VIEWPOINT


A MERCHANTS’ GUIDE TO FLAT ROOF INSULATION


Matt Neary, national sales manager, Knauf Insulation


FLAT ROOFS ARE increasingly multifunctional spaces, hosting solar PV, green roofs, terraces and mechanical plant. When designed and built well, they can deliver excellent thermal performance, long-term durability and support a variety of functions. This creates new demands for insulation, and new opportunities for merchants to supply solutions that meet these expanded requirements.


Offering the right insulation for the application


The first decision in any flat roof project is the build-up type. Most flat roofs fall into two categories – warm or cold – based on whether the insulation is placed above or below the roof deck. Warm flat roofs fix the insulation above the deck and the vapour control layer. The insulation is then covered by a single-ply membrane. Strong thermal continuity, minimal thermal bridging and reduced condensation risks mean this is increasingly the standard approach to flat roofs. Your customers will need to consider how the warm roof is being used to determine the compressive strength required for the insulation. For example, if only occasional access for maintenance is needed, then Rocksilk Flat Roof Slab’s compressive strength of 70 kPa is more than adequate. If the roof will have frequent foot traffic and additional weight, such as from solar PV panels or mechanical plant, then a product with even higher compressive strength will be needed.


A cold flat roof places insulation below the structural deck, within the ceiling void, leaving a space between the insulation and the deck for ventilation. Despite some drawbacks, cold flat roofs are still used in many commercial buildings, retrofits, older residential properties, and constructions where minimising the height of the roof is critical.


Both glass and rock mineral wool 16


are suitable for this application and provide comparable thermal, acoustic and fire performance. Your customers may prefer glass mineral wool in this application as it is easy to cut and handle. The flexibility of glass mineral wool also has a performance bonus: as it will adapt to minor imperfections in the surface of the joists to minimise air gaps and maximise thermal performance.


Meeting thermal regulations


Approved Document L sets limiting U-values at 0.16 W/m²K for residential and 0.18 W/m²K for non-residential flat roofs. The Future Homes Standard, expected this year, may bring further tightening and even in- use performance checks. As the focus shifts from theoretical to real performance, insulation that


is easy to install correctly becomes increasingly valuable.


Merchants who can provide accurate U-value guidance can help customers choose compliant build-ups without costly redesigns or delays.


Acoustic performance: the overlooked priority


Acoustic insulation is becoming a priority as heavier rainfall, rooftop equipment and growing urban density increase the risk of disturbance. Rain impact noise, vibration from plant and general environmental sound can all be transmitted into spaces below flat roofs, making acoustic insulation a performance priority. Mineral wool addresses noise through absorption, density and separation. In warm roofs, rock mineral wool slabs combine


compressive strength with acoustic absorption. In cold roofs, lighter glass mineral wool slabs are easy to handle while still providing acoustic performance. Prioritising acoustic insulation can help deliver buildings that are not just efficient but comfortable.


Supporting products and accessories Merchants can add value – and sales – by ensuring customers have access to the complementary products needed to deliver a high performing flat roof. Vapour control layers are essential in warm and cold roof systems to prevent interstitial condensation, and high-quality breather membranes help manage moisture movement. Mechanical fixings suited to the chosen insulation type, as well as appropriate adhesives and sealants for detailing around penetrations, are also vital for ensuring long- term performance.


Edge trims, upstands, and pre formed corners can simplify installation and improve detailing, while acoustic or thermal breaks for service penetrations help maintain envelope integrity. Stocking compatible flat roof accessories means merchants can provide a “one stop” solution, reducing the likelihood of site delays and ensuring that the insulation performs as intended.


Performing on every front


The opportunity for merchants lies in supplying flat roof insulation that performs on every front: thermal efficiency, compressive strength and acoustic comfort. This means engaging customers on project specifics, understanding regulatory drivers and positioning insulation as a multifunctional solution rather than a commodity. By doing so, merchants can meet current requirements and anticipate future ones, helping deliver roofs that are compliant, comfortable and built to last. BMJ


www.buildersmerchantsjournal.net September 2025


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