Mark Siddall explains how thermal bypass – or air movement around insulation layers – might impact a self-build or retrofit project.
Interview with Mark Siddall
Mark is a Passivhaus specialist who has been on the podcast before, talking about avoiding the risks of poor ventilation and his ‘forever home lifestyle’ framework.
His practice LEAP operates from County Durham and Newcastle. He is also a technical adviser to the Passivhaus Trust and the AECB. He has written a paper on thermal bypass risks for the Passivhaus Trust.
There are many types of performance gap – but energy is often the focus
For Passivhaus buildings, the focus tends to be energy performance rather than other factors like air quality. Passivhaus projects actually close gaps in predicted performance, which can’t be said for other approaches. The methodology has got better quality assurance processes, which avoids risks that might otherwise arise.
Insulation has to be specified correctly for it to deliver energy performance benefits
Dealing with energy performance often means the thermal performance of the building fabric. Three variables influence whether insulation works or not.
- The material itself.
- Whether the material is specified sympathetically for the construction method used.
- Whether the material is installed properly.
Get those variables aligned and the evidence shows everything works properly.
An example of an air gap when using blockwork and rigid PIR insulation
Choosing insulation for cavity walls
Cavity wall construction is very common in the UK. A Passivhaus designer will question if the usual foam insulation solution is sympathetic to the method of construction.
Blockwork features mortar joints, and mortar is squeezed during installation. That creates snots which project into the cavity and stop the insulation sitting against the face of the blockwork. The resulting small air gap is enough to completely undermine the thermal performance of the insulation.
Mortar snots can be cleaned off, but that might not happen all the time. A mineral wool solution is more flexible and capable of dealing with subtle deviations, so it’s a more appropriate insulation for a cavity wall. It’s also used to the full depth of the cavity, which helps to reduce the risk of air movement.
Mineral wool fully filled cavity at Steel Farm Passivhaus
Airtightness is about moisture as well as heat loss
Part L of the Building Regulations deals with heat loss and prescribes a specific level of airtightness. Part C talks about moisture and only says things should be fairly airtight. There’s no numerical target related to moisture, and that’s a problem.
The Passivhaus standard, and research conducted in Canada, both confirm that airtightness of 0.5 or 0.6 air changes per hour at 50 Pascals is needed to avoid damage to timber structures (such as roofs or framed walls). That’s an air permeability of around 0.5, compared to the Part L target of 8.0.
Good airtightness alone doesn’t guarantee insulation performance
From the external side of the wall, wind tightness stops the wind blowing through and around the insulation, which wicks away heat. It’s not something that can be tested on site, however. As a rule of thumb, the quality of workmanship needed for a wind tight layer is similar to that needed for the airtight layer.
Fully encapsulate insulation between the air and wind barriers
Convective loops can happen around insulation just from the temperature differences between the warm and cold sides of the wall. We can avoid that by properly encapsulating the insulation between a notional air barrier and wind barrier.
Using the same cavity wall example as before, a good build-up might be brickwork or stonework on the outside face. Then a 300mm cavity full filled with mineral wool insulation that's built up in two or three layers. The inner leaf is blockwork, which is plastered internally.
The plaster acts as the air barrier, and the brick or stone is the wind barrier. The cavity is fully filled, so the insulation is encapsulated and has no air gaps around it.
Rainwater is the dominant moisture problem to tackle
Cavities were introduced to walls to act as a capillary break and stop water being drawn through from the outside. Any water getting through the outer leaf drained down the inside face of the cavity and out through a weep hole.
A very narrow cavity fully filled with insulation carries a risk of rainwater bridging across to the inside of the building. That risk reduces with a wider cavity. Passivhaus design requires wide cavities and, with good workmanship and no mortar snots, any risk from driven rainwater is mitigated.
Installing the cavity insulation in layers means that if any water gets through one layer, it can still be pushed down and back towards the outside.
