Infiltration & Ventilation
You don’t have to look far to find some straightforward definitions of infiltration and ventilation.
Infiltration is the unwanted and unintentional flow of air through a building’s fabric. It’s caused by gaps and cracks, typically at junctions between different building elements. Some are big, for example a visible gap between a window frame and ill-fitting openable casement, and some are small, such as an almost imperceptible flow of air where an external wall meets a ground floor.
Ventilation is the deliberate movement of air to remove contaminants such as water vapour, airborne chemicals (VOCs), CO2, NOx, and odours from an indoor environment, and to keep everything feeling ‘fresh’. Kitchen and bathroom extractors and opening a window are simple forms of ventilation.
Infiltration causes heat loss, as warm internal air is replaced by colder external air. A building’s heating system then has to warm up that cooler air if a constant and comfortable temperature is to be maintained, increasing heating bills. Infiltration can also cause significant discomfort, as even if a room’s average temperature is warm, localised draughts are often noticeable.
in new buildings infiltration can be almost completely avoided through robust airtightness design and, perhaps more importantly, through high quality implementation of the design on site.
Indoor Air Quality
Unlike infiltration, ventilation is certainly desirable. It removes moisture generated by cooking, breathing, showering and drying clothes, consequently reducing the risk of mould growth, and gets rid of those unwanted smells from kitchens and bathrooms. Good ventilation also has a host of less well-known benefits, such as reducing internal CO2 concentrations (CO2 can cause drowsiness and a lack of concentration, so should be a particular concern in schools and workplaces) and removing volatile organic compounds (VOCs) and NOx, which can damage our health.
So, what’s the problem? Infiltration is bad, ventilation is good. Unfortunately, infiltration and ventilation have become intertwined. Even experienced construction professionals often confuse and combine the two without thinking about it.
If you live in a Victorian house, you might well have an extractor fan in the kitchen and perhaps in your bathrooms. These form your ventilation strategy, as they suck out unwanted moist air and odours. What happens next is where ventilation and infiltration merge. Removing air from your kitchen causes a pressure drop, so to restore equilibrium air from somewhere else has to flow into the room. This air comes from outside, sneaking in through gaps and cracks in the building fabric and making your heating system work that bit harder as it warms the new air up to room temperature. In this situation it’s great that we’ve got a well ventilated room, but the price we pay, usually without realising it, is a higher heating bill and more CO2 emissions.
In modern buildings, the design team and builders might have done a better job at reducing infiltration than their Victorian ancestors. As a result, conventional kitchen and bathroom extract fans won’t work quite so well. Even if they remove all unwanted air, there will be less fresh air arriving from outside to replace it. This is why you might see air bricks in your wall or trickle vents in your windows, and is where infiltration and ventilation are even more subtly combined. A trickle vent is essentially a deliberately made hole in a window frame to allow air to flow into a building. So, having built a nice airtight building to reduce draughts and heating bills, we now have to add a hole to allow an extract ventilation system to operate effectively. An air brick or trickle vent appears only to be a desirable part of the ventilation strategy, but is simultaneously a big source of infiltration, causing comfort issues and increasing heating requirements. Drilling some holes in an otherwise airtight window frame is no different from building a leaky house in the first place.
Trickle vents in window frames can usually be manually opened or closed. This controllability highlights the choice faced by occupants; do you want ventilation, or do you want to be warm? If you want both, you might have to turn the heating up. In reality, most occupants won’t remember to deliberately open or close trickle vents anyway. This means they can often be closed when they are needed or open when they aren’t.
Trickle vent technology has advanced with the introduction of demand control ventilation (DCV). This is where you use conventional extractor fans, but the trickle vents have humidity sensors and consequently only open when necessary. This is clearly an improvement on having no humidity sensors, as when they aren’t needed the vents will start to close and heat loss will be reduced. However, DCV vents never close completely, and a lot of the time function just like traditional trickle vents and our older leaky buildings.
There is a way of ventilating without bringing in cold air from outside. Mechanical ventilation with heat recovery (MVHR), is a system designed to work in airtight buildings with low levels of infiltration. A centralised unit extracts air from kitchens and bathrooms, but also mechanically supplies filtered, pre-warmed air to living areas and bedrooms. The extracted air will be at room temperature, or even higher if taken from a kitchen after cooking or bathroom after washing, and is passed through a heat exchanger in the MVHR unit before being expelled outside. Cold incoming air is also passed through the heat exchanger and is therefore warmed up by the extracted air before heading off to living areas and bedrooms. An MVHR heat exchanger efficiency is around 90%, meaning very little heat energy is wasted by the ventilation process. Supply and extract flows are balanced during design and commissioning, and the goal of good ventilation without relying on infiltration is achieved.
MVHR units also include filters which remove particles from the incoming outdoor air, another advantage compared to other systems, particularly in locations with high levels of pollution.
The cost of an MVHR system will be more than a more conventional extractor and trickle vent strategy, but in most cases will be a worthwhile investment. Ducts do have to run from the central unit to each room, which can be a challenge in existing buildings, but Enhabit’s designers are yet to be presented with a project where it’s completely impossible.
As with anything, if designed or installed badly then an MVHR won’t work well. Issues, such as those relating to noise, can almost always be traced back to either poor design or incompetent installation. It is therefore important to make sure that design and installation are carried out by specialist and experienced teams. Commissioning by a qualified professional is also essential, as this ensures that the system is working exactly as it should.
From a technical point of view, MVHR should be the preferred ventilation strategy for both new builds and renovations where good levels of airtightness (low infiltration) are being targeted. So start with MVHR, and only downgrade to extractor fans and trickle vents if budget or practical constraints are extreme.