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The Future Is Passive

by Anne McGowan on September 27, 2023

Two passive housing projects completed recently by architect Mel Reynolds demonstrate conclusively that a passive housing system is a winning formula for anyone intent on achieving an ultra-comfortable home with vastly superior energy-saving credentials. Originally developed in Germany at the beginning of the 1990s, the passive house standard is a building performance measurement that has proved its effectiveness repeatedly and consistently over a prolonged period. Buildings incorporating the passive house principles demonstrate exemplary comfort levels and superb energy efficiency credentials, significantly reduced household emissions and negligible energy bills.

Derived from the German word Passivhaus, these houses are built to the most exacting of standards, incorporating a high degree of insulation and prioritising the eradication of thermal bridges. Adding highly efficient heat recovery and ventilation systems, together with strictly controlled air infiltration rates, ensures that the property maintains a comfortable temperature during winter and summer while minimising energy consumption and delivering optimal air quality.

Other factors considered under the passive house system include the orientation of the building and the climatic and geographical conditions of each site, which are also accounted for, to maximise the potential of renewable energy options, including wind and solar and further reduce the building’s carbon footprint.

The unique attributes of a passive house are amply demonstrated by Cove Mews, a 120m2, 3 to 4 bed new build compact mews in Sandycove, Co Dublin, designed by renowned architect Mel Reynolds. Completed in the final months of 2020, Cove Mews achieved the Mel Reynolds, renowned architect and designer of Cove Mews coveted passive plus standard, one level higher than classic passive standard and the first one in Ireland to achieve the standard. The German passive house institute had earlier introduced new higher levels to the passive standard. The passive plus and passive premium levels, which essentially introduced higher levels of renewables on projects. At the time, only two buildings in the UK had achieved this accolade.



“The main benefit to using the passive standard is how detailed and how exact it is,” says Reynolds. “The entire process is modelled, and you have to input all your components and all your materials, and how they are configured. This way you develop a three-dimensional BIM model - basically a building information model detailing the performance of your building and this gives you a number at the back-end indicating how it’s going to perform.

 “It is extremely detailed, and nothing is left to chance. It will carry out condensation risk analysis and over-heating is also modelled. There are various performance targets involved, the simplest one being the amount of heating per square metre used per year. To meet the standard, you cannot overheat by more than 10%, but we try to target a little better than 5%. Overheating is stipulated as anything above 25 degrees, so you can model all this in advance, and that’s the real benefit of this system.” While Reynolds credits the Building Energy Rating (BER) system adopted to measure performance in Ireland as a valuable mechanism for promoting improved building quality and standards, he also points to a number of shortcomings that need to be eradicated.



“We know that the BER is very useful, but there are several flaws that will need to be addressed over time. For instance, there is no way to model overheating in the BER system, which is now becoming an issue, because as standards increase, our buildings become more insulated and ventilation is more controlled, so all of a sudden passive solar gain becomes an issue.”

 In addition, BER calculates bedroom temperature at 18 degrees and living space temperature at 21 degrees, but only for eight hours during the day. If those temperatures are recorded for eight hours, the property is deemed A1 compliant, the highest standard you can achieve under BER. However, Reynolds points to research carried out by Cork based architect John Morehead of Wain Morehead Architects which found that the existing Dwelling Energy Assessment Procedure (DEAP) used by BER assessors to calculate a property’s energy performance and carbon dioxide emissions, is insufficiently sensitive to various indicators, including location and orientation. As a result, it can frequently be misleading.

 “The DEAP software is not accurate or exact enough and one of the problems we’re going to have in doing the numbers on our retrofits, is you’re going to have people who think their heating is going to cost them a certain amount, but it might be double that to attain the level of comfort they want,” Reynolds explains. “Your building is A1 compliant if you hit that temperature for eight hours daily, but if you go in and model the average temperature in your building during the day using more accurate software, you will likely find that the average surface temperature could be less than 16 degrees, which is not comfortable.

 “The BER system is really good, but it does have its drawbacks and if you are retired and you want to have a constant temperature in your house for 24 hours or for longer than eight hours, you’re going to be using a lot more energy than you think. This is the difficulty. The passive standard is far more exact and accurate and most of all, it works.”

