What is a passive house & why should you want one?

A passive house is the ultimate eco build. It is built to a stringent set of standards to improve insulation and airtightness in order to reduce energy requirements by up to 90% so that a home does not require heating or air conditioning but is still comfortable to live in. The ideal passive home is also affordable and of course more sustainable. Who wouldn’t want that? 

Table of contents

What is a passive house? 

So, what is a passive house and what are the standards required?  According to enthusiast Hans-Jörn Eich, a passive house will have super insulation. It will aim to be completely airtight. With the right building materials in place to ensure insulation, like triple glazed windows, it will then use an efficient heat recovery system to circulate air, while warming it. Last but not least, it will need to be built facing the right way. 

Passive house insulation

A passive house needs the best insulation – ‘super’ insulation – to reach the required R-value. Insulation levels need to sit between R-40 & R-60 for walls, R-60 & R-90 for roofs, and R-30 & R-50 for footings and foundation. 

The passive house design needs to eliminate any thermal bridges. Thermal bridges are building materials that are good conductors, better than surrounding materials. Heat takes the easiest route out of the building, using the bridge. This can cause a cold spot in the interior which has the potential for dampness and mold issues. 

An example of a thermal bridge is a concrete slab that protrudes beyond an external wall for a verandah or balcony. The concrete will conduct heat better than surrounding materials like insulation in the walls. The solution is to build in thermal breaks which stop the material, in this case concrete, from being a bridge. 

Airtightness in passive houses

The fabric of a passive house needs to be airtight. One way to achieve this is super insulation with potentially very thick walls to meet the requirements. The requirements are different in every country. For example, in Sweden, the insulation on the outer walls needs to be a minimum of 33.5cm wide (Insulation Superstore). 

The specification for windows is also stringent. This can double the cost of windows for a passive house. U-values are a measure of thermal transmittance (or heat loss in other words). The recommended U-value for windows in a passive house is less than or equal to 0.15 or 0.8 W/m2/K. This is unlikely with standard windows in the United States, say Efficient Windows. The passive house architect might need to compromise with higher U-value windows and use more insulation to compensate. 

Heating in passive homes

Passive houses have to be built to the right orientation. Many of us have experienced living in a house or apartment that faces the sunlight and have felt the warmth of the sun streaming through the windows. Or we have lived in a perennially cold home because it is always in shade. A passive home takes advantage of the correct orientation to ensure the house is warmed in the winter and shaded in the summer. 

A heat recovery system is key to keeping a passive house warm or cool. The HRV or MHVR, as they are also called, uses a heat exchanger. This allows the cold air entering the house and the warm air leaving the house to pass each other, exchanging their heat without mixing. The now cold stale air leaves the building while the fresh new air is warmed and ready for circulating around the house. 

Heat recovery systems are well worth their installation cost, say BPC Ventilation. They improve air quality and reduce damp, condensation, mold and dust levels. They are however more efficiently installed into well insulated new builds as opposed to retrofitting to older homes. Older homes often don’t have good insulation or airtightness.

A passive house is designed to meet a minimum of 50% of its heat through passive solar gain.

Passive solar house

A passive solar house uses passive solar energy to heat the home (Williams College). Sunlight passes through an aperture (windows) and onto an absorber (flooring or a dark thermal storage wall) which absorbs the heat. A thermal mass underneath the absorber (sub-floor or less commonly water) holds the heat. 

Basic principle of passive solar house
passive solar house

The heat is distributed around the house via natural airflows and the goal is to reduce or eliminate the need for mechanical distribution via fans or pumps. A control, usually a shade vent or awning on the roof that juts out, controls the sunlight’s entry into the house on hot summer days when the sun is at its zenith. 

The thermal layer is cooled using ventilation overnight. Where cooling is required, ventilation at night is collected and then stopped during the day when temperatures rise. 

So now that we have covered what passive house standards are, what do they actually look like?

Passive house design

Is passive house design restricted by the standards required? What does it look like with all this insulation? The answer is, like any normal house!

In 1991 Dr Feist built the first passive house in Austria. This first house was white and looked very like an apartment block with south facing windows and balconies on all three floors and a shade vent running across the roof. 

Passive house designs differ depending on the homeowner. Some are design led and modern, others are ranch style with wrap around decking or balconies. This passive house under construction in Germany looks like any other home on the estate. 

Passive house
A passive house under construction in Germany

Can you build a passive house for yourself?  Yes, you can. Getting passive house certification is not required, it is optional. Of course, if your plan is to resell, having a certification may make it easier to sell. 

Passive house certification

Passive house certification is managed by the PassivHaus Trust in the UK and the Passive House Institute US (PHIUS). These organizations are the place to start if you are looking for a professional contractor to build a passive house for you. 

The Passive House Institute US is a non-profit organization that built the first passive house in the United States. They have certified more than 7,000 projects in over 42 states. PHIUS certification costs upwards of $1,500 and the price is dependent on the internal floor area. 

The PassivHaus Trust are affiliates to the International Passive House Association (iPHA). Over 1,300 homes in the UK have met the certification standards and globally 60,000 homes are passive homes. It costs around £1,500 to get a house certified by the PassivHaus Trust in the UK. 

A passive house is estimated to cost as much as 49% extra to build, according to the PassivHaus Trust in a 2019 report. However, they also say that it depends on the experience of the builder. Exeter City Council, who have years of experience, is building passive homes at 8% more than traditional construction costs. 

PHIUS estimates the costs to be 3%-5% more than standard construction and provide data on passive homes that were constructed for between $111 and $233 per square foot. The cost of passive houses for multi-family use are likely to be lower. 

Reasons to opt for a passive house

Starting a passive house project is not to be taken lightly. If the aim is certification, then the standards must be met. Choose your specialist carefully, both PHIUS and the PassivHaus Trust provide a database of options. 

A passive house may cost more to build, but there is a huge advantage to be had in savings on energy across the years and potential profit on resale if the house is certified. Additionally, there is the advantage of reducing the environmental impact of using fossil fuels to heat a home. 

Are you planning a passive house project?  Tell us about it.