Aside from their primary function of allowing natural light into a building and affording the occupants a view of the world outside, windows also contribute to the ventilation and insulation of a building. In order to fulfil the latter tasks, correct installation and perimeter sealing is imperative. 

What does window perimeter sealing refer to and why do we need it?

Usually, this would refer to the sealing of the head, jambs, sill, and along the entire interior and exterior casing of the window. 

anatomy of a window

The purpose of proper window perimeter sealing goes beyond just keeping the rain out, it also ensures that the building is airtight. Simply put, air and vapour control layers (AVCL) are important to improve the energy conservation of the building and control indoor environmental quality whilst extending the lifespan of the window and providing cost savings to the owner.

What products are used for sealing the window perimeter?

1. Sealants

Sealants have long been used as a form of low-cost and immediate prevention of water and air infiltration. Despite the ‘wet’ caulking, sealants cure to form a dry, elastomeric compound. There are three main types of sealants for window perimeter sealing: Polyurethane, Silicone, and Hybrid.

What are the differences between these 3 types of sealants?

TypeChemical CompositionUV ResponseWeather ResistanceUsagePrice
PolyurethaneOrganicProne to discolouring.GoodBest for porous substrates.Medium
SiliconeInorganicPrevents discolouration and deterioration.ExcellentBest for metal, glass.High
HybridInorganicLess likely to experience discolouration.ExcellentSuitable for all construction materials including metal, plastic, polycarbonate, and ceramic.Medium

2. Tapes and Membranes

Alternatively, tapes and membranes can also be used to effectively seal the window perimeter.  Below are two commonly used types:

Impregnated Foam Tape

Impregnated tapes are usually made from polyurethane (or similar) foam and are saturated in various synthetic resins including water repellants and flame retardants. These foams expand after installation, filling gaps, and providing weatherproofing, vapour diffusion as well as acoustic insulation. The impregnated foam tapes generally have a long life span because they continually expand and contract to accommodate movement while completely sealing the joints and perimeter. 


Sealing membranes are typically EPDM-based, Butyl-based, or a combination of polyethylene copolymers that provide an air-tight and water-tight seal for interior and exterior applications. Available in various widths and lengths, they offer applicators the option to apply before or after window installation. Membranes are normally used in conjunction with a sealant. 

3. Foams

Most of the time, foams are used as an airtight gap filler, especially in joints where movement is expected to occur. They also provide excellent acoustic properties. 

Air Foam 

Typically gun applied, air insulating foams fill and seal between window frameworks and rough openings easily and quickly. Once cured, they form an airtight, water-resistant barrier that provides excellent elasticity and sound insulation. However, they do not provide good UV protection, so they are typically used in combination with a sealant.

Self-Expanding Foam

These types of foam products are made from pre-compressed, self-expanding foam that are engineered to perform as a highly flexible, weather-tight, flame retardant seal in primary expansion, control, and construction joints. 

Tips For The Perfect Weathertight Seal

1.    For Sealants

  • Apply a primer, if necessary, before filling the joint. Typically apply a 2:1 width-to-depth ratio of sealant.
  • Dry tooling is preferred to ensure intimate contact of sealant with joint substrates.

Dymonic 100 window perimeter application

2.    For Tapes and Membranes

  • During membrane application: 
    • Always ensure that it is located in a tension-free state, and all adhesive bonds are consolidated with a seam roller. 
    • The recommended overlap width between membranes and porous materials (concrete, brick, etc.) is a minimum of 100mm. 
  • During impregnated foam tape application:
    • A suitable wedge should be used to hold the tape in position until it is expanded when working in damp or cold conditions. 
    • Remember, high temperatures (above 20°C) will accelerate tape expansion, so unused tape that is on site should be stored in a cool place, away from direct sunlight. 

3.    For Foams

  • Always lay plastic coverings on the ground before application. 
  • To assist adhesion and speed up the curing process, apply a light misting of clean water between each layer.  
  • On horizontal surfaces, always work away from the exuding bead, and work upwards for vertical surfaces.

While there are advantages and disadvantages to using any of the above, what is important to keep in mind is that it requires both good workmanship and the appropriate high-performance products to ensure that the window perimeter is effectively water-and-air-tight.

The best solution is to incorporate a trusted and proven system, one which may or may not offer a combination of sealants, tapes, membranes, and foams into the design. That way, you can be assured that every product used serves its purpose and works together to prevent leaks, mildew growth, air drafts, dirt, frame rotting, paint peeling, insects, and other damages for a long time to come.

Windows, Glazing and Façades Specialist

Tremco CPG makes the process of selecting and installing the right glazing system simple and efficient. We offer a variety of tested, proven transition solutions that can accommodate any design, from standardised to highly specialised. All our weatherproofing products provide the highest performance in adhesion, weatherability and durability to guarantee a safe and leak-free building. Our expert team is on hand to help you in any current or upcoming project you may have, get in touch with the team.

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