Transcript

Good morning everyone, my name is Keira Proctor, and welcome to our twelfth webinar of 2022. Our series of webinars has been running now since 2020, and if you’ve missed any you can go back and review them all on-demand right here on our YouTube channel.

Today we’re running our third RIBA-assessed webinar, this time looking 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 BS5250:2021, the code of practice for moisture management, 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 more detail and how roofing practice is adapting to meet the changing nature of these challenges.

BS5250 The starting point for a successful roof design must be keeping the building dry, and the relevant guidance can be found in the BS5250:2021 Standard.

The BS5250 standard was formerly the "code of practice for the control of condensation", but now in its 2021 edition covers the wider remit of “management of moisture” including the effects of a wider variety of moisture sources on building design.

This change helps facilitate a more holistic approach to the management of moisture, air leakage and energy performance.

The building regulations across the UK and 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.

The energy performance aspects of roofing systems are covered in England & Wales by Approved Document L, in Scotland by Section 6 of the technical standards. In northern Ireland, this is Technical Booklet F, and in the Republic of Ireland, Technical Guidance Document L.

For projects where the aim is to exceed the regulation energy performance standards, it's also worth considering the recommendation given in the Passive House standard, even if passive house certification is not a project requirement.

The performance of a pitched roof in relation to heat flows, moisture transfer and air leakage are closely linked, and a having good understanding of the principles outlined in BS5250:2021 as well the relevant regulations is important.

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.

“Secondary” protection in this context means temporary protection during the construction phase, and in case of in-service damage to the primary covering. It does not mean the specification of the outer covering can be reduced, or that slates/tiles can be used below their minimum recommended pitch.

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. The methodologies for calculating these forces and specifying an appropriate outer covering are detailed in the BS5534:2014 +A2:2018 standard.

Outer coverings such as slates and tiles are not a continuous homogeneous layer, so it’s possible on occasion that moisture may 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.

This should however be regarded as a temporary situation, and underlays of any specification do not allow for reduction in the minimum pitch at which slates or tiles can be used. 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. Suitable test methods and performance criteria are defined in BS EN 13859-1:2014.

BS5250 defines two primary types of roofing underlay, one of which can be split into two sub-types. 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: A membrane with a water vapour resistance (Sd) that is greater than 0.05 metres (0.25 mega newton seconds per gram) 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 BS EN 13707:2013 for bitumen felts. HR underlays can also be plastic sheets of various compositions and performance criteria for these are given in EN13859-1:2014, 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 their name.

LR Underlay: In contrast, LR underlays are defined in BS5250:2021 as: A membrane with a water vapour resistance (Sd) not exceeding 0.05 metres (0.25 mega newton seconds per gram) LR in this case meaning 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 majority of LR-Type underlay membranes used in the UK are airtight, meaning they do not allow significant air movement, however there is a second type of LR membrane available.

LR-Underlay (Air Open)

The second type of LR underlay is air open, which means air can flow through the membrane to some extent. These types of membrane are acknowledged in BS5250:2021 but not defined.

This airflow can have a significant effect on the condensation risks present in the roof construction, and we will discuss these effects later in the presentation. What important to note however is that such membranes can only take advantage of the effects of air openness if they are certified by an appropriate 3rd Party.

BS5250:2021 does not contain guidance on their use but simply refers specifiers to this 3rd party accreditation.

BS5250 Roof Types

BS5250:2021 defines four classes of pitched roof constructions, with a pitched roof being a roof over 10 degrees of pitch, but less than 75 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, most commonly found where there is a “room in the roof” such as in loft conversions, 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.

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

Structural Insulated Panel (SIP) 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 BS5250:2021 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 typically be from inside to outside. The level of vapour drive is 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 buildings 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.

Should subsequent repair or upgrade work, such as adding MVHR or a change of use for the building, have the potential to affect the indoor environment, the performance of the roof systems should be reassessed to account for this.

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 establishes the “question” of what condensation risk is present, the air leakage rates involved begin to 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 condensation. Similarly the air permeability of the outer covering must be considered as roofs with continuous coverings such metal sheets will have a significantly different hygrothermal performance from discontinuous roof coverings like tiles 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.

If insulation levels are altered during the buildings life, either as part of repair works or to improve the energy performance, then the effects on balance of the roof system as a whole must be reconsidered.

Insulation wind washing

Wind washing describes 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 may 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 conditions 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.

Another aspect to consider regarding weather protection 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 roof space 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 to other parts of the specification. For example, if 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.

