Insulating a loft conversion properly is one of the most important steps in creating a warm, comfortable, and energy-efficient living space. Whether you’re building a new loft room or upgrading an older conversion, getting the insulation right can save money on heating bills and make the space usable year-round.
This guide covers everything you need to know about how to insulate a loft conversion, from the best materials to step-by-step advice.
Why Loft Insulation Is Essential
Loft spaces are particularly prone to heat loss because warm air rises, and without proper insulation, it escapes quickly through the roof. Here’s why insulation matters:
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Energy efficiency – Reduces heat loss and lowers energy bills.
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Comfort – Keeps the room warm in winter and cooler in summer.
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Condensation control – Helps prevent damp and mould issues.
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Soundproofing – Adds an extra layer of noise reduction, especially useful if your loft backs onto a busy road.
Good insulation is also a requirement under UK building regulations, so it’s not just a luxury—it’s essential.
Types of Insulation for Loft Conversions
The type of insulation you choose will depend on your roof structure, budget, and the space available. Common options include:
1. Rigid Foam Board Insulation
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Brands like Celotex or Kingspan
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High thermal performance, ideal for between rafters
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Space-saving and easy to install
2. Mineral Wool (e.g. Rockwool)
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Excellent for soundproofing and fire resistance
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Typically used in between joists and studs
3. Multifoil Insulation
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Reflects heat rather than absorbing it
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Thin and lightweight, great for tight spaces
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Often used alongside other materials
4. Spray Foam Insulation
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Expands to fill every gap
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Great for irregular or hard-to-reach areas
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Must be installed by professionals
Each type has its pros and cons, and you may end up using a combination of materials in different parts of the loft.
Step-by-Step Guide to Insulating a Loft Conversion
Obviously not every loft is the same but here is a rough guide on how to insulate a loft conversion:
Step 1: Assess your space
Check whether the roof is pitched or flat and whether you’re insulating between, under, or over the rafters.
Step 2: Choose your insulation method
For pitched roofs, most people insulate between and under rafters. Flat roofs often require insulation above the roof deck.
Step 3: Install insulation between the rafters
Cut rigid foam boards to fit snugly between rafters. Leave a 50mm ventilation gap between the roof and insulation to prevent condensation.
Step 4: Add a second layer under the rafters (optional but recommended)
This “warm roof” approach improves thermal performance. Fix another layer of insulation board over the rafters and tape the joins.
Step 5: Insulate walls and floors
Use mineral wool between stud walls and floor joists to improve soundproofing and thermal insulation.
Step 6: Seal and finish
Tape all gaps, add a vapour barrier, and finish with plasterboard.
Note: Always check with Building Control or a qualified professional to ensure your loft conversion meets current insulation regulations.
Best Materials for Loft Insulation
In 2025, these insulation materials are considered top performers for loft conversions:
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Celotex GA4000 or XR4000 – Lightweight, effective, and widely used in roof and wall insulation.
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Kingspan Kooltherm – Ultra-thin with excellent thermal conductivity.
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Rockwool Flexi – Great for acoustic and thermal insulation between joists and studs.
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Actis Hybris or SuperFOIL – Multifoil options for tight spaces and layered performance.
Eco-conscious? Opt for more sustainable materials like sheep’s wool or recycled denim insulation, ideal for low-impact builds.
How Much Does Loft Insulation Cost?
Costs vary depending on the size of your loft, the type of insulation, and whether you DIY or hire a professional.
Here’s a rough breakdown:
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DIY rigid board insulation: £20–£30 per m²
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Mineral wool: £10–£15 per m²
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Multi-foil insulation: £10–£25 per m²
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Professional installation (including materials): £1,000–£3,000+ for a typical loft conversion
While insulation adds to the upfront cost of a loft conversion, it pays for itself over time through lower energy bills and increased comfort.
Loft Conversion Key Points
Proper loft insulation is key to making your conversion energy-efficient, comfortable, and regulation-compliant. Whether you’re insulating a newly converted loft or upgrading an existing one, investing in the right materials and learning how to insulate a loft conversion properly well worth it.
