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Warm Flat Roofs are Simply the Solution

Warm Flat Roofs are Simply the Solution when it comes to Quality and Performance

Article originally published in the October edition of Insulate Magazine. Receive every issue free via email by subscribing here.

A warm flat roof, correctly designed, specified and built, is one of the simplest construction elements of the building envelope, writes Paul Forrester, Technical Services Manager at PIR insulation manufacturer, Recticel. However, its performance remains reliant upon a basic, but essential value: high-quality workmanship.

Structural deck, vapour control layer, insulation and waterproofing combine to create a warm flat roof; the perfect example of a fabric-first approach in action. It’s a tried-and-tested solution, not least because it keeps the roof structure at or around the same temperature as the building interior. This eliminates the possibility of condensation occurring – which could otherwise cause deterioration of the structure – and ensures the roof performs for the building’s intended lifespan. And yet questions such as, Does a roof need a vapour barrier? continue to be asked, highlighting that misapprehensions remain about the ramifications of flat roof design. Too often, poor roofing performance results from a lack of awareness good-practice principles.

Space consideration

Compromises can also be caused by on-site constraints, a good example being that of hybrid roofs. Imagine a flat roof constructed on timber deck and joists: it’s not surprising that people sometimes look at the space between joists and wonder if it can be filled with additional insulation. If there is a restriction on the thickness of insulation that can be accommodated above the structure, then the temptation to utilise the extra space is even greater.

However, problems can occur if a standard warm roof is designed and constructed, but with additional insulation incorporated below the deck. Not only is a vapour barrier or vapour control layer (VCL), a vital component of the roof, it must be correctly positioned – on the warm (internal) side of all the insulation. There are numerous reasons why incorrect placement of insulation relative to the VCL should be avoided, perhaps best summarised in the foreword of BS 5250:2011 + A1:2016 Code of practice for the control of condensation in buildings, which states: Bearing in mind that occupants often fail to use buildings in the manner intended, be it by choice, lack of understanding or force of circumstance, designers are advised to err on the side of caution and adopt robust, fail-safe solutions.

Hybrid Roof Insulation Solution Warm Roof Insulation Solution

Label with care

To that end, section H.2 of the standard lists three acceptable scenarios for the placement of insulation in a flat roof: cold roof, warm roof and inverted roof. Nowhere does it support the design and construction of hybrid flat roofs. Some insulation manufacturers, however, are happy to advocate hybrid constructions, usually when the balance of the two insulation thicknesses is considered to be correct, i.e. the layer of insulation between the joists does not have higher thermal resistance than the layer of insulation installed over the deck.

A condensation risk carried out in accordance with the method detailed in BS EN ISO 13788 – Hygrothermal performance of building components and building elements – may appear to show no risk. However, the fact that BS 5250 does not support the construction type means that we believe insulation manufacturers should label any condensation risk analysis for a hybrid flat roof as not recommended.

Consider as well another reason to doubt the appropriateness of this analysis of a hybrid flat roof solution. Section 4.2 of BS 5250 states: BS EN ISO 13788 considers only the risks arising from the diffusion of water vapour through the building fabric; it does not take account of the much greater risk of condensation occurring as a result of air leakage, which transports water vapour through gaps, joints and cracks in the building fabric.

By its very nature, a hybrid roof that features insulation fitted between timber joists introduces the potential for air gaps. Section A.3 is clear that, the rate at which moisture is transported by air movement, where it occurs, is much greater than that of transportation by diffusion. A condensation risk analysis cannot adequately allow for the standard of workmanship, and a high level of workmanship in itself would not mitigate potential issues.

If anybody is to make a judgement on the levels of workmanship, and whether it makes the construction method any more acceptable for a particular project, it should be the designer, building inspector or end user. The insulation manufacturer can advise on the interpretation of a condensation risk analysis for a hybrid roof, but first and foremost should promote the fundamental principles of good roof design outlined in BS 5250.

Considering one of those principles is to ensure the correct VCL position, some might ask if a hybrid roof is acceptable if the VCL is moved to ceiling level behind the internal plasterboard finish. While technically that might satisfy the basic requirements, we would urge practical thinking: are light fittings or other services going to be installed in the ceiling? Can the continuity of the VCL – vital to its performance – be guaranteed?

It only takes the building’s next owner to want to change something and that VCL could be compromised as part of any works to the ceiling. Will anybody make sure it is restored to its original condition? If a VCL cannot be relied upon to mitigate the risk of condensation then the roof design cannot be considered appropriate, and there are precious few alternative solutions.

Install a Warm Roof With Confidence

Despite the best efforts of many to promote the advantages of service voids – airspaces between ceiling and VCL, where services can be installed and the ceiling altered without risking the integrity of the VCL – few seem willing to employ them. There are several reasons why: extra time and materials to construct, restrictions on headroom etc; all of which serve to highlight why it is best to keep things simple.

One of the positive aspects of a warm roof is that the VCL can be installed with the confidence that it will perform for as long as the roof performs – hopefully for the lifetime of the building. That’s why we referred to it as a good example of fabric first construction, and it’s why we believe it should always be preferred over a hybrid alternative.

What is your perspective on warm flat rooms, do you agree with the points raised in this article? Leave a comment below and join the debate

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  1. Hello,
    I am interested in a Hybrid flat roof principle and have a rudimentary understanding of the issue of condensation. However I can not find anywhere on the internet what would seem to me to be the obvious solution which might be to fill the void between the joists with a closed cell spray insulation thus removing all space for air to circulate, and if he joist were Eco joist with an open metal web it should reduce any cold bridging through the joist. This should surely remove and risk of condensation and almost create a site constructed SIP panel. I would be delighted to hear your thoughts on this.

  2. Hi Mark, Thanks for the question.

    We put your question to the authors, Recticel Insulation and their response is below..


    Web Team


    We would recommend that they speak to a spray foam manufacturer regarding this enquiry, to see what their approach is with hybrid flat roofs and seek advice on using a spray foam insulation in the proposed application.

    BS 5250 (British Standard for Control of Condensation) do not recommend any form of hybrid flat roof, therefore we do not recommend them either.

    I hope this is of assistance.

    Kind regards,

    Ashley Millington
    Technical Assistant
    BBA Competent Person CS/1003-6

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