The ‘performance gap’, which is the gap between the desired performance and the actual performance is effectively eliminated with the passive house system, according to Reynolds. “The great thing about using the passive standard and modelling software is that there is no performance gap. It is very detailed and accurate, which can be challenging, but you have to manage it as you build. You cannot directly compare the two systems, but to meet the same standard as the classic passive standard, you’re probably looking at 30 to 80% more energy being used in an A1 rated home for space heating to get the same level of comfort.”

Despite the passive house standard being purely voluntary rather than a measure for compliance, Reynolds urges anyone pursuing an A1 BER rating to adopt passive house principals while building, even if they have no intention of applying for certification. It virtually assures that you will obtain the highest BER rating and the additional costs incurred are minimal. “Even when not trying to get the certification, the system is excellent as a design tool for mainstream projects and is way more accurate and informative than the DEAP or BER system, which were never intended to be design tools,” he says.

“In terms of the actual building, we are doing a great deal of it already and we just need to assemble things slightly differently. For example, the eradication of thermal bridging, which is extremely important, can be done relatively simply by taking extra care of the junctions and making sure that you don’t have any cold spots. The more insulated a building gets, the more important that becomes, and while it sounds technical, it can often be accomplished just by moving window and door frames by three or four inches. In that instance, the position of the window and the door relative to the wall is what makes it thermal bridge free and does not require any additional financial outlay to get it right.”

 Reynolds uses the analogy of a boat, pointing out that air-tightness is a principal requirement for any seafaring vessel and making it really watertight requires a little bit of extra care and attention. “It is investment of time and effort rather than a substantial extra cost, and it needs to be done right during the initial build because it is really difficult and costly to do retrospectively,” he adds.



The current level of air tightness for the average A2 rated house is just under 2.7, which means that 2.7 volumes of air will be lost out of your building in an hour. In older dwellings, the figure will typically exceed 5. At the same time, it can reach as high as 7 or 10 in some properties which will result in high levels of air leakage and a draughty and uncomfortable environment. Meanwhile, the passive house standard requires a reading of 0.6, while the slightly less onerous requirement for retrofits under the passive house system requires a reading of one or less, still significantly more airtight than the requirements of the top BER- rated properties.

The more insulated a building gets, the more important that becomes, and while it sounds technical, it can often be accomplished just by moving window and door frames by three or four inches. In that instance, the position of the window and the door relative to the wall is what makes it thermal bridge free and does not require any additional financial outlay to get it right.

“A reading of 0.6, represents a really airtight building,” says Reynolds. “If you take an A2 rating under the BER standard which is fully compliant, it will have an airtightness level of around 2.7, or slightly less than that, but to get the same level of comfort as a passive standard home which requires eighteen degrees and twenty one degrees for 24 hours a day, you could easily be using a lot more energy for space heating. So, the number one thing that I recommend to anyone building a house is to try and get their air tightness as low as possible, because it is really expensive and really difficult to do retrospectively.”

 According to Reynolds, the three core tenets of the passive house standard are exemplary levels of comfort, unrivalled quality assurance and superb energy efficiency characteristics. The fundamental starting point for the passive standard over thirty years ago was the elimination of fuel poverty and the standards are so exacting that the occupant never has to turn off the heating because the heating bill is so low and the comfort level so high that they can simply forget about it. The space heat bill is negligible.

 “Comfort is the first priority and we’re aiming for a level of comfort which allows you to turn on your heat, turn on your stats and just forget about it,” says Reynolds. “Passive homes maintain a steady and consistent temperature Cove Mews contemporary bathroom with little variation throughout the year. Robert Tyan of Earthcycle Technologies, a consultant I use on my projects, tries to get overheating down to less than 1% a year, which means your temperature is only allowed to go above twenty five degrees for four days a year. After that, the second priority is quality and it is a rigorously controlled quality system, while the third factor is energy efficiency and that is achieved as a consequence of the prior emphasis on comfort and quality control.”

The significantly more stringent building regulations being implemented in Ireland and Europe-wide are already bringing buildings closer to the criteria encompassed in the passive house standard, and Reynolds is adamant that evolving EU legislation will require all new housing to be completed to this standard in ten years. He admits to a sense of frustration at what he views as a missed opportunity to incorporate the passive standard at a more advantageous early stage.

“It is only sensible to get in front of the curve and avoid making the mistakes as you go along,” Reynolds explains. “If you are building an A2 rated house, you’re almost there already and it is a shame, because there is really no excuse not to try and hit the passive standard, other than the fact that you’re not familiar with it. Many architects are not, in which case you will need to hire a separate consultant, but the cost is peanuts and what you’re really getting is someone to model your building to see if this will work the way you want it to work.”


