Why is 150mm PIR insulation better than standard 120mm on flat roofs in Torbay?

While 120mm PIR satisfies baseline Part L building regulations (achieving approx 0.18 W/m²K), our 150mm PIR specification lowers thermal transmittance to an ultra-efficient 0.14 W/m²K. In coastal Devon microclimates, this eliminates interstitial condensation risk and cuts thermal loss through extension and dormer ceilings by an additional 25%.

What is Carlisle Alutrix 600 and why is it essential?

Alutrix 600 is an aluminium-reinforced self-adhesive vapour barrier. Unlike flimsy polythene sheets, it bonds directly to the structural 18mm OSB3 deck to form an airtight seal that stops warm room moisture penetrating into the insulation, while providing immediate walk-on weatherproofing during construction.

What are timber hard edging grounds on a warm deck roof?

Timber hard edging grounds are solid pressure-treated timber battens installed around all outer perimeters and gutter drops to match the insulation stack height. They prevent crushing of the rigid PIR board edges and provide solid mechanical anchors for metal trim fixings and gutter brackets.

Warm Deck Insulation Torbay | 150mm PIR & Alutrix VCL

The Definitive 6-Stage Spec

Achieving a legal U-Value of 0.14–0.18 W/m²K requires a factual, multi-stage layering process that handles the extreme maritime humidity of Torbay.

At Raven Roofing Services, our warm deck protocol begins with a structural 18mm OSB3 conditioned deck with precision firrings to create 1:80 falls. We then install a heavyweight Carlisle Alutrix 600 aluminium VCL bonded directly to the deck to arrest moisture migration and provide immediate dry-in protection.

The core thermal layer is 150mm PIR insulation (or 120mm where door thresholds require), which is structurally supported at all perimeters and gutter channels by technical timber hard edging grounds.

This hard edging is a critical structural component that inferior contractors omit; it prevents compression of the rigid insulation and provides a permanent mechanical anchor for edge trims and gutter drip brackets. The build-up is completed with a high-tensile carrier underlay and a final weathering layer of 20-year spec mineral SBS bitumen felt. Every installation is engineered for Approved Document Part L compliance and undergoes an independent third-party inspection for signed-off building warranty records.

Technical Build-up

01

18mm OSB3 Deck

Structural foundation board laid across inspected joists with falls to gutters.

02

Alutrix 600 Aluminium VCL

Airtight, puncture-proof aluminium vapour control layer bonded directly to timber.

03

150mm PIR Rigid Core

High-density thermal core driving U-values down to 0.14 W/m²K, moving dew point safely outside.

04

Structural Hard Edging

Solid timber grounds milled to 150mm height to stop perimeter compression and secure drip trims.

05

Carrier Underlay & SBS Cap

Elastomeric high-tensile carrier capped with heavy-duty 20-year SBS mineral felt.

On-Site Technical Evidence | Live Devon Rebuild

Photographic Build Sequence: Alutrix 600 & 150mm PIR

This live residential flat roof rebuild demonstrates our uncompromising warm deck engineering in South Devon. Every installation is documented step-by-step to provide undeniable photographic proof for your property survey and Building Control sign-off.

The project highlights a continuous Carlisle Alutrix 600 self-adhesive vapour control layer dressed seamlessly over the structural deck, fully wrapped up vertical parapets, and hermetically sealed over the raised timber skylight upstand. 150mm rigid PIR insulation boards are laid with staggered zero-gap joints, terminating into solid timber hard edging grounds along the recessed gutter trough before receiving our high-performance SBS waterproofing capping.

Carlisle Alutrix 600 self-adhesive vapour control layer installed across 18mm OSB3 flat roof deck in Devon

Stage 01: Deck VCL

Continuous Alutrix 600 VCL

Directly bonded to the structural OSB3 deck. The puncture-proof aluminium foil core creates an airtight seal arresting internal moisture migration.

Airtight lap joints and parapet wall upstand dressing with Alutrix aluminium vapour barrier on flat roof

Stage 02: Parapet Seal

Parapet & Perimeter Dressing

Membrane dressed continuously up the vertical boundary parapets. Lap joints are pressure-sealed to eliminate thermal bypass.

Alutrix vapour control layer dressed against natural stone external house wall and raised skylight kerb

Stage 03: Stone Abutment

Heritage Stone Integration

Alutrix terminated high against natural stone masonry, elevated to receive Code 4 stepped lead flashings once the insulation stack is in position.

Structural timber roof lantern upstand kerb fully wrapped in Alutrix vapour control barrier

Stage 04: Aperture Wrap

Skylight Kerb Hermetic Wrap

Upstand timber frames wrapped completely in the vapour barrier, preventing moist room air from leaking into the cold side of the aperture frame.

150mm thick rigid PIR insulation board installed across deck terminating into recessed timber hard-edging gutter channel

Stage 05: 150mm PIR & Eaves

150mm PIR & Hard Edging

High-density 150mm rigid PIR boards bonded over the VCL, terminating into structural timber hard edging grounds along the gutter drip trough.

150mm PIR rigid insulation fitted tightly around raised skylight upstand on flat roof in Torbay

Stage 06: Zero-Gap Continuity

Precision Kerb Encapsulation

Rigid 150mm PIR cut tight to the raised window aperture frame, eliminating thermal bridges and preparing the surface for final SBS capping.

Aperture Integration

While structural thermal layers are the priority, the integration of roof windows and lanterns requires specialized technical foresight. In a warm deck rebuild, the thermal stack elevates the roof plane by 150mm.

As proven in our live project photographs above, we construct specialized structural timber upstands (kerbs) raised a minimum of 150mm proud of the finished insulation level. This allows for continuous vapour dressing and a definitive watertight seal between the window frame and the 3-layer SBS material stack.

Thermal Continuity Note

"Installing a Velux window or lantern during a warm deck rebuild is the optimal time for fitting. This ensures complete thermal continuity and prevents cold spots around the window frame that otherwise lead to internal condensation and mould growth."

Thermal Performance Metrics

System Metric	Legacy Cold Roof	Raven Warm Deck Spec
Thermal Core	Mineral Wool Between Joists (Cold)	150mm High-Density Rigid PIR (Warm)
Calculated U-Value	0.35 – 0.50 W/m²K (Severe Heat Loss)	0.14 W/m²K (Surpasses Part L 0.18 target)
Dew Point Position	Internal Timber Rafters (Rots Timbers)	External (Above Structural Deck)
Vapour Control Layer	None or Torn Polythene Sheet	Carlisle Alutrix 600 Aluminium Self-Adhesive
Perimeter Support	None (Trims Nailed into Soft Foam)	Technical Timber Hard Edging Grounds
Compliance & Sign-off	Non-Compliant with Part L	Part L Certified + 3rd Party Inspected

Factual technical data based on BS 6229:2018 and Approved Document Part L Building Regulations.

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Technical Warm Deck Contractors • Serving TQ1 to TQ12 • Paignton • Torquay • Brixham • Totnes

Warm Deck Insulation & Thermal Rebuilds