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How to Insulate a Flat Roof

How to Insulate a Flat Roof

When considering thermal insulation for flat roofs, there are two primary options - warm flat roofs and cold flat roofs. Each presents unique benefits and considerations, influencing the choice for building owners and developers. This guide explores the differences between warm and cold flat roof systems, installation methods, refurbishment options, waterproofing choices, and insulation board selections.

Warm Flat Roof vs Cold Flat Roof:

The fundamental distinction between warm and cold flat roof systems lies in the placement of the insulation layer. In a warm flat roof, insulation rests atop the roof structure, ensuring the interior remains warm. Conversely, a cold flat roof positions insulation between the rafters and roof deck, creating a cold gap beneath the solid board. Both options cater to various building configurations, from standalone flat roof structures to extensions of a pitched roof.

Choosing Between Warm and Cold Flat Roofs

The decision between warm and cold flat roofs hinges on specific considerations. Opting for a warm flat roof prioritises superior thermal performance and ease of installation. Conversely, cold flat roofs offer space-saving advantages, crucial for areas with balconies or doors opening onto flat roofs. Additionally, cold flat roofs provide enhanced support for foot traffic due to the direct contact with faced boards, minimising compression concerns associated with insulation boards.

Insulating a Hot Flat Roof:

A warm flat roof comprises six sections, with insulation applied externally. The waterproofing membrane forms the top layer, followed by insulation boards and a vapor control layer secured over the timber roof decking. Uninsulated roof joists allow for the installation of plasterboard, facilitating interior decoration. The vapor control layer prevents condensation and moisture ingress, while the waterproof membrane ensures protection against adverse weather conditions.

Insulating a Cold Flat Roof:

Conversely, a cold flat roof maintains the same structural elements but insulates internally. Only the waterproof membrane sits atop the timber deck, with insulation boards installed between the roof joists. A vapour barrier beneath the insulation boards and a plasterboard ceiling completes the setup, ensuring efficient thermal insulation from within.

Refurbishment Options

Refurbishing existing flat roofs involves assessing the structural integrity and waterproofing condition. Options range from replacing aged or damaged waterproof membranes to complete overhauls, including the removal and replacement of all components. Retrofitting insulation alone is impractical for warm or cold flat roofs, necessitating comprehensive refurbishment measures tailored to the roof's condition.

Waterproofing Choices

Upon constructing an insulated flat roof, selecting a suitable waterproofing membrane becomes crucial. Options include single-ply membranes, built-up felt, and mastic asphalt, each offering distinct advantages in durability, weather resistance, and ease of installation.

Insulation Boards for Flat Roofs:

Various rigid insulation boards cater to flat roof insulation requirements, such as Kingspan Thermaroof TR26 and Recticel Powerdeck F. These boards ensure compatibility with specific waterproofing systems while achieving desired U-values for thermal insulation.

Insulating a Dormer Flat Roof:

Insulating a dormer flat roof mirrors the approach for larger flat roofs, utilising warm or cold flat roof systems with insulation boards, vapour control layers, and waterproofing surfaces. This strategy effectively minimises heat loss through dormer flat roofs, optimising energy efficiency.

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