Pitched Roof Ventilation and Membranes: A Guide
Understanding ventilation is crucial when designing a pitched roof. Not only does it ensure the longevity of the roof, but it also prevents a myriad of problems that could arise from trapped moisture, condensation and poor air circulation.
Much of the guidance regarding ventilation to pitched roofs and different membranes can be found in BS5250:2021 Management of Moisture in Buildings – Code of Practice. Sadly the British Standard document is very expensive, and often unaffordable for small practices or sole traders. In this post we will try to summarise the main points of BS5250 in relation to pitched roof membranes and ventilation.
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Materials with the highest vapour resistance should be placed on the warm side of the thermal insulation and those with the lowest vapour resistance should be placed on the cold side of the thermal insulation.
At the heart of preventing condensation in roof voids is the principle of natural air ventilation. In line with the British Standard, BS5250, it’s vital for architects and designers to consider these moisture sources within buildings:
- Moisture introduced during the construction phase, which can also includes rainwater.
- Rain or precipitation post-construction.
- Moisture emanating from the occupants and their daily activities.
The key takeaway from BS5250 is the emphasis on the need for ceilings to be well sealed. This helps limit the transfer of moisture to the roof space, which might be carried by moisture-heavy air. So, what does this mean in practice?
- Ceilings should be free from gaps and holes.
- If you’re placing access doors or hatches that lead to the roof space, it’s wise to place them away from areas that produce a lot of moisture, ie places like your kitchen or bathroom.
- Ensure hatch covers are sealed properly.
- And of course, never compromise on the quality of workmanship.
Interestingly, air that seeps through gaps in a ceiling can transport a greater amount of heat and moisture into the roof through convection, compared to what might pass through the ceiling materials by diffusion. Hence, ensuring good airtightness at the ceiling is important when thinking about the roof’s design.
Key Design Pointers
- Keep construction gaps to a minimum.
- Avoid situating roof access doors or hatches in moisture-heavy rooms (bathrooms for example).
- Opt for proprietary sealed loft hatches and frames. Also, make sure you seal them correctly, adhering to the manufacturer’s guidance.
Air and Vapour Control Layer
If you’re considering the addition of an air and vapour control layer (AVCL), remember to position it on the insulation’s warmer side. If you place an AVCL at the ceiling level, this will heighten the necessity for adequate ventilation beneath it, especially during the phases where wet trades are drying out.
In summary, the right approach to roof ventilation and membrane installation can drastically influence the structural integrity and durability of a building. Remembering these key points will ensure your design is both effective and long-lasting.
Membranes / Roofing Underlay
There are two main types of roofing underlays, non vapour permeable and vapour permeable.
Non Vapour Permeable/Impermeable, High Resistance (Type HR), Non-Breathable Underlays
These non-breathable underlays range from the age-old bituminous products to the newer impermeable membranes, which are often made from materials such as polypropylene. They are known for their ability to offer a sturdy secondary shield against the elements, particularly wind pressure and rain.
Breathable membranes might be gaining traction in the construction world, but that doesn’t mean they are absolutely necessary. Historically, roofs have employed non-breathable underlays paired with low-level (at the eaves) and high-level (at the ridge) ventilation systems, a methodology that remains both effective and time-tested.
For certain roof structures, specifically where counter-battens are needed to aerate the batten space, low-resistance underlays might not offer any distinct advantage. In such scenarios, it’s not uncommon – and is often more economical – to opt for a non-breathable underlay. This can be either of a lightweight variant or a fortified bitumen membrane.
Cold Roof Construction and (HR) Impermeable Membranes
Underlays of the HR or impermeable kind are pivotal in ensuring high vapour resistance on the cooler side of the thermal insulation. Their role is to hinder the passage of water vapour from the loft to the batten area. For this reason, it is important that the loft area is adequately ventilated, in alignment with guidelines from BS 5250 and the LABC Technical manual.
For a standard cold pitched roof (pitch between 15 and 75 degrees) with an HR underlay a provision of 10,000 mm2/m should be provided at eaves level. This equates to 10mm clear vents at low level.
Warm Roof Construction
A warm roof construction is defined by insulation following the slope of the roof, rather than being at the line of the ceiling. If non vapour permeable or high resistance underlays are specified in a warm roof construction, ventilation to the counter batten void becomes a necessity according to the LABC.
For roofs utilising a non vapour permeable or HR underlay, there exists a danger of interstitial condensation forming on the underside of the underlay. To combat this, it’s imperative to integrate an AVCL on the insulation’s warmer side. Additionally, ventilated spaces should be created between the underside of the underlay and the insulation.
Remember, all materials and products used in a warm roof construction should possess valid third-party certification.
Vapour Permeable, Low Resistance (Type LR), Breathable Underlays
Breathable underlays represent a newer generation of roofing products. They’re commended for their lightweight nature and straightforward installation, although they tend to come with a higher price tag than their non-breathable counterparts.
