Nearly all new build houses use cavity wall construction, a well established system but has it reached the end of its useful life? Building regulations, energy costs, climate change, personal choice, lifestyle, affordability, availability and build-ability are just some of the many factors that have an affect on what we build and how we build but it all starts in the same place; on paper in an architect’s office and the way architects view construction and the delivery of their designs is vital to building durable healthy buildings into the future.
We asked Sandy Patience architect and editor of specification site Greenspec for his view on why we still build using cavities, (when there are many other viable options) what to watch out for and how we could improve building performance in the future.
Sandy begins with explaining how to calculate the performance of a building element, in this case a wall: he writes..............
How a wall, floor or roof will behave thermally is first calculated using software to predict how the building element will perform, it is of course an educated guess but is used (largely) to determine the amount of heat lost from a building element: the most common and accepted way of comparing performance is by comparing elemental U values in fact they form a central part of The Building Regulations Part L and of course so apply to cavity walls.
Back in the day when detailed heat loss calculations didn’t keep architects awake at night, U-values were quite enough to convey how good a building element or component was at contributing towards energy efficiency. Today, in more straightened times and with a greater emphasis on building performance, U-values are still of key importance, but in calculating heat loss there’s a bit more to think about and it’s not just sums.
What is a U-value?
When we talk about the U-Value of a particular component of a building such as a wall, roof or window, we’re describing how well or how badly that component transmits heat from the inside (usually) to the outside. On a cold day in the UK when we’re warm and cosy on the inside of the building, we will be happier the lower the U-Value is – because it means that our wall or roof or window is quiet good at holding-up the heat getting to the outside.
The technical name for which we use the shorthand ‘U-Value’ is Thermal Transmittance. Before we start looking at what that means, let’s sort out the units we use to define it. Energy flows along in watts (which is a measure of energy in ‘joules’ flowing over a period of time in ‘seconds’).
Temperature is measured in degrees Kelvin – which practically is degrees Celsius to the rest of us. The U value of a piece of building fabric like a wall, roof or window, measures the amount of energy (heat) lost through a square metre of that material for every degree difference in temperature between the inside and the outside.
The U value of a building element is the inverse of the total thermal resistance of that element, the actual equation involves a few more ‘values’ which, when put together the calculation incorporates heat flow into the section by radiation from all parts of the inside the building and by convection from the air inside the building. Equally additional thermal resistances should be taken into account associated with inside and outside surfaces of each individual element, all put together this gives us the U-value of our wall or window. We’ll look at the other accommodated factors in a moment, the full equation is shown in Figure 1 above but the essential equation is this:
U = 1/R in W/m2K or Watts per square metre per degree Kelvin
Example of how U-values are used
- The U-value of a single sheet of glass as found in a traditional window pane is 6.0W/m2K – which means that for every degree of temperature difference between the outside and the inside, a square metre of the glazing would lose 6 watts. So for example, if the temperature difference on a typical cold day was 15 degrees, then the amount of heat loss would be 15x6 = 90 watts per square metre. That’s a lot of heat!
- By comparison, the U-value of a modern piece of triple-glazing can be as low as 0.7W/m2K – which is not very much heat loss at all.
The ‘R-value’ and other components of the U value calculation
R (or the ‘R-value’) means the Thermal Resistance or how much of a fight the material puts up against the heat passing through it. The R-value is expressed as m2K/W the higher the R-value, the more thermal resistance the material has and therefore the better its insulating properties.
If the material consists of several elements, the overall resistance is the total of the resistances of each element. Hence the higher the R-value, the more efficient the insulation.
Example of R-values:
- 100mm of wood fibre insulation board would have has an R value of 2.6 m2K/W whereas in comparison
- 100mm of glass fibre insulation batt would have has an R value of 2.2 m2K/W – which makes the wood fibre more resistant to heat loss.
