CPD 02 2021: Ventilation requirements for roofing membranes

This CPD, sponsored by Glidevale Protect, explains how to take ventilation requirements into account when specifying roofing underlays.
DEADLINE TO COMPLETE: 16 April 2021

A membrane, required in virtually all tile and slate pitched roof constructions, offers a secondary line of defence against wind, rain and snow – but without compromising on additional ventilation that may be required, which needs to be factored into the specification.

Introduction

The government’s goal of reducing carbon emissions to net zero by 2050 is going to have a significant impact on the way homes and other buildings are constructed. As legislation develops, ensuring roofs afford protection from the elements while at the same time allowing for effective ventilation to ensure a comfortable indoor environment will be crucial.

This CPD will look at the topic of pitched roofing underlays and how they can be specified to meet building regulations and standards, securing the integrity of a building while also covering the roof ventilation requirements that go hand in hand with the membrane choice.

Specifying pitched roofing underlays for new-build or refurbishment projects is often a confusing subject with so many options available on the market, each offering a variety of technical attributes and performance credentials.

Required in virtually all tile and slate pitched roof construction, an underlay should be designed as a true hidden protector, operating as the secondary line of defence to the roof tiles, offering weather defence against wind driven rain or snow and wind uplift resistance to the loading pressures applied to the structure in accordance with BS 5534, the code of practice for slating and tiling.

Underlays have a further requirement to absorb a proportion of the wind load acting on a roof without ballooning and contacting the underside of the roof covering. Some modern underlays have severe restrictions on their use in this country, while some underlays brought in from Europe are often not designed for UK conditions.

However, with the roofing underlay often specified in isolation, it can be easy to overlook the integral link between the type of membrane specified and the controlled ventilation strategy that needs to be employed to manage condensation risk.

Roofing underlays and ventilation must work simultaneously to deliver a full solution that provides a watertight structure and avoids condensation build-up within the roof space, ultimately leading to a healthy indoor living environment.

Balancing ventilation against airtightness

The choice of underlay for any pitched roof impacts the amount of ventilation required to eliminate the risk of harmful condensation and to create a better building and healthy home. This means that selecting a manufacturer well versed in offering the appropriate guidance along with an array of solutions is paramount. Choice is the all-important factor here, rather than a one-size-fits-all approach, with the membrane an integral component as part of the overall solution.

Even if there are cost restraints for the overall roof package, using the cheapest membrane without factoring in additional ventilation requirements may result in problems and expensive structural repairs in the future. Getting it right first time means homes are future-proofed, maintaining their property value and ensuring reduced maintenance costs.

The need for controlled ventilation within housing has never been more important. With the onset of climate change and the ongoing commitment by the UK government to bring all greenhouse gas emissions to net zero by 2050, compared with the previous target of at least 80% reduction from 1990 levels, energy efficiency is top of the agenda.

Meanwhile building regulations are set to become more onerous by lowering U-value notional targets. As well as updates to Building Regulations Approved Document Part L (conservation of fuel and power) and Part F (ventilation), recently announced by the government following the initial consultation process, changes are also expected this year to BS 5250, the code of practice for control of condensation in buildings.

Improvements to these regulations in 2021 will facilitate the transition to the Future Homes Standard, set to vastly improve energy efficiency in new-build properties, which will be introduced by 2025. Additionally, the government’s recently launched new charter for social housing residents commits to reviewing the Decent Homes Standard to support the decarbonisation of social homes.

As these more stringent targets mean homes are being designed with a fabric-first approach to increase airtightness, it is vital to balance that with the correct ventilation strategy to protect against excessive build-up of moisture and pollutants. This includes the roof, where it is key to have appropriate ventilation provision that is aligned to the type of roofing underlay specified, particularly if the loft is a liveable space.

So, with the undoubted importance of ensuring ventilation is provided into dwellings, what different categories of membrane are available, and how do these impact on the ventilation requirements for each? And how is this affected by whether the construction is cold roof (where insulation is located immediately above or between ceiling joists) or warm roof (where insulation is placed in the rafter line either above or partially between the rafters)?

