Earthwool mineral wool is the cheapest per m² — approximately £3-6/m² for 100mm thickness. Rockwool costs £5-8/m² for 100mm. PIR boards are more expensive: Celotex GA4000 costs £8-14/m² (100mm), Kingspan Kooltherm £12-18/m² (100mm), Recticel Eurothane £8-12/m². However, PIR achieves better U-values in thinner buildups, saving space which has its own value.

Question 3

Which insulation is best for fire safety?

Accepted Answer

Rockwool and Knauf mineral wool are non-combustible (Euroclass A1) and can withstand temperatures above 1,000°C. They are required in many fire-rated wall and floor systems. PIR boards (Kingspan, Celotex, Recticel) are combustible with Euroclass C/D ratings and have foil facings. Following the Grenfell Tower inquiry, mineral wool is increasingly specified for external wall insulation on buildings over 18m.

Question 4

Can I mix insulation brands in one project?

Accepted Answer

Yes, this is common. You might use Kingspan Kooltherm in walls for maximum thermal performance in thin buildups, Rockwool in floors for acoustic performance, and Knauf Earthwool in the loft for cost-effective coverage. The key is that each application uses the correct product type and achieves the required U-value, regardless of brand.

Question 5

Which insulation is best for loft insulation?

Accepted Answer

For loft rolls between and over joists, Knauf Earthwool and Rockwool are the standard choices — cost-effective, easy to install, non-combustible and available in 100-200mm rolls. The government recommends 270mm total thickness for a cold loft. PIR boards (Kingspan, Celotex) are used for warm roof applications where boards are fitted between or over rafters in loft conversions.

Feature	Kingspan	Celotex	Knauf	Rockwool	Recticel
Type	PIR/Phenolic	PIR	Mineral wool	Stone wool	PIR
Best lambda (W/mK)	0.018	0.022	0.032	0.034	0.022
Cost (100mm, per m²)	£12-18	£8-14	£3-6	£5-8	£8-12
Fire rating	Class C/D	Class C/D	Class A1	Class A1	Class C/D
Acoustic benefit	Moderate	Moderate	Excellent	Excellent	Moderate
Availability	All merchants	All merchants	All merchants	All merchants	Specialist

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