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Pitched Roofing Design Considerations: Energy, Moisture and Air Permeability

Welcome to our webinar, Pitched Roofing Design Considerations: Energy, Moisture and Air Permeability. The presentation lasts for approximately 30 minutes, and is followed by a live Zoom Q & A session with the audience, hosted by our Managing Director, Keira Proctor, with members of our Sales & Technical team.

The webinar covers the following topics:

  • BS5250 Guidance and Consultation Updates
  • Material specifications in pitched roof applications
  • Effects on air permeability of roof design
  • Energy performance of warm pitched roofs
  • Moisture management in roofing
Webinar Transcript

Today, we're going to take a look at pitched roofing design and what strategies we can adopt to manage the movement of air and moisture to ensure our roofs are both energy efficient and free from condensation.

We'll start out with a look at BS5250, the code of practice for moisture control, how it relates to roof design and the building regulation, and some of the roof types it introduces.

We'll then look at the physics and design influences in a more detail and how roofing practice is adapting to meet the changing nature of these challenges.

Since the introduction of our Roofshield air and vapour permeable underlay membrane back in 1996, we've been involved in all aspects of pitched roof design, from working to build acceptance on non-ventilated cold pitched roofs to contributing to technical standards addressing wind uplift.

Throughout this, we've based our approach on testing and experience gained across all types of roof in the most challenging conditions around the world, building both the knowledge and the technology to meet whatever problems a site may pose.

BS5250

The starting point for a successful roof design must be keeping the building dry.

The BS5250 standard, formerly the code of practice for the control of condensation, but now in its draft 2020 edition for "management of moisture" covers all aspect of keeping buildings dry. The 2020 draft is currently open for consultation until September, so while we'll base our overview on the 2020 text, bear in mind there might be some changes to the details after the consultation is complete.

Of course, we'll keep updating our guidance to reflect the changes as they happen, so if it's of interest you can sign up for updates via our learning hub or subscribe to our YouTube channel and be notified of that straight away.

The building regulations across the UK are Ireland all base their moisture control guidance for roofing on BS5250.

Approved Document C in England and Wales, covering "site preparation and resistance to contaminants and moisture", The Scottish "Section 3: Environment", Northern Irish Technical Booklet C and Technical Guidance Document F in the Republic of Ireland all reference this guidance. This means that unlike for example, energy performance, moisture control practice in roofing is relatively similar across all four sets of regulations.

There are two main types of moisture that roofs of all types must control, moisture from internal sources, vapour and condensation, and moisture from external sources, rain and snow.

External moisture is assumed to be primarily deal with by the external weatherproofing once the building is complete, but in order to provide a secondary layer of weather protection, roofs are fitted with an underlay membrane beneath this outer layer.

These underlay membranes contribute to the performance of the roof systems in a variety of ways.

Firstly, they provide a degree of resistance to wind uplift forces, and the methodologies for calculating these forces and specifying an appropriate outer covering are detailed in the BS5534 standard.

Outer coverings such as slates and tiles are not a continuous homogeneous layer, so they are susceptible to moisture being driven through them by wind. An important function of the underlay is therefore to ensure this wind driven moisture does not penetrate into the structure of the roof.

The water resistance characteristics of the underlay should also be sufficient to allow it to also function as temporary weather protection for a limited time until the outer covering can be completed.

Related to this, the membrane and any supporting structure for the outer covering, such as battens and counterbattens, must ensure this moisture is able to run off into the gutters.

Lastly, the underlay must function as part of the roof system to limit the potential of damage to the roof fabrics caused by condensation from internal sources.

How they do that varies depending on the characteristics of the underlay.

BS5250 defines two primary types of roofing underlay, and a third sub-type. All these types are considered suitable for the role of secondary weather protection, so the main differentiation is in their characteristics regarding vapour and condensation.

HR Underlay:

membrane with water vapour resistance, Sd, greater than 0.05 m (0.25 MNs/g)

This definition covers traditional bitumen roofing felts, such as the old BS747:Type 1F felts that were a staple of tiles and slated roofing for many years. The BS747 standard has since been replaced by EN13707 for bitumen felts.

