Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 29 configuration of the building in question – not all older buildings can accommodate the ducting or fan units required for some ventilation systems. Other factors such as complexity and occupant type must also be considered, as they can be key determinants of success. TABLE 2 A simple overview of each of the principal ventilation options, and their main pros and cons INTERMITTENT EXTRACT (suitable band B–E) Pros • low cost • easy to install • easy to use Cons • fan noise • user can choose not to use PASSIVE STACK VENTILATION (suitable band B–C) Pros • low cost • easy to install (in top-floor wet rooms) • silent • continuous Cons • hard to accommodate vertical ducting (in ground-floor wet rooms) • summer ventilation may be insufficient CENTRALISED MECHANICAL EXTRACT (suitable band A–C) Pros • medium cost • potentially easy to install • easy to use • continuous • maintains background ventilation Cons • requires ducting, which may be hard to accommodate • uses electricity • potential fan noise DECENTRALISED MECHANICAL EXTRACT (suitable band A–C) Pros • low cost • easy to install • easy to use • continuous • maintains background ventilation • less ducting than centralised system Cons • uses electricity • room-side fan: increased potential for fan noise WHOLE-HOUSE MECHANICAL HEAT RECOVERY (suitable band A–B) Pros • air quality: intake air is filtered • comfort: air movement and exchange throughout home • efficiency: heat recovery reduces heat demand and tempers incoming air Cons • most expensive system • requires ducting to most rooms • uses electricity • potential fan noise • correct commissioning can be complex The suitable band information refers to Table 1 (Source: A Bristolian’s Guide to Solid Wall Insulation ). membranes) the greater the potential for application errors or future failure, in contrast to a simpler system (a wet lime plaster finish and/or backing coat for a wood-fibre insulation system, for example) which could be more robust in the longer term. • End-use and end-users – how is the building going to be used post- retrofit and who will be using it? How engaged are they? What, if any, behavioural changes may be needed? Are they tech-savvy or would a simple, passive ventilation system suit them better? All these questions need to be answered to inform the design of a suitable ventilation strategy. Considering all these aspects before embarking on building work is more likely to result in a successful retrofit. VENTILATION OPTIONS Once a thorough assessment of insulation, airtightness and ventilation has been made, an appropriate ventilation system needs to be identified. There are many different types of ventilation system, all with different pros and cons (see Table 2). Broadly these divide into passive and mechanical systems. Passive ventilation systems work on the basis that warm air rises and avoids the need for extractor fans. Ducts rise through the building to the roof, extracting moist air from bathrooms and kitchens. The extracted air is replaced by air drawn in from outside through adjacent rooms, so the whole house is naturally ventilated. These systems rely on changing air pressures and particularly on wind movement over roof vents, so the design of the system must be carefully considered and they are not always suitable or feasible. The simplest mechanical ventilation systems use the same system of air displacement but use fans to extract moist air from those rooms with the highest moisture level. These may be intermittent (a simple extractor fan in a bathroom for example, activated by a humidistat) or constantly operating at a low speed (a ‘decentralised mechanical extract ventilation system’ or DMEV). More complex systems have centralised fans and can be either continuous or demand controlled. In theory this makes the systems more reliable and the fans can be less noisy as they are located in loft spaces or cupboards. Mechanical ventilation and heat- recovery (MVHR) systems are the most complex mechanical systems, having a heat exchanger and two sets of ducts, one to extract moist air and the other to supply fresh air back to the adjacent rooms, preheated using the heat from the waste air. MVHR systems are generally best suited to comprehensive retrofits where a high level of airtightness is achieved. The type of system suitable for a building depends largely on its post-retrofit airtightness and on the