Another aspect to consider regarding weather protection 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 roof space 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 to other parts of the specification. For example, if 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, a properly designed roof system must enable internal moisture to escape and the building as a whole to dry out. This not only means 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 layer from inside to outside, and ensuring specifications which are designed and modelled remain fixed through to completion. There can be substantial differences in the performance of seemingly similar materials which product literature and different test methods do not always make obvious. While CE marking and the construction products directive went some way to clarifying and standardising testing and the presentation of information, 3rd party certifications specific to the proposed application remains the most effective way to ensure design criteria are met.

This is particularly important during the transition from CE to UKCA.

Roof ventilation Finally, we must consider whether ventilation is 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.

BS5250:2021 does not address situations where ventilation is omitted, but in section 12.5.3 refers designers to UKAS (or equivalent) accredited 3rd party technical approvals for guidance where a non-ventilated approach is taken. 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 buildings 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.

Ensuring these areas are adequately ventilated is important in reducing remedial action, as the retrofitting of additional ventilation to resolve issues can involve complex work at height which may impact on CDM regulations and considerations.

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.

Any changes to the ceiling assembly, for example adding downlighters, or in the longer term, maintenance to building services such as HVAC systems may affect how this moisture can enter the roofspace. The impact of any proposed alterations or maintenance work on the airtightness of the ceiling assembly should therefore be fully considered at the earliest possible stage.

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 they are not properly designed, openings can allow the ingress of windblown rain or snow. If they are not correctly installed, openings may not deliver 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. This leaves the roof at risk of moisture ingress until the last outer ridge tile is installed.

Finally, vents may be inadvertently blocked by stored items in the loft, or purposefully blocked by homeowners who may not fully appreciate the importance of this airflow, choosing instead to block up the “draughts” to warm the loft up. 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, 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 fail-safe hygrothermal envelope design more important than ever.

Adopting a fail safe approach reduces the potential for long term maintenance problems to arise in the roof construction.

If insulation levels are increased as part of energy performance related upgrade work, this can either block the ventilation openings at the eaves or if it is stopped short of the wall insulation, lead to a cold bridge at the wall/roof junction. Either way this can result in elevated condensation risk and attendant moisture problems.

In addition to the hygrothermal design criteria outlined in BS5250:2021, it's also important to understand the performance of the roof assembly in relation to fire.

In roofs, the primary requirement outlined in Approved Document B is to limit the spread of fire both from an external ignition source, and between adjoining roofs.

A roof is defined in Approved Document B as being less than 70 degrees from the horizontal, above 70 degrees is considered a wall, although AD:B clause 11.4 acknowledges there is "a matter of judgement" about whether features such as large runs of dormer windows should be counted as wall sections.

Internally, requirement B3 applies, requiring subdivision "of fire resisting construction" between adjoining buildings, and this should extend into the Roofspace, with such subdivisions being treated no differently than any other compartment wall.

Depending on the building type, ADB:2 may require additional separation between parts of a single building with different functions, and again this requirement extends to the roof spaces above these areas.

B3 Clause 4 also requires measure be taken to limit the "unseen spread of fire and smoke within concealed spaces".

to this end sections 8 and 9 of ADB:1 (9 and 10 of ADB:2) detail the provisions that should be made to ensure cavities and openings are adequately fire-stopped, for example with cavity barriers or other proprietary systems.

Externally, Requirement B4 requires the walls and roof to resist fire spread over the element, and between buildings.

Although Regulation 7 Paragraph 2 states that materials used in external walls should be "of European Classification A2-s1, d0 or A1 (classified in accordance with the reaction to fire classification)" paragraph 3 exempts most roofs from this requirement.

Regulation 7 paragraph 3 also provides an exemption for membranes such as roof underlays.

BS5250 Roof Type Guidance

Taking all these factors into account, the BS5250:2021 document offers various solutions depending on the roof configuration, underlay type and permeability of the outer roof covering.

The first factor to consider is the air permeability of the outer covering. If it 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, as slates and tiles usually offer sufficient air permeability. The standard for air permeability of the outer roof covering used in BS5250:2021 is defined in the BS5534:2014+A2:2018 standard as follows:

“The air permeability of an outer weatherproof covering comprising discontinuous units may be determined by testing in accordance with BS 5534:2014+A2:2018, Annex L, using equipment capable of measuring pressure differences of 10 pascals.

The outer weatherproof covering is deemed to allow sufficient air movement and be air permeable if the airflow in metres cubed per hour at a differential of 10 pascals is greater than 17.4 A R. Where A R is the area of the outer weatherproof covering under test in metres squared (as defined in BS 5534:2014+A2:2018).