HR underlays can also be plastic sheets of various compositions and performance criteria for these are given in EN13859-1, the definitions and characteristics for underlays.

The main characteristic of HR underlays is that they are impermeable to water vapour, this high vapour resistance gives them the name.

LR Underlay

In contrast, LR underlays, defined in as BS5250 as:

membrane with water vapour resistance, Sd, not exceeding 0.05 m (0.25 MN∙s/g)

LR in this case being LOW resistance to the passage of moisture vapour. The EN13859-1 standard also covers the performance criteria for LR underlays, which are typically composed of polyethylene or polypropylene.

The 2020 draft of BS5250 for the first time acknowledges a subtype of LR underlay, the Air Permeable LR or APLR underlay. Membranes of this type, such as our Roofshield, allow the passage of air as well as moisture vapour, and we'll move onto discuss the implications of that, along with the differences between these underlay types next.

BS5250 Roof Types

BS5250 defines four classes of pitched roof constructions, with a pitched roof being a roof over 10 degrees of pitch, but less than 70 degrees.

Cold Pitched Roofs

In this type of roof the insulation is placed horizontally across the ceiling ties, with the loft space and the structure of the roof being outwith the buildings heated envelope, hence cold.

This type of roof typically uses fibrous quilt insulation, which is both air and vapour permeable.

Warm Pitched Roofs

In this roof type the insulation follows the pitch of the roof, either between the structural members, over the top of the structure or a combination of both. This leaves the enclosed space under the pitch as part of the heated envelope.

In warm pitched roofs it's more common to see the use of rigid foam insulation, which tends to be more vapour and airtight than fibre insulation. This is primarily due to the thickness of insulation required to meet thermal standards however, and there's no fundamental reason permeable insulation types cannot be used.

Hybrid Pitched Roof

This type of roof, also known as a "room in the room" has a mix of horizontal, vertical and inclined insulation. This means some parts are cold roofs and some are warm, with a corresponding mixture of insulation types commonly used.

The term "hybrid warm roof" is sometimes used to describe a roof with insulation positioned both between and over the roof structure, however BS5250 does not use that particular designation.

The various junctions between the horizontal, vertical and inclined sections found int his kind of roof also pose additional challenges when it comes to detailing and specification.

Structural Insulated Panel Roof

SIP roofs use panels which combine the supporting structure and insulation layers into a single panel.

Typically, these panels are comprised of a rigid foam insulation and timber sheeting like OSB or plywood. All these component parts are relatively air and vapour tight.

Pitched Roof Design Criteria

The BS5250 standard gives detailed guidance specific to each combination of roof type and underlay type, to ensure the response of the roof system to moisture is within acceptable limits.

A roof that cannot adequately deal with moisture is susceptible to a variety of negative effects, from mould growth to structural damage and weakness.

These prescriptive roof types have been developed based on a range of design criteria which must be balanced and managed. These constructions should not however be considered as interchangeable as a variety of factors can influence the performance of the roof system.

Vapour drive through the roof structure

This is influenced by the internal and external environmental conditions present on the specific project. In the UK, and other "heating climates" where buildings are heated more often than cooled, this will typical be from inside to outside.

The level of vapour drive it determined by the building type, use and occupancy. A large, cool, low occupancy building such as a warehouse will have a low vapour load, which something like a swimming pool which is typically much warmer and more humid will have a correspondingly higher moisture load.

Offsite construction such timber frame or SIP buddings will also tend to have lower moisture loads as there are less wet trades to dry out, and the building is usually wind and watertight earlier in the construction process. This construction moisture can lead to significantly higher condensation risks in the short term after completion.

The vapour drive present determines the overall condensation risk that will be present, and as such is the start point of the hygrothermal assessment of the roof's performance.

Airtightness of the roof

The airtightness of the overall roof construction, as well as the individual components is also an important influencing factor.

While the vapour drive present in a project established the "question" of what condensation risk is present, the air leakage rates involved begin to help shape the best "answer".

If the energy performance assessed in SAP or SBEM calculations requires a low air leakage rate, that may influence the choice of roof construction or the components specified as some types of roof can achieve lower air leakage rates more easily, particularly if extremely low air leakage rates such as those found in passive homes are required.

