CPD 02 2021: Ventilation requirements for roofing membranes – buildings | Building design

This CPD, sponsored by Glidevale Protect, explains how ventilation requirements are considered when determining roofing underlay.
Deadline for completion: April 2, 2021

A membrane, which is required in virtually all tile and slate roof constructions, provides a secondary line of defense against wind, rain and snow – but without compromising the additional ventilation that the specification may require.


The government’s goal of reducing CO2 emissions to zero by 2050 will have a significant impact on the construction of homes and other buildings. As legislation evolves, it is crucial to ensure that roofs provide protection from the elements while also allowing effective ventilation to keep the indoor environment comfortable.

This CPD addresses the issue of pitched roof underlays and how they can be specified to meet building codes and standards, ensure the integrity of a building while meeting the roof ventilation requirements that come with the choice of membrane.

Specifying pitched roof documents for new build or refurbishment projects is often a confusing topic because there are so many options available in the market, each offering a variety of technical attributes and performance characteristics.

An underlay, required in virtually all tile and slate roof structures, should be designed as true hidden protection and act as a secondary line of defense for the roof tiles. Pressures applied to the structure under BS 5534, the Code of Conduct for Slate and Tile.

Underlays must absorb part of the wind load on a roof without inflating and touching the underside of the roofing. Some modern records have severe restrictions on their use in this country, while some records brought in from Europe are often not designed for UK conditions.

Since the roofing underlay is often specified in isolation, it is easy to overlook the integral relationship between the specified membrane type and the controlled ventilation strategy that must be used to control the risk of condensation.

Roofing and ventilation must work at the same time to achieve a complete solution that provides a watertight structure and avoids the formation of condensation in the roof space and ultimately leads to a healthy indoor living environment.

Balance between ventilation and airtightness

The choice of underlay for any pitched roof will affect the ventilation required to eliminate the risk of harmful condensation and create a better building and healthy home. This means that choosing a manufacturer who is knowledgeable about providing appropriate guidance and a range of solutions is paramount. Choice is the most important factor here, rather than a one-size-fits-all approach where the membrane is an integral part of the overall solution.

Even if there are cost constraints on the entire roof package, using the cheapest membrane without considering additional ventilation requirements can lead to problems and costly structural repairs in the future. If you get it right the first time, the houses are future-proof, retain their property value and ensure reduced maintenance costs.

The need for controlled ventilation in the case has never been more important. With the onset of climate change and the UK government’s continued commitment to zeroing all greenhouse gas emissions by 2050, compared to the previous target of at least 80% reductions from 1990, energy efficiency is high on the agenda.

In the meantime, the lowering of the fictitious U-value targets has made building regulations more burdensome. In addition to the updates to the Building Regulations Approved Document Part L (Fuel and Energy Saving) and Part F (Ventilation) recently announced by the government following the initial consultation process, changes to the BS 5250 Code of Conduct are also expected this year to control the Condensation in buildings.

Improvements to these regulations in 2021 will facilitate the transition to the Future Homes Standard, which aims to significantly improve the energy efficiency of new buildings and will be introduced by 2025. In addition, the government’s recently introduced new social housing charter commits to reviewing decent housing standards in support of the decarbonization of social housing.

Since these more stringent goals mean that homes are designed with a fabric-first approach to increasing air tightness, it’s important to balance this with the right ventilation strategy to protect against excessive build-up of moisture and pollutants. This includes the roof, which requires adequate ventilation based on the specified type of roof underlay, especially if the attic is a habitable space.

What are the different categories of membranes available and how do they affect each ventilation requirement as it is undoubtedly important to ensure the ventilation of homes? And how does this affect whether the construction is a cold roof (where the insulation is directly above or between ceiling joists) or a warm roof (where the insulation in the rafter line is either above or partially between the Rafter lies)?

The increasing emphasis on airtightness in home design makes it even more important to provide an adequate ventilation strategy to ensure a healthy home

Low Resistance Diaphragms (LR)

The two main categories are low resistance (LR) products, which are vapor permeable, and high resistance (HR) products, which are vapor impermeable. Starting with LR products, there are two main types of such pads in the market today.

The traditional LR product is airtight, but allows water vapor to be dispersed into the atmosphere. For a solution on an apartment-sized cold roof, BS 5250 recommends the combination of a vapor-permeable, airtight underlay with a high ventilation of 5,000 mm2 / m through tile or ridge ventilation, regardless of whether a well-sealed or normal ceiling is available.

Alternatively, a well-sealed ceiling requires ventilation of only 3,000 mm2 / m at a low level, e.g. via the eaves – but for roofing projects where a normal ceiling may be available and a well-tight ceiling cannot be achieved, Ventilation at a low level should be increased to 7,000 mm² / m.

In warm roof constructions, in which the attic was designed as a living space and the insulation is therefore laid at rafter level, the use of an airtight and vapor-permeable LR roof underlay does not require any additional ventilation.A well-sealed ceiling and an air and vapor control layer are created on the warm side of the insulation (AVCL) installed.

If there is any doubt about the ability to provide an effectively sealed ceiling and AVCL at the specification stage, additional ventilation should be incorporated into the design. The chosen manufacturer should provide appropriate guidance.

The second type of LR underlay is both air and vapor permeable and allows the roof space to be ventilated using a special meltblown technology built into the membrane core. This ensures air flow through the material and allows steam to escape, thereby controlling the risk of condensation.

There are two main types of underlay: high resistance (HR), vapor impermeable and airtight, and low resistance (LR), either vapor permeable and airtight or air and vapor permeable. A type LR underlay is shown, which is both vapor and air permeable and does not require any additional ventilation, as accepted by the NHBC for cold roof construction.
Image source: Prentice Roofing

For these product types, specifications should be given for underlays that are also treated hydrophobically to repel water and provide additional protection. While an air-permeable underlay can result in higher initial material costs, this is offset by the fact that no additional ventilation is required, which saves time and labor costs during installation.

Whether for a cold or warm roof, air and vapor permeable membranes can be used without additional ventilation. This is accepted by the NHBC in its document on the technical requirements R3 with regard to the cold roof construction.

The only exceptions are when the underlay is installed on a sarking board with closed joints or when an airtight roofing such as metal tile or fiber cement slate is used. In such cases it would be necessary to ventilate the lath area with 25 mm counter laths. The manufacturer chosen should always recommend a well-sealed ceiling and use a separate AVCL where possible.

Highly resistant (HR) membranes

The other main category of pitched roof underlay is high resistance (HR) membranes, which are both airtight and prevent the diffusion of water vapor through the material. HR vapor impermeable membranes were developed to overcome the disadvantages of conventional Type 1F felts. They are generally the most affordable if you are just looking to buy the membrane.

However, this has to be weighed against the requirement for additional ventilation at high and low levels for both a cold and a warm roof. This additional ventilation is essential to avoid the risk of interstitial condensation forming on the underside of the mat.

For example, cold roofs the size of an apartment with slopes of more than 15 ° require at least 10,000 mm2 / m ventilation at a low level and 5,000 mm2 / m at a high level. For distances over 35 ° or spans over 10 m, additional ventilation of 5000 mm2 / m is required. Warm roofs where HR underlays are used will also require high and low level ventilation. The selected manufacturer should provide project-specific relevant ventilation guidelines.

There are undoubtedly synergies that result from the correct selection of the roofing material and a suitable ventilation strategy. It is important that the selected manufacturer advise on the complete solution to ensure that the roof is future proof and contributes to a healthy health home for residents.

Ventilation from high-level areas such as the ridge, along with lower-level ventilation, is of the utmost importance when an impermeable (HR) roofing mat is used.

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