Not since the oil crises in the 1970s has there been such global concern about the upward spiral of energy prices. Back then the energy efficiency of new buildings was far from an exact science, but now it is fast becoming one of the key considerations for today’s house builders and specifiers. More and more new homebuyers are not just concerned about owning any home, but ensuring that their new home is comfortable, warm and affordable to heat.
The introduction of the EU Energy Performance of Buildings Directive will mean that superior energy performance in buildings now needs to be a standard feature and not an added extra.
Improving the energy efficiency of a building is not simple but there are many options to choose from during the design and construction phase that achieve better performances and reduce annual heating costs. A study by the globally renowned British Research Establishment (BRE) shows that choosing the right roofing system is critical to the energy performance of a building and in the new energy conscious era the days of the traditional cool pitched roof are numbered.
The BRE project was undertaken to evaluate the impact of ventilation upon energy performance and air leakage of cold-pitched roofs. Also examined were the potential savings that could be achieved in practice by adopting a non-ventilated roofing system such as that advocated by DuPont™ Tyvek®.
To ensure a completely unbiased result the work was carried out using two BRE 'matched pair' test houses. They were calibrated against each other by maintaining, temperature, humidity, energy use and external weather conditions for a month. Following this the second home then had its roof rebuilt using Tyvek® Supro from DuPont. The monitoring then resumed throughout the winter.
When properly installed over the top of rafters or trusses, Tyvek® Supro helps provide a vapor-permeable air and water barrier that stops airflow while still allowing attic moisture to escape by diffusion through the micro channels in the structure of the material. This simple roof system helps dissipate attic moisture in cold weather, and helps reduce heat build-up in warm weather, much more efficiently than traditional attic ventilation. The cooling effect is further enhanced by the use of Tyvek® Enercor® Roof, the latest generation of low emissivity membranes with a metalised surface. When installed facing downwards (into the attic), this feature helps reduce the radiant heat buildup that is normally emitted through the roofing materials. It also helps reflect radiant heat back into the living space in cold climates. This reduction in radiant heat flow cuts down the energy costs to condition the living space below.
The results of the BRE experiments will come as a surprise to aficionados of the conventional cold-pitched roof. In the roof fitted with Tyvek® Supro, air leakage was reduced by 74%, a decrease from around 6 air changes an hour to 1.7 air changes an hour. Also, the test house with the sealed roof system used significantly less energy for space heating, in fact there was a daily saving of 4.2kwh, or 927kwh over a full heating season.
An infrared survey was also conducted by the BRE to monitor the thermal performance of the test houses. In the first month the path of the loft ventilation air could be clearly distinguished in both houses as it entered the roof space. However when house two was fitted with the Tyvek® Supro non-ventilated roof system it was clear that no air left the roof space at the ridge vents. As no changes had been made to the insulation it can be concluded that the thermal performance is derived from improved performance in the roof construction, proving air tightness is just as important to a roof structure as insulation.
From this BRE report it is evident that reducing the flow of air through the roof space by installing the Tyvek® Supro non-ventilated roof system increased the effectiveness of the ceiling insulation, thus reducing the overall heat loss through the roof. Also by constructing a much tighter cold-pitched roof, the convection heat transfer between the heated envelope and the loft had also reduced.
DuPont has been awarded a BBA certificate (Certificate number 04/4101) and is the first to be given this accolade. This certificate guarantees that the type of roof covering and air permeability of the roof has no effect whatsoever on the moisture performance of a Tyvek® non-ventilated, cold pitched roof system.
Knowing how the non-ventilated attic system works is easy, but the concept can be hard to grasp. That’s because we are used to thinking about wall systems that have an air barrier, house wrap, and a thermal barrier, insulation, in one contiguous assembly that we call the "building envelope."
A non-ventilated attic, which is vital to completing a continuous, whole-house building envelope, has an air barrier at the roof line, while the thermal barrier typically exists in the ceiling plane. In this configuration, the two still work together to help reduce heat and moisture migration, yet are separated by the open area of "semi-conditioned" attic space.
Even when a polyethylene air/vapor barrier is installed on the ceiling, numerous penetrations - such as cracks along interior walls, holes for electrical lines routed through the attic, plumbing vent penetrations, can lights, chimneys, and others - breach the ceiling air seal. In short, ceilings leak like sieves, with the air driven under pressure by the "stack effect." As buoyant hot air rises, it creates a positive pressure against the ceiling that pushes air through all the holes and into the vented attic. This in turn creates a negative pressure at the lower levels of a house, which pulls unconditioned air through cracks near the foundation and floors. The result is a powerful pressure driver that pushes warm air out of the home and pulls cold air inside. The warmer the air, the more moisture it can carry, so all that escaping air carries a significant moisture load. Moisture from cooking, breathing, showering, and other indoor sources is at risk of condensing on attic surfaces, leading to severe moisture damage in the attic.
Because of these problems, roof ventilation is a way to remove moisture in the attic, and is vital for maintaining a cold roof that minimises the formation of ice dams. Roof ventilation is also required by roofing manufacturers to maintain product warranties. Heat degrades asphalt-based materials, and roof ventilation helps dissipate the heat build-up in roofing materials. Ventilation not only extends the life of the roofing, but helps reduce cooling loads wherever air conditioning is used.
Another advantage of Tyvek® Supro is the substantial cost saving in home energy bills that can be made. The BRE study shows that by using a non-ventilated roof, reductions of up to €635 in energy costs per annum can be achieved.
A non-ventilated roof system proves, not only to be cheaper at the outset, but the savings continue after construction. Energy savings from using a sealed roof system can be more than 30% per annum and the results of accelerated ageing tests allow DuPont to confidently predict that Tyvek® Supro will last the lifetime of the building. Contrasted with the huge cost and disruption of replacing bituminous felts, in some cases after just 15 years, the advantages are clearly evident.
The result of all this analysis and experiment points to the need for a resounding change in the way in which the construction industry choose to manufacture roofs. With not only ecological and financial benefits to a non-ventilated roof system, but also huge practical advantages it may well be time to ditch the traditional cold-pitched roof.
For further information please contact:
Peggy Beicht - Media Relations
DuPont Building Innovations
L-2984 Luxembourg
Tel: (352) 3666 5945
Fax: (352) 3666 5021
Email: peggy.beicht@lux.dupont.com
