Proper installation of window joinery on the base profile should include three essential stages:
- mechanical fixing and transfer of loads,
- installation of thermal insulation,
- sealing the joint and providing waterproofing.
Each of these stages affects the durability, airtightness and stability of the entire assembly.
1. Mechanical installation
Start by cutting the base profile beams to the required dimensions. If the base profile consists of several elements, their joints should be thoroughly glued and sealed. There must be no gaps between individual beams that could allow air, water or moisture to penetrate.
Next, apply suitable adhesive to the surface where the base profile meets the window frame. Bond the base profile to the frame and then additionally secure it mechanically with screws.
An exception are some aluminum thresholds for HS lift-and-slide doors, where the manufacturer forbids making holes. In such cases the method of attaching the base profile should follow the threshold system manufacturer’s instructions.
After preparing the complete assembly you can place it in the installation opening.
The window should be levelled on support blocks designed to carry high loads. Supports should be located at points where loads from the window are transferred to the building structure, in accordance with proper installation practice and RAL guidelines.
Once levelled, mechanically fasten the assembly to the substrate. Use mounting brackets matched to the height of the base profile and the expected loads to stabilise it.
The brackets protect the base profile from shifting or overturning, especially during internal screed work. The taller the base profile, the more important the correct choice of length, stiffness and number of fasteners becomes.
Simply bonding the base profile or filling the space with assembly foam cannot replace proper support and mechanical fixing.
2. Thermal insulation
After fixing the window, fill the space under the base profile and at its sides with insulating material.
Insulation must be continuous, without voids or gaps. The insulating layer under the window should connect with the insulation at the sides and above the frame, forming an unbroken perimeter.
Lack of continuity in the insulation can lead to:
- formation of thermal bridges,
- cooling of the lower part of the frame,
- condensation of water vapor,
- moisture in the joint,
- deterioration of the thermal performance of the entire installation.
The insulating material should be selected according to the gap width, the type of substrate and the conditions in which the joint will operate.
3. Sealing the joint and applying waterproofing
The lower connection between the window and the building must be protected against:
- capillary rise of water,
- rainwater,
- standing water,
- construction moisture from screeds and other wet works,
- penetration of water vapor from the room side.
Bitumen felt is often used on foundation benches to protect walls against capillary moisture rise. At the window installation area, however, it is recommended to properly prepare the substrate and apply a tight, rigid mass waterproofing.
Why might bitumen felt need to be removed at the installation area?
The bitumen felt under a wall is ultimately pressed down by the weight of the masonry. This pressure reduces the chance of water seeping beneath the waterproofing layer.
For window assemblies the situation is different. The window is supported point-wise on support blocks, so the bitumen felt is not evenly pressed to the substrate over the whole area. In spaces between supports there is a risk of water seeping underneath.
Placing support blocks directly on bitumen felt can also cause:
- local crushing of the felt layer,
- loss of support stability,
- progressive settling of the assembly,
- changes in the window level,
- excessive loading of the frame or threshold.
For this reason the substrate under the support blocks should be hard, stable and resistant to long-term loading.
External waterproofing
The lower part of the joint is particularly exposed to rainwater, standing water and building movements relative to the substrate. The protection should therefore remain watertight even with small structural movements.
Use materials designed for such conditions for external waterproofing, for example:
- EPDM membrane in tape form,
- flexible waterproofing known as “liquid EPDM”,
- other system materials approved by the manufacturer for contact with standing water and for use on construction joints.
Waterproofing should be tightly connected both to the base profile or frame and to the prepared building substrate. Pay attention to correct execution of corners and joints between material sections.
Internal sealing
On the room side, create an airtight layer that limits water vapor penetration into the insulation layer.
You can use:
- vapor-tight tapes,
- liquid membranes,
- flexible sealants designed for window installation.
The internal layer should be less permeable to water vapor than the external protection.
Why you should not use random tapes and sealants at the bottom of the window
Not every product intended to seal the gap around the frame can be used as waterproofing for the lower joint.
Typical tapes and sealants used at the sides and top may not be suitable for:
- long-term contact with standing water,
- mechanical loads,
- contact with ground or foundation substrates,
- building movement,
Therefore, before using a material check whether its manufacturer permits use in the lower window joint and whether the product is resistant to standing water and the expected substrate movements.
Summary
Proper installation of a window on the base profile requires ensuring at the same time:
- stable and durable support,
- appropriate mechanical fixing,
- continuity of thermal insulation,
- protection against water and moisture,
- airtightness on the room side.
Omitting any of these elements can cause the window to go out of level, create thermal bridges, leaks, moisture in the insulation layer, or damage the joint during the building’s use.