100mm Loft Insulation

100mm of loft insulation is a commonly used thickness in many loft insulation build-ups. It is substantial enough to help reduce heat loss and can be installed without requiring major structural redesign in many UK homes.

What is loft insulation 100mm?

Loft insulation 100mm is manufactured to a nominal thickness of 100mm. It comes in boards, slabs, and rolls and is suitable for roof-space applications.

100mm insulation depth provides a measurable level of thermal resistance, but the exact resistance depends entirely on the material, such as Isover or Knauf insulation.

• PIR rigid boards have low thermal conductivity, commonly around 0.022 W/mK
• Glass mineral wool rolls or slabs typically have a thermal conductivity of around 0.040 – 0.044 W/mK, depending on product grade, such as a 100mm Knauf loft roll.
• Rock mineral wool slabs often have around 0.034–0.037 W/mK

What is 100mm thick loft insulation used for?

Between loft joists (cold lofts)

100mm glass or rock wool is commonly installed between timber ceiling joists. This acts as the base layer in many cases. Additional insulation is then laid crosswise above to help achieve current Building Regulations guidance (Approved Document L).

100mm mineral wool on its own will not normally achieve the commonly targeted U-values of around 0.16 W/m²K for existing loft upgrades, but it forms a strong first layer.

Loft floor upgrades

Where lofts are used only for storage, 100mm PIR between joists can help maintain thermal performance while supporting the use of raised boarding above. Because PIR offers higher thermal resistance per millimetre, it’s often selected for use where height is restricted.

Pitched roof insulation (warm roof construction)

In rafter-level builds, 100mm PIR is commonly fixed either between or across rafters. In many cases, it forms part of a layered system, combined with an additional continuous layer to reduce thermal bridging.

Loft framing walls and internal masonry walls

In loft conversions, knee walls or dwarf walls often need insulation to enhance energy efficiency. 100mm boards or mineral wool slabs are used within timber framing to maintain continuity of the thermal envelope. Vapour control and airtightness detailing become critical here to prevent interstitial condensation.

Types of 100mm loft insulation

We stock three main categories of 100mm loft insulation:

PIR insulation

These are high-performing, foil-faced boards.

• Low thermal conductivity – around 0.022 W/mK
• They are ideal for tight spaces and areas with demanding target U-values
• They require accurate cutting and a tight fit

Glass wool insulation (rolls or slabs)

Both slabs and loft insulation rolls 100mm are light and flexible.

• They friction-fit easily between joists, making installation straightforward.
• Provide effective acoustic absorption
• Lower cost per m²
• Needs greater thickness than PIR to achieve similar thermal insulation results

Rock mineral wool (slabs)

• Typically classified as Euroclass A1 reaction to fire (non-combustible)
• Often chosen where fire separation or sound reduction is important

Consider the product’s performance per millimetre, fire rating, acoustic insulation needs, and budget when choosing between materials.

Benefits of 100mm loft insulation thickness

• Noticeable thermal performance improvement compared with thinner insulation layers
• Fits standard joist and rafter depths without major structural change
• Works as a dependable base layer in multi-layer systems
• Balances cost and performance effectively
• Mineral wool versions improve sound absorption
• Rock mineral wool provides high fire resistance

100mm thick insulation loft roll 44 can significantly reduce heat loss through the roof when correctly installed.

Why choose our thick 100mm loft insulation?

At Insulation Wholesale, we keep supplies simple and efficient by:

• Hold strong UK stock levels of popular items, such as the 100mm Knauf loft roll.
• Offer competitive pricing and clear bulk rates
• Provide pallet deals for larger projects
• Deliver directly to your site or home
• Provide fast quotes for trade accounts
• Support both DIY customers and contractors
• Keep our 100mm loft insulation prices transparent and trade-friendly

Our team understands U-values, detailing, and compliance, and is always ready to provide guidance when needed.