Off-Site Kit Houses: A Greener Build?

ABOVE: This stunning eco house is by German manufacturer Baufritz. Learn more about this house

Off-site manufacture (or OSM) is where the whole house – walls, floors, roof, windows, doors, electrics, plumbing, the lot – is manufactured in a factory and then effectively assembled on site. The method is exemplified by manufacturers like Baufritz, Hanse Haus, Huf Haus and, the newest entrant to the UK market, Kampa Haus — one of the biggest OSM companies in Europe, having built over 100,000 houses.

The raison d’etre of OSMs is to build more energy-efficient houses. As Baufritz puts it, “Efficiency: it’s a German thing.” And most of the OSMs are German, but we also have to add UK manufacturers into the mix: most notably Potton – famous for its Lighthouse but now launching the Zenit – and Rural - ZED, a company set up specifically to provide sustainable housing.

Of course, much self-build housing involves some off-site manufacture — most notably structural insulated panels (SIPs), where the panels are delivered to site with voids for windows pre-cut. OSM, however, delivers a finished house, up to and potent ially including carpets and curtains. Diana Burns of Kampa Haus says, “We are stress-relievers. The client gives us a bare site and we give back a finished house. On time and on budget, every time. No stress, no pain.”

U-values

According to the Energy Saving Trust, a traditional brick and block house built to Building Regulations stand ards will consume up to 85kWh of energy per year, per square metre of floor area for space heating and hot water. The off-site manufactured house can get that figure down below 30kWh per year.

U-values are a measure of the rate at which heat escapes through a structure. So the lower the figure, the better. The Building Regulations call for a maximum U-value of 0.35 for walls, which would equate to brick and block construction with 90mm insulation in the cavity. Kampa gets that figure down to just 0.11 with single-skin timber frame construct ion with 300mm of insulation. Baufritz makes similar claims with 0.13 while Hanse offers around 0.15, and Build It Green 0.17.

Triple-glazed windows, heat pumps, heatrecovery systems, solar panels and pellet boilers are either included as standard or available options. In all cases the levels of airtightness are excellent, all achieving around 5m3/hr or better — twice as good as the current Building Regulations standards call for.

Taken together, these measures add up to huge energy savings. Hanse Haus suggests a typical annual energy consumption level of 43kWh/m², Baufritz 40kWh/m² and Kampa 24kWh/m², which takes into account a heat pump and heat-recovery ventilation system (compared to around 250kWh/m² in a typical house). Potton Zenit and RuralZED do not publish these figures but their insulation levels are very similar and, therefore, energy consumption should be about the same.

This translates in cash terms to the Building Regulations standard house costing around £50 per month to heat and the OSM house less than £20 per month.

ABOVE, clockwise from top: A traditional style self-build by Hanse Haus; Potton’s new Zenit concept; The Haus Basel design built by German company Kampa Haus

Build Cost

Comparing like-for-like is difficult as OSMs quote prices to include the finishings, fixtures and fittings. The client chooses what to install and that directly impacts on cost. In addition, fluctuating exchange rates are affecting prices from German companies. As an example, Kampa was quoting £1,200/m² but is saying that this figure might need to rise to as much as £1,500/m² to account for the exchange rate. Potton suggests about £1,250/m² for its Zenit house and Baufritz quotes prices in Euros — €2,100 to €2,500 (£1,900 to £2,260 at current rates) for its bespoke turnkey service. RuralZED quotes £1,150/m² for a Code Level 4 home and £1,500/m2 for Level 6, and Hanse Haus £700 to £900/m² for the shell only. There is also variation in what is included in the package. Kampa includes a heat pump and heat-recovery system as standard, but all the companies have their own list of standard features and optional extras.

By way of comparison, the H&R Average Build Cost Guide shows average prices across the country varying from £900 to £1,600 for a medium-quality finish using a main contractor. While the figures do assume the delivery of a finished house, they do not, however, take into account budget over-runs, which are still a feature of the UK housebuilding industry. It seems that a traditional build is a little – but not significantly – cheaper than an OSM house. Maybe the price comes in bite-size chunks rather than all at once and so just seems smaller. The exchange rate is also having an impact.

Amanda Politzer of Baufritz says, “We offer a low-risk option and that includes the price. Right now we are encouraging clients to take the shell-only option and use lower-cost UK labour to complete the project.” This option, available from most of the OSMs, gives the benefits of energy efficiency and lower cost.

How Off-Site Works

1. The walls are built in exact accordance with the computerised measurements.
2. Windows and other openings are created in the factory.
3. Each element is checked over in the factory before being packaged up for delivery to site.
4. The house is delivered to site by lorry.
5. The front ground floor wall is craned on site, on top of the foundations.
6. Each remaining wall is craned in.
7. The team slot in smaller, craned-in sections. The house fits together like a huge piece of flat-pack furniture, and is simply built upon in parts.
8. Once the first floor has been added, work on the roof can begin.
9. The finished house by Baufritz.

Why Build an OSM House?

From an energy-efficiency point of view there is no contest: OSMs beat brick and block every time. If what you want is the hands-on involvement of building your own home then OSM is not for you. If you want an energyefficient house, delivered on time and on budget, then OSM may be the way to go. The reality is that by 2016, when zero-carbon houses are mandatory, we will all be doing something similar.

Kit house Case Studies:

• A Timber Frame Self-build
• A Kit House from Germany
• A Super-E Airtight Kit House
• An Imported American Kit house

Further reading:

• How to Choose Your Timber Frame
• SIPs: Faster, Greener, Warmer?
• A Guide to Timber Frame