Breathable membranes are available in two forms:
1. Vapour Permeable Membranes
This type is the more prevalent of breathable underlays. However, a common misconception is viewing it as a replacement for roof ventilation. Contrarily, using low-level and high-level ventilation alongside these membranes is essential for effective cross-flow ventilation. While some might lean on vapour permeable underlays as the only means of ventilation, both British Standards and the NHBC emphasise the necessity of high-level ventilation to ensure adequate cross-flow, facilitating the expulsion of water vapour from buildings.
2. Vapour and Air Permeable (Air Open) Membranes
The BS 5250 does not yet officially acknowledge this specific underlay type, nor its singular use for ventilation. Often, manufacturers of this underlay type seek independent assessments to demonstrate their product’s efficacy. Despite this, many in the industry, notably housebuilders, prefer these products, particularly for their streamlined ventilation capabilities.
Close fitting roof coverings and vapour permeable underlays
When working with close-fitting, relatively airtight roof coverings like fibre cement slates, the potential risk of condensation forming on the underlay’s underside and the external covering emerges. Addressing this risk requires the batten space to be ventilated, aligned with B5250, applying to both warm and cold roof constructions.
Underlays facilitating the transfer of moisture into the batten space can inadvertently result in detrimental condensation if the batten space isn’t well-ventilated. This ventilation can either be intentionally integrated or occur naturally via a suitably breathable roof covering.
Cold pitched roof with (LR) Vapour Permeable Underlay
For cold roofs utilising vapour permeable underlays, the likelihood of interstitial condensation diminishes significantly, as long as the ceiling is effectively sealed, and the eaves allow a minimal continuous ventilation opening of 3mm. If the ceiling is not sealed well, larger openings (equivalent to 7mm) are recommended. A 10mm eaves vent should suffice for both criteria to be on the safe side.
Cold pitched roof with a (LR) vapour permeable underlay and close fitting roof covering
Utilising an LR/vapour permeable underlay could allow designers to minimise loft ventilation compared to what BS 5250 suggests for HR underlays. However, adherence to the manufacturer’s third-party accreditation remains crucial.
Moisture movement can occur through both diffusion and convection from the loft to the batten space with some breathable underlays. To counter potential condensation, it’s essential to ventilate the batten space using counter battens to create a 25mm void and vents:
- Low-level vents: Equivalent to a continuous 25mm deep slot along the eaves.
- High-level vents: Matching a continuous 5mm deep slot along the ridge, consistent with BS 5250.
If batten space ventilation is omitted, elevated roof space ventilation becomes necessary, following BS 5250 and manufacturer guidelines. Identifying an underlay as HR or LR purely by sight can be challenging, making it imperative to consult the manufacturer’s third-party accreditation for clarity.
Warm pitched roof with (LR) vapour permeable underlay
In a warm pitched roof that features a LR or vapour permeable underlay, an AVCL should be provided at ceiling line. If the external cover is suitably permeable, the vapour will be released into the atmosphere and there will be no requirement to ventilate the batten space.
Warm pitched roof with (LR) vapour permeable underlay and close fitting roof covering
In warm pitched roofs featuring a breathable underlay, it’s advisable to incorporate an AVCL at the ceiling level. When using external coverings that are relatively airtight, the risk of condensation forming on the underlay and external covering arises. To mitigate this, ventilation of the batten space becomes indispensable.
Insulation may be provided above the rafter and between rafters to form a warm roof construction. The position of insulation and air vapour control layers (AVCL) must strictly adhere to the insulation manufacturer’s recommendations. All warm roof construction products must have appropriate third-party certification.
Sarking boards, primarily constructed from softwood and typically around 150 mm in width, are affixed to the rafters of pitched roofs. Their main function is to enhance the structural integrity of a building.
While sarking boards are more common in places subject to harsh weather conditions like heavy winds or relentless rain, they’re not as prevalent in regions like England and Wales. However, in Scotland, they are frequently used due to the distinct weather patterns and specific building regulations in place.
The application of sarking boards is more common in new builds rather than refurbishment projects. This is largely because, in renovation processes, the focus tends to be on placing insulation both between and beneath the rafters. Incorporating sarking boards in such a scenario would necessitate a complete re-roofing procedure where roof tiles or slates are removed and subsequently repositioned.
A typical layering, from the innermost layer outward, would be:
- Insulation, placed either between and below or between and above the rafters to achieve the desired U-value.
- Sarking boards, laid with a slight gap of about 2-3 mm between each board.
- Breathable membranes, like Nilvent, which negates the necessity for additional roof ventilation.
- Roofing slates.
It’s worth noting that if one opts for tiles over slates, the structure would demand the addition of counter battens and tiling battens above the breathable membrane. This adaptation allows for effective water drainage and facilitates tile attachment.
In areas of Scotland with severe weather exposure, slate roofs are typically secured directly onto the sarking board, bypassing the use of battens.
In addition, when an underlay is positioned atop rigid sarking boards, it should possess low vapour resistance (LR) or be vapour permeable. If the underlay has a high vapour resistance (HR), there must be augmented ventilation either in the roof space or between the underlay and the sarking to maintain optimum conditions. Once again manufacturer guidance should be followed to ensure correct installation.
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