When Warm Roofs Go Wrong

Flat roofs are among the simplest construction elements that can feature in a building. A warm roof features a structural deck, vapour control layer, insulation and waterproofing – a tried and tested solution that the industry knows works.

Designed, specified and constructed correctly, a warm roof will keep the roof structure at the same temperature as the building’s interior, eliminating the possibility of condensation occurring and ensuring the roof performs for the life of the building.

Why, then, do issues still occur with warm flat roofs?

Not so simple?

“Does the roof need a vapour barrier?” remains a common question, highlighting that plenty of misapprehensions and misunderstandings still exist about flat roof designs. Too often, designers and contractors alike are unaware of basic, good practice principles that ensure a roof performs.

Compromises can result from on-site constraints too. Maybe there is a height restriction on the roof (especially if it’s an extension to an existing building), or the aesthetics of deep fascia boards are considered undesirable.

On other occasions, pressing on with a roof installation before the specification has been finalised, or working to out-of-date specifications, results in a misjudgement of the insulation thickness. Or the preferred insulation stockist only carries certain thicknesses, so what’s available is bought rather than what is needed to achieve the necessary U-value.

Hybrid flat roofs

Imagine a flat roof constructed on timber deck and joists. When issues like those above occur, it’s not surprising that people look at the space between the joists and wonder if they can use it for more insulation. If the basic warm roof construction is followed before extra insulation is installed, problems can arise.

Not only is a vapour barrier, or vapour control layer (VCL), a vital component of the roof, it must also be correctly positioned: on the warm (internal) side of all the insulation. The British Standard for the control of condensation in buildings (BS 5250) makes the best argument for why the incorrect placement of insulation should be avoided: in short, because occupants can’t be relied upon to use buildings in the intended way!

Designers should therefore take a cautious approach and adopt fail-safe solutions. To that end, the standard lists three acceptable scenarios for the placement of flat roof insulation:

• Cold roof,
• Warm roof,
• Inverted roof.

Nowhere does it support the design and construction of hybrid roofs.

Condensation risk

As we’ve seen though, hybrid roofs are as likely – if not more likely – to occur because of on site issues, without the input of designers. If work has progressed too far, there is likely to be a greater willingness to accept a hybrid solution.

This is particularly true 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 a higher thermal resistance than the layer of insulation installed over the deck.

Sometimes this is referred to as the ‘two-thirds’ rule: two-thirds of the total insulation placed above the joists, with the other third is between the joists, is considered an acceptable solution. And a typical condensation risk carried out using the Glaser method (detailed in BS EN ISO 13788) may appear to show no risk.

But don’t forget that BS 5250 does not support the construction type. It also explains that the Glaser method can’t take account of the 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.

Quality of work

A condensation risk analysis is unable to allow for varying standards of workmanship, but 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.

Insulation manufacturers 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, practical thinking should be urged: are light fittings or other services going to be installed in the ceiling? Can the VCL’s continuity – vital to its performance – be guaranteed?

Stick to good principles

All of which serves to highlight why it is best to keep things simple in the first place. It only takes the next owner to want to change something and even a well installed VCL could be compromised as part of any works to the ceiling. Will anybody make sure it is restored to its original condition?

Despite efforts to promote the advantages of service voids, few seem willing to employ them. There are several reasons why: extra time and materials to construct, restrictions on headroom etc. But they should be considered where compromises have to be accepted, because providing an air space between ceiling and VCL where services can be installed means the ceiling finish can be altered without risking the integrity of the VCL.

If a VCL cannot be relied upon to minimise the risk of condensation then the roof design cannot be considered appropriate. Which is where we come back to the warm roof design: when the VCL is installed as a continuous layer on the deck, it will be left untouched and the internal ceiling can be modified as often as necessary.

Source: By EAL Consult