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External Wall Insulation for Passive Houses?

Last modified: 13.09.2022
Illustration of a Passive House design showing components like thermal insulation, airtight construction, ventilation strategy, solar panels, triple-glazed windows, and thermal bridge reduction, all aimed at improving energy efficiency and indoor air quality.

Passive Houses and external wall insulation are two hot topics within the energy efficiency and insulation industries. Passive House is a construction concept that originally began in Germany; initially coined ‘passivhaus,' it is generally known as an energy efficient, environmentally friendly house that practically requires no heating. The building construction creates a thermal envelope which allows very little heat to escape. Essentially, a room can be warmed simply by sunlight, body heat or the heat from electrical appliances. 

Four factors affect the passive house status:

  • Primary energy demands (such as warm water, heating, house appliances) lower than 120 kWh/m2a
  • Heating demands lower than 15 kWh per square meter annually.
  • Pass a pressure test and the pressure must be limited to 50 Pascals with the pressure differential not exceeding 0.6 times a room’s volume per hour.
  • Thermal comfort must be met for all living areas during winter as well as in summer, with not more than 10 % of the hours in a given year over 25 °C.

A Passive House has a high level of insulation. That includes the doors, windows (triple glazed), walls, floors and roof. However, as we know, houses need proper ventilation – therefore Passive Houses also require a mechanical ventilation system.

The excellent air quality associated with passivhaus builds relies on mechanical ventilation systems. Warm air is exchanged for cooler air entering the house. These are great for being energy efficient, because during the process of heat exchange the ventilators can transfer the fresh air into the room at the same time as heating up/cooling down the fresh air. This means that Passive Houses are able to exist without the frequent or constant use of conventional heating systems, many only have one radiator in the bathroom for drying off towels.

So, as previously mentioned, Passive Houses require an outstanding amount of insulation; typically a Passive House wall will have a U-value of 0.15m/m2k. Luckily, external wall insulation would do the job perfectly if you get it at the right thickness. EPS, Mineral Wool and Wood Fibre have low enough U-values to be able to obtain the required level of insulation for a passive house.

However, the insulation needs to be totally sealed against thermal bridging. This is because gaps in the insulation break the thermal envelope. In highly insulated houses, thermal bridges reduce the effectiveness of the insulation, as heat actively finds avenue to escape outside. Thermal Bridging can cause damp and condensation - which can be far more detrimental to a passive house than a regular house.

270mm of Mineral Wool would be required to achieve Passive House U-values on a 215mm solid brick wall. Mineral Wool is highly breathable so it’s great for a Passive House construction where ventilation and air quality are key in order to prevent damp. Due to the fact that Mineral Wool is made of a renewable material (volcanic rock), it’s also in keeping with the ethos of the Passive House. Rockwool is made by spinning volcanic rock in a furnace at a high temperature to create a cotton candy-like texture. It is then compressed into dense, flat insulation boards. Rock is not a very good heat conductor, and the fibres from the spun rock provide air pockets, enhancing insulation capabilities.

The EWI-225 Premium Adhesive, designed for use in Rockwool systems, is also ideal because it is breathable, strong and waterproof.  

To finish off the system, you will want to use a Silicone Render. Again, this works really well in conjunction with the Rockwool system because of its breathability. Silicone Render is hydrophobic (water repelling) so moisture will essentially bounce off its surface, safeguarding the system against damp.

The most environmentally friendly insulation material is Wood Fibre insulation. To achieve Passive House U-values with Wood Fibre on a 215mm solid brick wall, you would need 240mm of insulation. Wood Fibre is a highly versatile insulation material,  offering excellent thermal performance. Wood Fibre insulation is completely recyclable and holds the prestigious title of being NaturePlus certified. Being highly renewable and environmentally friendly fits in with the ethos of Passive House builds. A high level of breathability, and Wood Fibre is the most recommended insulation material to achieve this.

Building a Passive House is a highly technical procedure, and external wall insulation plays a crucial part. Our sales team are on hand to answer any questions about our products. You can get yourself a free materials quote using our calculator!