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+44 (0)1268 777871Choosing the Right Insulation for Flat Roofs
Ensuring the thermal performance of flat roofs complies with current UK Building Regulations is essential for energy efficiency, reducing heat loss, and preventing issues like condensation. The guidance below is based on Approved Document L (2021 edition incorporating 2023 amendments), which covers conservation of fuel and power. This includes requirements for both new build and refurbishment projects, distinguishing between dwellings (Approved Document L, Volume 1) and buildings other than dwellings (Approved Document L, Volume 2). These are minimum standards; best practice often involves exceeding them to achieve better energy performance, such as incorporating continuous insulation to minimise thermal bridging.
New Build Dwellings
For new dwellings, flat roofs must meet limiting U-values to limit heat gains and losses through the building fabric. The maximum (limiting) U-value for flat roofs is 0.16 W/m²K (area-weighted average).
- Best practice: Design to the notional building specification, which uses a U-value of 0.11 W/m²K for flat roofs, to help meet overall targets for primary energy rate, emission rate, and fabric energy efficiency. Ensure insulation is continuous, tightly fitted without air gaps, and extends to meet wall insulation. Assess load-bearing capacity before adding insulation.
New Build Buildings Other Than Dwellings
For new non-domestic buildings, the limiting U-value for flat roofs is 0.18 W/m²K (area-weighted average). These standards apply to new thermal elements, with the overall building needing to comply with primary energy and emission rate targets.
- Best practice: Limit thermal bridging and ensure insulation continuity across the roof structure. Consider airtightness to reduce infiltration, sealing around service penetrations as per Approved Document F. Notional specifications suggest aiming for lower U-values, such as 0.11 W/m²K for flat roofs, to enhance performance.
Extensions, Replacements, and Refurbishment of Existing Dwellings
When extending an existing dwelling or replacing a flat roof element (e.g., during waterproofing renewal), new fabric elements must meet limiting U-values as new thermal elements. The maximum U-value for flat roofs is 0.15 W/m²K (area-weighted average). This applies to elements in extensions and new or replacement elements in existing dwellings.
For refurbishment of an existing flat roof (renovating a thermal element, e.g., replacing waterproofing or renovating more than 50% of the element's surface, or during a major renovation affecting over 25% of the building envelope), check the existing U-value. If it is worse than the threshold of 0.35 W/m²K, the roof must be upgraded where technically, functionally, and economically feasible (e.g., simple payback of 15 years or less).
- Improved U-value target for renovated flat roofs: 0.16 W/m²K (area-weighted average, with specific considerations for insulation placement, such as over joists).
- If the target is not feasible: Upgrade to the best achievable U-value with a payback not exceeding 15 years, but no worse than 0.70 W/m²K.
- If the existing U-value is 0.35 W/m²K or better: No mandatory upgrade, and the roof can often be overlaid with a waterproofing membrane alone, provided it avoids condensation risks.
- Best practice: Assess load-bearing capacity before adding insulation. Always conduct a condensation risk analysis, especially where vapour-permeable constructions are involved, to prevent long-term fabric deterioration. If headroom is limited, a slightly worse U-value may be acceptable, but insulate to the lowest practicable U-value.
Extensions, Replacements, and Refurbishment of Existing Buildings Other Than Dwellings
For extensions or replacements in non-domestic buildings, new thermal elements (including flat roofs) must meet limiting U-values of 0.18 W/m²K (area-weighted average). This is particularly relevant for buildings over 1,000 m² where consequential improvements may be triggered.
For refurbishment (renovating existing flat roofs, e.g., if more than 50% of the surface is renovated or during a major renovation), if the existing U-value exceeds the threshold of 0.35 W/m²K, upgrading is required where feasible.
- Improved U-value target for renovated flat roofs: 0.18 W/m²K.
- If the target is not feasible: Aim for the best U-value with a 15-year payback, not exceeding 0.70 W/m²K.
- If the existing U-value is 0.35 W/m²K or better: Upgrading is not required, allowing for waterproofing overlay without additional insulation, subject to condensation checks.
In all cases, if there is little or no existing insulation, upgrading to the relevant improved U-value is recommended where feasible to enhance energy efficiency and avoid condensation. Let our technical team advise you on the most suitable insulation and waterproofing solutions tailored to your project.
View our Technical Insulation Solutions products
Proteus Pro-Therm
Our in house technical design team can overcome challenging project criteria with innovative design solutions and hybrid options.
Cold Melt®
Cold Melt® incorporates recycled rubber crumb and other organically grown products to create an elastomeric, seamless, cold applied membrane that can be applied to a wide range of substrates including concrete, asphalt and timber.
Proteus Hot Melt®
Proteus Hot Melt® is a hot-applied modified bitumen, reinforced with reinforced with Proteus Hot Melt Fleece, used in conjunction with a bitumen roofing.
Proteus Pro-Cold®
Proteus Pro-Cold® is a seamless and cold-applied waterproofing membrane for flat roofs, pitched roofs and a waterproofing coating to Proteus’ Built-Up Roof systems.
Pro-BW® Plus
Pro-BW® Plus is a liquid-applied, rapid curing protective coating that provides a colourfast, UV stable and skid-inhibiting surface for balconies and walkways.
FAQs
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How can Proteus Waterproofing help me choose the right insulation?
Proteus Waterproofing have a range of insulation solutions and can provide the right specification and products for a variety of projects – even those in areas where the risk of fire can be a major concern.
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What are the benefits of using Proteus Waterproofing for my technical insulation needs?
Proteus Waterproofing offer a wide range of insulation materials, design solutions and technical expertise to help you achieve the desired performance and meet building regulation requirements.
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What types of technical insulation solutions does Proteus Waterproofing offer?
We offer a range of technical insulation solutions, such as:
• Hybrid insulation, which combines different types of insulation to achieve the desired performance and meet specific project requirements. For example, hybrid insulation can be used to create a continuous insulation layer on party walls, where PUR insulation cannot meet the required performance standards
• Additionally, we provide insulation materials like PIR and phenolic foam, known for their thermal insulation
• Our in-house technical design team can develop design solutions and hybrid options
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Why is it important to choose the right insulation?
Choosing the right insulation is essential for achieving the desired performance and to meet building regulations. Current building regulations state that when refurbishing an existing roof, the minimum insulation threshold of U-Value 0.35 W/m²K must be achieved. This can be achieved by overlaying the roof with a waterproofing membrane only if the existing insulation meets this threshold. If the existing insulation does not meet this threshold, the roof must be upgraded with roof insulation to meet a minimum U-Value of 0.18 W/m2K.
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What are technical insulation solutions?
Technical insulation solutions are designed to meet the projects specific needs, such as those with fire safety concerns or intricate design requirements.
Case studies
Basildon Fire Station, Essex
Basildon Fire Station's roofs and terrace were showing signs they were at the end of their life and needing replacement.
