Roof space ventilation is important in warm weather to dry out moisture that may have accumulated in the roof space. It is important to reduce the movement of moisture from the living areas to the roof space and to ventilate the latter by means of vents distributed between the upper and lower parts of theroof to take advantage of both wind action and stack action.
Attic ventilation is compared with other means of ceiling heat flux reduction in low cost housing. A simple steady state mathematical model has been run with climatic data for a summer day of Porto Alegre, Brazil. The increase inceiling thermal resistance has proved to be the best improvement, but it is expensive. The greatest proportion in ceiling heat flux reduction is in the natural ventilation range and forced ventilation adds little to it. As natural ventilation does not imply extra cost, it is very important in low cost housing and should be optimised.
A major cooperative study of the effect of ventilation of timber flat (cold) roofs on combatting condensation and moisture accumulation has been undertaken in Denmark. Field measurements of moisture content in a number of test roofs over long periods and under different conditions are evaluated and conclusions drawn. They include the advice that, where moisture accumulation is a problem, it can be aggravated if roof vents are installed.
Concentrates on low energy housing construction in Scandinavia, and Sweden in particular, where typical new detached houses with a floor area of 140 m2 now use less energy for space heating than water heating.
The multiple tracer gas technique developed at UMIST has been applied to the measurement of roof-space ventilation rates and house to roof-space air movement, for various types and combinations of roof-space ventilation. It has been shown that ridge tile ventilators, whilst increasing roofspace ventilation rates at low wind speeds, also significantly increase house to roof-space airflows over the whole range of wind speeds. This has implications not only in terms of energy wastage, but, more significantly , in terms of increased moisture rates to the roof-space.
Recent work has demonstrated the existence of daily and seasonal cycles in attic moisture parameters. Over the course of a day, the attic air humidity may vary by a factor of three, and during the course of a winter there isstorage of perhaps
This is the third item in a series on methods for predicting condensation risks within structures. It answers criticisms made of the method described in NO 1729, on the basis that the method does not give the same answers, nor does it take account of the effect of the occurrence of condensation on the vapour pressure gradient within the structure, as does the graphical method described in NO 1728.
Describes in detail a computer-based technique for predicting the risk of condensation occurring in building structures. The technique not only indicates the position at which condensation is likely to occur, but also puts a figure on the risk of decay in timber within the structure. In the case of ventilated roofs or walls it gives the minimum sizes for ventilation openings.
Discusses insulation of lofts, roofs, walls, windows and floors, natural ventilation of dwellings and mechanical ventilation with heat recovery in dwellings. Considers cost benefits of weatherstripping and constant-flow ventilators for naturally ventilated houses. Concludes that installation of mechanical ventilation with heat recovery is uneconomic, but adding a heatexchanger to an existing mechanical ventilation system has economic benefits.
With correct application of vapour barriers the ventilation of building structures is in general not necessary, unless such barriers prevent the escape of trapped moisture from moisture-sensitive - especially organic - materials. Indoor and outd