The following article is a reflection on the use of stone in the first certified “Passivhaus” for any railway company worldwide. A Passivhaus is an ultra-low energy building. For example, even if it is freezing outside, a typical living room could be heated with seven candles alone.

One minute to midnight and counting

It is often said that if you represent the time since the creation of the earth up to now as one day, then man arrived at just a few minutes to midnight. Yet, having come so late to the planet, no one can deny man’s impact.

The use of insulation in building construction has had a similarly late arrival, really only appearing after the first oil crisis of the 1970s and not really in earnest until the 21st century and the focus on rising energy costs, CO2 emissions and climate change.

I believe insulation will be a total game changer in how we construct buildings, along with other so called “passive” techniques such as air tightness and the controlled use of solar gains.

So, the question is, where does this leave the use of natural stone in modern construction build-ups?

Deconstructing the wall

The use of stone in building has a very long tradition. But how will its use change as we now “deconstruct” the traditional wall and look at how we should adapt its construction to this new paradigm?

I would argue that the properties that made people seek out stone for building were its durability, aesthetic qualities and good resistance to the passage of moisture. I believe these are still the reasons to choose stone, but the way we use it will be more reasoned, scientific and performance-driven.

Going back to the resistance of traditional stone walls to moisture, this characteristic was nearly an accidental benefit as walls built from stone had to be load bearing and hence quite thick — which in turn meant moisture had further to travel, keeping the interior drier.

Well-built Ashlar dimension stonework was in essence watertight, whereas traditional vernacular random rubble stone walls often needed a render coat, but both exploited the overall thickness of the wall to keep the interior dry.

However nowadays, with space being at a premium, many designers, myself included, look at the amount of plan area taken up by traditional wall thickness and wonder how much of that overall thickness we can convert to an insulating layer in the construction and how we should rethink the way we construct buildings to allow for the modern understanding of building physics.

Making the use of stone honest again

Personally, I always “struggled” with the use of stone cladding (veneer) in modern buildings. Often, stone is fixed to a substrate that is load bearing, weathertight and basically sound. So in essence, the use of stone has been relegated to a largely cosmetic or aesthetic role. As an architect, you ask yourself, “When stone is used in a non-structural way, do you simulate traditional jointing, or do you highlight its non-structural use by using stack-bonded jointing patterns?”

Dealing specifically with the use of stone in this building, the question is whether the use of stone is still purely aesthetic, or whether it has a more active role to play in the performance of the wall and the building in general. As you will read, I believe that, in modern construction build-up, we will be able to “square the circle” and find a common or converging approach to using stone technically and aesthetically — making its use “honest” once again.

The recently completed Train Drivers Building at Portlaoise gives some examples of a performance-driven approach, seeking very high levels of insulation (0.07 W/sq.m K, R value 14.28) and harnessing the latest thinking in building physics — specifically hygrothermal analysis — to deliver the first “certified commercial Passivhaus for the state sector” in Ireland and the first “Passivhaus” for any railway company in the world.

Stone and building physics

Modern building physics teaches us that there are two drivers to moisture. One is vapor pressure, which typically goes from an area of higher pressure (read high temperature), to one of lower pressure (read lower temperature).

Typically, this means that in northern climates, it moves from inside to outside in winter. In summer, however, this can be reversed, depending on the climate and construction build-up. (It also means the direction is from outside to inside in tropical climates, so understanding the climate characteristics one builds in is vital to putting together the appropriate construction build-up).

The second driver is relative humidity. Again, liquid moisture is driven, in this case from areas of higher relative humidity (read lower temperature) to lower relative humidity (read higher temperature).

There are additional factors, such as the capillary movement of moisture and the absorption of moisture due to the surface characteristics of the material, externally and internally, as well but I want to mainly deal with the first two.

If you have been paying attention, you will see that the two main drivers are generally in opposite directions: moisture from outside to inside and vapor from inside to outside.

So, an ideal wall will prevent or significantly reduce the amount of external moisture — i.e. rain — from entering the build-up, but also allow sufficient breathability for vapor to diffuse without being trapped.

In this case, an external ventilated rain screen cladding of natural granite was chosen to provide these two functions.

