Prefabricated Passivhaus – is it possible to have Passive Houses manufactured in a Factory?

Perhaps not everybody knows that a Passive House is more than just a low-energy building.
Many think that Passive Houses are very high tech contructions implementing complex technical installations and using renewable Energy instead of gas or grid electricity.

Well, if you think so… you couldn’t be more wrong.

The term “Passivhaus” was first used by Germans in the 70s to describe a building that can keep up its temperature by using almost no technical installation at all, other than a ventilation system which primary use is to ensure a continuous supply of clean and fresh air.
So there is no “high-tech” in Passivhaus, it’s instead a rather simple and dated concept.

The Short Answer

The short answer to the question in the title is: “yes, we can deliver a building envleope that meets requirements set by the Passivhaus Standard, even though it is very demanding“.
But… are you sure you are asking the right question?

A Passivhaus is not only about the building envelope… so, if you give too much attention to that, you are surely missing something.
Moreover, if you think that you will get a Passivhaus by assembling together components that are specificaly designed to be used in a Passivhaus… you have it all wrong. Perhaps, you have to rethink your approcach and learn a bit more about the Passivhaus Standard.

Simplicity is the key

The Germans established precise guidelines on how to develope such buildings and, in the course of three decades, these guidelines became more and more strict and effective, giving birth to what is today known as “Passivhaus Standard”.
The Passivhaus Standard is a sort of Building Code that regulates how a Passivhaus should be planned, designed and built. Today the Passivhaus Standard is the most strict building Guildeline on our Planet.

From a conceptual standpoint you should understand that the Passivhaus Standard has been developed to allow systematic achievement of the following goals:

1) minimize heat loss
2) maximize solar gains
3) optimize building’s arecitecture to “trap” energy in winter and to project protective shade in summer

While the Passivhaus Standard tries to use as less Energy as possible, its primary objective was to deliver a better indoor experience for the inhabitants of the house. It was first noticed that keeping all the internal surfaces at high temperature (by minimizing heat losses and thermal bridges) the indoor environment becomes much more user-friendly: no more cold from the windows, no more breeze inside the house, no more condensation or molds.

Today indoor comfort and Energy performance are the basis of many Standards and have been adopted by several Building Codes. However, what makes the Passivhaus Standard still stand out, is its German-like strictness on some key indicators that make very hard, even today, to build a real – certifiable – Passivhaus.

What gives a Passivhaus

According the Passivhaus Standard, a Passivhaus works thanks to the perfect Energy balance of its building envelope and its small (but highly efficient) technical installations.
This balance is so perfect that the building has to be designed from the beginning as a single entity otherwise the excellent indoor climate can turn very wrong (overheating or too cold temps).

This results in a very important fact: no one can deliver a Passivhaus if the building was not designed as a Passivhaus in the first place.
In other words: delivering a building that meets all the criteria of the Passivhaus Standard, does not automatically entitles the thing to the Passivhaus heaven… it very much depends on other conditions as well, such as:

1) orientation of the building and overall solar gains
2) layout, efficiency, quality of build of the ventilation system
3) care and accuracy in the implementation of the whole construction

So, as you can see, it is much more complicated that ordering a house on a catalogue.
Summarizing, the key indicators addressed by the Passivhaus Standard are the following:

a) U-value limit for building envelope (walls, roof, foundation, windows)
b) minimize thermal bridges
c) guarantee air-tightness n50<0.6 1/h
d) implement a balanced mechaninc ventilation system with overal heat recovery efficiency >75% and very low acoustic emission (only certified VCM units are allowed in principle)
e) guarantee that the Energy balance works theoretically on paper (simulation through PHPP – Passive House Planning Package) and respects the maximum Specific Heat Demand of 15 kWh/m2a
f) guarantee that maximum consumption of Primary Energy is below given limit

All these points are conditions “sine qua non“: you wont get a Passivhaus Certificate for your building if you fail to meet one of the items on that checklist!

Now, while points a,b anc c are strictly related to the building envelope, points d, e and f involve also technical installations… and here you should get once and for all that a Producer of Prefab Houses – no matter how good he is – cannot help you to solve all the issues related to the construction of a Passivhaus.

The bottleneck

Having very clear guidelines, the Passivhaus Standard makes possible an accurate planning of all construction details, of all Energy matters of the building, of all the implementation aspects. One would say that everything can be planned so nothing can go wrong.
Well, reality is a little more complicated than that, especially when it comes to prefab buildings.

Yes, it sould be easier to get everything right in a controlled environment as a Factory… and as a matter of fact it is!
We can get walls’ U-values right, windows installed at perfection, optimize construction details, optimized cut of wood that reduces waste and environmental impact… and the list goes on and on…

But there is one thing we cannot do in Factory: get the whole building air-tight.

As a matter of fact  n50<0.6 1/h is quite a tight number. For reference you must know that in Norway the limit is 2.5 (according TEK10), in UK is about 4.0 and in many other Countries in EU is not even set!

Now, while it is relatively easy to guarantee a “perfect” air-tightness when the whole building is assembled on site, when delivering dozens of prefab elements we have the problem that all of those parts have to be joined together with an air-tight connection!
So, in this regard, the prefab construction makes things more complicated.

Unfortunately there is no way to say beforehand what is gonna be the air-tightness of the building once closed up in its final shape. It all relies on the experience of the Producer and on its his track of records on air-tightness. On this matter the know-how of the Producer does really make the difference.

Apples and Oranges

Choosing a Producer that understands your needs and has a proven record of success in what you are trying to accomplish, is moving a step closer to the solution of your problems.
However, you probably should sort out first if you want to buy Apples or Oranges.

In fact many Clients come to us asking for a Passivhaus then, when things start to get too technical, they usually realize that they just want a very low-Energy building and not a Passivhaus.
The term is so popular these days that everyone get sucked in by its magic.

As a matter of fact a nearly zero-Energy building can be more cost effective than a Passivhaus and can give you much less headaches.
However, we are not here to tell you if you shall buy apples or oranges BUT- be informed – when you come asking for oranges your preferred Producer should be able to present you the alternative of apples and to give you all the elements to support your choice.

When that doesn’t happen… well, you better change Producer