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Tests: ventilated versus non- ventilated cold pitched roofs

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Tests Show How to Stop Condensation in Roofs

Comparative construction phase testing shows that combining an advanced breather membrane at roof level with an AVCL at ceiling level provides a solution for how to stop condensation in roofs. The combination of breather membrane and AVCL creates warmer temperatures, while rafters dry out quicker, in a non-ventilated system. This means greater thermal efficiency and moisture control for the long term.

Ventilated Versus Non-Ventilated Roofs
For at least 13 years, there have been two schools of thought as to which type of cold pitched roof system is more efficient. Energy efficiency and moisture management have been two main areas of debate. The latter also concerns the risk of condensation occurring within the roof space. This has led to decisions to solve a short-term problem – unfortunately, at the cost of long-term performance and energy efficiency. DuPont Building Innovations has conducted a brief on-site study that is enlightening – and also indicates the need for further in-depth testing and analysis within the industry.

Over the last 10 years, during the winter months, the initial drying out phase of new buildings has led to a number of reports of condensation. In some quarters, a portion of blame has been levelled at modern breather membrane systems. To help us see what’s going on in our roofs, DuPont Building Innovations carried out an on-site test programme from January to March 2012. The test compared two cold roof systems – one ventilated, the other non-ventilated – that both used DuPont™ Tyvek® Supro, a Type LR underlay complying to BS5250:2011.

South West Contractors MiSpace kindly allowed access to the roof spaces of houses they were constructing on a site in Bodmin, for the purposes of measuring the environmental conditions inside. One plot incorporated high-level ventilation as per current NHBC Standards, while the other plot was constructed with no eaves or ridge ventilation, in accordance with Tyvek® Supro BBA certificate 08/4548. This non-ventilated system also incorporated DuPont™ AirGuard® Control as the AVCL at the horizontal ceiling line. The rest of the roof construction in both plots was the same.

Tyvek® Supro installed on January 2012

Measured temperature and Relative Humidity (RH) hourly within the roof space at ridge line and in the eaves

Measured temperature and Relative Humidity (RH) hourly within the roof space at ridge line and in the eaves

DuPont™ Tyvek® Supro was installed on both plots in January 2012. This allowed follow-on trades to carry on working in a dry environment while the roof work continued. Data loggers that recorded temperature and Relative Humidity (RH) at hourly intervals were placed within the roof space at ridge line, and in the eaves. The moisture content of the timber was recorded at the start, mid-point and at the end of the test period.

The Results:
Non-ventilated roof
Average temperature at ridge = 12.7°C
Average temperature at eaves = 10.6°C
Average RH at ridge = 86.4%
Average RH at eaves = 100%

Ventilated roof
Average temperature at ridge = 12.5°C
Average temperature at eaves = 10.6°C
Average RH at ridge = 74.6%
Average RH at eaves = 84.8%

External
Average temperature = 9.15°C
Average RH = 75%

Over the recording period, the accumulated data provided a great deal of information on the relative performance of the two different roof build-ups. The results showed the following:

Average ridge temperatures were similar; the non-ventilated ridge was 0.2°C warmer.
Eaves temperature of both systems were on average identical, although the ventilated eaves dropped to a temperature nearly 1°C colder.
Average eaves RH 100% in the non-ventilated roof.
Average eaves RH 84% in the ventilated roof.

 

While all the data above have great significance, the moisture content of the rafters is more revealing in terms of how the different roof types perform:

Rafter moisture content when the loggers were installed (Feb 8th 2012):
Non-ventilated: 19.1%
Ventilated: 17.1%

Rafter moisture content at the midpoint of the test (March 6th 2012:
Non-ventilated: 16.8% (-2.3%)
Ventilated: 15.8% (-1.3%)

Rafter moisture content The day the loggers were removed (April 12th 2012):
Non-ventilated: 16.8% (-2.3%)
Ventilated: 14.8% (-2.3%)

The moisture content of the rafters was taken in the same location in both roofs. On the face of it, it appears as though evaporation of moisture within the timbers was similar. Not so – the drying-out process of the non-ventilated system was more rapid, with the ventilated roof taking almost a full month more to drop by an equal percentage. This not only suggests that vapour transfer from the building interior is reduced by DuPont™ AirGuard® Control, but also that the roof underlay (DuPont™ Tyvek® Supro) is diffusing this vapour to the outside atmosphere at an acceptable rate.

The study is interesting for this reason alone, and it is one that can be built upon, especially when considering overall performance in the long term. This energy-efficient, non-ventilated system, using a good quality breathable roof underlay in conjunction with an airtight membrane at ceiling line, will provide a balanced solution to moisture migration and heat retention.

If we continue to build cold pitched roof systems in the UK, we should build them with both short- and long-term performance in mind.