Ensuring the transfer of warmer, more humid internal air is limited is also important as air leakage can transfer moisture directly into colder areas leading to localised moisture and condensation.

Similarly the air permeability of the outer covering must be considered as roofs with continuous coverings such metal sheets will have significantly different hygrothermal performance from discontinuous roof covering like tile or slates.

Insulation Specification

The dew point temperature associated with roof design is the point at which the warm moist internal air cools sufficiently to lose it's ability to carry moisture. This is the point at which liquid condensation will form.

The location of the dew point within the roof structure is therefore a critical factor in assessing the performance of the roof. As this is influenced by both the temperature gradient and the vapour drive, the specification of the insulation has a significant effect.

As the level of thermal insulation increases, the outer layers of the roof will become colder with correspondingly higher condensation risk all else being equal.

Conversely, a more vapour resistant insulation will limit the quantity of moisture able to reach these colder areas, lowering the risk.

Achieving a good understanding of the hygrothermal performance of the insulation is important, as part of the role of construction membranes in roofing is to help balance these characteristics, limiting the risk of harmful moisture issues.

Without good information and solid specifications for the thermal envelope, ensuring vapour control layers and underlays with the correct performance characteristics are chosen for the project can be difficult.

Insulation wind washing

Wind washing described the process of weather driven airflow inducing additional heat transfer through the insulation layers. In extreme cases this airflow can also affect the dew points by moving colder air further inwards.

Although this issue is more prevalent with permeable, fibrous insulation, it's worth bearing in mind that simply friction fitting rigid vapour and air tight insulation boards between structural members may not be convection tight.

If the joints and interfaces around rigid board are not sealed and tolerances tightly controlled, there may still be gaps allowing both wind washing and convection driven airflow around the board, reducing thermal performance and introducing additional moisture loads which not be accounted for in the hygrothermal design.

Comprehensive Weather Protection

A successful roof design must, of course, provide weather protection, and this is true not just throughout the life of the building, but also during construction.

That the outer roof covering must be able to resist extremes of weather is an obvious consideration, but in such extreme condition the performance of the secondary weather protection measures is also important.

In the event of storm damage to tiles, slates, or ridge tiles, having a robust and durable secondary layer to protect the roof until repairs are completed can be the difference between a relatively simple and limited repair and something far more disruptive to the building's occupants.

Under such conditions, another aspect to consider is how wind driven rain or even snow can enter a roof structure. Roof ventilation, if specified, should have openings carefully designed and positioned such that rain and snow cannot enter the roofspace or saturate insulation and roof timbers.

Ensuring all parts of the roof structure can properly resist wind loads without damage is also important, and as with many aspects of roof design this may have a knock-on effect on other part of the specification.

If for example, a particularly exposed location requires the use of additional wind bracing or sarking boards, or necessitates changes to the specified outer covering, this may require additional or different condensation control measures.

Escape of Moisture

As well as preventing the ingress of moisture from external sources, the a properly designed roof system must enable internal moisture to escape and the building as a whole to dry out.

This not only mean ensuring condensation problems are mitigated, but is also important in ensuring acceptable indoor air quality is maintained.

This necessitates a good understanding of the hygrothermal properties of every later from inside to outside, and ensuring specifications which are designed and modelled remain fixed through to completion.

There can be substantial difference in the performance of apparently similar materials, which product literature and different test methods do not always make obvious. While CE marking and the construction products directive have gone some way to clarifying and standardising testing and the presentation of information, 3rd party certification specific to the proposed application remains the most effective way to ensure design criteria are met.

Roof Ventilation

Finally, we must consider the ventilation provided in the roof, or not.

Whether or not roofs and lofts require ventilation has been the subject of a long running but now largely settled debate across the industry since vapour permeable underlays first gained mainstream acceptance in the 1990s.

Traditionally roofs of all kinds required ventilation to ensure moisture could escape as bitumen felt underlays did not allow moisture transfer to any significant extent. While there's nothing wrong with this approach and it has been used successfully for a very long time, as building become increasing well sealed and moisture loadings increase, it's not always the optimal solution.

