Installation Position in the Wall Cross-Section
The position of the window frame within the wall cross-section has a direct effect on thermal bridge formation at the perimeter junction. In conventional installation, the frame is placed centrally in the reveal or close to the interior face. In passive house construction, the window is typically moved toward the exterior face and placed at or within the insulation layer.
This approach — described in Passivhaus Institut documentation and commonly referenced as "box installation" or "insulation-level mounting" — minimises the uninsulated reveal area exposed to the exterior. The thermal bridge correction value (Ψinstallation) for this type of connection can reach values close to zero or even slightly negative when the frame overlaps the insulation layer sufficiently.
Moving the window frame into the thermal envelope reduces perimeter thermal bridges and lowers effective Uw of the installed assembly compared to a centrally-placed identical window.
Structural Fixing Methods
Windows in Poland must be fixed according to the frame manufacturer's specifications and comply with the structural requirements of PN-EN 14351-1. For masonry and block construction — the dominant residential construction method in Poland — the standard fixing approaches are:
- Frame plugs and screws through the frame — standard for standard-weight masonry, practical but creates localised thermal bridges at each fixing point
- Hammer-set anchors in the reveal — allows installation without drilling through the frame, reducing direct conductive paths
- Bonded connection (adhesive angle brackets) — used in passive house construction; brackets are fixed to the exterior face of the masonry and to the frame, allowing the frame to be positioned within the insulation zone without penetrating the wall
- Structural foam bonding — used for lightweight frame situations or supplementary fixing
For heavy timber frame construction (increasingly used in Poland for energy-efficient single-family homes), the fixing is typically into solid timber elements using lag screws, with insulation closing the perimeter.
The Three-Layer Joint: RAL Principle
The RAL guideline for window installation (RAL GZ 695, developed in Germany and widely referenced in Polish construction practice) defines the connection between frame and wall as a three-zone system:
| Zone | Position | Function | Typical material |
|---|---|---|---|
| Inner layer | Interior side | Airtight, vapour-retarding | Pre-compressed tape or vapour-control film |
| Middle layer | Core of joint | Thermal insulation and gap filling | Low-expanding PU foam, mineral wool rod |
| Outer layer | Exterior side | Weathertight, vapour-open | Pre-compressed expanding tape (EPDM or foam) |
The principle — interior layer more vapour-resistant than exterior layer — prevents moisture accumulation within the joint. Failure to follow this principle is a common source of condensation and mould at window reveals in Polish renovated buildings.
Airtightness Requirements
Passive house standards require an airtightness level of n50 ≤ 0.6 h⁻¹ (air changes per hour at 50 Pa pressure difference), as defined by the PHI criteria. The window-to-wall connection is one of the most critical junctions for achieving this target. Standard PU foam alone is insufficient to provide reliable long-term airtightness.
Interior airtightness membranes or pre-applied adhesive tapes are used to create a continuous airtight layer from the interior window frame surface across the reveal and connecting to the wall interior vapour control layer. This connection must accommodate thermal movement of the frame (PVC frames in particular can move several millimetres over a temperature range) without tearing or detaching.
Common materials for airtight connection in Polish practice:
- Pre-compressed expanding tapes (EPDM or foam-based) with defined vapour resistance
- Adhesive joint tapes (single or double-sided) rated for adhesion to UPVC, wood and concrete
- Reinforced plaster over mineral wool insulation for reveal finishing combined with tape at the frame face
- Proprietary liquid-applied membranes at complex geometry junctions (e.g., rounded or bay window reveals)
Reveal Insulation
The window reveal — the visible interior surface of the wall opening between the frame and the room surface — is a significant thermal bridge location in most standard Polish construction. With a 36 cm aerated concrete (ytong-type) block wall, the reveal depth typically runs 15–20 cm without insulation applied to it.
In passive construction, the reveal is insulated using one of two approaches:
- Rigid insulation board (EPS or XPS, 20–40 mm) applied to the reveal surface before plastering — reduces thermal bridge but requires careful connection to the frame
- Frame overlap into the insulation — when the exterior insulation layer wraps past the frame edge, it covers part or all of the reveal, eliminating the cold bridge at that location
Handling and Protection During Construction
Windows installed at the passive house stage are typically in place before rendering, roofing completion or interior finishing. During this period, several precautions apply:
- Glass surfaces must be protected from mortar and adhesive contamination — hard-set mortar on low-E coated glass can cause permanent damage
- Temporary protective film on the frame should be removed within the manufacturer's specified time to prevent adhesive residue bonding under UV exposure
- The joint must not be compressed by formwork or scaffold planks before foam or tape has fully cured
- Drainage channels in the frame sill section must remain clear during construction — blocked drainage causes water accumulation in the frame profile
Quality Control and Documentation
For buildings seeking Passivhaus certification in Poland, the PHI requires photographic documentation of window installation details at each junction type. Blower door testing is mandatory before completion to verify n50 target achievement. If the target is missed, the installation junctions are the first area for inspection using pressure-differential combined with smoke pencil or infrared thermography.
For standard new construction under WT 2021, an energy performance certificate (świadectwo charakterystyki energetycznej) is required. The U-value of windows entered into the energy calculation must correspond to the installed product's declared performance, supported by the CE declaration of performance supplied by the manufacturer.
References: Passive House Institute · RAL GZ 695 — Window Installation Quality Guideline · PN-EN 14351-1:2006+A2:2016