PIR insulation
Designing a passive house around the site, the building envelope and the people who live there
Passive house design starts with the site and the layout of the rooms. These decisions guide the choice of insulation, windows and building services. A compact shape helps reduce heat loss, but it also has to suit the way people live. A bungalow, a two-storey house and a home with rooms in the roof each call for a different approach. All three need continuous insulation, airtight joints and protection against overheating. Check orientation, shade and vehicle access. Large south-facing windows can admit useful winter sun and need shading in summer. Neighbouring buildings and tall trees change the amount of sunlight that reaches the house. Use the conditions on your own plot when positioning the building.
In this article
What should you check on the plot first?
Map out how each room will actually be used. A bedroom has different needs from a workspace with several appliances giving off heat. Decide which windows will open and how the shading will operate. If street noise makes night ventilation impractical, allow for that when assessing summer comfort.
Plan utility connections and the positions of outdoor equipment too. Short, direct service runs simplify installation and maintenance. Coordinate the outdoor air intake and exhaust outlet with the windows and surroundings. Adding them after the facade is finished may require new penetrations through the insulation.
What changes with a single-storey layout?
A single-storey home connects all the rooms without stairs. For the same usable floor area, it may need a larger roof and ground floor slab than a compact two-storey house. The larger envelope affects material quantities and heat loss. Include both effects when comparing layouts.
In a house with an unheated loft, the ceiling below it can form the boundary of the heated space. Check whether insulating the roof slopes serves a purpose in that design. Show the insulation and airtight layers clearly on the drawings. Insulating both levels without planning how the loft will ventilate can make moisture harder to control and increase costs.
How can a two-storey house make the most of a compact shape?
Two storeys can reduce the envelope area per square metre of usable floor space. That is a good starting point for low heating demand. Setbacks, projecting bays and a complex roof can erode the advantage. Each extra corner needs its own structural, insulation and finishing details.
Pay particular attention to balconies, canopies and supports for external components. Where they join the heated building, they can form thermal bridges. A suitable structural thermal break or a separately supported structure may help. Assess the load capacity and thermal effect of the connection alongside ease of installation.
What should you allow for when planning rooms in the roof?
Rooms in the roof need enough headroom after the roof layers are installed. Over-rafter PIR insulation can help preserve space inside. Its low thermal conductivity is useful where the build-up depth is limited. Calculate the thermal performance of the complete roof, including the fasteners and the other layers.
Roof windows, dormers and junctions at the knee wall need careful detailing. They can be difficult to seal and shade. The lining and the rest of the assembly must meet the fire and acoustic requirements. Follow the installation instructions for the boards and roof covering specified for the project.
How do you coordinate the structure, insulation and services?
Draw one continuous airtight layer and show how it connects at the foundation, windows, floors and roof. Installers need to know where penetrations are permitted and how to seal them. A service cavity inside the airtight envelope can help keep wiring and pipes clear of this layer.
In a steel frame, heat flow through the steel sections needs particular attention. Sandwich panels can form the enclosure. Their declared performance describes the panel itself, whereas a corner or foundation junction needs a calculation for its actual geometry. Design the facing fixings, fire protection and load paths as well.
Which checks belong at the end of the design stage?
Assess heating demand and summer conditions for the same version of the design. Specify the actual window, insulation and ventilation performance used in the calculations. Check that the products can be obtained and installed on the site. Even a promising calculation is of limited use if the details are difficult to build.
Arrange for layers to be inspected before they are covered, and schedule an airtightness test while the connections can still be reached. In the cost estimate, separate the effect of changing the building shape from that of improving individual components. This shows where the money goes and helps you choose details that justify the work involved.