Planning for the ventilation of a building is crucial to ensure the comfort, health, and safety of a building's occupants as well as the longevity of the building itself. It is also a principle componant of the "fabric first" approach of passive house design where the energy (warm air) within the house is controlled and conserved to ensure it's energy efficiency.


Large sliding doors together with an airtight construction can give light, comforable conditions throughout the year.


What are the types of ventilation within a building?


Ventilation is the process of extracting stale air, from active rooms such as bathrooms, utility and living spaces to the outside. All buildings need both 'background ventilation' (a continuous trickle of clean air) and 'purge ventilation' which can respond to changing conditions (eg cooking or showers) and extract a large volume of air quickly. As well as this, clients may wish to choose between 'natural' and 'mechanical' ventilation options. Natural is the most common form of ventilation for a typical house as the materials and the construction strategy allow for a building to be ventilated by natural air movement. This can involve trickle vents positioned in doors and windows in order for fresh air to enter the interior as well as openable windows which can purge the space.

Traditionally, opening and closing a combination of windows and doors allows fresh air into a building. This in collaboration with the building fabric allows the building to breath, although in most cases not adequately.



This may sound like the sustainable approach, but in fact natural ventilation can be somewhat difficult to control, either by occupants forgetting to open and close windows, or especially in older houses, where the venilation relies upon gaps within the building fabric which are difficult to seal. This can lead to uncomfortable drafts and energy loss on cold windy days, but also the builds up of moisture which can cause condensation and mould growth. This is often combated with the use of a dehumidifier. Better still, mechanically ventilating the home can solve these issues whilst providing a host of other benefits.



What is mechanical ventilation and why is it being used?


The other form of ventilation is mechanical which is more commonly used in modern buildings that are well sealed & airtight. Mechanical ventilation is the process by which fresh air is brought into a building interior by way of a machine that can be controlled and adjusted as needed. This machine essentially consists of a fan that draws fresh air inward whilst another fan exhausts stale air outward. This is known as an air exchange.


The volume of fresh air required for an efficient level of ventilation of a space is determined by factoring in the size, the number of inhabitants and the use of a space. The quantity of air supplied to a space, expressed in terms of the number of times the total volume of air in the space is replaced in a specific period of time, is typically one hour. Issues of condensation are also resolved with the implementation of a Mechanical Ventilation with Heat Recovery (MVHR) unit as the outgoing stale air, in which water vapour sits, is extracted from the interior spaces whilst also passively heating the incoming fresh air. As the stale air is cooled upon extraction, a small reservoir captures any condensation and drains it away; the same is also true for cool incoming air being passively heated. Smart controls could also be implemented along with sensors to determine when a space is in need of more air changes either due to a temperature rise, an increase in occupants or other factors.


As we develop ever more efficient and airtight buildings, the requirement of mechanical ventilation becomes a sensible consideration in order to control air quality, ventilation rate and heat removal from our interior inhabited spaces. The mechanical ventilation system of choice today is one with heat recovery which further improves the comfort to the inhabitants as well as boosting the efficiency credentials of a building. This ventilation strategy is being used to counteract heat loss in highly insulated homes via the ventilation system and is far more efficient than earlier variations of mechanical ventilation as heat is recovered from stale outgoing air without ever mixing with the fresh supply. Over time, this technology has further developed to include filters on the incoming air supply to remove pollen and other elements from the incoming air which has been proven to create a safer habitat for those with respiratory issues.


The sketch below details how a Mechanical Ventilation with Heat Recovery system (MVHR) operates.

The process of a Mechanical Ventilation Heat Recovery System

  1. Cool, fresh air from outside is drawn inside via a fan and filtered to remove dust and other fine particles

  2. Incoming cool air is heated passively in the heat exchanger and distributed throughout the building

  3. Warm, stale air is drawn from interior spaces

  4. Outgoing stale air is cooled passively in the heat exchanger and exhausted from the building. Cooling the outgoing stale air removes the moisture which is drained away

  5. Filtering the incoming air helps people with respiratory issues such as asthma.


Project Focus

In a previous article on airtightness, we discussed how Wellington Lane in Bristol was constructed to airtight standards. As a result, there are no unintentional drafts or air leaks and so a mechanical form of ventilation was integrated into the build to provide a continuous and constant supply of fresh air to the building interior. Simultaneously, the stale interior air is exhausted outward and the heat from this air is exchanged to the incoming air supply. This benefits the inhabitants as they are receiving fresh air that is at a temperature that feels comfortable and so creates a comfortable interior environment throughout the day. Without this heat exchange, the incoming fresh air would feel cool and affect the comfort of the interior environment.


The continuous exchange of air is also beneficial to the building structure as moisture is wicked away from the interior spaces and eliminates the chance of rot and mould growth. This is especially true in high moisture environments such as the bathroom and kitchen.