The cladding has a ventilated air gap behind the stone. This allows any vapor that has been driven through the breathable insulating construction to diffuse to the atmosphere.

The Fraunhofer Institute of Building Physics in Germany and Oak Ridge National Laboratory in the U.S. have developed software (WUFI) for simulating how construction behaves in the real world. Its usefulness is that it allows the designer to go through various ‘what if’ scenarios in assembling a wall construction or build-up and then run a simulation to see how it would perform.

Figure 9 shows “screen shot” from WUFI’s Pro 5.1 software. WUFI deals with many parameters, but the key one is the amount of liquid moisture entering any build-up. WUFI shows that up to 70% of wind-driven rain adheres to exposed elevations. As a result, this amount of moisture is very significant, and any techniques that reduce or eliminate it will go a long way towards arriving at a successful construction build-up.

Thus, the second function the external stone facade provides is the reduction of wind-driven rain adhering to the inner, more vulnerable construction build-up. The relatively high-density and low-porosity properties of natural granite and the fact that the stone has a ventilated air gap behind it ensure that there is a good initial resistance to wind-driven rain. Even if totally saturated, the accumulated moisture will simply drain away, leaving the more vulnerable insulated build-up behind the stone as dry as possible.

Cost, tradition and sustainable sourcing

In addition to satisfying the above performance criteria, natural granite was quite cost-competitive with mineral cement boarding, but had a much higher aesthetic value and a much lower embodied energy in terms of its production. There is quite a tradition of using stone in railway architecture. This use of stone complements and updates the use of stone for the needs of the 21st century. Finally, granite is the most abundant stone in the world. Thus from a sustainable point of view, using granite did not mean depleting a scarce resource.

Use of stone for passive solar design

In addition to external cladding, the second use of stone in this particular building is as an internal floor finish. It may be unusual to see stone laid on what is, in essence, a suspended timber ground floor, but again, the principle behind passive solar design illustrates the logic of using stone here.

Passivhaus buildings attempt to strike a balance between heat losses through the building fabric and ventilation, with heat gains from people, appliances, lighting and particularly solar gains in the winter months.

In buildings of low thermal mass, any solar gains can potentially overheat the building when the sun is shining and lose heat quickly on cloudy days or at night. The use of a stone floor in the bay area window of the building allowed any solar gains to be absorbed by the stone — thus using stone as a “heat sink.” This helps iron out the diurnal change in temperature of the building and any spikes in direct solar heat gains through the glazing of the building.

Design for Deconstruction (DfD)

DfD is a growing “trend” in the sustainable design of buildings. Even if a material is abundant, instead of looking at material use from cradle to grave — i.e. source to disposal at the end of its life in the original building — materials need to be considered in a cradle-to-cradle context so that they can be successfully recycled and reused (or reborn) if their original use changes or reaches the end of its useful life.

This building paid particular attention to DfD, and the stone cladding is no exception. All of the stone units — except for the corners and details to windows — are exactly the same size. This made ordering and processing the original material more efficient, but will also make the reuse of the material easier offering the new user a consistent dimension for the material.

The stone units are dry-mounted on a stainless steel hanging system which uses the dead weight of the stone and some concealed dowels to fix the stone on a totally independent self-supporting fixing system.

No silicone is used in the joints for several reasons. First, from a DfD point of view, the material is close to pristine. Technically, the joints allow for the pressure equalization of the ventilated air layer behind the stone. Aesthetically, omitting silicone avoids the staining to the edge of the stone that can often be seen where silicone is used to close stone joints.

Conclusion

Convergence is a familiar trend in the technological world, but it seems that in advanced construction, detailing it is equally so.

Stone is a precious material, no matter how abundant. Like any precious material, the way we use it and the reasons for using it need to be informed and reasoned. In modern, complex, multi-layered construction build-ups, the use of stone is poised to take on a more performance-led role, which will complement its long-admired aesthetic and durability qualities.

This use of stone cladding, particularly with DfD in mind, will give the architect and designer a new way to interpret the expression of stone to communicate and bring to the fore ideas of quality and sustainable design as an integral part of creating beautiful architecture.