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What is a ‘fabric first’ approach to creating eco homes?

This is also referred to as the ‘building envelope’.

Principles and measurements like airtightness, U-values, thermal bridging, triple glazing and solar gain are a big part of the fabric first approach and contribute to minimising the energy usage required in a home.

What is fabric first?

The fabric first approach involves carefully considering the design and construction of the building envelope in order to lower the home's energy consumption.

Put simply - it is creating an airtight, insulated building that reduces the amount of energy required to heat the home. It prioritises the performance of the building fabric.

A Facit Home designed using the fabric first approach

Even though it might be one of the simplest, most effective and affordable ways to create an environmentally optimised 'eco home', the fabric first approach is a term that is relatively unknown outside of the industry. Passivhaus is perhaps better known - but even that is firmly based on the principles of fabric first.

Digitising the design and construction process means that we can rigorously ensure each Facit Home follows these principles. Prioritising energy preservation over generation, we use digital manufacturing technology to create homes that exceed building regulations requirements and meet some important Passivhaus standards.

What is airtightness?

Airtightness is exactly what it sounds like. An airtight building envelope doesn’t allow air to escape, or enter, where it is not intended to. Leaking precious warm air out into the world and feeling cold drafts from ill-fitting doors and windows are effectively eradicated.

Measured by how much air escapes from gaps in the building fabric, building regulations state that the air leakage can be no more than 10 cubic metres of air escaping per hour for every square metre of the building envelope, expressed as 10m3/h/m2@50pa or 10m3/hr.

Our homes regularly come in at around 1.5m3/hr, far exceeding building regulations and just over the Passivhaus score of ‘less than 1m3/hr’. In the industry as a whole, 5m3/hr is becoming fairly standard, especially as more traditional house builders adopted more modern approaches.

Some of the more complex Facit Chassis™ components

Our digital approach allows us to precisely design and manufacture the timber superstructure of each Facit Home, which we call the Facit Chassis™. Assembled from digitally manufactured plywood ‘components’, each custom designed chassis creates an essentially airtight building envelope.

To bring the airtightness score down even further, improving the building’s performance, the chassis is then wrapped with an airtight vapour membrane that keeps moisture out and warm air in.

How do you ventilate an airtight home?

Airtight homes still need fresh air, so they normally have a Mechanical Ventilation Heat Recovery (MVHR) system. This simple and efficient system brings in fresh air whilst recovering and reusing the heat from the stale air it extracts from spaces like kitchens and bathrooms. Our systems retain up to 94% of this heat, transferring it to the fresh air coming in via a heat exchanger in the main unit.

Mechanical Ventilation Heat Recovery is a mouthful, and one of the most commonly misunderstood terms in sustainable construction. It really should be rebranded as a ‘fresh air’ system!

A diagram of the MVHR system in a Facit Home

Stack ventilation is another method that we often employ to bring fresh cooler air into the home during the night in warmer months. It involves including a central rooflight above the stair that opens to allow warm air to pass up and out of the building, drawing in cooler air from ground floor windows set to a locked ventilation opening.

How does insulation contribute?

Insulating the building envelope will keep the cold out and the warmth in, just like putting on a big winter coat. Different building methods will provide different levels of insulation, but our timber structural components are all super-insulated. Each component is filled with a pressure injected EPS insulation that is chosen for its low embodied energy (the sum of all the energy required to produce it).

The Facit Chassis™ has 316mm thick components that contain 280mm of insulation - meaning that 89% of the wall is actually insulation, keeping things nice and toasty inside our homes.

All external walls and the roof are made up of the same super-insulated components and our insulated raft foundation system then acts like a pair of slippers for the home. Thermally efficient, technically advanced and quick to install, it uses a minimal amount of concrete and provides the maximum amount of support.

Our levels of insulation create excellent U-values across the building envelope which meet the stringent Passivhaus standards.

What is a U-value?

Also known as thermal transmittance, the U-value is the rate of heat transfer through a structure. It measures the heat loss of a home and, similar to the airtightness score, the better insulated a structure is, the lower the U-value.

A Facit Home has U-values of 0.12W/mk2 for external walls and roof, compared with a building regulations requirement of 0.26W/mk2, and 0.1W/m2K for the foundations.

VELFAC windows in one of our past projects

Triple glazed windows

As a necessary gap in a building envelope, window openings should be designed and detailed precisely so they can be as airtight as possible. High performance windows are then essential to reduce heat loss.

Digitally designing our homes means that we can send exact specifications to our suppliers and ensure that the wall components are manufactured so that the windows fit perfectly.

VELFAC triple-glazed composite windows are our product of choice for their durability, low maintenance, thermal performance and aesthetic appeal. Triple glazed windows retain more heat and are an important part of the fabric first approach, with clear benefits over standard double glazed units.

What is solar gain?

Designing south facing windows into a home where possible means that you can harness more of the sun’s warmth to heat the home during the winter. Solar gain is the result of heat from the low angle sun being used to gently warm a building.

It maximises the use of a natural, endlessly sustainable resource (the sun) and the homes then retain that heat (through airtightness and insulation). In order to avoid overheating in summer, we typically use some sort of shading above the windows to deflect the powerful high angle rays.

Solar shading on a Facit Home

What is thermal bridging?

Thermal bridging is the transfer of heat from inside to outside via a conductive material. This usually occurs at weak spots such as junctions between walls, floors and roofs, and around windows and doors. Timber is not as conductive as other materials, which is why it is our material of choice for the frame.

The depth of our chassis (316mm) means that each home loses next to no heat via thermal bridging. We carefully detail around any more conductive structural steel elements to keep them within the timber envelope of the building and avoid making a thermal bridge between this material and the outside.

Have any more questions?

Get in touch by emailing or calling us on 020 3034 0720.


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