An Energy Efficient Alternative to Traditional Roof Ventilation
Traditional roof ventilation, although recommended for reducing condensation, can lead to a loss of energy efficiency in the building structure. This is why DuPont™ Tyvek® recommends a non-ventilated but sealed alternative to traditional roof ventilation that not only reduces heat losses, cutting harmful CO2 and improving energy efficiency, but also prevents condensation.
Part L of UK building regulations (conservation of fuel and power) requires the energy efficiency of whole buildings to be assessed. It is now standard to measure air leakage rates and thermal bridging at junctions of building elements in all building types. Uncontrolled air exchange is the biggest contributor to heat losses today, and we must take a fresh look at the building fabric and the various ways in which energy is transferred – not only via heat conduction through materials, but also the effects of heat exchange via radiation and via convection through joints, gaps, cracks and penetrations.
Air Leakage Vs Ventilation
It is essential to differentiate between air leakage and ventilation. Nearly half of all heat lost from the average home is a result of uncontrolled air leakage through gaps in the structure. Air leakage happens where the building has not been properly sealed and so air “escapes” from the building interior. This is distinctly different from natural ventilation through vents designed as part of the building structure – such as windows and ventilators – that can be controlled by occupants. However, both combating air leakage and providing effective ventilation require a suitably airtight building.
“Leaky” constructions have been exposed as wasteful of energy, and are major contributors to the CO2 emissions damaging our atmosphere. Studies by the BRE (Building Research Establishment in the UK) show the amount of airborne heat and moisture vapour carried by convection through gaps in the building fabric is far greater than by conduction. Heat energy in the air is lost, while the moisture vapour condenses on cold surfaces within cavities – and in the case of a cold pitched roof, that means the loft space.
Eliminating Uncontrolled Air Leakage
An energy efficient roof is a good starting point, as the roof is almost always the main source of air and energy leakage because of its large exposed surface area, orientation and relatively low mass structure. But despite the need to reduce air leakage, many still insist on using traditional roof ventilation as a way of controlling condensation. Roof vents waste a lot of heat by convection, from the loft and also from the heated living space beneath. In addition, introducing cold external air into roofs:
- Reduces the effectiveness of the building envelope
- Promotes warm air leakage from the building into the roof space
- Increases air infiltration into the heated building
- Introduces dirt, dust and insects into the roof construction
- Introduces external moisture laden air into the construction
The best way to eliminate uncontrolled air leakage is to form a sealed building envelope that consists of an airtight internal layer such as DuPont™ AirGuard® air and vapour control layers (AVCLs) and a breathable but watertight external membrane like DuPont™ Tyvek® breathable membranes.
Cold Pitched Roofs
While on the one hand increased house insulation has allowed improved energy efficiency, much of this can be undone by simply not using appropriate roof insulation and allowing condensation to build up in the roof cavity or loft space. Warm air from general building use rises up and collects in the roof cavity where the moisture meets with the cooler external surfaces resulting in condensation. This problem is especially common once construction is completed during the temporary “drying out” phase, and afterwards during any harsh cold spells.
Moisture management is vital in order to avoid structural damage to the building, health problems in building occupants and reducing the building’s energy efficiency.
With these two products, DuPont offers an effective and unique solution for cold pitched roofs. It’s a solution that ensures an airtight construction, condensation protection and improved thermal performance – while also meeting UK building regulations, saving energy and reducing CO2 emissions.
Non-Ventilated Roofs Are More Efficient
Tests by the BRE using real houses show a non-ventilated roof system is four times more efficient than a traditional ventilated system. Improved thermal efficiency and permanent control of condensation in all seasons is achieved by eliminating roof space ventilation and allowing vapour to escape through a high-efficiency permeable membrane located on the cold side of the insulation. Performance is optimised by using an integrated vapour control/air barrier layer on the warm side of the insulation. This is where DuPont™ Tyvek® breather membrane products and DuPont™ AirGuard® AVCL products come into their own.
The airtight and watertight qualities of DuPont™ Tyvek® membranes means they prevent interstitial condensation and improve energy efficiency, saving money as well as reducing CO2 emissions.