The Passive House standard can't be diluted and here are the maths to prove it

“As debate ramps up in Ireland about whether local authorities in Dublin should adopt the passive house standard, and the UK government scraps its plans for zero carbon homes, Dr Shane Colclough urges passive house advocates to prepare for the lobbying battles ahead by remembering the basic science behind the standard.”

This week, I am reminded of the first meeting I had with Prof Wolfgang Feist when he met the Irish delegation attending the international passive house con­ference in Dresden. One of the delegation raised the point to Prof Feist that the Irish climate is mild, and therefore asked if a case could be made for reducing the requirements of 10 W/m2 (for heat load) or 15 kW/m2a (for space heating demand) in the case of buildings constructed to the passive house standard in Ireland.

Prof Feist paused and countered with a question himself: "What would your response be if I were to ask you if the ratio of the circumference of a circle to the diameter [Pi] could be altered?" He paused again. "Unfortunately, I have no way of changing the definition of a passive house, irrespective of the climate."

“The passive house movement's greatest strength is our ability to highlight the building physics basis & evidence-led approach of the standard"

We often lose sight of our roots and the strength of the concept of the passive house standard. Let's remind ourselves. The passive house standard is defined based on sound building physics and has as its core the objective of ensuring that occupants enjoy thermal comfort with good indoor air quality: "A passive house is a building in which thermal comfort (as defined by the standard ISO 7730) can be guaranteed by post­-heating or post-cooling the fresh-air flow required for a good indoor air quality without additional recirculation."

So, at its core is reducing the space heating demand to facilitate the use of air as the heat transfer mechanism ­and, if this is achieved, the traditional heating system can be dispensed with. The mathematics of the standard pro­posed by Wolfgang Feist are explained on Passipedia as follows:

To ensure good indoor air quality, one person needs about 30 m3 of fresh air per hour. This supply air can only be heated up to 50C to avoid the scorching of dust. The specific heat capacity of air is 0.33 Wh/(m3K) at normal pressure and a temperature of approx. 21 C. From this the heat flow can be calculated:

Heat Load: 30m3/hr/pers*0.33Wh/(m2K) * (50-20) k = 300 Wipers

Hence: Fresh air heating can supply 300 Watt per person. Assuming 30 m2 of living space per person the maximum heating load at a given point of time may not exceed 10 Watt per square metre of living space - independent of the climate. As these figures refer to that day of the year where the maximum amount of heat needs to be supplied to the building (heating load), passive houses require different levels of in­sulation depending on the individual climate: more insulation in extreme climates, less insulation in milder ones.

So, at the air change rate required to ensure good indoor air quality, air can be used to heat a dwelling, thereby eliminating the need for a conventional heating system. This was the concept in all its purity proposed by Wolfgang Feist, and the contribution to the world­wide body of knowledge which gained him his PhD.

The real beauty of the passive house standard is that the sound theoretical basis on which the concept was founded has not only been proven with the dwellings built in Darmstadt, but has also been reinforced by countless other academic publications and studies such as the CEPHEUS ('Cost Effective Passive Houses as European standards') study where hundreds of houses have been moni­tored and results reported on. This en­ables the evidence-based approach which is the bedrock of the passive house standard.

When Wolfgang Feist answered the Irish delegate comparing the definition of Pi to the requirements to achieve 10 Watts per metre squared to meet the passive house standard, he reminded me of the beauty of maths and the elegance of his solution.

As we enter the fray of discussions about the adoption of the passive house standard in Dun Laoghaire-Rathdown County Council and Dublin City Council, and elsewhere, we must always be mind­ful of the sound theoretical basis on which the passive house standard is founded. While others have opinions on the best approach to achieve near zero energy buildings and meet the building regulations, the passive house movement's greatest strength is our ability to highlight the building physics basis and evidence-led approach of the standard which must remain at the core of all that we do.

Due to this sound theoretical basis and evidence-led approach, we know that passive house occupants will benefit from good indoor air quality, good ther­mal comfort and the virtual elimination of fuel poverty.

This makes it all the sadder to see the success of the various lobby groups in their obfuscation of the truth, and success in getting environment minister Alan Kelly and housing minister Paudie Coffey to instruct the four Dublin local authorities not to introduce tougher building standards locally.

The only reason I'm involved with the Passive House Association of Ireland is a firm belief in the ability of the passive house standard to make a real contri­bution to the welfare of our citizens. When I bought my home, I simply trusted that the house was as it should be. Had I the knowledge of what a passive house is, and had the option to buy one, with­out doubt I would have been living in a passive house for the last 20 years.

As a group of industry professionals, we have a moral obligation to do all we can to ensure the passive house standard is adopted in Ireland, and that we must do all in our power to ensure that the right decisions are made on behalf of the citizens of Ireland, irrespective of the might of the lobbyists. The truth is on our side. Pi is and always will be 3.14159265...

Dr Shane Colclough is chairperson of the Passive House Association of Ireland.

Colclough, Dr S. (2015) ‘The passive house standard can't be diluted and here are the maths to prove it’, Passive House + , Issue 12 , pp. 22