Passivhaus - Closing the performance gap with SIPs

16 April 2020 Kingspan Insulation UK

The gap between the design and as-built performance of our buildings is one of the most significant and pressing challenges facing the construction industry today. Buildings which fail to perform as expected undermine confidence in energy efficiency work and mean that, rather than becoming part of the solution to the climate crisis, these new structures actually add to the problem. For example, a study of 49 non-domestic buildings from Innovate UK showed that the average carbon emissions were 3.8 times the design estimate and that only one project met its estimated emissions level.

It’s not all doom and gloom, however. Over the past three decades, projects built to the voluntary Passivhaus Standard have not only routinely met their expected performance targets, but actually out-performed them. In this piece we’ll look at what makes this approach so successful, and explore how offsite approaches are now allowing it to be applied at scale.

What is Passivhaus?

The Passivhaus Standard was developed in the late 1980s as a method of constructing buildings which require very little energy to heat or cool, whilst also providing a high level of comfort for occupants. To achieve this, it sets clear energy performance targets which a building must meet:

Primary energy demand

≤ 120 kWh/m2/yr

Space heating/cooling demand

≤ 15 kWh/m2/yr

Specific cooling load

≤ 10 W/m2

 Table 1: Performance targets for a European climate

To put these figures in context, the maximum space heating demand for a Passivhaus building is around 10% of that of an average home (estimated to be 140 kWh/m2/yr). As such, whilst these criteria do not specifically address a building’s carbon emissions, in practice they should significantly limit emissions when compared with a property built to current Building Regulations/Standards.

The Standard is backed by a rigorous certification process and post-occupancy monitoring suggests that, on average, actual energy performance of these properties meets or outperforms the design prediction.

The external fabric of a Passivhaus property typically needs to be insulated to meet a U-value of 0.15 W/m2K, or lower, and be fundamentally ‘thermal bridge free’. Close attention to detailing is therefore crucial to ensure that potential thermal bridges around openings and at junctions are properly addressed. In addition, air-leakage rates must be no higher than 0.6 ach@50 Pa. This is typically achieved by installing an airtight layer, such as oriented strand board (OSB), and airtight tape, which is applied to seal all junctions.

We explored the Passivhaus Standard in more detail in a recent episode of the Insight podcast.
Whilst it is possible to meet these envelope requirements with traditional construction methods, offsite construction approaches such as Structural Insulated Panels (SIPs) can provide a simpler, faster and more adaptable solution.

Why use SIPs for Passivhaus?

Alexandra Road - Passivhaus Project

SIPs are fabricated by bonding two sheets of particle board or oriented strand board (OSB) either side of an insulation core and can deliver excellent thermal performance out-of-the-box. For example, our Kingspan TEK SIPs are available in two thicknesses — 142 mm and 172 mm — and can achieve whole wall and roof U-values of 0.20 W/m2K and 0.17 W/m2K or better with no additional insulation. These values can easily be improved further with the addition of an insulated lining.

The panels are precision cut to each project’s specific requirements using CNC cutting machinery. This ensures that openings for windows and doors precisely match the finished units, helping installers to achieve accurate detailing to eliminate thermal bridges and limit air-leakage around these junctions. Our Kingspan TEK SIPs also feature a proprietary cassette joint which slots into a routed channel between the panels, ensuring excellent insulation continuity across the envelope and further limiting air-leakage. Once an airtight membrane is fitted internally and tape is applied to junctions, the air leakage rate can be reduced to the 0.6 ach @ 50 Pa required by the Passivhaus Standard.

In addition to supporting simpler detailing on-site, the offsite production process for SIPs also helps to provide scalable scheduling benefits. The factory cutting process allows identical panel kits to be produced for multiple units without the errors and inaccuracies common with traditional on-site processes. Production timescales can be easily predicted, allowing for accurate scheduling and improved site logistics.

The shells of individual housing units can typically be erected in around two weeks by a small team of site operatives with lightweight lifting machinery. Once a breather membrane is applied and windows and doors fitted, work can then begin on internal fit-out, further speeding up the construction process.

Passivhaus case studies

River Studio - Passivhaus EnerPHit Project

These diverse benefits have led to the use of SIPs on a variety of projects over recent years.

For example, at the River Studio, Sjölander da Cruz Architects used Kingspan TEK Cladding Panels to refurbish an old engineering workshop into an architectural studio which meets the slightly relaxed criteria of the Passivhaus retrofit standard — EnerPHit.

What is EnerPHit?

Existing buildings present a number of challenges not present on new builds such as fixed building orientation, quirky architectural features and limitations on the original build quality. There may also be planning restrictions on the type and scale of work which can be carried out on these properties. All these factors may make it infeasible or cost prohibitive to raise some existing properties to the full Passivhaus Standard. The EnerPHit Standard has been developed to provide a more practical model for these retrofit projects using the following performance targets for European properties:

  EnerPHit Parameters
Primary energy demand ≤ 120 kWh/m2/ yr + heat load factor
Space heating/cooling demand ≤ 25 kWh/m2/yr
Specific cooling load ≤ 10 W/m2
Airtightness ≤ 1 ach @ 50 Pa

As the original building stood on greenbelt, it wasn’t possible to simply demolish it and start afresh. Instead, the firm stripped away the original asbestos cladding and installed the Kingspan TEK Cladding Panels on the original steel frame. 100 mm Kingspan Thermawall TW55 was fitted on the outer face of the panels behind a western red timber façade whilst Kingspan Kooltherm K3 Floorboard (now superseded with Kooltherm K103 Floorboard) was fitted beneath the barn’s screed floor. The completed building has a specific energy demand of just 18 kWh/m2yr, far surpassing the level of performance required by the EnerPHit standard (25 kWh/m2yr).

Rayne Park - Sterling Prize Winner

In addition to smaller one-off projects, a growing number of developers are now utilising SIPs to meet the Passivhaus Standard across larger estates. The Norwich Regeneration Company, for example, has been applying this approach across a number of developments, including the Sterling Prize winning Goldsmith Street Estate.

At another of its schemes — Rayne Park — Kingspan TEK Building System panels were used to help construct an estate of 172 properties with 112 of these expected to be certified to the full Passivhaus Standard. The offsite production process supported a fast and efficient construction programme whilst still allowing designers the freedom to create diverse and colourful home designs.

The first 40 units received full Passivhaus certification in September 2019 and are expected to have a heating demand of just 11 kWh/m2/yr and a primary energy requirement of 77 kWh/m2/yr.

You can read more about how a range of Kingspan products have been used to help projects to meet the Passivhaus Standard in our Passivhaus brochure.

Other articles in this series

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