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The Importance of Ventilation Design When Planning a Self-Build Project


By EnviroVent Jun 05, 2019

When you’re planning your dream home, ventilation may not be at the top of your list of priorities. By definition, when ventilation is working well, it’s completely invisible. However, the best-designed home in the world can be rendered unliveable by poor ventilation, and by far the most effective way to guarantee a well-ventilated property is to incorporate your ventilation system at the design stage.

What is ventilation?

In its simplest form, ventilation is the process of removing air from your home and replacing it with air from outside. Without a ventilation system in place, stale air builds up in the interior of your house, and it can have some unpleasant consequences. The most common result of poor ventilation is a build-up of condensation from everyday activities such as cooking, washing and laundry. Excess condensation can quickly cause damp and mould to develop, with potentially serious consequences for your health and the integrity of your house. In the most serious cases, condensation damp can destroy the home you’ve worked to build.

Proper ventilation also has a number of benefits. A good ventilation system will remove cooking smells as well as airborne particles of fat and oil quickly and efficiently, preventing a build-up of cooking odours and contaminants on your surfaces and furnishing. Fuel-burning appliances such as wood-burning stoves require a steady supply of oxygenated air to be efficient, and without it they may reduce the oxygen content of the air in your house, making you sluggish and drowsy. Proper ventilation can also prevent the build-up in your house of chemicals which may cause allergies, such as cigarette smoke, mould spores, pet dander and pollen.

Approaches to ventilation

Self-builders have a range of ventilation options to choose from, and there are a number of factors to take into consideration. Historically, builders have tended to rely on uncontrolled ventilation: built-in draughts, air bricks, trickle vents and other systems that simply permit air to flow into a house from outside. The benefits of uncontrolled ventilation are cost and ease. Most builders can easily install an uncontrolled ventilation system, and they won’t charge the earth for it. In many cases, uncontrolled ventilation will provide an acceptable minimum of airflow, so long as the system is installed in compliance with Building Regulations. However, there are a number of stark downsides.

Uncontrolled ventilation is just that: uncontrolled. A still day can cause a build-up of stale air, and every windy day is going to hit your energy bills. Blockages can occur very easily, and may be difficult to spot, especially in the case of air bricks. Uncontrolled ventilation is completely ineffective when internal doors are closed, which can be a problem if you like privacy! Worst of all, uncontrolled ventilation does not systemically exchange internal air for external air. Instead, the movement of air is largely randomised, and condensation, allergens and pollutants may remain in the home rather than being removed.

Controlled ventilation

Most self-builders now opt for some form of controlled ventilation. The main reason for this is that Building Regulations now require an airtightness of  5m3/m2/hr@50pa for energy-efficiency purposes. At that level of airtightness, stale air and condensation build-up is nearly unavoidable without controlled ventilation.

The most popular and effective form of controlled ventilation for self-builders is Mechanical Ventilation with Heat Recovery (MVHR). Although MVHR is not suitable for all homes, where it is appropriate, it provides not only efficient ventilation but long-term cost savings.

An MVHR system consists of a heat exchange unit and two sets of ducts, each with a fan to control air pressure. One set of ducts extracts air from inside the house and directs it to the heat exchange, where its heat can be recovered, so that the house’s internal heat is not lost when stale air is expelled. The second duct system draws external air into the house, filters out any pollutants, and directs it to the heat exchange, so that it enters the rooms of your house as warm air, regardless of the temperature outside. Heat recovery means that you do not have to reheat the air in your house with every air change, which translates to huge savings on heating compared to even the most effective ventilation system without heat recovery.

One note: MVHR is not suitable for every self-build. To work effectively, heat recovery requires an airtightness of 5ach (air changes per hour) or less, and to be cost effective, something close to 3m³/hr/m² is advisable. 

If, like many self-builders, you feel that an MVHR system would be optimal for your project, you should aim to design the system into your build. It can be difficult and often expensive to install an MVHR system after the fact.

The Building Research Establishment recently found that 9 out of 10 heat recovery units in UK new builds may have been incorrectly installed. Compared to uncontrolled ventilation systems, MVHR are relatively technical, and it’s advisable to seek expert advice.

A PIV, or Positive Input Ventilation unit, works similarly to a MVHR, without the heat recovery system. A PIV is still a much more effective form of ventilation than an uncontrolled system. Although it does not have the cost-saving benefits of an MVHR, a PIV can still be cost-effective. EnviroVent’s PIV units are powered by ultra-low-watt motors, so they use minimal power, and every unit is designed to last for the whole life-cycle of a property. Components can be individually repaired rather than replaced. Every EnviroVent PIV is backed by a five-year guarantee.

Some self-builders are reluctant to choose controlled ventilation due to noise. However, if properly installed, MVHR and PIV systems are very quiet. Horror stories generally arise from badly installed systems, or when self-builders have tried to cut costs by installing a unit which is too small for the house.

What ventilation system is right for your self-build?

Scotland’s Building Control agency offers a useful set of guidelines for ventilation in domestic new-builds. In the most airtight buildings (0-3 m3/(h m2) @50Pa or less), Building Control suggests that the only effective form of ventilation is MVHR. They recommend that uncontrolled ventilation be relied upon only in buildings with an airtightness of 5-7 m3/(h m2) @50Pa.

It’s worth sounding a note of caution here, however. It’s commonly the case with self-build projects that the measured airtightness of the finished home is substantially different from the target set out in planning. It’s impossible to calibrate the finished airtightness ahead of time: there are too many factors which arise during building, including the skill of the tradesmen involved. The problem is that, as mentioned earlier, the most effective ventilation systems (i.e. MVHR) are very difficult and expensive to install in a finished building. If the finished airtightness of your self-build turns out to be much higher than planned, you may find yourself forced to retro-fit MVHR, which on a tight budget can be a disaster. For that reason, wherever possible, it’s probably best to implement an MVHR system from the planning stage onwards, rather than leaving it to chance. 

Implementing MVHR at the planning stage

Most self-builders ultimately opt for MVHR, but there are still a number of factors to be considered. One question is to find out how much filtration is required. The UK Pollution Map offers a snapshot of air pollution in your area, which will help you to identify how much money you need to spend on filtration. It’s not simply a question of health. Efficient filtration makes your ventilation system and your heat recovery system more effective.

The main reason MVHR systems are hard to retrofit is that in addition to the heat exchange unit, a complex ducting system is required, which reaches every part of your home. A powerful and efficient unit is useless unless ducts are properly installed, airtight, and the correct diameter for your unit. Badly-installed ducting is also the most common cause of noise in the ventilation system.

It’s important to check that the MVHR system you’ve chosen will comply with Building Regulations. Mechanical Ventilation with Heat Recovery is covered under System 4 of the Building Regulations, Approved Document F. Consult this document to check that the airflow rate in your property will meet requirements.

Due to the technicality of installing MVHR systems correctly, it’s a good idea to take professional advice. EnviroVent has more than 30 years’ experience working on self-build projects and we will design your ventilation free of charge.