Thermal Bridge vs
Whether inspired by passive house principles or not, most of the industry is now attempting to design for continuity of the thermal envelope, thus removing cold bridges, increasing both thermal performance and building energy efficiency over time.
Any break in the insulation layer of the thermal envelope will allow heat energy to escape. Thermal bridges such as roof joists, structural and services roof penetrations should be minimised. The simpler the roof design, the better.
The simple test for continuity is to trace or draw a line of the insulation products over a proposed section drawing (of the whole building). If you have to stop or lift the pen because there are uninsulated elements, then the continuity is broken.
In a warm roof configuration, a vapour barrier layer is recommended to achieve the highest performance standards. Because the insulation is kept dry between the membrane and the vapour barrier, it will retain the R-Value performance. In contrast, cold roof insulation products often degrade over time.
The vapour control layer prevents warm moist air from reaching the dew point where it will form condensation. It also acts as an air barrier by preventing air leakage through the roof system.
The vapour barrier to the roof membrane must be properly sealed at roof penetrations and at the roof perimeter. If not, there will be an opportunity for moisture to be drawn into the insulation envelope. If moisture does enter the insulation, it can create vapour expansion and compromise the whole membrane outcome. Condensation underneath the membrane is a failed attempt at a warm roof. Without vapour control, it is no longer a true warm roof.
The provision of insulation alone is not enough to prevent condensation. If the insulation is permeable to water vapour, the vapour will pass upwards through it and condense on the underside of the waterproof membrane.
To prevent this from occurring, a vapour control layer should be provided directly below the insulating layer. At Nuralite, we recommend using a Nuratherm Warm Roof system installed with an IKO ALU Vapour Barrier/Blocker. This Nuralite vapour barrier provides temporary waterproofing during construction and a third layer of defence.
Designing ventilated cold roofs or airtight warm roofs without careful attention to risks of condensation can compromise the whole building fabric and integrity. The performance of a building over time can seem less relevant when the budget and schedule are immediate. Design and specification with attention to condensation and airtightness can help extend the building’s lifespan.