If the airflow is not greater than 17.4 A R (in metres cubed per hour), then the outer weatherproof covering is deemed air impermeable.”

If the roof covering is later replaced as part of repair or upgrade work, any changes that may affect it's air permeability should be taken into account.

In some specific situations, such as where wind loads may be 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 a ventilated batten and counter-batten space directly below 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 12.5.4.1, Table 5 of BS5250:2021. 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. BS5250:2021 does not however go into detail about this additional ventilation.

The requirements in table 5 are broken into classes. For roofs of 10-15 degrees pitch with an HR underlay, ventilation openings of equivalent area to 25 time the longest horizontal dimension of the roof in millimetres should be provided.

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

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 square millimetres per metre squared requires an opening of 7 times the horizontal dimension, while the opening of a “well sealed” ceiling conforming to BS9250:2007 with a permeability not more than 30 square millimetres per metre squared can be reduced to 3 times the horizontal dimension.

A further option where the ceiling is well sealed is to have ventilation at the ridge only, equivalent to 5 millimetres time the longest horizontal dimension. This is commonly required on NHBC projects and is referenced in technical standard 7.2.15.

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.

Construction (Design and Management) Regulations 2015 set out the duties placed on designers, and these must be referenced by both the architect specifying the system and the product manufacturer and installer who are designing the system. Responsibilities include “the duty to eliminate, reduce or control foreseeable health and safety risks through the design process, such as those that may arrive during construction work or in maintaining and using the building once built” (CDM 2015).

The CDM framework aims to improve health and safety in the industry by helping to:

  • sensibly plan the work so the risks involved are managed from start to finish
  • have the right people for the right job at the right time
  • cooperate and coordinate your work with others
  • have the right information about the risks and how they are being managed
  • communicate this information effectively to those who need to know
  • consult and engage with workers about the risks and how they are being managed

The faster and simpler installation inherent in reducing or removing the ventilation requirements reduces the need for work at height and for communicating detailed specification guidance to contractors, especially where multiple different roof designs may be present in a development. It also simplifies the overall design thus requiring less verification and oversight of the installation.

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 section 12.5.4.1, Table 5 , however if the underlay membrane is covered by appropriate third-party certification it may be permissible. In this case it is assumed the vapour drive from the heated space alone 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 temporary use of dehumidifiers in the 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 in the specification that might cause problems if any special conditions in the certification are not adhered to.

A good example of this is when we consider the air permeability of membranes.

While specific guidance regarding air permeability was included in the initial drafts of the 2020 BS5250 it was latterly removed, leaving these important additional considerations to be dealt with via third party certification only.

The final 5250:2021 text simply includes the following footnote:

"Most LR underlays are airtight, however some are designed to be air permeable; this property can allow air movement from the loft space through the underlay into the batten space thereby reducing the risk of condensation on the underlay and cold structure"

"Air Permeable" or "Air Open" underlays provide, in the opinion of the BBA "a significant additional mechanism for water vapour egress by convection" and therefore sometimes have less restrictions on their use, for example some air open membranes may not require the use of a well sealed ceiling or vapour control layer.

Because the third part assessment criteria are not standardised, the exact requirements might vary between membranes. Its therefore important designers and installers are aware of these differences. While it may seem that one "Type LR" membrane can simply be swapped for another, and indeed this may arguably be true of airtight LR underlays where ventilation is used, for completely non ventilated constructions the picture is more complex.

Not all membranes are the same, particularly where an air open underlay is specified.

So how do we define whether a membrane is air permeable or not? Again the helpful common definition in the BS5250 drafts has been removed, leaving amore complex picture. There are currently two definitions commonly used, one given in BS5534:2014+A2:2018, and another used by the NHBC, outlined in "Technical Extra 24, November 2018".

BS5534 gives the figure of "not less than 20 metres cubed per metre squared per hour at 50 pascals" but this is primarily intended for use when considering wind uplift on tile assemblies, and may not provide sufficient airflow to adequately reduce condensation risks as is intended in the BBAs guidance.

The NHBC takes this position, and therefore uses a higher figure of 34 metres cubed per metre squared per hour at 50 pascals or more" as the minimum necessary to provide the ventilation effects required to minimise condensation.

Both use the same BS EN 12114 test method however, which simplifies matters. As is the case with vapour permeability, ensuring the test data is directly comparable is important.