Think about how wall ties affect performance too
Cavity wall ties help to provide structural stability, but are also a repeating thermal bridge through the insulation layer. Mark specifies Teplo wall ties instead of stainless steel ones. Stainless steel ties have a larger physical dimension and a higher conductivity, whereas Teplo ties are thermally broken.
Design calculations usually assume perfect installation
On most projects, architects contact insulation manufacturers for U-value calculations. The manufacturer never asks what quality of workmanship is being specified. Instead, they assume perfect installation, which requires certain tolerances on site.
When those tolerances aren’t achieved and workmanship is less than perfect, which is the case on every building site, then air gaps are the result. Materials haven’t been specified to work sympathetically with one another and the predicted U-value isn’t achieved.
Timber frame construction can help to address performance gaps
From inside to outside, a typical timber frame construction capable of producing good results would be:
- Service void, which may or may not be insulated (ideally not).
- Airtight layer.
- Engineered I-beam filled with cellulose insulation.
- Wood fibre insulation on the outside.
- Wind barrier membrane.
- Drained cavity, acting as a capillary break.
- Rain screen on the outside face.
Compared to this build-up, performance gaps can occur because standard industry timber frames don't have the wood fibre layer, don’t use engineered I-beams, and are less likely to feature a service void. Services penetrating the air barrier will allow moisture from inside the house to get into the timber frame, potentially causing moisture damage.
One area they tend to do well in is using 150mm thick insulation in the 140mm frame. Because the insulation is that little bit bigger, it’s compressed slightly and ends up being snugly sandwiched between the air barrier and wind barrier. There’s less risk of convective loops.
Improvements can always be made to reduce performance gaps
Research from places like Sweden and Canada is showing that, in the construction described above, moving the wind barrier behind the wood fibre (which might be just 20mm thick) could significantly reduce the risk of condensation.
The climates are different to the UK, so the conclusion might not be the same here, but it shows there are still refinements that can be done in Passivhaus timber construction to make it even more resilient than it already is.
Air movement is the issue across all build systems
All construction technologies need to address the key issues of building physics. With straw bale, for example, a lime or clay plaster on the inside can provide the airtight layer. There are other subtleties and complexities, but it’s a case of limiting air movement through, and within, the wall construction so the building performs as predicted.
An example of a warm roof (Shepherd's Barn)
The same principles apply to roofs…
…although it depends what type of roof it is. A warm roof construction is very similar to the timber frame walls, just at an angle. The tiles or other roof covering replaces the rain screen.
A cold roof, with insulation at ceiling level and the attic space above is risky to deal with from a moisture perspective. Information Mark has seen suggests one-third of the moisture that transfers into the roof space could condense on the roof structure. It has to be possible to remove that, using a vapour permeable underlay.
Ventilation can also be used, but there are reasons why that may or may not be preferable or doable. Then it’s necessary to look at the eaves detail and the risk of wind washing. Air comes into the loft at higher speed, so special details are needed to help protect the insulation at the edges.
The important advice is to keep things simple
Details have to be simple to make them buildable. Then they can be replicated from person to person, site to site, and house to house.
Make sure you have an air barrier and a wind barrier, and that you encapsulate the insulation. Do that and you’re on the right path.
The ‘Thermal Bypass Risks’ paper on the Passivhaus Trust website provides solutions. It includes tips and tricks for designers and architects to help mitigate thermal bypass. Each section features accessible takeaways that could also be used by self-builders.
Find out more
Visit the website of LEAP
Read Mark Siddall's paper on Thermal Bypass Risks
Watch the documentary case studies at PassivhausSecrets.co.uk
Dear Evil Engineer
Thank you to Stephen Miller in Oxford for sharing this with us. The Institution of Engineering and Technology has a bit of fun from time to time with their Dear Evil Engineer series. Villians write in for advice on their diabolical plans.
So how many leopards do I need to heat my home? And they even consider homes that are built to Passivhaus standard!
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