The Passivhaus building standard, which dramatically reduces heating and energy demand through strict performance targets by prioritising excellent insulation and airtightness, is a good path to creating a highly energy-efficient home. To achieve the low-energy demands of Passivhaus standard, floors, walls and ceilings must have a thermal performance (U value) of a minimum of 0.15, and in many cases it is necessary to reduce this further, potentially requiring a significant thickness of insulation.

Thermal bridging (breaks in the insulation layer typically found at the junction between walls and floors or around windows) must also be effectively eliminated.

The choice of insulant therefore clearly plays a key role in maximising available space, cost-effectiveness and long-term performance. All insulation can achieve the Passivhaus standard, but rigid thermoset phenolic insulation can provide premium levels of thermal performance while being thinner than most other commonly used insulants. This allows wall, floor and roof constructions to be kept to a slim thickness.

In addition to the insulation decision, the choice of structural walling system plays a key role — particularly in terms of achieving low airtightness levels. Traditional masonry construction approaches do not lend themselves naturally to the demanding fabric requirements of Passivhaus (although the first Passivhaus in the UK was a masonry build — albeit with 500mm-thick walls). In order to get the most from the available construction space it is therefore worth considering alternatives such as structural insulated panels (SIPs), which can often provide a simpler, faster and more reliable option.

Structural Insulated Panels (SIPs)

SIPs are easily capable of achieving a U value of 0.16 without any additional insulation. Their jointing arrangement also ensures that the oriented strand board (OSB) facing and insulation core run continuously throughout the walls and roof of the building. This helps to keep thermal bridging to a minimum and can help reduce air loss to as low as 1.0 air changes per hour @ 50 Pascals (Pa).

Raising this performance to Passivhaus levels is then relatively simple. For example, if a ventilated cladding option is chosen for the walls of the property and slates or tiles for the roof, then installing a 60mm thickness build-up of phenolic insulation on the internal wall of 172mm-thick SIPs, on both the walls and roof, will achieve a U value of just 0.10.

Assuming the property has a solid floor construction with a floor perimeter/area ratio of 0.7, 75mm and 100mm layers of phenolic insulation should also be fitted beneath a concrete slab and a layer of screed to match this level of thermal performance. An airtight layer should be taped internally within the structure, with care taken to ensure all joints are properly sealed.

Problems (and a Solution) for Renovators

Homeowners retrofitting to the EnerPHit standard (the Passivhaus standard for existing homes) can find it very challenging, particularly when dealing with certain areas where available space is extremely limited. It is possible to compensate for these areas by insulating to a higher level elsewhere, providing the overall average U value for the component (i.e. walls) meets the EnerPHit requirements. In addition, for certain areas where the entire surface may be difficult to insulate, such as solid floors, EnerPHit allows the overall heating target to be exceeded providing the area meets certain relaxed criteria.

Vacuum insulation panels (VIPs) offer an alternative solution. As their name suggests, they feature a microporous core which is evacuated, encased and sealed in a thin, gas-tight envelope. This allows them to reach levels of thermal performance up to five times better than other commonly used insulation. As a result, in several challenging applications (i.e. balconies, dormer windows and solid floors) they can help achieve U values of 0.15 or below, with a minimal thickness.

Tony MillichapAbout the Author

Tony Millichap is Head of Technical at Kingspan Insulation and is one of the UK’s leading experts on insulation

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