A key part of any retrofit is reducing drafts and improving ventilation. It is important to distinguish between draughts; unwanted air leakage and heat loss through the gaps building fabric, for example around loft hatches, windows and doors. And ventilation which involves removing stale air from cooking, showering etc and providing fresh air for us to breath. As part of a retrofit we want to reduce draughts but it’s equally important to ensure we have a ventilation strategy in place too. This ensures we improve indoor air quality as part of the retrofit and helps deal with mould or condensation problems. The 4 main types of ventilation systems are:
- Natural ventilation. Though this kind of system does not meet building regulations for new dwellings, it’s not uncommon to find it in older homes. The home is ventilated through a combination of air leakage and opening windows to remove stale air and provide fresh air. The ventilation rate is difficult to control and relays heavily on the homeowner to open and close windows. It tends to result in under ventilation in the winter months when people are less inclined to open the windows and is not suitable for air tight homes with minimal draughts.
- Intermittent extract fans. This system uses extract fans to remove stale fir from the wet rooms, often set to run for 30 minutes when a light is turned. If fresh air is planned for in this kind of system it can be through trickle vents in the windows or through wall vents. These systems work OK in draughty buildings but can lead to over ventilation or under ventilation. As draughts are reduced, (by retrofit works for example) these kinds of systems become inadequate.
- Demand controlled ventilation (DCV or MEV). There are a number of variations to this kind of system. The general principle is that you extract stale air continuously at a very low rate which is boosted, by certain triggers, when required. This boost function is usually triggered when humidity reaches a certain level in the room. The extract fans can either be single room units, that look very similar to intermittent extract fans or a centralized unit for the whole house which has ducting pipes to grills all the wet rooms. These kinds of systems also use demand control for fresh air supply, with humidity sensitive inlet vents through the walls or windows. These kind of systems are great for a retrofit project where draughts are being reduced significantly or where there is already a problem with high levels of humidity, condensation or mould. The motors used in the extract fans are externally efficient and usually cost less than €5 a year to run.
- Mechanical ventilation with heat recovery, (MVHR). This is the gold standard for ventilation and originates in Passivhaus buildings. The system comprises a centralised heat recovery unit with ducting to extract air from the wet rooms. This air is then passed through a heat exchanger in the heat recovery unit and this is used to pre heat incoming filtered fresh air which is supplied to bedrooms and living rooms through a separate set of ducts. Heat recovery with a system like this can be up to 90%. These kind of systems are more complex and expensive to install, particular in a retrofit where duct routes need to be found around the existing building structure. They are most effective in buildings which have been made particularity air tight with very few draughts, to maximize the efficiency of the unit and balance the cost of the electricity to run the fans, (around €40 a year). MVHR is great for people with asthma or dust allergies as all fresh air is filtered. There is also the added bonus of reducing your heating demand by pre-heating the fresh air supply with the heat from the stale warm air that is otherwise wasted in all other ventilation systems.
We hope you have found this useful. In our next blog we will talk about reducing draughts and air tightness