The detailed modelling is one of the most impressive and beneficial aspects ofthe passive house system and Reynolds recommends its use for anyone embarking on a new build or renovations. Regarding resources, the most significant part of this is the modelling - the passive house planning software that comes with the passive standard, which he urges anyone to use. “Whether you’re going to try to hit the standard or not, get the modelling done because it’s a brilliant performance model for your building and will tell you exactly what you’ll get. It’s like an audit on your building. In the end you will have a comprehensive and detailed Excel programme that goes through all the components and how they are put together,” Reynolds argued.

There have been approximately 10,000 licenses issued for people to use the software and applicants are only permitted to use it once they have completed the training course. However, uniquely, a client can employ the services of anyone worldwide to carry out their modelling, provided they have completed the course and been issued a license. The software feature’s location specific zones, which is a further distinction that BER doesn’t have, and the testing can be carried out from anywhere in the world by entering the inputs appropriate to your location.

 Third party checks are required to obtain certification and all the calculations and modelling for the property are dispatched to a separate designer who will review the information before sending it to the Passive House Institute, which decides whether certification is granted or otherwise.

“Kym Mead Architects did the third party checking of my mews in the UK. I’d never met him, but he had done the course and provided the service,” Reynolds points out. Mindful of continually evolving EU building standards, Reynolds set himself the task of achieving the passive house plus standards at Cove Mews with a budget equivalent to that required to achieve an A2 rated home under the BER.

The initial design for Cove Mews, a sleek modern building that features a south-facing pair of full height, double doors, standing 2.6m high and 5m wide, included an overhang of around 1.27m deep. However, the modelling indicated that it needed to be altered to a depth of 880mm deep to shade the midday sun in summer, which was fully overshadowed, while in winter it would be narrow enough to get as much of the winter sun as possible.

“If it were deeper, it wouldn’t have got as much of the low-lying sun in winter so we changed the physical shape of the building to optimise that,” Reynolds explains. “There was also an expensive roof light that I took out because they use so much energy, which saved about €2,000. You are essentially trying to get this energy balance where you offset the energy losses with the energy gains 365 days a year.”

The intention was to design a contemporary, flexible family dwelling, with pleasant outdoor private spaces and spacious living areas which also just happened to be as energy Cove Mews efficient and as future-proof as possible. Given very low levels of heat loss, excellent air tightness and a carefully designed mechanical ventilation system, there was no need for a primary heating system to maintain a 21 degree average temperature in living areas and 18 degrees in bedrooms. The cost saving from the omission of the usual underfloor heating system offset the extra cost of a larger PV array.

“Basically, we had a finite budget, and I wanted to see how far we could go. Early on we discovered that if we hit the passive standard, we wouldn’t need a primary heating system. Normally what you would be looking at, is putting in some kind of underfloor heating system and a heat pump and the fact that I didn’t need any of that because it was so compact and the insulation so well done, saved about €7,000. So, I spent around €3,000 extra increasing the glass specification and put the balance towards installing PV on the roof. Adding renewable energy into the mix brought the property up to the Passive House Plus standard.

“I saved money on our build cost by going passive because we didn’t need the primary heating system and I got the photovoltaics on the roof for free. At that stage we ended up with energy expenses of about €400 for the year, and that’s for heating hot water and for powering the car, which is exceptional.”


Throughout the project Reynolds consulted with Tim Cooper, who installed the first photovoltaic installation on the Green Building in Temple Bar thirty years ago. He expressed concerns regarding the amount of surplus energy generated by the thirty two solar panels, which would simply be lost to the grid. However, thirty two panels were required to meet the passive house plus standard, and the challenge for Reynolds was how to integrate them seamlessly into the project and avoid looking like a space satellite.

“It was a system of approximately 7kw, about double the volume that most people would put in. There was no feed-in tariff then, and the problem was what to do with all the surplus energy being produced. So, we ended up storing that energy in hot water tanks on site but we were still losing quite a bit of energy to the grid. Then we worked out that if we plugged an electric car into the house for two hours a day, five days a week, we could get ten thousand kilometres of driving for free.

