The Essential Role of Weather Sealing in Glazing

Weather sealing encompasses all the components and techniques used to prevent the passage of air (draughts), moisture (rain or condensation), and noise through the junctions between the frame and the wall, and within the opening mechanism of the window or door itself. Its importance extends far beyond mere comfort; it is fundamental to energy efficiency and structural longevity.

Why Effective Sealing Matters

• Energy Efficiency: Poor seals lead to significant heat loss via convection and infiltration. Effective sealing is critical for meeting the thermal performance targets required by UK Building Regulations Part L (Conservation of Fuel and Power).
• Moisture Management: Preventing water ingress protects the building fabric (plaster, timber, insulation) from damp, mould growth, and structural damage.
• Acoustic Performance: A continuous seal is essential for minimising noise pollution, particularly important for homes near busy roads or airports.
• Durability: Sealing protects moving parts, hardware, and the frame material itself from exposure to the elements, extending the lifespan of the unit.

Key Weather Sealing Components and Materials

Modern glazing systems use a combination of materials tailored to specific locations and movement tolerances. These materials must remain flexible, UV resistant, and durable over decades of exposure to temperature fluctuations.

1. Perimeter Sealing (Frame to Wall Junction)

This is the seal between the installed window or door frame and the surrounding structural opening. This junction is typically managed using a combination of expanding foams, backer rods, and external sealants.

• Expanding Foam: Often used to fill the primary gap between the frame and the reveal. Low-expansion, closed-cell foams are preferred as they offer good insulation and structural support without distorting the frame.
• External Sealants (Mastic): Applied externally to provide the primary barrier against driving rain. High-quality neutral cure silicone or polyurethane sealants are standard. Silicone offers excellent UV resistance and flexibility, crucial for accommodating the slight movement between the frame and the brickwork.
• Compriband Tapes: Pre-compressed, impregnated foam tapes that expand slowly to fill the gap. These are highly effective as they provide a continuous, vapour-permeable seal, allowing moisture to escape while blocking water ingress.

2. Operational Sealing (Sashes and Doors)

These seals manage the interface between the moving sash (the opening part) and the fixed frame. They must compress tightly when closed but allow smooth operation when opened.

• Gaskets and Weatherstrips: These are typically made from EPDM rubber (Ethylene Propylene Diene Monomer) or TPE (Thermoplastic Elastomer). EPDM is highly durable and resistant to temperature extremes. Modern systems often use multiple layers of gaskets (e.g., primary, secondary, and drainage gaskets) to create a multi-level barrier system.
• Brush Seals: Primarily used in sliding doors or sash windows where a tight compression seal is impractical. While effective against air infiltration, they offer less robust water resistance than compression gaskets.
• Drainage Systems: Essential for managing the small amount of water that inevitably breaches the outer seal. Frames are designed with internal chambers and weep holes (drainage slots) to channel water safely back outside.

UK Building Regulations and Standards

Compliance with UK Building Regulations ensures that weather sealing meets minimum standards for performance and durability. The primary regulation impacting sealing is related to thermal performance and resistance to the elements.

Part L: Thermal Performance

While Part L focuses on U-values (thermal transmittance), poor sealing can negate the performance of a high-specification window. The installation must ensure that the junction between the frame and the wall does not create a significant thermal bridge or allow excessive air leakage (infiltration).

Air Permeability Standards

The standard measure for air tightness is defined by BS EN 12207. Windows and doors are classified from Class 1 (lowest) to Class 4 (highest). Reputable suppliers aim for Class 3 or 4 performance. This testing involves subjecting the unit to varying air pressures to measure the volume of air passing through the closed unit.

Common Weather Sealing Challenges and Solutions

Even the best materials can fail if installation is poor or if the wrong product is used for the application. Homeowners should be aware of these potential pitfalls.

Challenge 1: Installation Movement

Frames, especially uPVC and aluminium, expand and contract significantly with temperature changes. If the sealant used at the perimeter is too rigid (e.g., cheap acrylic caulk), it will crack, creating pathways for water and air.

Solution: Always use flexible, high-modulus sealants (silicone or polyurethane) designed for construction joints. The sealant bead should be applied correctly, ensuring good adhesion to both the frame and the masonry.

Challenge 2: Gasket Compression Failure

Over time, rubber gaskets can harden, shrink, or lose their elasticity (known as 'compression set'). This means they no longer fill the gap adequately when the window is closed.

Solution: Regular cleaning and occasional treatment with a silicone spray can help maintain the pliability of EPDM gaskets. If gaskets are visibly cracked or brittle, they must be replaced. This is often a simple job that can drastically improve performance.

Challenge 3: Weep Hole Blockage

Weep holes are crucial for drainage but are often overlooked. If they become blocked by dirt, debris, or paint, water can build up inside the frame profile, eventually finding its way into the building interior.

Solution: Regular inspection and clearing of weep holes using a thin wire or small brush. Ensure they are visible and unobstructed both internally and externally.

Comparison of Perimeter Sealing Methods

The choice of sealing method at the frame-to-wall junction significantly impacts the long-term performance and thermal efficiency of the installation.

Long-Term Benefits of Advanced Weather Sealing

Investing in superior weather sealing technology during installation pays dividends over the lifespan of the property. It moves the performance of the glazing system from 'adequate' to 'exceptional'.

Maintenance and Ensuring Longevity

Even the best seals require periodic maintenance. Homeowners should incorporate these checks into their annual property maintenance schedule:

1. Inspect External Mastic: Look for cracks, peeling, or separation where the sealant meets the brickwork or frame. If failure is visible, the old sealant must be carefully removed and replaced with a high-quality, flexible exterior sealant.
2. Check Gaskets: Open and close the windows and doors, checking that the gaskets compress evenly. Clean them gently and apply a silicone lubricant annually to prevent drying and cracking.
3. Test Drainage: Pour a small amount of water into the drainage channel (the bottom track of the frame) to ensure it flows freely out of the weep holes.
4. Hardware Adjustment: Ensure locks and hinges are correctly adjusted. If a window is sagging or the lock mechanism is misaligned, the gasket compression will be uneven, leading to leaks.

Note on Shard AG: Understanding the nuances of weather sealing is paramount to delivering long-lasting performance. At Shard AG, we prioritise the use of multi-chambered EPDM gaskets, advanced MS Polymer perimeter sealants, and rigorous installation protocols that adhere strictly to BS EN standards, ensuring every installation achieves optimal air and water tightness for the demanding UK climate.