building regulations and U-values, EWI, external wall insulation and U values

What is a Passive House?

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 is constructed in such a way that it is tightly sealed in a thermal envelope which allows very little heat to escape, so that essentially a room can be warmed simply by light from the sun, body heat or the heat from small electrical appliances. 

There are three things that a house needs to achieve before it can be considered a Passive House:

  • A Passive House needs to have primary energy demands (such as warm water, heating, house appliances) lower than 120 kWh/m2a
  • A Passive House needs to have heating demands lower than 15 kWh per square meter annually.
  • A Passive House must 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.

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.

Excellent indoor air quality is a characteristic that Passive Houses possess, and this is achieved through the mechanical ventilator exchanging the air from inside with the air from outside. 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.

External Wall Insulation on Passive Houses

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– which means absolutely no gaps. This is because gaps in the insulation break the thermal envelope and, especially in highly insulated houses, reduce the effectiveness of the insulation. Thermal Bridging can cause damp and condensation – which can be far more detrimental to a passive house than a regular house.

Which EWI System is Best for a Passive House?

To achieve Passive House U-values on a 215mm solid brick wall, 270mm of Mineral Wool would be required. 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, and then it’s compressed into dense, flat insulation boards. Rock is not a very good heat conductor, and the fibres from the spun rock provide air pockets, which is what makes it a great insulation material.

In keeping with the Passive House rules, 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, for a Passive House 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.

Wood Fibre Insulation for Passive Houses

Our most environmentally friendly insulation material is our Wood Fibre insulation, which we source from Pavatex. 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. Our Wood Fibre insulation is completely recyclable and holds the prestigious title of being NaturePlus certified. The fact that this insulation is so highly renewable and environmentally friendly means that really it’s ideal for Passive Houses where environmental consciousness is at the forefront of the building design. Passive Houses also require 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 could definitely play a part in helping to mimic the Passive House standards. Our sales team are experts on all of our products and will be able to give you any technical advice about their technical specifications and usage. Feel free to contact us for any questions regarding our materials or get yourself a free materials quote using our calculator!

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