This is particularity the case where complex roof shapes, and rooms in the roof are present, creating the potential for dead spots where air cannot properly circulate leading to localised damp and mould.

If not carefully designed, the pressure drop between a ventilated loft, particularly if ventilated at the ridge only, can set up additional suction forces on the ceiling, drawing both heat and moisture from the habitable spaces below.

There's also the potential for issues arising from the ventilation openings themselves, which if they can let moisture out, can also let moisture in. If not properly designed then these openings can allow the ingress of windblown rain or snow, and if not correctly installed they may not provide the design airflow rates on which a dry roof depends.

Ridge vent systems may also be more susceptible to moisture ingress during construction as there may be an opening present at the ridge even once the underlay is in place.

Finally, any items stored in the loft may block vents, or homeowners may not fully appreciate the importance of this airflow and block up the "draughts" to warm up the loft. Any subsequent loft conversion may also be complicated by the need to maintain good airflow.

With more people currently looking at their options to work from home that ever before, home offices being created in lofts, whether done properly with a building warrant or on a more ad-hoc basis is likely to be increasingly common, making a robust and failsafe hygrothermal envelope design more important than ever.

BS5250 Roof Type Guidance

Taking all these factors into account, the BS5250 document offers various solutions dependent on the roof configuration, underlay type and permeability of the outer roofcovering.

The first factor to consider is the air permeability of the out covering, as if this is not permeable, then some degree of ventilation will be required. In most cases, this requirement to ventilate will apply to coverings such as metal sheeting, and slates and tiles usually offer sufficient air permeability.

In some specific situations, such as where wind loads maybe particularly high, or where traditional Scottish slating practice is being used, there may be sarking materials present on the roof. In such cases it's always a good idea to check what effect the specific sarking type has on the ventilation requirements for the roof, particularly if reduced or no ventilation is proposed.

If ventilation is required, this typically consists of ventilated batten and counter-batten space directly under the outer covering. This is a separate consideration from ventilation of, for example, the loft space, although if the loft space is ventilated it's usually not necessary to ventilate the batten space too.

Cold Roof (Ventilated)

The first of the roof types we'll consider is a ventilated cold roof, this is the traditional form of pitched roofing used all over the UK, and is historically the standard against which more modern alternatives are judged in terms of condensation risk and timber moisture contents.

The guidance on acceptable ventilation for loft spaces is given in section L5.5 of the 2020 draft document, and in table L2.

In every case airflow paths within the roof should be a minimum of 25mm deep and openings protected with a mesh to prevent the ingress of birds and insects

For a simple roof of less than 10m eaves to eaves, vent openings at the eaves only will be sufficient, but for more complex or larger roofs, additional high-level ventilation might be required.

BS5250 does not however go into detail about this additional ventilation.

The requirements in table L2 are broken into four classes.

For roofs of 10-15 degrees pitch with an HR underlay, ventilation opening of equivalent area to 25x the longest horizontal dimension of the roof in mm should be provided.

In roofs between 15 and 75 degrees this can be reduced to 10x the longest dimension, with all opening area requirements being in square mm.

If the underlay specified is of an LR type, these requirements can be reduced further depending on the air permeability of the ceiling. A "normal" ceiling with an air permeability of 300 mm2/m2, requires opening of 7x the horizontal dimension, while a "well sealed" ceiling conforming to BS9250 with a permeability not more than 30mm2/m2 can reduce this to 3x the horizontal dimension.

A further option where the ceiling is well sealed is to have ventilation at the ridge only, equivalent to 5mm time the longest horizontal dimension. It's not clear however how effective this type of ventilation is given no airflow into the loft is possible through the "well sealed" ceiling assembly or the airtight underlay.

Given the various categories and differing guidance for ventilation, and the lack of clear specification advice for more complex roof types and shapes, a simpler option is often to omit the ventilation altogether and rely instead on the vapour permeability of the underlay to remove moisture.

Cold Roof (Non-Ventilated)

Where an airtight LR underlay is specified on the roof, the BS5250 standard does not directly address situations where ventilation is reduced below the levels given in L5.5, however if the underlay membrane is covered by appropriate third-party certification it may be permissible.