“In September that year we were completely off-grid every two days, and we only needed to use the grid once to fill the car. Everything else was being powered off the roof, so we got an abundance of hot water and power for our car for normal domestic mileage for free. I think we got a running cost saving in the house of about

€900 a year, and when you add the car into the mix, that energy saving doubled. But then when you look at your carbon emissions as compared to a household with a normal diesel car and a normal house, the reduction was off the scale.

“The great thing about photovoltaics, which a lot of people in the industry don’t realise, is that they require little to no maintenance. We don’t have harsh climatic extremes which can degrade the panel and the solar installation on the Green Building, which has been there for 28 years, has only been cleaned once, and they’re still producing the same energy as they did in the first week. Since China got involved in producing PV panels, the price has also plunged, and I couldn’t get over how cheap they were. They have come down in price dramatically over the last decade.

“We worked out at the time that the cost for all energy used, heating, hot water and consumer items such as tv, washers, dryers, lights and the like, was circa €300 per annum, and if used to power an electric car, the cost was €100 extra per year for normal 15,000km driving. That is for all energy used in the family home plus a full year’s driving.” 


Meanwhile, outside, buried underground is a 5,000 litre rainwater harvesting tank that supplies WC’s and the garden, so when combined with low-flow internal sanitary fittings, reduces potable water consumption by approximately 50%. Although not beneficial cost-wise, this is something that Reynolds believes should be mandatory for all new and refurbishment projects, given the pressures on our water supply and drainage infrastructure due to global warming.

The compact design meant that garden space was at a premium, so a green wall was installed to enhance bio-diversity with specific plants selected. The green wall not only enhanced biodiversity but became a central design feature of the enclosed private south facing roof garden of the house.


Following the completion of Cove Mews, Reynolds embarked on a deep refurbishment of a 1935, four-bedroom, Victorian property in Rathgar, South County Dublin. A year after the upgrade was completed, the owners decided to seek Passive House certification and the property is currently undergoing the rigorous testing required to ensure compliance with the standard. Marginally less onerous criteria are applied to retrofits which the Passive House institute categorises under a separate EnerPHit label, which Reynolds equates to an A1 BER rating or higher.

“The retrofit standard is slightly different from the passive classic standard,” Reynolds says. “It’s more forgiving because the passive standard is so hard to hit, particularly in existing buildings with walls that you can’t upgrade and areas where thermal bridging is impossible to address. Then, you might have a brick frontage to the front of your house that you can’t externally insulate, so you have to find an internal solution, which will not be as good as 6 inches of insulation placed on the outside. You have to make more compromises when upgrading an existing building.”

The extensive refurbishment of the property, which had not been upgraded for some time, entailed the demolition of a rundown garage to the side which was replaced by a two-storey side extension and a single-storey extension to the rear. The design plan also features open- plan living areas and the provision of additional bathrooms as well as a self-contained studio apartment/office space.

The house which occupies a floor area of some 190m2 was designed to meet the passive standard, which Reynolds calculated added an extra 1% to the budget, a pittance in the context of the energy savings achieved by the upgrade. Much of the additional cost pertained to high performance glazing specified for the project. “It is very well insulated and critically the air tightness in that house is less than one, which was difficult to achieve and it took us about two weeks extra on-site. We had to strip all the walls, remove the bathrooms and the kitchens and start from scratch,” says Reynolds.

Where possible the property was insulated internally, and a blower-proof, liquid airtight membrane was applied to the internal plaster- work in the existing house, while a suspended timber floor was replaced with an insulated concrete slab. A Lunos decentralised heat ventilation unit was installed, and an eight panel PV array was added to the roof.

A full twelve months total energy consumption record for three adults in the house is nearing completion, and according to Reynolds, the projected space heating demand for that house is around 900 kW for the year. The house is being pre-heated at night for about two hours and during the day for two to four hours depending on what is required, which amounts to a cost of less than €160 for the year at current rates. Putting it in context, Reynolds points out that it is equivalent to the energy required to power an electric Aga cooker for four or five weeks.

“We’re measuring the temperatures in all the rooms, and the interesting thing is that for the entire year, the bedrooms oscillate between slightly less than 18 degrees to around 19 degrees,” he says. “It’s a really narrow range of temperature, and the living rooms are the same, ranging between 21 to 21.5 degrees, and that’s 24 hours a day. If that were an A2 rated house or an A1 rated house, you’d see this much bigger oscillation where during the day the temperature would be at a certain level and as soon as the heating is turned off, it will drop quickly because it’s a leaky building.

