Kingspan Kooltherm vs Celotex PIR for Insulating a Warm Flat Roof to 0.18 U-Value

Two boards, two thicknesses, one target

The 0.18 W/m2K figure comes from the notional target used across most domestic warm roof refurbishment work under Approved Document L. To hit it above a timber or concrete deck, the insulation carries almost the entire thermal load. A single-ply or torch-on membrane, an 18mm ply deck, and internal plaster contribute a combined thermal resistance under 0.3 m2K/W, so the boards do the rest.

Kingspan Kooltherm K107 declares a thermal conductivity of 0.018 W/mK for the thicknesses common on flat roofs. Celotex TB4000 and the equivalent EcoTherm Eco-Versal sit at 0.022 W/mK. That 4-point gap in lambda translates directly into board height. A Kooltherm buildup reaching 0.18 typically lands at 120mm to 130mm. The PIR equivalent runs 150mm to 160mm. On a roof with a fixed parapet height and existing door thresholds, 30mm of difference decides whether the finished surface clears the damp-proof course.

The boards also differ in facing. Kooltherm uses a glass tissue facing bonded to a phenolic core. Celotex and most PIR products use a foil facing on a polyisocyanurate core. The facing matters for adhesive compatibility with hot-melt and cold-applied membranes.

Worked calculation for a 4m by 5m extension roof

Take a rear extension roof, 20m2, timber joist deck with 18mm WBP plywood, cold-applied liquid membrane on top, and 12.5mm plasterboard with skim below. The target is a maximum U-value of 0.18 W/m2K, meaning a total thermal resistance of at least 5.56 m2K/W once the standard surface resistances of about 0.14 m2K/W are added.

Fixed layers contribute roughly: external surface 0.04, membrane and ply deck combined near 0.14, plasterboard 0.06, internal surface 0.10. That accounts for about 0.34 m2K/W. The insulation must supply the remaining 5.22 m2K/W.

With Kooltherm at 0.018 W/mK, the required thickness is 5.22 multiplied by 0.018, giving 0.094m, so 100mm nominally, though repeating thermal bridging through fixings and the timber-to-insulation ratio pushes the practical specification to 120mm. With PIR at 0.022 W/mK, the sum is 5.22 multiplied by 0.022, giving 0.115m, so 120mm before bridging corrections, and 140mm to 150mm once they are applied. The 25mm to 30mm real-world delta holds across most warm roof geometries.

On a 20m2 roof, 120mm Kooltherm at recent merchant pricing runs materially higher per board than 150mm PIR. The phenolic board costs more per square metre and the extra PIR thickness partly offsets the saving. The height saving is what justifies Kooltherm when upstand clearance is tight.

Where the vapour control layer sits

A warm roof puts insulation above the structural deck, which keeps the deck warm and the dew point outside the timber. A vapour control layer must sit on the warm side of the insulation, directly on top of the structural deck, before the boards go down.

Skip it and moisture from the rooms below migrates up through the plaster and joists, reaches the cold underside of the waterproofing, and condenses. On a phenolic buildup this is more critical because Kooltherm boards absorb moisture more readily than closed-cell PIR if the facing is breached. A 500-gauge polyethylene VCL or a self-adhesive vapour barrier taped at laps is the standard detail. The taping at penetrations, soil vents and rooflight upstands is where most warm roofs fail damp inspection.

Upstand and threshold clearance

Height is the deciding factor on refurbishment. A warm roof abutting a door needs the finished waterproofing 150mm below the internal floor level, or a compliant threshold detail with a drainage channel. With 150mm PIR plus deck plus membrane, the total buildup can exceed 190mm above the existing structural deck. That frequently pushes the finished level above the door cill.

Kooltherm at 120mm brings the same buildup down to around 160mm total. On a bungalow flat roof extension where the internal ceiling and external door were set decades ago, that 30mm is the difference between reusing the existing door and lowering a threshold. Upstands at the parapet follow the same logic: the insulation must continue up the wall to the same thickness to avoid a thermal bridge at the perimeter, and a taller PIR upstand can foul a coping overhang.

The tapered scheme most estimates leave out

Flat roofs are rarely flat. Building Regulations expect a minimum finished fall of 1:80 to move water to outlets, which in practice means designing to 1:40 to allow for deck deflection and construction tolerance. On a 5m roof run, a 1:40 fall adds 125mm of height at the high point. That interacts directly with the U-value calculation, because the average insulation thickness across a tapered scheme, not the thinnest point, sets the mean U-value.

Both Kingspan and the PIR manufacturers cut tapered systems to a scheme drawing. A tapered Kooltherm scheme starts thinner at the outlet and still averages the thickness needed for 0.18, so the thinnest board at the drain might be 60mm while the ridge reaches 150mm. Miss the tapered layout and a flat-thickness estimate either overspends at the outlet or fails the fall requirement.

The tapered cut also changes the ratio of full-thickness board to cut pieces, which raises the effective cost per square metre above the flat-board rate. On a small extension the tapered premium can be steep enough that a firring-timber fall under a flat insulation layer becomes cheaper, at the cost of a thermal bridge through the firrings that the U-value sum must account for. That timber bridging is exactly what pushes the nominal 100mm Kooltherm calculation up to a 120mm specification.

Compatibility between the tapered scheme and the membrane matters too. A cold-applied liquid system needs a primer suited to the board facing. Foil-faced PIR and tissue-faced phenolic prime differently, and the manufacturer scheme states which primer the warranty requires.

Fire and the phenolic argument

Kooltherm K107 achieves better reaction-to-fire performance than standard PIR, which is one reason phenolic boards feature on projects with tighter fire requirements. For a domestic single-storey flat roof this rarely drives the choice. The height saving and the moisture behaviour do most of the deciding.

Fixing pattern and wind uplift

Mechanical fixing counts on a flat roof scale with wind zone, roof height and the exposure of the corners and perimeter. A warm roof buildup fixed through the insulation into the deck uses telescopic tube fixings sized to the total board depth, so a 150mm PIR scheme needs longer tubes than a 120mm Kooltherm one, and the fixing schedule increases density at the perimeter and corners where uplift concentrates.

Bonded systems avoid penetrating the VCL, which suits the warm roof principle better because every mechanical fixing through the vapour barrier is a potential moisture path. Hot-melt and cold-applied adhesives bond the insulation to the VCL and the membrane to the insulation, keeping the barrier continuous. The adhesive choice again returns to the facing: the glass tissue on Kooltherm and the foil on PIR each carry a specific bonding recommendation from the board manufacturer, and mixing an incompatible adhesive with either facing voids the system warranty and risks delamination under thermal cycling.

The practical outcome is that a fully bonded warm roof tends to favour whichever board the chosen membrane system was tested against. A single-ply supplier that warranties its membrane over foil-faced PIR may not extend the same cover over phenolic tissue, and that constraint can override the height argument entirely.

What the U-value sum does not tell you

The calculation that lands on 120mm Kooltherm or 150mm PIR assumes the boards stay dry and the facings stay intact for the life of the roof. A warm roof that traps construction moisture, or one where the VCL laps were never taped at the rooflight upstand, will drift from its designed 0.18 within a few winters as the phenolic core takes up water and its lambda climbs.

The open question on any of these buildups is not which board hits the number on paper. It is whether the detailing at the three or four penetrations on a small roof was executed as carefully as the flat field of boards, because that is where the calculated U-value and the roof that actually gets built start to part company.