Passivhaus – What’s the problem?

Submitted by CathHassell on Thu, 10/01/2015 - 01:00

Leigh J Caller | Kendall Kingscott

Passivhaus – What’s the problem?

Post occupancy studies have shown that Passivhaus delivers on its core aims of delivering extremely high fabric energy efficiency with superior levels of thermal comfort time and again with few exceptions. So, why are Registered Social Landlord’s (RSL’s) not universally adopting Passivhaus for some or all new affordable housing schemes?

Some organisations, including Hastoe and Exeter City council, are rolling out Passivhaus schemes because of the advantages it brings to tenants, including tackling fuel poverty. But they are few and far between. So why is this? Obviously the elephant in the room is Cost (with a capital C). But there are other barriers to the Passivhaus utopia as well.

It’s complicated!

Let’s not kid ourselves, Passivhaus is a complicated business, but then nothing worth achieving is ever easy. Not only does it require a team of experienced consultants from the very early stages of a project, but it also requires commitment from the contractor. The attention to detail required is a step change from what we are used to dealing with in the UK house building industry.

Every detail must be carefully considered, not only in terms of thermal efficiency and air leakage, but also in terms of buildability and future maintenance. The external envelope of a new dwelling procured with HCA grant funding must have a design life of at least 60 years. However, major components such as the windows and external doors will almost certainly need to be replaced within 30-35 years. Therefore air tightness membranes and thermal continuity must be carefully considered in the context of future work and any replacement thermal elements assessed in terms of how they might impact the energy balance.

Maintenance and Replacement Costs

A Passivhaus requires a whole-house mechanical ventilation system with heat recovery in order to meet the maximum peak space heating demand. For a modest two bedroom four person home, MVHR is likely to cost somewhere in the region of £3,800 (including ductwork, attenuators and insulation) plus installation costs. On-going filter changes and future replacement costs for key components need to be taken into account. Replacement costs for the fan(s) and heat exchanger are £150 and £325 according to data provided by the Green Building Store, with filters costing around £25 every six months.

Having residents change the filters themselves will drastically reduce the cost as pointed out in a whole-life cost study by Encraft, but would this be acceptable to an RSL on the basis that they are ultimately responsible for ensuring the system is working correctly and is maintaining good IAQ? Should landlords cover the cost of filter replacement or expect the tenant to pay for what is essentially a consumable item, much like a lightbulb?

Capital cost

Given that a Passivhaus delivers drastic reductions in annual energy consumption for space heating and superior levels of thermal comfort, it is perhaps not surprising that it costs more. For RSLs it is both capital cost and on-going maintenance costs that are the main drivers, not the decrease in long term running costs, which will ultimately benefit the tenant.

Studies have shown that, by and large, a Passivhaus can cost anywhere between 0-30% more to build than a ‘conventional’ home built to Part L. The problem therefore is cost uncertainty and the risks attached to building a Passivhaus under design and build procurement, which dominates the affordable housing market. Until the recent Government announcement, it was argued the cost gap between Passivhaus and Part L compliant homes would be reduced in the run up to 2016 and the Zero Carbon Homes policy. It was felt the additional costs associated with achieving FEES and Carbon Compliance would mean taking the next step to Passivhaus would be less painful. However, the Government U-turn on ZCH means this cost gap is likely to remain.

A recent study by AECOM put the average post tender build cost at around £1,823/m2 (Q4 2014) Meanwhile a smaller study by Encraft found that a timber frame superstructure (Beattie Passive) with gas boiler was the most economical way to achieve the Passivhaus standard, with just a 4% uplift from the affordable norm of £1,000/m2. What is clear from trying to make sense of the cost data out there is that it’s difficult to make a like for like comparison because there is no standard format for the reporting cost information, i.e. what’s included/excluded.

How can we move forward?

Passivhaus is still relatively new in the UK and the number of units completed to date is low. With 26,498 new affordable housing starts between 1 April 2014 and 31 March 2015, less than 10 new homes achieved Passivhaus certification in the same period (1).

To overcome some of the barriers outlined above we firstly need to start incentivising RSL’s to develop Passivhaus schemes so knowledge and skills become more widespread. This could be in the form of increased grant funding for RSL’s that wish to try Passivhaus for the first time. Secondly, RSL’s need to learn from each other and share information from past projects in order to minimise the risk and cost premium. There is a steep learning curve for everyone involved in the design and delivery of a Passivhaus project the first time around, so sharing information and experience is key. Finally, greater emphasis must be placed on tenant support and aftercare for those lucky enough to be given the keys to a Passivhaus to ensure the dwelling is being used and systems maintained appropriately, particularly the MVHR system.

(1) From a search of the Passive House Database and UK Passivhaus Trust UK Buildings Database