Loft Insulation Calculator UK – Rolls & Depth Guide

Project Details

UK Standard: For optimal energy efficiency, a total depth of 270mm is recommended for mineral wool insulation.

Loft Length (m):

Loft Width (m):

Total Loft Area (m²): Calculated automatically from dimensions, or enter manually.

Required Insulation Layer:

Roll Coverage (m²): Check packaging for exact "m² coverage". Typical rolls are 6-9m².

Results

Estimated Material Cost: £0

(Based on UK averages of ~£25 per roll)

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How to Calculate & Install Loft Insulation

Insulating your loft is one of the single most cost-effective ways to reduce your energy bills. According to the Energy Saving Trust, an uninsulated home loses roughly 25% of its heat through the roof. By installing the correct depth of insulation, you can shave hundreds of pounds off your heating costs every year.

Our Loft Insulation Calculator helps you determine exactly how many rolls of mineral wool or glass wool you need to buy to meet modern energy efficiency standards.

What is the Recommended Depth?

In the UK, building regulations and energy experts currently recommend a total insulation depth of 270mm. This is usually achieved in two layers:

The Base Layer (100mm): This layer is laid first, fitting tightly between the wooden ceiling joists.
The Top-Up Layer (170mm or 200mm): This layer is laid at a 90-degree angle (cross-legged) right over the top of the joists. This prevents "cold bridging," where heat escapes through the timber joists themselves.

Types of Insulation Materials

While mineral wool rolls are the most common, there are several options for the home DIYer:

Glass Mineral Wool (Quilts): The standard flexible rolls found in most DIY shops. It is lightweight and easy to handle.
Sheep's Wool: A natural, sustainable alternative that is breathable and doesn't cause the skin irritation often associated with glass fibers.
PIR Rigid Boards: Highly efficient but expensive. Usually used for insulating "warm roofs" or when loft height is limited.

Safety First

Laying insulation is a messy and potentially hazardous job. Always wear a FFP2 or FFP3 dust mask, safety goggles, and protective gloves. Ensure you only step on the timber joists or use "crawling boards" (sturdy planks of wood) to avoid falling through the plasterboard ceiling below. Never cover electrical cables with insulation; they should be left clear or clipped above the material to prevent overheating.

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UK Building Regulations: Loft Insulation

Under current Part L Building Regulations and Energy Saving Trust recommendations for the UK, loft insulation must achieve a specific thermal performance:

Recommended Depth: 270mm (Standard mineral wool/glass wool).
Target U-value: 0.16 W/m²K or better.
Ventilation: You must not block the eaves (the edges of the roof). Air must be allowed to flow from the outside into the loft space to prevent condensation and timber rot.
Electrical Safety: Cables should be moved above the insulation or left clear to prevent overheating.

View Energy Saving Trust guidance →

Frequently Asked Questions

What is the recommended depth for loft insulation?

The UK government recommends a minimum depth of 270mm of mineral wool insulation to maximise energy efficiency and reduce heating bills.

Do I need gloves to lay loft insulation?

Yes. Mineral wool insulation contains fine glass fibers that can severely irritate the skin, eyes, and lungs. Always wear a dust mask, goggles, gloves, and long sleeves.

Should I insulate between or over the joists?

Both. To achieve the 270mm depth, a base layer (usually 100mm) is laid tightly between the joists, and a top-up layer (usually 170mm or 200mm) is laid at a 90-degree angle right across the top of the joists to prevent cold bridging.

Can I compress loft insulation under boards?

No. Compressing insulation squeezes out the trapped air pockets that actually provide the heat retention. If you want to board your loft for storage, you must use raised loft legs.