Windows are essential for the energy demand of a building. If not properly installed, it results in high thermal bridges, reducing the overall performance of the building's insulation and leading to unnecessary heat loss.

By properly accounting for and managing thermal bridges in the planning process and during window installations, we can:

• Improve Energy Efficiency: Efficiently installed windows with reduced thermal bridges can significantly cut energy consumption and operating costs.
• Enhance Comfort: Reducing thermal bridges contributes to achieving higher average surface temperatures and avoidance of local discomfort
• Thermal protection: Thermal bridges can lead to condensation and mould growth, which can damage the building structure. Proper installation can prevent that.

A general introduction to the detailing and calculation of window installations can be found here (German).
To make a realistic assessment and consider the thermal losses in the energy balance, the following procedures are recommended:

• Lookup window installation values of similar installation situations, or
• Simulation / calculation of the specific installation situation

In cases where the actual installation resembles the recommendations of documented window installation thermal bridges, these figures can be used for energy calculations. Therefore, a recommended first step would be to look up the installation Psi-values of certified frames, as shown below:

Each window certificate contains at least three installation situations, and in the case of window systems up to ten installation situationss that achieve the climate-specific requirements. In addition to that, custom situations can be requested by the manufacturer. The certificates contain installation drawings with the corresponding thermal brides for the sill, head and jamb sections within three different wall assemblies See example in Component Database

Wall systems and construction systems also include installation Psi-values. In cases where a specific building system is used, the values from the building system documents can be checked and used, as long as they match the reality. Details can be found in the connection details in the certificate. See all construction system in Component Database.

Furthermore, the window-independent calculation based on the installation detail can be viewed in the database.

The respective installation elements are taken into account and various components are determined using 2D and 3D-simulations. In particular, pre-wall mounting systems from various manufacturers can be found here.

Dummy windows for each climate zone were developed for this purpose, which represent the main window materials (vinyl windows, wooden windows and wood-aluminum constructions, integrated windows and aluminum windows) For more details on the dummy windows specific to each climate zone, the inputs and models can be found in the criteria for the certification of components here on page 5.

The U-values used for these dummy windows are included in the table below.

Component Database: Passive House certified roller shutters and external venetian blinds

Installation thermal bridges in connection with sun protection systems require special consideration. A wide variety of certified systems can be found on the component database.

In case no comparable thermal bridge calculations or simulations are available, the heat loss value can be determined by conducting a finite element method (FEM) simulation with suitable software. In case no frame model is available, the methodology of frame modelling in accordance to the ISO 10077 part 1 and 2 is required. Fittings and hardware can be considered with replacement models. Documents to be submitted for calculation of window profiles and window installation situations for the Passive House building certification

The installation situation itself should reflect the actual position of the frame, considering the installation joint, as well as insulation and façade covering main frame, as well at least an indication of drainage and sill, and a realistic assembly (e.g. insulation should not cover the sash or IGU, sill section should not be impaired regarding drainage). Small mounting elements like small window brackets can usually be neglected. Material conductivities and assignments should be in line with Table D.1 of the ISO EN 10077-2 , the values tabulated in ISO 10456, or technical approvals from a recognized national body, ETA or comparable.

The steps to carry out this calculation are described here: Documents to be submitted for calculation of window profiles and window installation situations for the Passive House building certification

A suitable approach for window modelling and to determine the thermal characteristics of a frame with the software THERM can be found here: B.C. Reference Predecure for using THERM to Determine Window Performance Values for Use with Passive House Planning Package

In case the simulation contains consideration of shading elements, the brackets and mounting elements should be considered via a 3D FEM Simulation or throughout a simplified 2D FEM Simulation of the brackets (only applicable for non-metallic boxes). The following equation can be used to determine the effective heat flow:
Q₁ = Heat flow through section without bracket
Q₂ = Heat flow through section with bracket
w = width of ALL brackets needed combined
b = width of the window
Q = (1,5 x w x Q2 + (b – 1,5 x w) x Q1)/b
The psi value (installation header) can be calculated from the heat flow Q.

Thermal bridge calculation must always match PHPP! Recommendation for 3 different situations are shown here:

A) Window installation (common)
With an installation gap of 1 cm or smaller the shell dimension (clear rough opening) can be used in PHPP instead of the outer edge of the frame.

B) Frame extension (often on thresholds) or support profile
Doors often require a support profile of insulation materials with compressive strength, mounting elements or brackets. The dimension reference should be the lower edge of the threshold profile.

C)Roller shutters/Venetian blinds etc. like A)

For building certification it is not a valid option to use a replacement model representing the UW-value of the window; the installation values would be dependent on the performance of the insulated glass unit (IGU). Frame values tend to have much higher heat transmission coefficients compared to the glazing properties and might lead to an underestimation of thermal bridges.

Substitute models representing the Uf-value that match the thickness of the frame can be a suitable option for EIFS, cavity walls or ventilated facades. This should only be used where the window is installed in line with the insulation layer.

A valid procedure to use substitute models representing the frame is described here: The simplified model depicts the frame as a panel which varies in terms of its materials and Uf value. Three different methods for various frame types are outlined in Figure 1. In this figure, aluminum shells with excellent thermal conductivity are shown as a separate layer. When calculating the equivalent thermal conductivity, the aluminum layers can be left out. This procedure is applicable for PH suitable frames installed in front of the load bearing strucuture of the wall and should not be used in combination with shutter systems.

This method provides a practical and fast way of calculating thermal bridges. The installation of various frame types was investigated by performing comparative calculations using a simplified model and a model with a precise frame geometry. The results obtained for aluminum frames or frames with aluminum shells match those obtained using the precise calculation method, while the deviations in the calculations made for timber and PVC frames also have some measurement uncertainty (around 0.005 W/(mK)). Nevertheless, it seems safe to say that the simplified method can achieve greater accuracy. Using the simplified thermal bridge calculation model offers a straightforward means of better determining energy balances according to PHPP, even during the early stages of the planning process. The model should, however, always be created with building physics in mind. Precise installation values for certified frames can be found on the certificates.

For further information see: Simplified calculation for window installations

Another option is using already simulated values from manufacturers, suppliers and institutes etc. as long as they are calculated based on EN ISO 10211.

• Window Installation Thermal Bridges Catalogue

• Timber construction thermal bridge catalogue and tool

Manufacturers of certified PH components are invited to send us a link to their catalogues or tools

• Wiegand Fensterbau GmbH

Thermal bridge calculation

Building envelope

Certified Passive House Components

Building Certification