A Passivhaus haven of timber tranquility

A timber-framed Passivhaus in Warwickshire, Larch Corner boasts some truly impressive credentials.

Mark Siddall, of architecture practice LEAP, took on a challenging project and delivered an extraordinarily efficient building. It is 195 times more airtight than 2023 UK building regulations require. It is also a haven of timber-based tranquillity – a ‘forever home’ that Mark’s client is delighted with.

Re-thinking the existing design

The land for Larch Corner had been purchased with planning permission in place at auction. The initial planning statement declared that “the building has been designed to meet Passivhaus standards, taking advantage of the southerly aspect for solar gain and natural light.”

Craning the walls into place

Craning the engineered walls into place; © Mark Siddall, LEAP: Lovingly Engineered Architectural Practice

But it soon became clear that the original architects had known little about Passivhaus, considering it to be more a question of construction rather than design. Siddall had to transform their design into something viable.

For one thing, the design had poor form factor and was unnecessarily complicated for a single-storey building. The floor plan was muddled, with some confusion between public and private areas.

Form factor and building performance

“We had to rethink things and started by considering the form factor – the surface area to floor area ratio,” Mark Siddall explains. “The lower the form factor, the more compact the building is – and the lower are the energy losses for a given floor space.”

A larger form factor requires more insulation and Larch Corner’s form factor is 3.9 – quite large. This was largely due to planning constraints over the building’s height and how it would affect views from a neighbour.

“The heat loss form factor is the heat loss area divided by the treated floor area. If we halved heat loss area we could reduce the thickness of insulation.”

Opportunities for improvement

The client wanted a three-bedroom house that would suit his day-to-day lifestyle while also accommodating visits from his family and grandchildren.

Exterior of Larch House, clad in timber

The timber-clad exterior of Larch House; © Mark Siddall

Mark overhauled the design to make the house more efficient. He retained the building a ‘sawtooth’ profile but squaring off some protuberances in the design. He also improved the floor plan and articulated the ceiling levels to help differentiate between the more private and public areas within the house. He designed in lots of flexibility – a large sliding door allows the office space to change its function to a bedroom (complete with folding down bed) and there’s plenty of wall storage.

The original design had extensive glazed areas and to mitigate overheating risks, the original architects had designed in extensive solar shading. Mark’s design reconsidered the amount of glazing, by addressing the AECB (Association for Environment Conscious Building) Daylight Standard and using daylight calculations. The fact that windows cost more than walls was also a consideration. The finished house has a bright interior, with window seats making the most of the glazing.

An MVHR (mechanical ventilation heat recovery) system wasn’t originally accommodated within the former architects design, so dropping some ceiling heights helped  with the distribution of ductwork. Mark favours MVHR over trickle vents, because available evidence shows that as airtight homes with trickle vents are, he points out, more likely to lead to toxic buildings. This is because occupants prefer having them closed so that they don’t experience uncomfortable drafts.

A timber aesthetic runs throughout the interior; © Mark Siddall.

A mass timber Passivhaus solution

Everything above the concrete slab at Larch Corner is timber-based, including the insulation. Providing timber is sourced responsibly and sustainably, timber’s low upfront carbon emissions make it a good material to use, Mark believes.

The house used a mass timber frame system by Novatop, based in the Czech Republic. In fact, Mark qualifies, while he refers to it as CLT, Novatop themselves describe their system as “a solid wood system based on CLT” and  “CLT at a higher level”. Mark says that there are subtle differences in terms of the number of layers and laminates Novatop use, but that the principles are the same.

He states that he wouldn’t typically advocate CLT for a building of this size – timber I-beams would be more resource-efficient – but that it was a client stipulation because they wanted the timber finish (which could have been achieved by using large format timber boards). He has calculated that I-beams would have reduced embodied carbon emissions by 51.1 kgCO2/m2.

A healthy, timber-based home

“We used CLT that is Natureplus certified and achieves a formaldehyde emission class of E1 (EN717-1),” Mark reveals. Natureplus is an environmental label for building products that is fully compliant to ISO 14024.

Internally, all the CLT is exposed. The dining-room table is even made from offcuts of CLT.

The installation of a domestic sprinkler system to control surface spread of flame in the event of a fire meant that the timber did not need to be encapsulated or treated with a fire-retardant coating. Low-toxicity paint finishes were used throughout.

The overall formaldehyde quantity in the house is less than 0.1 mg/m3 – better than BREAMM standards – while the total VOCs (volatile organic compounds) are less than 0.3mg/m3, well below Approved Document F.

A house on a raft

Larch Corner is a house that essentially sits on a raft made of XPS (extruded polystyrene), which has very low conductivity. This transfers the load from the slab into the ground in elegant fashion. The raft, from Jackon’s Atlas range,  is 260mm thick with a 200mm upstand.

The Novatop system arrives; © Mark Siddall

The arrival of the Novatop CLT system; © Mark Siddall

“It means there is thermal bridge-free construction when we mount the CLT on top of the concrete,” Mark states. “It’s all prefabricated, with no cutting onsite so waste  is mitigated. It just slots together like a very easy jigsaw.”

The CLT stands on a sole plate, which avoids complicated bolting into L-shaped brackets that would have protruded into the rooms or necessitated putting a screed over the floor.

A fast and efficient build

Novatop’s timber frame system proved an effective one. Mark also speaks glowingly of the project’s joiner and carpenter Andy Mackay of Mac Eye Projects, who became a certified Passivhaus tradesperson over the course of the project.

On the first day the walls were craned into position. On day 2, the roof cassettes were lowered into place. These were pre-insulated: not quite to the level required but “a good way there”, Mark tells us. Thermal taping could then begin.

A windtight barrier – Pavatex DSB2- went on over the roof on Day 3 to protect the house from rain. All joints were taped and sealed that day. The external walls of CLT were taped, providing the house its primary air barrier. All the taping was on the warm side of the insulation but on the outside of the house, allowing timber finishes to be exposed inside.

All the insulation above the damp proof course was made from timber sourced from forest thinnings or as a sawmill by-product. The house was, in fact, the first UK installation of Steicozell – a blown wood fibre insulation. This worked out cheaper than cellulose, because it could be introduced at a lower density to achieve same U-values. The walls had to be heavily insulated because of the form factor but ultimately achieved a U-value of 0.092 W/m2K, while the roof achieved 0.12 W/m2K.

Rear of Larch Corner

Rear of Larch Corner; © Mark Siddall

The outside face of the insulation was wrapped in a wind barrier and all joints were taped, then Siberian Larch rainscreen cladding placed on the face of that to ensure water cannot track through to the inside. The framework on the roof extrudes to form gargoyles, to run-off rainwater.

A forever home

Larch Corner was completed in June 2019. It generates more power than it uses, thanks to its solar array and air source heat pump. With a heating demand of just 14 kWh/m², its occupant is very happy with it and has been cooperative in keeping Mark updated about its performance.

Screwed together using some 21000 screws, it should prove to be more than just a forever home for Mark’s client: many of its materials will be kept for reuse at the end of its life.