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Vapour Control
Control of Condensation
Satisfactory performance in respect of the control of condensation, both on the surface of and within the roof system, is essential if thermal and durability targets are to be realised.
All designs should be checked in terms of condensation risk for the intended building function (and any future change of use). The Building Regulations Approved Documents F (F-1, Means of ventilation and F-2 Condensation in roofs) sets mandatory requirements in respect of the control of condensation.
Condensation in a roof construction occurs when moist air is cooled below its dew point. The greater the moisture content of the air (relative humidity, RH), the lower the dew point temperature. In cold external conditions, as moisture vapour from a heated interior moves upwards through a typical roof system, its temperature drops. Correct design against interstitial (within the system) condensation, ensures that either a vapour control layer (warm roofs) or ventilation (cold roofs) is provided to control this process. The former works by acting as a barrier, the latter by dispersal. Building uses such as kitchens, swimming pools or shower rooms are at particularly high risk. Buildings such as school classrooms or community centres that are heated intermittently and then closed for security reasons, are also of significant risk.
Conversely, low RH buildings such as warehouses with only background heating, or offices with air management systems are at very low risk. Cold stores can be assessed in similar ways but in reverse, with the external waterproofing also being required control effectively moisture vapour transmission into the roof system from the exterior. Increased thickness of insulation in roofs helps to reduce the risk of surface condensation on ceilings, but it does not in itself reduce the risk of interstitial condensation. Indeed it may increase that risk. The correct design of vapour control is therefore vital for effective roof performance.
In a warm roof, the vapour control layer (VCL) is placed on the underside of the insulation. However, the VCL is never totally resistant to moisture vapour transmission or air convection. Small quantities of water vapour passing through the membrane, will pass through the insulation system and condense on the cold underside of the waterproof membrane. Design calculation takes account of this process, by ensuring that there is no significant accumulation of condensate within the system, over a complete annual cycle of Winter condensation and Summer evaporation. Calculation may indicate that no VCL is required for certain low-risk buildings. In this situation, an unsealed metal deck may provide sufficient control.
Guidance is available in Building Research Establishment ‘Thermal insulation- avoiding risks’, BS 5250 : 1989 ‘Code of practice for control of condensation in buildings’ see footnote and Chartered Institute of Building Services Engineers (CIBSE) Guide - Volume A – Design Data.
BS5250 describes a methodology of quantifying the accumulation and removal of condensate during hypothetical Winter and Summer conditions respectively. This method of calculation has also been adopted for all roof coverings within the scope of BS6229, which additionally advises maximum levels of annual accumulation in Kg/m2. However, such calculation is theoretical, because it is based upon steady state conditions and nominal performance data for roof components. Particular consideration should be given to the following:
Warm Roofs
- Avoidance of cold-bridging across components with high thermal resistance
- Avoidance of air movement through and across the roof system
- Continuity of the vapour control layer at upstands and details generally
- The effect of penetrations through the vapour control layer
Inverted Warm Roofs
- Avoidance of surface condensation on lightweight decks
- Avoidance of cold bridging due to gaps in loose-laid insulation
Cold Deck Roofs
- Clear routes for through-ventilation
- A minimum 50mm gap between the underside of the deck and the top of the insulation
- Adequate openings for ventilation at each end of the roof
Note: Following independent research, SPRA has adopted a policy in respect of the need for a separation layer between steel decks and certain insulation types in warm roof construction, irrespective of whether calculation shows that a VCL is necessary.
© 2006 Sarnafil Ltd. Legal Disclaimer
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