What is a passive house based on and what role does its roof play?

No. The requirements concern the energy performance and comfort, not one method of masonry or type of covering. A house can have a different construction if the whole project meets the adopted criteria. The name of the material on the invoice is not confirmation that the building will achieve the expected standard.

Savings are sought first in reducing demand. Lower heat loss means a lower need to supply it. Heating appliances are selected for a house designed in this way. Installing an efficient heat source in a leaky building does not repair defects in its building envelope.

Every fold in the roof and wall increases the number of connections requiring attention. A dormer has not only a window but also side walls, corners and a junction with the roof slope. A simple roof usually offers fewer opportunities for errors and an easier run of insulation. This does not mean that more complex architecture is prohibited.

The project must be assessed as a whole. Large glazing can provide solar gains in winter but requires protection against overheating in summer. Enlarging windows without checking their orientation and shading is not a universal way to reduce bills. Comfort matters throughout the year.

The insulation layer should be continuous and have parameters resulting from calculations. PIR can be helpful where the depth of the assembly is limited. Above-rafter insulation reduces recurring interruptions caused by timber. In every solution, however, the eaves, ridge, chimneys and roof windows must be taken into account.

Do not compare a roof solely on the basis of the insulation lambda. Two assemblies with the same board can have different losses through the connections. In a building with low demand, weak details become proportionally more visible. It is therefore worth giving them as much attention as the thickness of material over a large area.

Airtightness limits uncontrolled flow through the building envelope. It also protects the assembly from moisture carried with the air. It is not the same as the absence of rain leaks. A roof can retain water and at the same time let warm air pass through the junction with the wall.

The airtight layer must be defined on the sections and run without interruptions. It is worth grouping service penetrations so that they are easier to seal. A pressure test carried out before the interior is completed allows faults to be found. The team’s mere conviction that everything has been carefully taped does not provide such control.

Air exchange should be provided by a planned ventilation system. It should not be based on gaps in walls and roof. Mechanical ventilation with heat recovery reduces losses associated with removing stale air. Its operation depends on the design, commissioning and subsequent maintenance.

The vapour permeability of materials is a different property from the ventilation of rooms. The flow of vapour through the assembly does not supply the required amount of fresh air. With PIR having a gas-tight facing, the moisture conditions of the layers must be designed. This is neither an automatic defect nor an exemption from analysis.

Cut or modular tile-effect metal roofing can cover a correctly designed warm roof. The low weight of the covering facilitates some structural solutions, but does not by itself deliver savings on heating. Thermal insulation and airtightness are located in other layers. It is their parameters that must be compared.

The installation plan on the roof is also important. The mountings for photovoltaic panels and the cable penetrations should be taken into account in advance. Drilling at random through a completed building envelope can impair its continuity. Generating energy on the roof does not justify leaks in the same place.

Before purchase, ask for the building calculations and a description of the details, not merely a list of modern materials. Separate the standard of the design from the confirmed result of execution. If certification is envisaged, establish its scope, documents and stage of verification. Do not assume that the name of the project alone guarantees it.

Include in the cost estimate the airtightness test, correct commissioning of the ventilation and documentation of the concealed works. These are the elements that help achieve the planned result. They are harder to show in a visualisation than the roof or windows, but they have a direct influence on the daily use of the house.

On the house drawing, draw two closed lines: that of thermal insulation and that of airtightness. Every place where they cannot be run without interruption requires a specific detail. This is a good way to assess the design before ordering materials.

The passive standard results from the interaction of the building envelope, windows and services. A metal roof and PIR insulation can support this goal if they form a correct assembly. Quality is determined by calculations, careful connections and verification of the execution.