Warm Roof Constructions

In warm roof construction, the lack of a large cold void in which moisture can collect makes moisture control a little simpler, but care must still be taken 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 in one of two ways. The first method is to use a dedicated vapour control layer on the warm side of the insulation.

The second options is to use insulation that is itself convection tight, such as seal foil-faced rigid foam.

With both methods, the resulting convection tightness ensures 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 may 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.

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 may 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.

One of the benefits of a non-ventilated roof is its lack of maintenance, if installed correctly there should be very little, if anything, required in terms of long term maintenance over and above good roof housekeeping such as clearing gutters regularly and general outer covering maintenance such replacing broken/slipped tile and slates.

Most modern vapour permeable underlays are sufficiently durable that they will be virtually unaffected by the normal conditions found in a roof space and will have a life comparable to that of traditional roof tile underlays, provided they are not exposed to sunlight for long periods, for example if the outer covering is damaged and not repaired in good time.

As the underlay is confined within the roof system and has the suitable durability, maintenance is not required. However any damage occurring during installation should be repaired, usually by inserting a patch of new material over the damaged area, ensuring the joins are detailed in order to shed water outwards.

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 25000 square millimetres per metre at low level and 5000 square millimetres per metre 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, ventilation 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 using 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. In roofing applications 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 an effective airtight layer. On the outside of the building there are far fewer issues of this nature.

Using an external air barrier membrane across the roof and walls of the building means the air tightness layer can simply be wrapped around the entire envelope with the roof membrane sealed onto the walls. Providing airtightness externally also limits the awkward detailing of internal air barriers that 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 sections 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 sealed, but the actual junctions between the vertical, inclined and horizontal structures must also 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 if the membrane is appropriately certified..

Air Leakage and Energy Performance When considering the correct roof type and underlay type, the affects 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 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 reducing 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 vapour permeable membrane which is also air permeable presents a far simpler, more reliable and fail-safe means of ensuring condensation risks are mitigated in cold and hybrid roof applications. This type of construction is also better able to accommodate future changes such as increased insulation thickness without requiring revisions to the ventilation, vapour control or air leakage design.

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.

The simple 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 an external self-adhered vapour permeable air barrier around the entire structure will facilitate the passage of moisture, while providing an effective air barrier. The benefits of using a fully self-adhesive product, where the material bonds to its substrate, are in 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 (CLT) or structural insulated panels (SIP), a self-adhering external membrane could be used to seal the panels immediately after erection, providing weather protection and a wind and watertight envelope to begin internal works.

Additional insulation could then be fitted externally as necessary providing a flexible and adaptable construction method to accommodate a variety of thermal performance targets.

This combination of CLT or SIP structure with an intermediate air barrier and additional insulation externally is a good fit form project which aim to significantly exceed the energy performance targets in Part L (England/Wales), Section 6 (Scotland), Part F (Northern Ireland) or Part L (Republic of Ireland).

This type of construction is commonly used in the delivery of Passive House projects.

Finally, sustainability of design and material is an important consideration.

Roof specification can have a major effect on the overall heat loss from a building, but increasing thermal insulation levels must be accounted for in the hygrothermal design. Ensuring the roof can accommodate upgraded thicknesses of insulation in future helps futureproof the roof against unintended consequences of such upgrades.

Similarly, the recyclability of products and systems should be considered. There is often a balance to be struck between technical in-service performance and end-of-life disposal.

For example, the environmental impact of plastic based membranes may be offset over the life of the building by the reduced heat loss or more durable building envelope they facilitate.

Manufacturers "Environmental Product Declarations", where available should be consulted to provide insight into the embodied energy and carbon of specific products. They can also provide important information regarding the recyclability of the material.

In most cases, all types of roofing underlay is heavily punctured with a variety of fixings during installation, so such systems are unlikely to be suitable for re-use once installed.

Polypropylene, which is a commonly used material for roofing underlays, is an easily recyclable type of plastic. Such plastics are also less hazardous and potentially polluting as a waste material than bitumen based underlays.

Some commonly used materials, particularly traditional felts and some types of roof covering, may require specialist disposal during demolition to be safely re-used or recycled.

That brings us to the end of todays presentation, and we’ll now move on to the Q&A session.

This Webinar Includes
  • Understanding of rooftypes outlined in BS 5250:2021 Code of Practice
  • Recognition of different rooftypes and associated hygrothermal strategies
  • Understanding of pitched roof underlay types
  • Appreciation of the effects of membrane properties on design requirements
  • Wider context overview of factors affecting pitched roof moisture control