Passive House and the waterbed effect

What is Passive House?

This is a building standard for new and refurbished buildings.  They are airtight and watertight, free of draughts and damp.  Passive House buildings require up to 90% less energy to heat and light than conventional ones.

This results in buildings that are healthier for those living and working in them.  They are comfortable, with constant, even temperatures.  Air is fresh and warm.  There is no damp or mould and humidity levels are low. They cost much less to run.

The second passive house home I visited had been renovated by a young couple.  They regretted installing central heating, as they'd only had the system running for 2 hours in the previous 18 months.  Effectively the home could be heated by a couple of electric towel rails.  Their tenants were delighted to avoid problems with dry skin and rollercoaster variations in temperature of standard heating systems.  The mechanical ventilation and heat recovery system kept the air fresh and warm by heating incoming fresh air and removing cooking and bathroom moist air.

How is it done?

The key to reaching passive house standards is to put time and effort into the plans.  The essential tool used in the process is the PHPP package.  This is based on an Excel spreadsheet.

Data from the design of the new build or retrofit is entered into the PHPP spreadsheet.  These calculations enable designers to estimate whether the build will meet the standards (for Passive House or EnerPHit for older buildings being refurbished:

Energy performance targets and air changes per hour:

Criteria                                                                                          Passive House       EnerPHit                            

Specific Heating Demand                                                             ≤ 15 kWh/m². yr          ≤ 25 kWh/m².yr

Specific Cooling Demand                                                             ≤ 15 kWh/m². yr         ≤ 120 kWh/m².yr 
                                                                                     * PE ≤ 120 kWh/m².yr + ((SHD - 15 kWh/ m².yr) x1.2)
Specific Heating Load                                                                  ≤ 10 W/m²

Specific Primary Energy Demand                                                ≤ 120 kWh/m². Yr

Air Changes Per Hour                                                                  ≤ 0.6 @ n50

Limiting Value                                                                              n₅₀  ≤0.6^-1                  n₅₀  ≤1.0^-1

Calculations include the Form Factor, derived from the external surface area and the Treated Floor Area.  The size and shape of the building affect the energy demand of the building and the amount of insulation required.  Data is included from the U values (thermal efficiency) of all construction material including windows.  Thermal bridges, the junctions and links between parts of the building, are potential areas for the greatest loss of heat.  These must be reduced or eliminated as far as possible and data is added to the spreadsheet to demonstrate this.

Airtightness is key to effective thermal insultation and airtightness tests are usually carried out several times during the build to ensure that detailed finish is done to a high standard.  Simple mistakes, such as poorly applied insulation tape, may negatively affect airtightness.  Summer temperatures are included in the spreadsheet and methods to prevent overheating, such as shading.  Data for mechanical ventilation and heat recovery is included.  Primary energy appliances are also calculated for heating, ventilation, hot water and cooking.  The orientation of the building and its geographical location are also included in the calculations.  Solar gain may reduce winter energy use, but may require additional shading to prevent summer overheating.  Windier, colder areas at higher altitude may face bigger challenges than buildings located in sheltered, mild valleys.

What has PHPP software got to do with a waterbed?

The spreadsheet provides a framework for the designer to use, against which everything is measured. Rather than guessing and producing a more or less well insulated house, PHPP channels the work into a disciplined approach, which achieves a specific result.

On paper, building designs look straightforward.  In practice, building takes place in messy, complex situations.  Typically people create problems or require certain amendments to the plan.  A house may lie in a conservation area, which prohibits use of modern, highly efficient, single expanse, triple glazed window units.  The local authority may insist that the facade matches its neighbours, increasing the heat loss from one part of the building envelope significantly.  PHPP helps the designer increase insulation in other parts of the building to compensate for heat loss at the front.

The customer may insist on a particular design element that includes a thermal bridge, which may be difficult to reduce or eliminate.

The client may refuse to spend as much money as the designer suggests on insulation, focussing instead on state of the art facilities in bathroom and kitchen.

The builder may not be able to source material of the required U value for a part of the build, which may mean that PHPP has to be recalculated to make up lost ground elsewhere, if the project is to be complete on time and to budget.

Push down the energy gains in one area, then lift up insulation/reduce the energy loss in others.  PHPP enables a dynamic design process with constant iterations throughout the project.

Why bother?

The road to hell is paved with good intentions, and, as the above link indicates, politicians may rush into new schemes that look as if they will solve all of our housing and energy problems at a stroke.  I see new and renovated buildings that are completed without using a rigorous and dynamic standard: medical centres with noisy reception areas because of 100% hard surfaces, where confidential information is heard by all; public buildings which are too hot in Summer and too cold in Winter; heating vents on outside walls that pump valuable heat into the street.

PHPP and the passive house standard enables designers to achieve a level of energy efficiency and comfort in buildings that will have a long life and fewer problems than those of conventional design.  Whether they meet the standard or not, the discipline helps ensure a quality finish and customer satisfaction.  Bob Prewett's 80% house, above, didn't meet the standard, but achieved a comfortable and energy efficient level, which fitted well with the rest of the street.

PHPP is not an easy or straightforward tool to use.  A new book is designed to assist the designer in getting to grips with the process.  PHPP is cost effective and can be used when standards change or a designer decides to include heat recovery from water and other Passive House Plus elements.

I wouldn't employ an architect or designer who DIDN'T use PHPP, even if their work didn't fully hit the target.