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| Pitched Roofs Insulation - Design Considerations |
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| UNVENTILATED
AND VENTILATED CONSTRUCTIONS Unventilated roofs are characterised by the use of a breathable sarking membrane and have no deliberately introduced ventilation below the membrane. Ventilated roofs use traditional sarking felt and the Building Regulations /Standards require a 50 mm ventilation air gap between the insulation and the sarking felt, so as to avoid condensation. There is generally a choice between either approach, except in the case of refurbishment / loft conversions. In these instances, unless the whole roof is to be stripped, it is impossible to use an unventilated roof, because the necessary breathable sarking membrane cannot be installed. UNVENTILATED ROOF – VENTILATION CONSIDERATIONS Unventilated roof approaches create a warm pitched roof space, which does not require cross ventilation. Recent research suggests that sealing an unventilated roof, yields a more energy efficient roof as the impacts of ventilation and incidental infiltrating cold air are negated. Therefore, if creating an unventilated roof, it is preferable to fully seal all joints in the breathable sarking membrane with tape. Any water vapour reaching the breathable sarking membrane escapes without condensing. There is then adequate air movement beneath the tiles or slates to dissipate this water vapour to the outside atmosphere. UNVENTILATED ROOF – POSITION OF BREATHABLE SARKING MEMBRANE The taping of breathable sarking membrane joints is considerably easier to achieve if the membrane is installed on a continuous surface. In these cases, the breathable sarking membrane is installed under counter battens, which provide a channel for water drainage, or in situations with a sarking board under a slated roof, directly under the slates (as neither tile battens nor counter battens are used). Generally, when a continuous surface is available, it will prove easier to install the breathable sarking membrane in horizontal runs, whilst still enabling easy sealing between runs. In some cases with a continuous surface, (when counter battens, tiling battens and tiles replace slates nailed directly into the sarking board) the breathable sarking membrane can be installed over the counter battens. This yields a marginally better design U–value but it may be more difficult to seal the breathable sarking membrane joints effectively, as the membrane must be draped over the counter battens in horizontal runs so as to provide a water drainage channel. The air movement allowed by the unsealed membrane may negate the benefit of putting the membrane above the counter battens. In situations where there is no continuous surface, the breathable sarking membrane can be draped over the rafters in horizontal runs to provide a channel for water drainage. In this situation, sealing of the breathable sarking membrane joints will prove difficult. It is preferable, though more difficult, to install the breathable sarking membrane in vertical runs with junctions between runs sealed by counter battens placed over the laps in rafter positions. The breathable sarking membrane is installed taut as the counter batten provides a space for water drainage. POSITION OF INSULATION Dependent on the designed insulation value of the construction and the available rafter depth and headroom, different approaches can be taken. The choice of approach may be influenced concerns over the depth of bargeboards, pattern staining and available headroom. Approaches with a layer of insulation over rafter are likely to yield very large fascia boards. Pattern staining in the position of rafters can be caused if rafters are left as uninterrupted cold bridges. For this reason, solutions relying solely on insulation between rafters should be avoided. All solutions shown in this guide minimise the risk of pattern staining. Because of the above two issues, between and under rafter insulation approaches are probably more desirable. Headroom reduction can be minimised by placing most of the required insulation between rafters and a minimum amount below the rafters. RECOMMENDED SOLUTIONS FOR NEW BUILD/RE–ROOFING The ideal solution for new build or re–roofing projects is, therefore, between and under rafter insulation with a continuous surface for the breathable sarking membrane so that it can be installed in horizontal runs under counter battens with laps sealed. The next best solution is, therefore, between and under rafter insulation with no continuous surface for the breathable sarking membrane, and the breathable sarking membrane installed in vertical runs with laps sealed under counter battens. |
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