The ‘R-Value’ too has its own equation that picks up on yet another ‘value’:
R = t/ λ where‘t’ is the thickness of the material in metres and λ is the Thermal Conductivity (sometimes known as the ‘k-value)
The lambda (λ) value or the Thermal Conductivity of a material is a value that indicates how well a material conducts heat. It indicates the quantity of heat (W), which is conducted through 1 m² wall, in a thickness of 1 m, when the difference in temperature between the opposite surfaces of this wall equals 1 K (or 1 ºC). In practice λ is a numerical value expressed in terms of W/ (mK). The lower the λ value, the better the insulation property of the material.
Examples of Thermal Conductivity
- Wood fibre insulation has Thermal Conductivity of 0.038 W/mK
- Glass fibre insulation has Thermal Conductivity of 0.044 W/mK
- And the Thermal Conductivity of dense concrete is around 1.5 W/mK
- In comparison, the Thermal Conductivity of copper is a whopping 401 W/mK – which is why some of your kitchen pans might have copper bottoms.
Why knowing all this important?
The calculation of a building element’s U-value is sufficiently complex to be beyond the scope of all but construction professionals utilising purpose designed software and certainly would confuse and confound the average lay reader of this article, put simply the values described above must be known or at least estimated for the building element’s thermal transmittance to be calculated: This is critical in establishing on paper performance to meet design targets and building regulations this is the place where compliance begins, however up to this point it is theoretical as we haven’t built anything in real life yet and despite on paper predictions delivering on these paper promises is where it most often all comes unstuck.
So, what are the other issues if building with a cavity wall?
The bottom line is that cavity walls occur not just in the heat-loss calculations on the architect’s desk- they’re actually built in the real world where other factors can compete with the modern performance requirements we make (or predict) of our walls.
The development of the cavity wall from the solid types we know from pre-1920 was perfectly understandable. Our forbears set out to stop driving rain soaking through the walls and ruining the expensive wallpaper the middle classes were decking their homes out with after the First World War. Putting in a cavity that could drain the moisture penetrating the outer leaf of brickwork was just the job – dry wallpaper and no mushrooms growing on the walls. But it was a limiting technical solution and the precursor to the problems we have with cavity walls today. Indeed, if we were to set out to design the perfect wall today, we’d be insane to come up with the cavity wall. But it’s what the construction industry has been bequeathed, and it takes time and argument to persuade it otherwise.
Thermal Bridging (formally known as ‘Cold Bridging’).
In the decades before 1980 books of details (such as produced by the GLC architects department) demonstrated how to design using lintels of steel or concrete that ‘bridged’ cavity walls. These books are still to be found on bookshelves. To modern eyes, the details are horrendous – a bit like seeing old photographs of motorcyclists without crash helmets. The lintels were materially homogeneous passing from the inside to the outside through the cavity without break or interruption: a clear route for conducting heat. ‘Cold Bridging’, as it became known, was of little concern to a country with plenty of oil to pump from the North Sea and when the expression ‘climate change’ meant that you were holidaying on the Costa's.
What we now recognise as and call ‘Thermal Bridging’ still occurs - most commonly where the needs for a cavity wall to be thermally efficient as well as structural collide. The two leaves of the wall, the outside and the inside, need to be tied together. In practice this means wall ties. Wall ties are most commonly made from steel – which is, of course, super-conductive. So despite the best of intentions including stuffing the cavity with insulation, those dozens of little metal thermal bridges are doing their damage by transferring heat from the inside to the outside.
The more recognisable conduction of heat through lintels is addressed through the ‘thermal breaking’ of the material. Layers of insulation are ingeniously inserted into the lintel to interrupt the flow of heat. The results are successful to a degree, but because most lintels are now made from steel, it’s hard work to design for perfection.
An alternative is to use lintels fabricated from GRP. Though there are few companies producing them, the material is far superior to steel in reducing conduction. Likewise, wall ties are available that are made from the very low conductive fibres of basalt. Both materials attract a premium for their manufacturers, so they are rarely used by the volume house builders.
We’ve seen above how Thermal Bridging overcomes insulation by driving a conductive material like wall ties right through it. Another disrupter of calculated U-values is the effect of ‘Thermal Bypass’. In principle a wall cavity is fully filled with insulation, but in practice it isn’t. As workmen insert the insulation batts air pockets are left between the batts and between the batts and the wall. Nearly all air moves to a certain extent from convection as warm rises upwards to be replaced by cold air. In an enclosed environment this leads to the air going round and round the insulation material transferring heat from one leaf of the wall across the cavity to the other leaf – hence the epithet ‘Closed Loop’.