In this case the vapour drive from the heated space is sufficient to drive moisture out of the roof space. In practice however, membranes of this type can struggle to deal with the elevated vapour loadings during the drying out period, which may require the use of dehumidifiers int he building in extreme cases.

There's a also good number of airtight LR membranes on the market, and while it might appear that they are all reasonably interchangeable, there can be differences int he specification that might cause problems if any special conditions in the certification are not adhered to.

One way such uncertainly can be avoided however is to use an air permeable membrane.

The inclusion of such membranes as a specific construction type is one of the principle additions in the 2020 BS5250, and this type of roof requires no additional ventilation.

The particular advantage of this type of underlay, referred to as Air Permeable Low Resistance or APLR underlays, is that in additional to allowing vapour to escape by diffusion, they also allow a degree of airflow to boost the transfer of moisture vapour.

Membranes that are used in this type of construction must meet specific criteria however to ensure no problems arise.

They must have an air permeability of not less that 6m3/m2/hr at 10 pascals when tested as required by BS EN 13859 parts 1 and 2, and also a vapour resistance not greater than 0.07 MNs/g, or sd-0.014m when tested to BS EN ISO 12572 Method C.

Membranes meeting this specification can however be used in all circumstances with no ventilation, regardless of the size and shape of the roof. They also do not require a well-sealed ceiling, making this type of underlay a good choice in refurbishment applications where the precise ceiling specification may be unknown.

Taken together, these factors make APLR underlays which meet the performance criteria the simplest solution to address condensation in cold pitched roof application, and the only solution that can be specified across multiple projects with no changes.

Warm Roof Constructions

In warm roof construction, the lack of a large cold void in which moisture can collect makes the moisture control a little simpler, but care must still be to ensure the correct specifications are followed.

Warm roof constructions are those where the insulation follows the line of the roof from eaves to ridge, the spaces within the roof and structure itself are within the heated envelope of the building.

In all roofs of this kind, ensuring convection tightness is important, and this can typically be achieved either thought the use of a dedicates vapour control layer on the warm side of the insulation, or by using insulation that is itself convection tight, such as seal foil faced rigid foam.

This convection tightness ensure moisture laden air cannot bypass the insulation into colder areas of the roof.

It's also important with this configuration of roof that any moisture passing through the outer waterproofing layers can drain properly. This may occur where there is damage to slates or tiles, or where windblown rain passes up under the outer covering.

If tiles or slate on battens are used on the roof, there must be a gap under the battens in order to ensure this drainage. Where possible, this is usually achieved by draping the membrane between the roof timber, in the same way as in a cold roof.

However, in this type of roof, the underlay may be fully supported on the insulation layers, which can be placed over the rafters, not just between. If the membrane cannot be draped, it my be necessary to use counter battens to introduce this gap, similarly to if ventilation were to be required in the case of an airtight outer covering.

HR Underlays

As with cold roof constructions, warm roofs with impermeable HR-type underlays will always require ventilation below the underlay to ensure the escape of moisture vapour is facilitated. This is in addition to a vapour control layer at ceiling level.

The depth of the ventilated void between the insulation and the underlay should be a minimum of 25mm deep, plus the maximum allowable underlay drape of 15mm, if applicable. Ventilation openings must be provided at high and low levels, with 25000mm2/m at low level and 5000mm2/m at high level. Openings should also be provided at any features like dormers or roof lights, so the ventilation is unobstructed across the entire roof.

LR Underlays

As with cold roofs, this can be simplified by using an LR underlay. In the case of warm roofs this allows the ventilation to be removed provided the internal layers are convection tight. If this can't be guaranteed, or if the outer covering is not air permeable, then ventilation would be needed.

In a purely warm roof situation, where the insulation follows the pitch of the roof from the eaves all the way to the ridge, the lack of large voids in the roof means there is less benefit to the use of an air permeable underlay. This correspondingly makes this type of roof a good choice for buildings where low air leakage rates are a priority.

This allows the airtightness of the roof assembly to be maximised, in turn lowering the air leakage rate of the building as a whole.