The cross section of Wellington Lane below highlights the location of the MVHR unit and the supply of fresh air to the interior spaces. The exchanger is highlighted in purple which is where the heat recovery occurs from the stale outgoing air (not shown) and the incoming supply of fresh air (highlighted in blue). We can also see how the ducting is hidden in the floor/ceiling assembly and out of sight in the finished construction.


A cross section of Wellington Lane, Bristol showing how an MVHR was integrated into the design and final build



In this post we’ll explore SIPs, a modern high performance building system which is becoming an increasingly common method for construction across the globe due to its simplicity in construction and its many inherent benefits that create higher quality buildings.



What are SIPs?

Wood is the most widely used building material across the globe. It’s a sustainable material, flexible in design and easy to work with. Structurally Insulated Panels are an advanced form of timber construction. SIPs are a type of high performance building system used in residential, light-weight commercial, and even small industrial buildings. A SIP is made up of two structural OSB faces which both sandwich an insulating foam core as pictured below. SIPs are incredibly versatile and can be used to construct walls, floors, and even roofs which speeds up the construction process whilst delivering a higher quality of build in comparison to other traditionally used materials such as masonry.


A closer look at the SIPs construction system shows how the insulation is sandwiched between two large OSB3 boards to create a complete insulating and structural component.







Should I consider using SIPs in my project?

Due to the ever increasing carbon footprint of the construction industry, the UK’s building regulations are becoming increasingly stringent. Part L of the building regulations focuses on the conservation of fuel and power as reducing carbon emissions for new build properties remains high on The Government's agenda, and plays a big part in the design and construction of new build properties. SIPs are in a league of their own when it comes to meeting these stringent guidelines. The sheer thickness and continuous line of insulation sandwiched within the structure of a SIPs building leads to a very energy efficient and comfortable internal environment. The carbon footprint of a SIP building is reduced immensely as energy demand is significantly reduced leading to a more economical and environmentally friendly building in the long run.

This detail shows how the various materials would come togther along with the continuous line of insulation in the interior to increase overall u-values

The benefits of using SIPs

As SIPs are made in a factory environment and to specified shapes and sizes, there is minimal waste in comparison to traditional construction materials. The speed in which a SIPs building can be erected equates to labour savings from shorter construction times and the prefabricated nature of this system can effectively make this building method financially more viable than other traditional construction materials. This is particularly attractive for self-builders, where the ability to erect the structure quickly and make it watertight means that an element of self-finish can be completed by the client, or other contractors.

Further savings occur as the need for heating and cooling systems are drastically reduced due to the thermal characteristics and airtightness of this technology. The inherent air tightness of SIPs necessitates in the need to integrate an air exchange system such as a Mechanical Ventilation Heat Recovery system (MVHR) to exchange stale air for clean fresh air as touched on in our previous posts on Airtightness This allows for a controlled internal environment which is especially beneficial, in combination with air filters, for those with allergies. This topic will be explored in detail in a future post on Mechanical Ventilation.


What about the external/internal finish of a SIP building?

As the name suggests, SIPs are generally used for their inherent structural properties but can also be used with other structures to form an insulation and airtight layer to a building. A SIPs structure can be clad in a variety of different materials depending on the desired aesthetic. Brick work is often used as the exterior face to fit the local context of many UK cities. Timber cladding, render boards, metal cladding amongst many other options are available as the finished structure is easy to apply finishes to. The interior can be finished with plasterboard or plywood, again depending on the desired aesthetic.


We explored using SIPs on our project at Glebe barn, due to the speed of construction and the possibility of incorporating an element of self-build. For the exterior finish, ideas of vertical timber boards meeting the stone plinth looked like a great juxtaposition of materials and expressed the modernity of the new build whilst honouring the material palette of the existing barn. The conceptual 3D technical details above and below show how the construction would come together on site as well as some thoughts on the interior and exterior finishes mentioned earlier.

This conceptual detail shows the exterior finish we are trying to achieve and how the timber cladding would meet the stone wall.



If you are interested learning more about SIPs, or have a project in mind where you are considering using this method of construction, then please do get in touch.



Updated: Jul 6, 2020


Earlier this month we gained planning consent for a new single storey extension to a previously converted threshing barn in North Devon.


The design incorporates strategies for sustainability, aiming to reduce energy consumption during construction and throughout the lifetime of the building. We have decided to use materials with a low embodied energy: the structure will be timber framed, clad with lime rendered wood fibre boards and locally sourced timber cladding. We are also looking to specify insulation levels which are better than the minimum required for building regulations, so as to improve the performance of the building for years to come.


We are looking forward to developing the detailed design as we work towards getting the project on site.


Learn more about my other residential and eco projects.

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