Sealants
Sealing the joints in window installation
The joint between frame and wall is where heat loss, condensation and leaks most often start. Getting it right with the three-layer principle.
Short answer
In window installation the joint between frame and wall is sealed in three layers: an airtight, vapour-tight layer on the inside, thermal insulation in the middle and, on the outside, a layer that keeps rain out but lets vapour escape. On the outer joint, a backer rod and an elastic, paintable PU sealant give a lasting result.
1The three-layer principle
The installation gap between frame and wall is the weakest point of a window. To prevent heat loss, condensation and leaks, the widely accepted principle is three-layer sealing: closing this joint with three layers, each with its own job. In Türkiye this is also known as RAL installation.
- Outer layer
- Middle layer
- Inner layer
- Wall
- Frame
Tighter inside than outside: moisture can leave the joint, rain cannot get in.
- Inner layer: airtight and vapour-tight. It stops humid indoor air from entering the joint and condensing.
- Middle layer: thermal and acoustic insulation, usually installation foam or insulating tapes.
- Outer layer: keeps rain and wind out but lets moisture in the joint escape.
The basic rule: the inner layer must be tighter than the outer one. Otherwise moisture that gets into the joint cannot escape, the insulation gets wet, and over time mould and cold bridges appear.
2Prepare the installation gap
- Leave enough gap between frame and wall for the frame material’s thermal movement.
- Clean the gap of dust, mortar residue and loose material.
- Surfaces must be dry and sound; on porous surfaces use the primer the product recommends.
- Fix the frame with anchors and packers as the maker instructs; foam is not a fixing.
3Middle layer: foam done right
Installation foam should fill the gap completely without overflowing. Once cured, the excess is cut off; because cutting opens its cells, that face must be sealed against the weather. Foam itself does not stand up to rain and sun and cannot replace the outer layer.
4Outer joint: backer rod and elastic sealant
The most common outer layer is an elastic sealant over a backer rod. The rod sets the sealant depth and stops the sealant from bonding to the foam or the joint base, so the sealant bonds only to the frame and the wall and takes the movement. The general rule for joint geometry is 2:1 (width to depth).
If the facade will be painted, choose a paintable PU sealant. On PVC and aluminium frames, check adhesion and compatibility with the frame material in the product’s data sheet, and use the recommended primer for the frame if needed. Joints between glass and frame are a separate matter, usually done with glazing sealants.
- Fit the backer rod
Press a rod slightly wider than the joint to the required depth without twisting or piercing it.
- Mask the edges
Tape both sides of the joint for a clean line.
- Apply the sealant
Fill without gaps, drawing the cartridge at a steady speed.
- Tool and remove the tape
Tool the surface by pressing it with a spatula, and remove the tape before the sealant skins.
Do not seal outer joints in cold, damp or rainy weather. The application temperature and surface dryness are given in the product’s data sheet; sealant applied to a wet surface does not hold.
5Common mistakes
- Leaving cut foam exposed to the weather
- Filling the gap with sealant without a backer rod (three-sided adhesion)
- Leaving the inside open so moisture gets into the joint
- Fixing the frame with foam alone
- Using a non-paintable sealant on a facade that will be painted
See our Vickers PU Sealant page for window and facade joints, and the PU sealant or silicone guide for sealant types.
Frequently asked questions
Silicone or PU sealant around a window?
If the facade will be painted and the joint bonds to concrete or render, a paintable PU sealant suits it. Joints between glass and frame take glazing sealants.
Is installation foam enough on its own?
No. Foam insulates but does not stand up to sun and rain. It must also be sealed on the inside and the outside.
What is RAL installation?
The principle of sealing the window installation joint with three layers: inner, middle and outer. It is based on airtightness inside and rain tightness outside.
How deep should the outer joint be?
As a general rule the sealant depth is about half the joint width (2:1). Exact values are in the product’s data sheet.