PassivHaus Explained

PassivHaus Explained

When developers build houses, they tend to build as cheaply as possible, as they do not have to pay for the running costs for years afterwards. As a self-builder, there are financial incentives to invest some money up front to save money in the long run. Given the likely increase in energy prices in the future, designing a building to have low energy bills, but at a slightly higher build cost, is usually a good idea if you plan to stay there for any length of time.

 

Any architect or energy consultant can help you design a building which has lower running costs than standard, it just involves more insulation! But how far do you go? What is feasible? These are the questions that some German physicists asked themselves in the late 1980’s when they were looking at the energy performances of houses. As proper scientists, they threw away their pre-conceptions about how houses are built, and what the market currently provides, and looked at how good they could get houses without going over the top in terms of cost. What came out of the process was a design philosophy and a set of performance criteria called PassivHaus.

 

The PassivHaus performance criteria are mainly to do with comfort (not overheating, no drafts, no condensation, pleasant air quality) but the hardest one to achieve is an energy efficiency requirement for space heating. A Passivhaus can require no more than 15 kWh of heating per square metre per year to retain a pleasant 20oC internal temperature. For a typical 2-bed home of 120m2, this equates to a heating bill of £80 per year (with an efficient gas boiler). Of course, hot water is on top of this, but it is an impressively low figure!

 

The Passivhaus design philosophy is to consider the function of a building before the aesthetics. This is, unfortunately, the opposite of what normally happens in house building, and it’s a symptom of the mind-set of the scientists who developed Passivhaus. A building (orientation, shape, window sizes, window locations, etc) is designed to make best use of the available free energy from the sun (without getting too hot!), and then made to look good.

 

The impacts that aiming for the Passivhaus standards can have are:

-          Orientation south

-          Southern bias on the windows (quite small windows on the north face)

-          ‘squarish’ house shape

-          Good insulation (quite thick walls)

-          No (or very few) dormers or roof lights

-          Triple glazed windows

-          Overhang shading on the southern windows

-          An air tight build to avoid drafts

-          Mechanical ventilation with heat recycling

 

The other way in which Passivhaus differs from normal house building is its quality assurance. Products need to go through rigorous tests before being included in Passivhaus, and the building is tested in several ways after construction to ensure that the builders have done their job well. Additionally, hundreds of Passivhaus’s across Europe have been monitored for many years, and the data show that the 15 kWh m-2 yr-1 is achieved or bettered in reality in the vast majority of cases.

 

People building Passivhaus’s often go beyond the energy efficiency criterium and add solar PV and/or solar thermal to reduce their energy use even further.

 

What about cost? The additional cost of going to Passivhaus depends strongly on how early you make the decision to go down that route, and how flexible you are.  It can be no more expensive to build to Passivhaus standards than an average Building Regs home if the physics is allowed to take full precedence over the design. Designing it to suit both your needs and any design parameters required by planning authorities can potentially add cost. 10% is a figure often quoted, but in reality, each project is so different that it’s impossible to determine a good estimate. Either way, the additional costs are offset by reductions in running costs.

 

Regardless of cost and flexibility, if you are interested in Passivhaus, the sooner you get an expert into the design team the better! If they are not in from the very start, then build costs almost automatically increase. There are Passivhaus-qualified architects, and Passivhaus-qualified design consultants who can help advise architects.

 

Information provided by Ecofirst Consult, visit their website here for further information.

Ecofirst Consult

Armitage House, Victor Jackson Avenue, Poundbury, Dorchester, Dorset, DT1 3GY

E: info@ecofirstconsult.co.uk T: 01305 344 011

 

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