In the fourth and final webinar of the #TDChallenge23 series, three retrofit experts got into the nitty gritty of detailing for timber frame buildings.
Gervase Mangwana, Florence Collier and Beth Williams spoke about airtightness and moisture, M&E ventilation, and structural considerations.
After presenting at last week’s talk on the AECB standard, director of Waxwing Energy Gervase Mangwana returned to look more closely at airtightness.
Air tightness is important because of heat loss, which leaks out through all the holes in a property. This affects comfort, energy consumption and cost, but, Gervase explained, it also has hidden effects on the structural integrity of the building. Where the warm air leaves the building through the fabric, it becomes colder and condenses. This can lead to rot and other problems associated with moisture.
For this reason, it is important, during the design process, to specify and design in an airtightness layer. This allows everyone involved to be clear about what the strategy is and where it is, and means that when the builders come to look at the drawings, they can see how they are going to build it.
Although it doesn’t cost much more to detail and cost in the membranes, Gervase said that it does require attention to detail and care: to get the best performance out of the extra cost of materials. And, crucially, it needs all the trades to buy in to be implemented properly.
According to Gervase, air testing is traditionally in the UK done using blower doors. This is where air is blown in or sucked out of the house and the flow that results from this is measured, via one of two different methods, to give a metric of how leaky the building might be. In the UK, this testing is only typically required for new builds and is usually done after completion – at which point there is no longer anything that can be done about it.
In low energy building, however, diagnostic testing is done, sometimes beforehand, but almost always during the process – when the air tightness layer is still visible, and it is possible to rectify problems.
The two metrics for air leakage:
Permeability is air leakage (m3/hr.) divided by surface area (m2).
The AECB retrofit standard uses permeability to measure the air leakage of buildings.
Air Changes per Hour (ACH) is air leakage (m3/hr.) divided by volume (m3).
The Passivhaus standards use ACH.
For buildings in which the surface area and volume are fairly equal, the measurements created will be similar. However, Gervase explained, larger buildings (with greater volume) are favoured by ACH and it is harder for small houses to meet ACH targets. For this reason, he argued that permeability is a more useful representation of leakage, however heat loss is better measured with ACH.
Test, improve, retest, improve!
Having been to the site and air tested the #TDChallenge23 building – the Widemarsh pavilion – using both metrics, Gervase put its permeability at 8.3 m3/m2.a and its ACH at 9.4. The leakage points were found to be particularly around the doors, the single glazed windows and the sockets.
Advising participants on how they might solve leakage issues in the pavilion, Gervase proposed that any added extensions be designed to be super airtight – ideally to passivhaus newbuild standards. This will help to improve overall air tightness. He also recommended a second air test be done whilst the air tightness layer is still visible.
Finally, he emphasized the importance in ensuring everyone involved in the project is engaged with what the air tightness target means and is motivated by meeting it.
Picking up where Gervase left off, mechanical engineer and passivhaus designer Florence Collier focused on systems of ventilation.
As a building is closed up, adequate ventilation must then be provided in the right places, to keep materials intact, and to prevent mould – all the while minimizing the heat loss associated with bringing cold air in.
The best way to do this, Florence advised, is to use mechanical heat recovery ventilation (MVHR), rather than just extraction ventilation.
MVHR – what it is it and why use it
Florence explained that MVHR is a machine with two fans and a heat exchanger. Fresh air comes into the machine from outside. It is filtered, then supplied to the living and sleeping rooms in the house. It is then extracted from the wet rooms, getting rid of moisture at source. Before it is thrown out, heat is exchanged between the two streams.
This exchange means that the air coming in is close to room temperature, making it a more pleasant and energy efficient option, which also lowers the relative humidity of the space. In the summer MVHR can also recover cool as well as heat; a factor which is becoming increasingly important with rising summer temperatures.
The MVHR unit can be located within or outside of the thermal envelope of the building, but Florence suggested that it is best placed within. This decision then accordingly affects insulation of the exhausts and intake/supply and extracts.
She then described the two systems of distributing the air: the branch layout or radial layout. The radial (or octopus) layout often works well for retrofit, Florence explained, because of the small diameter, semi-rigid, plastic pipes that can be threaded between spaces, finding routes throughout the building.
The next low energy, systems-based decision Florence examined was heating. Reminding us that the only way to decarbonize heating is to switch to electricity-based heating, she gave an overview of how heat pumps work, and the different types there are.
Like a fridge cycle in reverse, a heat pump takes energy out of a source and, through a process of expansions and compressions, releases that heat into the heat sink (i.e., the building, or the water that circulates through the building).
An air source heat pump extracts heat from ambient air.
A ground source heat pump extracts heat from the ground.
The basic difference between them, according to Florence, is that a ground source heat pump is easier to get higher efficiencies from. This is because the performance of the heat pump depends on the relative temperatures of the heat source and the heat sink.
The last services decision that Florence covered was heat emitters, and the choice between radiators or underfloor heating. Here, she noted that if the AECB standard is being aimed for, it is likely that a high surface area will be needed – making underfloor heating a good option.
Beth Williams, structural engineer at Build Collective finished up by diving into some of the key structural considerations in designing low energy houses.
Thermal bridges are the points in the building fabric where heat loss is greater than the rest of the wall or planar element around them. They usually occur at junctions between two different planar elements (such as floors to walls, or walls to roofs) or at junctions with penetrations through the main fabric (such as at windows or doors).
Thermal bridges only occur if a building has insulation because they are the difference between the heat loss of the area versus the heat loss of that specific point of surface. Uninsulated buildings don’t have them because the surfaces aren’t any better than junctions.
Noting some of the things to look out for when identifying thermal bridges, Beth indicated steel frames, timber frame connections, metal lintels, and structures that run through, cross or break insulation.
How to identify and design out thermal bridges
For Beth, this involves marking out, early on, where the insulation will go in relation to where the structure will go. This is drawn up in plans and sections that show where the structure is and where the insulation is – any areas where these intersect are highlighted. These tricky junctions are the points which must be detailed and problem solved.
The process is similar for designing in airtightness. The air barrier should be drawn into plans, in relation to the structure. This barrier, Beth explained, should be either inside or outside the structure, and ideally this should be consistent all the way round. Junctions where it might be tricky to keep the air tightness membrane on the inside – such as around ridge beams, or beams supporting rafters; where floors come into walls; or at thresholds – can be identified in these plans.
Echoing Gervase’s earlier assertion, Beth underlined the importance in working through the levels of detail as a multidisciplinary team in order to develop a cohesive and complementary strategy.
As with all the other speakers in the series, the generous insights of Gervase, Florence and Beth provided huge value to participants considering the challenge of retrofitting the Widemarsh Pavilion, as well the greater challenge of widening the retrofit skills that we need across the country.
Thank you to all for sharing your knowledge with us!
Watch a recording of the webinar on our youtube channel here