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Flat Roof Insulation 

Insulating a flat roof is a task that must be done during construction. Retrofitting flat roof insulation is a difficult task and often a messy one that involves pulling up the waterproof layer that finishes off the deck so as to put flat roof insulation in underneath. You will be unable to pass your building or extension through required regulations without flat roof insulation. Insulating a flat roof in the first instance is the best way to achieve a well-insulated, competent, and good-looking structure.

A warm roof deck is the most common and most preferred type of flat roof build up in the UK. A flat roof where the insulation sits below the waterproof membrane and above the structural deck is a warm roof deck. Cold flat roofs are typically avoided as you would need to install ventilation, which can prove problematic for many reasons relating to space, labour costs, etc. Warm flat roofs do not need to be ventilated as the insulation is above the structure.

Benefit of the flat roof insulation

  1. Warm flat roofs create little or no thermal bridging. This ensures there is no unnecessary heat-loss through joints or fixings.
  2. A warm roof is breathable, allowing moisture out. A breathable roof ensures there is no moisture build up which can lead to fungal or mould growth.
  3. A warm flat roof can be retrofitted without too many complications. This means that in some cases (not all) the roof can be fitted right over the existing roof.
  4. Warm roofs allows heat to be retained within the property. Warm roofs are simply insulated more efficiently than cold roofs because of the way they are formed.
  5. When retrofitted, warm roofs can be considerably cheaper to install than a cold roof. This means saving money that can be spent on improving other areas of the building.

How to insulate flat roof

How do you insulate a flat roof?

Insulating a flat roof is relatively simple in terms of what materials you need, and the steps taken to complete the job.

  1. Lay 12mm of plywood or OSB board across the timber joists as a base for the flat roof build up
  2. Install a vapour control layer to reduce the risk of condensation and moisture compromising the flat roof structure
  3. Mark out on the vapour control layer where fixings will pierce the membrane (at rafter centers and at the correct axial spacing)
  4. Place butyl tape at each point where the vapour control layer will be pierced so the tape forms a seal after penetration
  5. Often dictated by the height available thanks to guttering, upstands and other adornments, lay rigid insulation board to the desired thickness
  6. Install a second layer of plywood or OSB board
  7. Fix the deck together with helical fixings, piercing each layer of the buildup including the flat roof insulation
  8. Finally install a waterproof membrane to finish off the deck and protect the build up from wind and driven rain

Insulating a flat roof with Kingspan Thermaroof

Kingspan has two somewhat similar flat roofing insulation products that come highly recommended from contractors and roofers alike. Kingspan Thermaroof TR26 and Kingspan Thermaroof TR27 are both high performance PIR boards which are both approved for steel flat roof deck assemblies.

Thermaroof TR26 is designed for use with a mechanically fixed single-ply waterproofing membrane thanks to foil facings on both sides. It achieves a u-value of 0.022 and can be used in conjunction with green roof systems.

Thermaroof TR27 has glass tissue facings on either side of the flat roof insulation board which allow the product to offer u-values as low as 0.024. Excellent compressive strength and the coated glass tissue facings combine to make Thermaroof TR27 compatible with mechanically fixed single-ply waterproofing membranes and partially bonded built up felt.

How to install flat or tapered roof insulation over a plywood deck

How to install flat or tapered roof insulation over a concrete deck

How to install flat or tapered roof insulation over a metal deck


Most Common Flat Roof Insulation Products


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