“The numbers are great, and the thing that caught my eye was just how stable and how comfortable the house is 24 hours a day. That tells me that the owner isn’t turning down the thermostat or turning down the heater when they were going out at all; they just leave it on and let it happen.

“It hasn’t been designed to be energy efficient overtly, we took out a bedroom floor, so you walk in and you’ve got a double height dining space which looks lovely. It’s open-plan and very spacious, and they haven’t compromised on the design in any way in order to achieve these numbers.


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Thermally insulating the exterior of a building provides a well- insulated and protective blanket. The worst effects of thermal bridging are nullified and the detailing to achieve high thermal performance is greatly simplified. The use of insulation on the outside of the wall also makes sense from a condensation point of view. It ensures the wall is kept ‘warm’ for the majority of its thickness from inside to out. This maintains the building fabric’s temperature above the ‘dew point’, effectively dealing with mould growth and other environmental issues. EWI protects the fabric of the building and helps to reduce additional heating requirements.

The Thermo10 EWI system comprises starter tracks and EPS insulation boards which are adhesively fixed to the substrate with Thermo10Contact, a mineralic adhesive. For masonry, concrete and panel backgrounds, supplementary fixings are also available. A reinforcement coat, strengthening mesh and Thermo10Primer are then applied, before the finishing coat of Therm10 Silcotop is applied. This high-performance silicone render provides additional resistance to weathering and water penetration, and reduces the likelihood of algae growth and soiling of the surface. The stippled texture gives the finished substrate helps to promote effective water shedding in rainy weather. The result is a beautiful and seamless façade, with crack resistant protection that can survive even the harshest climates.

“The owners wanted underfloor heating installed throughout the property, but after the modelling indicated that it was unnecessary upstairs, I spoke to them, but they decided they wanted it installed regardless because they were accustomed to it. Six weeks after they moved in, they turned it off, and they haven’t used it since.

“With the airtightness at 1 or less, there are no draughts, the temperature throughout the home is steady and consistent and the level of comfort is exceptional. All of the ventilation is controlled through a centralised system, where the extract preheats the air coming in without mixing it, so you’ve about a three to five percent energy loss. You’re not opening windows in winter, you’re not letting all this lovely hot air out and the big qualitative difference that I’ve found any time I’m in a passive dwelling, is that the air quality is phenomenal,” Reynolds adds.

“They’ve been developing these tools for thirty years and getting them more and more exact, and their modeling is brilliant as a design tool. Knowing how to do something to passive house standards gives you a competitive advantage, and the cost differential is negligible. It’s just about doing the same thing in a slightly different way and spending a little extra on getting your building modelled - this will actually save money in the long run. In many ways for me as a designer and a specifier, its simpler for me to try and hit that standard than trying to hit an intermediate one because it’s just too complicated. It’s far easier to say we’re hitting 0.6, and this system will work, and I know it will work for me. And there’s no cost penalty, all you have to do is buy into it.”


Reynolds advises anyone seeking to build or upgrade a house to passive standards to first go to the Passive House Institute of Ireland’s website, read Passive+ magazine, then employ a builder who has completed one of the excellent training courses hosted by MosArt Architects or the Passive House Academy and who has experience in building to passive house standards. Most builders trained in the standard understand and appreciate the importance of making the building airtight and will agree that the passive house approach is the most effective method of achieving a comfortable, low-energy home.

 Before he set himself the challenge of delivering a Passive House Plus standard dwelling at Cove Mews at an equivalent cost to that of an A1 rated home, Reynolds confesses to harbouring some initial doubts and wondering whether the boasts of passive house enthusiasts might be exaggerated. Any such doubts have now been entirely dispelled. “I can tell you that if you’re looking at 900 kw per m2, per annum heating bill for a retrofit and the exceptional comfort level achieved, you can’t argue - it just works. Let’s say you’re spending €70,000 doing up your house, and you end up spending €71,000 to get that standard and that level of performance; you win in the long run. “The standard is thirty years old and is tried and tested, the software and modelling tools are well proven now. This is also where building standards are heading or converging, so it makes sense to get ahead of the curve and future-proof new designs. My own experience in Cove Mews demonstrated that adopting the standard actually reduced build costs, simplified the design and improved build quality. The passive standard is all about comfort, quality assurance and energy efficiency. Higher standards at lower costs, its a win-win,” Reynolds concluded.



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