Structural designers guide
The passive house concept sets quality requirements for the design and features of structures and building systems. The objective of energy design of a house is to achieve a small need of energy and power. As a result, the design of the energy solution must be able to rely on the objectives set for the solutions. In structural design, the total heat transfer coefficient must be defined using calculations. Often, it is useful to use numerical calculation tools because the heat bridges of structures must be taken into account.
The habitability of a building consists of the quality of the building’s indoor facilities, microclimate, appearance and outdoor areas. Heat insulation, air tightness and ventilation have an impact on thermal habitability. The lack of draughts and warm inner surfaces are the basic features of an energy efficient house. The air tightness of a building’s outer shell is based on proper and solid air seals. Wind seals prevent the access of cold draughts into the insulation layer.
In a standard building the low inner surface temperatures of the of outer shell structures and windows causes the feeling of cold draught. This is compensated for using comfort heating, in which case the room temperature is higher than required. The inner surfaces of the outer shell structures in a passive house are warm. As a result, good thermal habitability can be achieved using lower room temperature levels. The room temperature objective in the design process is generally 20-21°C.
U-values of Passive House structures
A passive house can be built using different structural systems (Figures). However, the small heat need requires a heat insulation level significantly better than normal. Indicative objective values for the total heat transfer coefficient and features in the outer shell are given below.
- Exterior wall 0.07–0.1 W/m2K
- Base floor 0.08–0.1 W/m2K
- Roof 0.06–0.09 W/m2K
- Window 0.7–0.9 W/m2K
- Solid window 0.6–0.8 W/m2K
- Entrance door 0.4-0.7 W/m2K
Figure: Indicative structural solutions in a wood-structure passive house. The insulation thickness and U-value required for the exterior wall, base floor and roof depend on the total heat transfer coefficients of windows and entrance doors and the annual utilisation rate of ventilation heat intake.
Figure: Structural solutions in a German stone-structure passive house (photos Fraunhofer Institut für Bauphysik).
Frost insulation of foundations
Keeping foundations unfrozen is usually based on frost insulation in foundations and heat loss in the ground-supported base floor. Thermal insulation in the base floor of a passive house is so good that heat loss in the base floor does not help in frost protection. The frost risk of the construction site must be identified using soil studies, and frost insulation of foundations must be measured to correspond to the risk.
Cold bridges
The design of connections of building elements, such as the base floor and roof, exterior wall and inlets in corners, windows and doors must be aimed at reducing the effect of cold bridges. When the thermal insulation thicknesses of structures are great, the moisture behaviour in structures and the effects of cold bridges must be taken into account. The relative impact of a cold bridge on the heat loss of a structure increases as thermal insulation of the structure increases. The significance of cold bridges may be critical for the functionality of a building.
The size of windows in frame-structure houses should be suitable for the dimensions of the building’s frame structures. As a result, excessive frame structures can be narrowed down. Inner cladding in window and door connections can be supported by frames with narrower dimensions installed onto the side of the supporting frame.
Air tightness
The air tightness of the outer shell in a passive house has a limit value which must be verified through measurements. The maximum air leakage figure can be n50 = 0.6 1/h. The air leakage figure of Finnish buildings is typically 1.5-5. Air tightness has an influence on the heating energy need and the moisture-technical functionality of structures. When the air leakage figure is small, the building’s location and wind conditions in the building’s surroundings will have no major impact on the heating energy need in the building. In addition, the landscape can be utilised more freely in placing the building on the site.
The following factors have an impact on the functionality of the structure of a good air seal layer:
- The air seal layer is solid and its maximum air permeability, including all seams, is 1 x 10-6 m3/m2 s Pa.
- A plastic foil inside the heat insulation acts as an air seal if its seams are sealed.
- The air seal must be solid throughout the outer shell. Connections of different inlets must particularly be sealed thoroughly.
- The air seal and vapour barrier layer can be located at a maximum depth of 50 mm from the inner surface of the thermal insulation layer. The use of installation facilities inside the air seal and vapour barrier is recommended if electrical fittings are to be installed on exterior walls.
- The seams of window and door connections should be heat insulated and sealed on the inner and outer surfaces. Connecting solid windows directly to frame structures will reduce the connection’s cold bridge effect.
- Ventilation ducts must be located on the inside of the air seal. Only the air supply and extract air ducts access through the air seal.
- All HVAC inlets must be sealed thoroughly.
- Jointed concrete elements, sheet structures with sealed joints, masonry and coated inside walls and other such structures act as air seals, provided that the inlets are sealed.
Figure: When the air leakage figure of a building’s outer shell reduces, the building’s location and the wind conditions will have no great impact on the heating energy need (Pallari et al. VTT 1995).
Wind protection
A wind barrier protects the thermal insulation layer against cold outdoor air flows. The wind barrier must also be intact throughout the outer shell. Connections, inlets and different detail structures must be designed and implemented thoroughly. In principle, all sheet-like heat insulation and blown or sprayed insulation require a wind barrier and it can be one of the following:
- A material layer, the air permeability of which is a maximum of 10 x 10-6 m3/m2 s Pa including the seams
- A fibre board, gypsum board or other sheet structure, the seams of which are sealed
- Plastering with sealed connections on top of the thermal insulation layer
- Mineral wool insulation coated using air-tight cladding with sealed seams
- The moisture-technical functionality of well-insulated structures requires a vapour barrier in the structures. The minimum level of the vapour resistance of the material layer inside the insulation shall be five-times more resistant compared to the material layer outside the insulation. The majority of moisture damage in wood-structure exterior walls is caused by rain water accessing the structures. A rainproof exterior face must be secured using detail solutions.
Checklist for designer
A good design and implementation of a passive house’s outer shell are based on the following factors:
- Design cooperation to identify and solve structural problems
- Area requirements for building system routes
- Prevention of cold bridges through design measures in cooperation with the architect
- Optimisation of structural solutions in supporting structures so that the number of frame structures is the lowest possible
- The use of module dimensions as the starting point of design
- Air seal design
- Sealed inlets
- Wind barrier design
- Consideration of the work order in structural design
- Consideration of the drying capacity of structures in structural design Avoiding double vapour barriers