Roof Insulation Board

What Is a Roof Insulation Board?

A roof insulation board is a rigid panel of closed-cell foam, most often polyisocyanurate (PIR) or phenolic, fitted within the roof structure to slow heat transfer out of the building. The roof is one of the largest sources of heat loss in a house, and a rigid board delivers a high level of thermal resistance for its thickness, which is what makes it the standard choice where rafter depth or headroom is limited.

You will also see these products listed as roof insulation panels or roof insulation sheets; they are the same thing. They form part of the wider insulation board range and sit alongside mineral wool and multifoil in the full roof insulation collection.

Roof Insulation Types: PIR, Phenolic and Flat Roof Boards

Roof insulation boards split by material and by the roof construction they are made for. Thermal conductivity across this range runs from 0.019 W/mK for phenolic to 0.027 W/mK for the ply-faced flat roof boards, and that figure, together with thickness, decides how much depth you need to hit a given U-value.

PIR roof insulation boards are the most widely specified option in UK roofing. Polyisocyanurate gives strong thermal performance for its thickness, typically 0.022 W/mK, at a lower cost than phenolic, and most boards carry foil facings on both sides. In this collection PIR is mainly represented by flat roof products: Kingspan Thermaroof TR26 and TR27, and Recticel Powerdeck U, Powerdeck F and Eurothane Eurodeck. The full PIR insulation range extends beyond roofs into floors and walls.

Phenolic roof insulation boards have a lower thermal conductivity than PIR, so a thinner board reaches the same U-value target. Kingspan Kooltherm K107 is the pitched roof board in this category, stocked from 25mm to 120mm, and is the one to look at where every millimetre of rafter depth or headroom counts.

Flat roof insulation boards are engineered for warm deck construction, where the board sits above the deck and below the waterproofing. Facings vary with the covering: some boards are made for torch-on felt, others for single ply or built-up systems, and the Tekwarm Roofdek Thermal Ply bonds PIR to a ply facing so the board provides insulation and a fixing surface in one layer. All of these, along with Kingspan flat roof insulation and the Recticel Powerdeck range, sit in the flat roof insulation collection.

Insulation Board for a Pitched Roof

On a pitched roof, rigid roof insulation boards are fitted between the rafters, over the rafters, or as a combination of both to reach the required U-value. Fitting between rafters keeps the build-up thin; adding a continuous layer over or under the rafters cuts thermal bridging through the timber and usually gives the better overall result. Boards cut cleanly to width, which suits loft conversions and room-in-roof projects where the space will be lived in and a warm roof is the goal. Where the roof design calls for a ventilated gap between the board and the underlay, that gap must be kept clear. The pitched roof insulation collection narrows the range to boards suited to this work.

Insulation Board for a Flat Roof

Flat roofs use boards designed for the specific build-up: warm deck, cold deck or inverted roof. In a warm deck roof, the board sits above the deck with a vapour control layer beneath it, keeping the structure on the warm side of the insulation and removing the need for ventilation. It is the standard approach for new build and refurbishment flat roofs in the UK and the construction most flat roof insulation boards are designed for. The board, the deck type and the waterproofing system all need to be compatible, so check the manufacturer's guidance on facings and fixing before ordering.

How Rigid Roof Insulation Works

Rigid roof insulation works on two fronts. The closed-cell foam core has a low thermal conductivity, so heat moves through it slowly. The foil facings on PIR and phenolic boards are low emissivity, which improves the thermal resistance of any adjacent airspace in the build-up. The facings are not a vapour control layer, so a separate VCL is still needed where the roof design requires one. Thermal performance is expressed as an R-value in m²K/W, which rises with thickness, and the whole roof build-up is assessed as a U-value in W/m²K. A continuous, well-fitted layer with taped joints performs closer to its declared figures than a board with gaps, so accurate cutting matters as much as the product chosen.

What Thickness of Roof Insulation Board Do You Need?

Thickness sets the R-value, but the material's thermal conductivity sets how much thickness you need. A thicker board gives more resistance, and a lower-conductivity material gets there sooner. The right thickness depends on the U-value the project has to meet under Part L, the depth available in the rafters or roof build-up, and whether the board is used alone or in two layers. The range runs from 25mm to 150mm, with 100mm insulation board the most common single-layer choice and 150mm insulation board used where a warm roof insulation thickness target cannot be met in less. Where space is tight, a phenolic board can hit the same target as PIR at a reduced thickness. Check the manufacturer's U-value tables for the exact build-up before ordering.

Why Buy Your Roof Insulation Board from Insulation UK

Trusted brands: A focused range of roof insulation boards from Kingspan, Recticel and Tekwarm, all held in stock.

Fast UK-wide delivery: Free delivery on orders over £500 ex VAT, with reliable nationwide delivery from our Rotherham distribution centre.

Competitive pricing: Strong trade prices across the full range of roof boards, backed by good stock availability.

Expert help and advice: Our team is on hand to help you choose the right board and thickness for your roof. Call 03003 034 578.