In recent years, there has been no consensus on which “cold” pitched roof systems are more effective. The two main areas of discussion are energy efficiency and moisture control, the latter being particularly relevant to the risk of condensation forming in the roof structure.
A comparative test carried out during the construction phase showed that the use of an improved breathable roof membrane in combination with a vapor and air barrier layer for the ceiling can prevent condensation. This combination ensures a higher temperature in the rafters, and in an unventilated system, the rafters dry faster, allowing for better thermal performance and proper moisture control.
Test procedure
From January to March, DuPont Performance Building Solutions conducted on-site tests comparing two “cold” roof systems:
- Ventilated system – high-level ventilation according to NHBC standards.
- Unventilated system – no eaves or ridge ventilation, using DuPont™ AirGuard® Control vapor and airflow barrier.
Both systems used DuPont™ Tyvek® Supro LR roof diffusion membranes, in accordance with BS5250:2011. During the tests, the temperature and relative humidity were recorded at the eaves and cornice projections. The moisture content of the wood was recorded at the beginning, middle, and end of the test.
Results
Temperature and relative humidity
| System | Average temperature at the roof | Average temperature at the eaves | Average relative humidity at the roof | Average relative humidity at the eaves |
| Unventilated | 12,7 °C | 10,6 °C | 86,4 % | 100 % |
| Ventilated | 12,5 °C | 10,6 °C | 74,6 % | 84,8 % |
| Outside | 9,15 °C | – | 75 % | – |
Wood moisture content in rafters
| Date | Unventilated | Ventilated |
| 02-08 (beginning) | 19,1 % | 17,1 % |
| 03-06 (middle) | 16,8 % (-2,3 %) | 15,8 % (-1,3 %) |
| 04-12 (end) | 16,8 % (-2,3 %) | 14,8 % (-2,3 %) |
Analysis:
- The average temperature between the roof ridges was similar (0.2 °C higher in the unventilated roof).
- The temperature at the eaves was practically the same in both systems.
- In the unventilated roof, the relative humidity at the eaves was 100%, while in the ventilated roof it was 84%.
- Wood drying was faster in the unventilated system: the same moisture reduction was achieved in one month as in the ventilated system.
This shows that the DuPont™ AirGuard® Control vapor barrier effectively reduces vapor transfer, while the Tyvek® Supro roof membrane allows moisture to be quickly removed into the environment.
Conclusions
- An unventilated system using high-quality breathable roofing and an airtight ceiling membrane effectively removes moisture and retains heat.
- Moisture control in ventilated systems is slower but reduces the risk of condensation.
- Further long-term studies are needed to assess operational reliability and energy efficiency.
When planning “cold” pitched roof systems in the future, it is necessary to consider both their short-term and long-term reliability.