IMPORTANT: On 1 September 2017, Celotex took the precautionary measure to temporarily suspend the supply of Celotex FR5000, Celotex CG5000, Celotex CF5000 and Celotex SL5000 while we investigate the results of recent tests (Parts 6 and 7 of British Standard 476). In addition, we have recently identified a compliance issue relating to our calculation and testing of the declared lambda value of products in the 4000 and 5000 ranges and the Crown-Bond and Crown-Fix products within the Crown Flat Roofing range. Due to this issue, the suspension of the 5000 range will continue and now includes the FI5000 and GD5000 products. Materials relating to the 5000 range products are for information only. Please note that all products in the TB4000, GA4000 below 100mm, PL4000 and CW4000 ranges manufactured after 15 December 2017 will be marketed from January as Celotex 3000 with a declared lambda value of 0.023 W/mK.

Please use the link below to read our full statement regarding declared lambda:

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Please use the link below to read our latest statement regarding Class 0:

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This post will discuss why hybrid flat roofs can pose the risk of harmful condensation if used in an attempt to meet insulation requirements.

As U-value requirements have become stricter over time, thicknesses of insulation required to meet the building regulations have increased.

This provides a challenge to building designers, who have had to accommodate ever greater thicknesses of insulation in to the building fabric.

This can be a particular issue for refurbishment and conversion projects, where the position and dimensions of various elements may be fixed.

Warm and cold roofs for insulation purposes

Easily the most frequently asked question to the Celotex Technical Centre is with reference to hybrid flat roofs. In this context, the term hybrid flat roof refers to a timber flat roof construction that is insulated both between and over the joists.

Traditionally flat roof constructions fall in to two camps: warm flat roofs and cold flat roofs. In each case the name refers to the position of the insulation in relation to the roof deck.

  • A warm flat roof is defined by insulation being installed on top of the roof deck. Here the roof deck is maintained at the same temperature as the internal conditions (hence warm deck roof).
  • A cold flat roof is defined by insulation being installed between (and if necessary underneath) the timber joists. Here the roof deck is maintained at the same temperature as the external conditions (hence cold deck roof).

With a cold flat roof construction, it is critical that a minimum of 50mm of cross ventilation is provided underneath the roof deck. This is to minimise any risk of harmful condensation.

Each of these constructions have draw backs to the building designer. A warm flat roof may require anywhere from 120 to 200mm of insulation on top of the roof – depending upon the U-value requirements of the project. This requires a larger fascia detail and may make it difficult to meet planning requirements, as the roof grows in height.

A cold flat roof construction requires ventilation. This can be particularly problematic where the joists do not run to a clear fascia, or where a lantern light obstructs the clear flow of air.

For many designers, a perfect solution would seem to be the hybrid flat roof. A thinner insulation over the top of the roof gives the benefit of a warm design (no ventilation) while accommodating the majority of the insulation between the joists keeps the profile as thin as possible.

Unfortunately such a design can pose a serious risk of harmful condensation.

Reducing the risk of harmful condensation

Building regulations in the UK require building designers to minimise the risk of harmful condensation. The most common route to compliance is to follow the guidelines laid down in British Standard BS 5250:2011  – Code of practice for control of condensation in buildings.

This document describes flat roofs as adhering to either a warm flat roof design or a cold flat roof design. It also makes other recommendations – such as the requirement to provide an AVCL (air and vapour control layer) in each case.

It does not specifically prohibit a hybrid approach, and this has been used by some companies as justification of a hybrid solution.

Why should a hybrid flat roof be avoided?

Simply put, a warm roof design maintains the roof deck at a warm temperature – it does not present a cold surface for condensate to form upon.

A cold roof design uses a different mechanism to control the risk of condensation – fresh air circulating under the roof deck takes moisture vapour away before it can form as condensate.

A hybrid approach means that you have neither of these fundamental protections against the risk of harmful condensation.

A popular argument for a hybrid approach is that with a continuous vapour control layer the risk of condensation is minimised. A condensation risk analysis is then provided to justify this stance.

Unfortunately a condensation risk analysis makes certain assumptions that may not reflect real life conditions. For example, a condensation risk analysis will assume that any vapour control layers are installed to a perfect standard of workmanship and are not penetrated by services or fixings. This is unlikely to be the case in reality.

A condensation risk performed to BS EN ISO 13788 has other limitations. For example it does not take in to account thermal bridging within the structure. In addition it only models the movement of water vapour by diffusion and not by convection.

There is no doubt that best practice is to follow the guidance laid out in British Standards. A fully warm roof construction is recommended and where this is not practical, a cold roof can also be considered.

In the opinion of Celotex, a hybrid solution presents an unacceptable risk of harmful condensation and should be avoided by building designers.

View our range of insulation products for flat roof applications here.