Neither the retrofit industry – nor its clients – understand the purpose of ventilation, and why a ventilation strategy is crucial for all energy upgrade projects, writes Peter Rickaby.
In the US there is political maxim: “No taxation with representation”. The equivalent for retrofit should be “No insulation with ventilation”. Traditionally in the UK and Ireland, we have relied on a benign climate and the leakiness of dwellings to supply fresh air through wind-driven infiltration, and to displace moist, stale air through leakage. Sometimes we have reinforced this process with fans in kitchens and bathrooms but often there is limited provision for a balancing supply of fresh air. So when we insulate a dwelling (in any way), replace the windows and improve airtightness, infiltration and air leakage levels are reduced, moisture and carbon dioxide concentrations rise and the consequences are poor indoor air quality (IAQ), condensation and mould. That’s why we must improve ventilation whenever we insulate.
However, evaluation of the UK’s Retrofit for the Future projects demonstrated poor understanding of ventilation. Neither the retrofit industry nor its clients understand the purpose of ventilation, why a ventilation strategy is important, how to design, specify, install and commission ventilation, or how to explain the importance of ventilation to occupants. There is inadequate knowledge of simple ventilation, let alone techniques such as demand controlled ventilation (DCV), even though such systems are common in Europe.
There are many good ventilation products, but some suppliers lose interest when their products leave the factory gates. When IAQ problems occur, contractors, installers, landlords and occupants often blame each other. It is usually during design, installation or handover that things go wrong, because there are few properly trained designers or installers of domestic ventilation systems.
Another maxim, also learned from Retrofit for the Future, is “Don’t default to MVHR”. Almost all design teams in that programme, and many retrofit teams since, specified mechanical ventilation with heat recovery (MVHR) because of their background in passive house. But passive house is a standard for energy efficient new dwellings in which MVHR has three roles: providing good IAQ; recovering the ventilation loss (to meet the challenging energy standard); and distributing the very small required heat input around the dwelling. MVHR works well in new passive houses, where the envelope is airtight, where the heat exchanger and the ductwork are within the insulated envelope, where ducts can be large, round, smooth and straight, with no branches and minimum bends, and where filters are readily accessible. In existing Victorian terraced houses, 1930s semi-detached houses and 1960s flats these requirements can rarely be met. It is not just that the envelope can be very challenging to make sufficiently airtight for MVHR to work properly. MVHR doesn’t easily fit, and the compromises to make it fit – cramming heat exchangers into cupboards, convoluted arrangements of flexible ductwork, routing ductwork through unheated lofts – all increase resistance, fan-power and noise, and reduce efficiency. Hence the rise of decentralised units and approaches to house building services in prefabricated external insulation panels. Fuel-poor occupants, who perceive ventilation as noisy, draughty and expensive, often turn the systems off, though this may be reflective of a more intractable problem – the evidence shows that even holes in walls and trickle vents are routinely blocked, with the isolator switches on intermittent extract fans turned off.
For retrofit projects the passive house and Enerphit standards are often too challenging. If we can reduce average emissions by approximately 60% we will be doing well, and the supply side (decarbonisation of the electricity grid, etc.) will contribute the other 20% needed to meet our national emissions targets. In that context, heat recovery is an unnecessary luxury, and dispensing with it makes life simpler – half as many ducts for a start! We should not be defaulting to MVHR, but considering all the options and finding the best system for each project.
Continuous mechanical extract ventilation (MEV) is often a good option: it can be centralised (cMEV) with ductwork and a single fan or decentralised (dMEV) with several fans – though the very poor findings on dMEV from the as yet unpublished Aecom study for the Department of the Communities reported by Kate de Selincourt in issue 18 of Passive House Plus are real cause for concern. Adding demand control via relative humidity (RH) sensors or RH sensitive air inlets combined with constant pressure fans reduces over-ventilation, ensures that ventilation is provided only where and when required, and improves energy efficiency to a level competitive with MVHR.
We are making progress. At Thamesmead, Peabody is working to eliminate condensation, damp and mould (CDM) and mitigate fuel poverty. Their excellent CDM strategy is essentially a ventilation programme, combined with improved heating controls and energy advice, but finding affordable ventilation systems that are quiet, effective and fit into 1970s flats is proving a challenge. The Retrofit Academy, whose acclaimed eight-day Retrofit Coordination and Risk Management training programme includes a whole day on airtightness and ventilation, is seeking to upskill the industry on a broader front. The BSI Retrofit Standards Task Group has adopted the “No insulation without ventilation” maxim and introduced requirements for ventilation upgrades into the consultation draft of PAS 2030: 2017. But there is still a long way to go.
Source: Passive House + Magazine, Issue 19 – article by Peter Rickaby