The winter is fast approaching and it is important to make sure your roof and loft insulation is topped up to keep your heating bills down and ultimately keep your house warm. There are 2 main types of roof’s and 2 main types of insulated roofs. These are Warm flat Roof, Warm Pitched Roof, Cold Flat Roof, and Cold Pitched Roof. The average home owner will not know what type of insulated roof they have and how they can improve it. Below is a diagram for each scenario describing how the roof construction is insulated.
What Is A Warm Roof?Warm roofs are increasingly popular today as we aim to maximise our living space and the available accommodation in any construction. This will be the predominant form of pitched roofing construction.
A breather roofing membrane must be used in a warm roof configuration, to overcome the condensation risk. In a warm roof configuration, the insulation is positioned directly under the external waterproofing (e.g. tiles or slates), following the rafter line. A vapour control layer is usually placed beneath the insulation (on the warm side), to limit the passage of moisture vapour into the insulation.
The roofing breather membrane is placed on top of the insulation, with overlaps, penetrations and perimeters. Counter battens/tiling battens are used to install the tile or slate finishes.
Using roofing breather membrane there is no need to ventilate the void space below the breather membrane, as this void space will always be at a similar temperature to the habitable portion of the building, minimising the possibility for condensation. The void space that would have previously been a storage loft, is now a usable space.
In addition, a warm roof contributes to increased airtightness, reducing heat loss by uncontrolled air movements, and aiding compliance with the revised Part L Building Regulations (April 2006).
Warm Pitched Roof
When insulation is positioned between rafters but does not fill the whole rafter depth, an airspace must be maintained between the underside of breather membrane and the top of the insulation to allow a 10-15mm drape of the membrane, unless 38mm counter battens are used. Other requirements are as for when the insulation is laid above or fully filled between the rafters.
With rigid insulation boards, roofing breather membrane should be laid directly onto the insulation. Counterbattens (minimum 38mm deep) should be fixed according to insulation manufacturer’s instructions.If insulation between rafters is ‘full fill’, the membrane should be laid directly over the rafters and insulation. Counterbattens (minimum 25mm deep) must be nailed directly into the rafters.A vapour control layer should be installed on the warm side of the insulation as detailed in BS 5250: 2002 Code of Practice for control of condensation in buildings.No ventilation of any void space is required below the breather membrane membrane.With clay or concrete tiles or natural slates, air movement is usually sufficient in the space above the membrane (below the tiles/slates) to negate the requirement for additional eaves and ridge ventilation. However, when using close fitting fabricated slates (e.g. fibre cement) additional eaves and ridge ventilation is recommended above the membrane.
Warm Flat Roof
This is the most modern method of construction and should be used were possible (although not all situations are able to accommodate this design).From the room ceiling up the construction is as follows: The roof joists support the roof construction the internal ceiling is attached to these. On top of the joists the roof deck is fixed. A vapour barrier is laid on to the deck and the appropriate thickness of insulation is fixed to the roof deck. The waterproof roof cover is then installed.This method of insulating the roof alleviates the formation of condensation in the roof void, and complies with the current building regulation.
What Is A Cold Roof?
There are two main types of cold roof:
- A traditionally ventilated void space, where all types of under slating membranes can be used.
- An unventilated void space where only certain certified breather membranes can be used.
Cold Pitched Roof
A Cold Ventilated Void Roof configuration is the traditional form of pitched roof build-up. The insulation layer is placed horizontally, directly above the ceiling of the upper story of the habitable section of the building, usually positioned between the joists in the loft space. The Rubershield-Pro breather membrane or any type of proprietary underslating material is placed at rafter level, either draped between the rafters, or supported on a sarking board (usually Scotland). In a traditional cold roof system, any void space below the underslating membrane has to be ventilated. In this roof type, a breather membrane or non-breathing underslating membrane would be equally suitable. When an underslating membrane is installed in a cold roof configuration draped between open rafters, the use of counterbattens is not required.
Where a roof underlay or breather membrane is to be laid over open rafters, a drape of between 10 to 15mm between the rafters is desirable to guide any rainwater penetrating the main roof finish away from the rafters to the drainage point.Provide for ventilation of the void space in accordance with BS 5534 (BS 5250: 2002), with the inclusion of a proprietary eaves and ridge (high level) vents, fixed in accordance with manufacturer’s instructions.
Use with sarking boards
As Scottish practice where using tiling battens, roofing membrane must be laid beneath counterbattens of a minimum 25mm depth. In some instances tiles or slates can be installed by nailing through the membrane directly into the sarking board. However, the tile manufacturer’s instructions must be strictly adhered to.
All Roofing breather membranes fully comply with requirements in BS 5534: 2003, for use with sarking board installations in ventilated cold roof configurations.
In a Cold – Un-ventilated Void Roof configuration the insulation layer is also placed horizontally, directly above the ceiling of the upper story of the habitable section of the building, with the vapour control layer placed below the insulation at ceiling level. However, in this situation, only a breather membrane certified for this configuration can be used. Rubershield-Pro breather membrane is approved for this purpose and should be placed at rafter level, with overlaps, penetrations and perimeters sealed with Rubershield Jointing Tape, draped between the rafters. However, there is no ventilation of any roof void space below the membrane; the only escape route for moisture vapour is through the breather membrane.
Where a roof underlay or breather membrane is to be laid over open rafters, a drape of between 10 to 15mm between the rafters is desirable to guide any rainwater penetrating the main roof finish away from the rafters to the drainage point. (The membrane must not be pulled tight against the underside of the tiling battens.)Using roofing breather membrane in this type of cold roof requires no ventilation of any void space below the membrane. Also, when using clay or concrete tiles or natural slates, air movement is usually sufficient in the void space above the membrane to negate the requirement for additional eaves and ridge ventilation. However, when using man-made slates (or some close fitting interlocking tile systems) additional ventilation above the membrane in accordance with BS 5534: 2003 (BS 5250: 2002) is recommended to avoid potential damage to battens and fixings caused by condensation. It is recommended that guidance be sought from the tile or slate manufacturers concerned.
To minimise the risk of condensation in cold unventilated roofs, all penetrations into the roof space must be properly sealed to maintain the integrity of the vapour control layer or external seal, and loft hatches should be made convection tight. All water tanks in the loft space must be covered and pipework lagged. The occupied building below must be ventilated in accordance with Building Regulations, and rooms that experience high humidity levels must have provision for separate air extraction.
Use with sarking boards
As Scottish practice, where using proper timber sarking planks (usually 150mm wide) laid with a 2mm gap between the boards, there is no need to provide additional ventilation below the membrane. However, where using sarking boards comprising sheets of plywood or OSB, this type of cold unventilated roof configuration is not recommended, and a traditional ventilated void approach should be adopted.
Cold Flat Roof
This is the most common construction and can be the least thermally efficient. This roof design can also be prone to the manifestation of condensation within the roof void. From the room ceiling up the build is as follows: the ceiling is attached to the roof joists that support the roof construction. A polyethylene vapour barrier is laid into the roof void and mineral insulation is loose laid between the rafters. The roof deck is then fixed to the rafters and the water proof membrane is then installed. In some cases this construction may have to be ventilated to avoid condensation.
For all your insulation roofing needs please contact Layton roofing & Building Specialists via our contact formShare