‘Open loop’ is another way where air convection can cause heat loss. Where the outer leaf of a wall experiences air gaps like that between bricks where there is a weep hole or where the mortar is missing, air is driven through the gaps by the force of an increment wind pressure. Air pockets left over in the cavity between the insulation and the inner face of the external cavity wall leaf can be penetrated by external air. In this way, convection again is set up and heat can be easily stolen through the gaps in the wall. In reality ‘Open Loops’ and ‘Closed Loops’ can occur together. All put together, heat losses of up to 160% are known to occur in research conditions through the effects of Thermal bypass!
If you think this is all getting a bit nerdy – let’s return to some really big ways of losing heat: holes in the wall. There goes the brickie, proud of the new wall he’s just built. Brick carefully laid on brick, wall ties secure and insulation firmly enclosed in the cavity - a job well done.
The effect is usually short lived because within minutes there’s likely to be some other worker who has a job to do involving getting a pipe or wiring through this immaculate construction from the inside to the outside. The solution is a hole, 2 holes or 3 if you include the insulation. If that plumber, electrician, comms. man or anyone for that matter with a hammer and cold chisel is conscientious, he might just give a quick squirt of mastic in the direction of the gap left around the pipe– but it’s just as likely that you’ll be seeing daylight through the wall for the foreseeable future. In a similar way, the sequence of trades can confound predicted thermal efficiency through windows and doors not fitting the openings exactly. Expensive window systems can be rendered worthless through gaps left between frame and surround. The gaps are sealed but often casually and with materials causing their own thermal bridging or worse, shrinking to cause air leakage over time. There are dozens more ways that air can leak out, but it’s through careless workmanship like this that air leakage has continued to defy predictions of thermal performance in new buildings – mostly in housing'
Good thermal performance is not irretrievable – through training site workers to be conscientious together with inclusion of a continuous airtightness barrier within the wall build-up, actual heat losses can come close to those calculated ahead of construction.
So what’s with the U values now?
Knowing and understanding what the U-value is of an element of construction, whether it be wall, roof, window, door or floor continues to be the most important part of the thermal efficiency calculation.
We are seeing an incremental rise in the importance of energy conservation on how we build and its potential impact on how we design and delivery is coming in for much closer scrutiny. The owners of buildings are demanding that the performance as designed is met by the constructed building and there are more and more ways to check. One such avenue to test design against performance is Post Occupancy Evaluation (POE) which is still an evolving discipline where construction experts look at the difference between how the architect expected the building to perform and its performance in the real world. It’s turning out that the two usually don’t match, sometimes dramatically so. The real issue here is that it’s better to get it right first time than have to fix it later this is why more forgiving and more appropriate building systems need to be more widely deployed.
The obvious conclusion is.....
As you have read this far, you will already have a pretty good idea that in reality the on-paper design and as-built performances don’t match. There’s the simplistic piece of software (often no more than a calculator) that the architect uses that outputs the u-value, but with the practical variations experienced in reality, assessing the ultimate performance of a building has become very complex indeed – often involving advanced software operated by specialists. The calculated u values are unlikely to be true and almost certain to be higher when built as more and more examples are coming to light every day clearly illustrating this ‘performance gap’ in reality.
And the cavity wall?
There are so many alternative ways to construct a wall. Nearly all the alternatives involve methodologies that avoid thermal bridging and thermal bypass and provide a simple way of achieving airtightness. All of them are in use today and have been tried and tested. Often the solution becomes a wall that is super-thermally efficient as well as environmentally sound in its use of low impact materials. Certainly many more architects are moving towards these alternative technologies and are happily designing buildings that live-up to their promises - thermally, environmentally and comfortably.
Sandy Patience is an architect, journalist and speaker. He is the editor of GreenSpec at www.greenspec.co.uk - a site dedicated to delivering information about the design and construction of Green Buildings.