Tech Feature - Airtightness & MVHR
Research shows that we can lose as much as half of all the energy used to heat our buildings through unwanted draughts, inefficient and poor ventilation. We now generally insulate new houses very well but the proportion of energy lost to draughts has increased and in some cases around half of all heat losses are due to air leakage across the building fabric. In the UK we are not exposed to extremes in low temperatures, however our climate is exposed to high wind pressure, particularly in coastal regions.
The only way to achieve low carbon and zero carbon building is to ensure a high level of airtightness is attained. This means designing and installing a continuous seal around the internal fabric of the external envelope to eliminate unwanted draughts. Airtightness is the control of air leakage, i.e. the elimination of unwanted draughts through the external fabric of the building envelope. The WrightWall and WrightRoof encapsulated panel system is designed with airtightness in mind. The system includes meticulously sealing every joint with a high-tech butyl tape. Oakwrights’ panel system is designed to work with the oak frame, to allow good access to those otherwise hard to seal junctions.
While ventilation is intended, air leakage is not. It is desirable and necessary to have controlled ventilation, such as MVHR systems for healthy, comfortable buildings.
What is MVHR?
Mechanical Ventilation with Heat Recovery (MVHR) is an essential component of ultra low energy buildings including Passivhaus and is the solution to the ventilation needs of energy efficient buildings. Comfort Ventilation, mechanical ventilation heat recovery (MVHR), or heat recovery ventilation (HRV) are all names for the same thing. A properly fitted MVHR system provides a constant supply of fresh filtered air maintaining the air quality whilst being practically imperceptible.
An MVHR system ensures high air quality in very airtight buildings, whilst also conserving energy by recovering heat from extracted air and transferring it to the incoming air. In ultra low energy buildings like Passivhaus, the level of efficiency of this heat recovery is very important. Amongst other MVHR units, Green Building Store supplies the Novus 300 MVHR (mechanical ventilation with heat recovery) unit from PAUL in Germany which has the highest Passivhaus-certified MVHR heat recovery rates currently available in the world, with a heat recovery efficiency of 94.4/93%*. The unit's outstanding heat recovery efficiency offers designers greater flexibility when designing Passivhaus buildings in PassivHaus Planning Package (PHPP).
MVHR works quite simply by extracting the air from the ‘polluted’ sources e.g. kitchen, bathroom, toilets and utility rooms and supplying air to the ‘living’ rooms e.g. bedrooms, living rooms, studies etc. The extracted air is taken through a central heat exchanger and heat is recovered into the supply air. This works both ways, if the air temperature inside the building is colder than the outside air temperature then the cool environment is maintained in the building.
Novus 300 Unit
MVHR & Airtightness
Although MVHR could be installed in any building, there is a rule of thumb that its use is not justified unless the airtightness of the thermal envelope is at or below 3 air changes per hour when tested at 50 Pascal (equivalent approximately to 3 m3/m2.h @ 50 Pa for average dwellings). If this level of airtightness is not achieved then alternative strategies to ventilation are more appropriate. Through Oakwrights WrightWall and WrightRoof encapsulated panel system, an air tightness of less than 3 air changes per hour at 50 Pascals can be achieved, making WrightWall and WrightRoof a winner when it comes to MVHR.
MVHR system design
If you are intending to fit MVHR comfort ventilation into a new build or refurbishment then it is pertinent to consider the system early in the planning stage. For any MVHR comfort ventilation system to give its best performance the devil is in the detail. System design will make the difference between a highly efficient system working unnoticed in the background and one that inhabitants are constantly aware of, wasting both heat and electrical energy. These systems require ducts to be run through the building and the routing is important both from the point of view of the building and the oak frame, and the efficiency of the ventilation system. Indeed, layout of rooms can be influenced by ventilation needs.
Oakwrights & Green Building Store
Oakwrights has worked with Green Building Store on the Low Energy House that we exhibited at the Homebuilding & Renovating Show at the Birmingham NEC in March this year. Green building Store supplied us with the Eco-plus, oak framed triple glazed windows. One of our clients has chosen to use Eco-plus windows on their low-energy self-build project in Suffolk – watch this space for more details as the oak frame and our WrightWall system comes to fruition!
Green Building Store offers a specialist Passivhaus MVHR design service which helps ensure that the PAUL mechanical ventilation and heat recovery systems work at their optimum for Passivhaus and low energy buildings. Good MVHR design optimises the efficiency of the heat recovery, prevents noise, mechanical vibration and internal turbulence problems and maximises the energy efficiency of the system. Green Building Store’s MVHR design service offers technical support from design to commissioning.
*94.4% [145m3/h] or 93% [200 m3/h] Passivhaus Institut figures