The increasing emphasis on airtightness in housing design brings an even greater need to provide an adequate ventilation strategy to ensure healthy homes

Low-resistance (LR) membranes

The two main categories are low-resistance (LR) products, which are vapour-permeable, and high-resistance (HR) products, which are vapour impermeable. Starting with LR products, there are two main types of such underlay available in today’s market.

The traditional LR product is airtight but allows water vapour to disperse into the atmosphere. For a solution on a dwelling-sized cold roof, BS 5250 recommends the combination of a vapour-permeable, airtight underlay with high-level ventilation of 5,000mm2/m through tile or ridge vents, whether a well-sealed or normal ceiling is in place.

Alternatively, where there is a well-sealed ceiling, a ventilation provision of only 3,000mm2/m at low level, for example, via the eaves is required – although for re-roofing projects where a normal ceiling may be in place and a well-sealed ceiling cannot be achieved, ventilation at low level should be increased to 7,000mm²/m.

In warm-roof construction, where the loft area has been designed as a habitable space and thus the insulation is laid at rafter level, the use of an LR roofing underlay that is airtight and vapour-permeable will not require any additional ventilation, subject to a well-sealed ceiling and an air and vapour control layer (AVCL) being installed on the warm side of the insulation.

If there is doubt about the ability to provide an effectively sealed ceiling and an AVCL at specification stage, further ventilation should be planned into the design; the chosen manufacturer should provide suitable guidance.

The second type of LR underlay is both air and vapour permeable, allowing ventilation to be delivered into the roof space through special melt-blown technology built into the membrane core. This ensures airflow through the material and allows vapour to escape, helping control the risk of condensation.

Underlays come in two main types: high-resistance (HR), which are vapour-impermeable and airtight, and low-resistance (LR), which are either vapour permeable and airtight or air and vapour permeable. Pictured is a type LR underlay which is both vapour- and air-permeable, requiring no additional ventilation provision, as accepted by the NHBC for cold roof construction.
Image source: Prentice Roofing

For these product types, specification should be made for underlays that are also hydrophobically treated in order to repel water to offer added protection. While an air-permeable underlay may involve higher initial material costs, this is offset by there being no additional requirement to ventilate, saving time and labour costs when it comes to installation.

Whether for a cold or warm roof, air and vapour permeable membranes can be used without additional ventilation. This is accepted by the NHBC in its Technical Requirements R3 Document, related to cold-roof construction.

The only exceptions to this are where the underlay is installed on closed-jointed sarking board, or if an airtight roof covering is used, such as metal tiles or fibre cement slates. In such cases, it would be necessary to ventilate the batten space with 25mm counter battens. The chosen manufacturer should always recommend a well-sealed ceiling and separate AVCL is used, where achievable.

High-resistance (HR) membranes

The other main category of pitched roofing underlays is high-resistance (HR) membranes, which are both airtight and where the diffusion of water vapour is prevented from travelling through the material. Designed to overcome the disadvantages of traditional type 1F felts, vapour-impermeable HR membranes are generally the most cost-effective in terms of purchasing the membrane alone.

However, this must be balanced against the requirement for additional high- and low-level ventilation for both a cold and warm roof. This extra ventilation provision is imperative to avoid the risk of interstitial condensation forming on the underside of the underlay.

As an example, for dwelling-sized cold roofs with pitches in excess of 15°, a minimum of 10,000mm2/m of ventilation is required at low level, in addition to 5,000mm2/m at high level. For pitches over 35° or spans in excess of 10m an additional 5000mm2/m of ventilation is required. For warm roofs, where HR underlays are being used, both high- and low- level ventilation is also needed. The chosen manufacturer should provide relevant ventilation guidance on a project specific basis.

There are undoubted synergies to be gained from the right choice of roofing underlay material and with an appropriate ventilation strategy, and it is vital that the selected manufacturer can advise on the full solution, ensuring the roof is future-proofed and can contribute to a healthy home for occupants.

Ventilation delivered from high level areas such as the ridge is paramount alongside lower level ventilation when an impermeable (HR) roofing underlay is used.

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