If we extend this principle out to the building as a whole, and airtight warm roof construction can be easily integrated into a wall assembly with an external air barrier membrane. We've covered these types of membrane, like our Wraptite, in previous webinars, but in a roofing application this can simplify achieving a good airtight interface at the wall and roof junctions.

When internal membranes are used to control air leakage, there are numerous penetrations that must be sealed to provide and effective airtight layer. On the outside of the building there are far fewer issues of this nature.

Using a membrane such as Wraptite across the roof and walls of the building means this air barrier layer can simply be wrapped around the entire envelope, with a roof membrane seal onto the walls, limited any awkward dealing where and internal air barrier must negotiate structural members and junctions between floor and roof elements.

In a warm roof situation with a convection tight insulation, this provides the optimal balance of air and moisture control, while simplifying the construction process and thus limiting defects and remediation works.

This in turn allows very low air leakage rates to be used at the design stage, allowing highly energy efficient building to be constructed with resorting to excessive insulation thickness in the walls and roof.

Hybrid Roofs

This type of roof construction combines both warm and cold roof elements, and is sometime referred to as "room in the roof" construction.

In this type of roof, there is a mix of cold roof voids, usually at the eaves and ridge, with section of inclined insulation, sometimes called coombs, in between.

Because of the different hygrothermal characteristics of warm and cold roof constructions, this type of roof poses some specific challenges. If, for example, the roof voids require ventilation, then additional mid-level vent openings may be required at the top and bottom of the inclined sections. This might be the case even if airflow can be provided through the coombed section, as it may be difficult to design in dependable air movement under all weather conditions.

Achieving a reliable convection tight air and vapour control layer can also be difficult in this type of construction, as not only must service runs be seals, but the actual junctions between the vertical, inclined and horizontal structures must be addressed.

The simplest way to overcome this is often to run the insulation from eaves to ridge, making the roof entirely warm, particularly if it is part of an overall design strategy focusing low air leakage. however, if this not possible, for example when refurbishing an existing roof, another option is to specify an air permeable lr underlay which will greatly simplify the entire construction as ventilation will not be required into large roof voids.

Air Leakage and Energy Performance

When considering the correct roof type and underlay type, the effects on the overall energy strategy and anticipated air leakage rates should be considered carefully.

While are air permeable lr underlay clearly presents the most effective solutions to ensuring moisture can escape from a roof structure, this does make the sealing of an internal air barrier, be it an air and vapour control membrane or a well-sealed ceiling, of considerable importance.

While it may be tempting to consider an airtight LR underlay a good step to reduce the air leakage, such membrane will often require a degree of ventilation, such as the "ridge only" openings. In practice this means an airtight underlay does not necessarily mean an airtight cold roof.

If the ceiling or air barrier requirements are more or less the same either way, an APLR membrane such as our Roofshield present a far simpler, more reliable and fail-safe means of ensuring condensation risks are mitigated in cold and hybrid roof application.

A more fundamental consideration is the general suitability of cold roof constructions in buildings where airtightness and energy use are the primary focus, such as passive and low energy homes.

We've discussed these types of homes, and also the modern methods of construction used to build them in some of our recent webinars, so there's a lot more information to go back and review either at our learning hub at www.proctorgroup.com or here on our YouTube channel.

In terms of today's discussion though, the simplified answer is to consider that for very airtight buildings, a warm pitched roof with an external air barrier layer presents a far more optimal solution.

This is particularly true where this air barrier solution is also applied to the wall constructions.

Using a continuous layer of our Wraptite external air barrier around the entire structure will facilitate the passage of moisture, while providing an effective air barrier.

Being fully self-adhesive, the material also bonds to the substrate, reducing the likelihood of damage from extreme weather conditions during construction, and making for a simplified installation process compared to a mechanically fixed external membrane. It also reduces the sealing requirements around internal services.

In buildings constructed using cross laminated timber or structural insulated panels, the Wraptite can be used to seal the panels immediately after erection, providing weather protection and an wind and watertight envelope to begin internal works. It can then have additional insulation fitted externally as necessary providing a flexible and adaptable construction method that can accommodate a variety of thermal performance targets.

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