Why is Expanding Foam Tape so Important in EWI Systems?

Expanding foam tape is an often overlooked yet essential component in External Wall Insulation (EWI) systems. It plays a crucial role in maintaining the integrity, thermal efficiency, and longevity of an EWI installation. Let’s explore why expanding foam tape is so important, where it should be used, its benefits, and how to install it effectively to ensure the best performance.

What is Expanding Foam Tape?

Expanding foam tape, or pre-compressed foam tape, is a flexible, foam-based sealing material designed to expand and fill gaps or joints. Typically made from polyurethane or similar foam, the tape is compressed during manufacturing. It expands to fill spaces when applied, providing an adaptable, tight seal. It’s highly resistant to moisture, UV rays, and temperature changes, making it ideal for areas exposed to the elements, such as around windows, doors, and roofing components.

Unlike silicone, which can break down over time, expanding foam tape maintains its structure and flexibility, accommodating any natural expansion and contraction of the building materials. Expanding foam tape is used in various places within an EWI system, including between insulation boards, around window frames and sills, and at the roofline. It prevents thermal bridging, stops water ingress, and creates a stable seal across these vulnerable points. Expanding foam tape typically has a lifespan of up to 30 years, making it a durable option compared to other sealing materials like silicone, which may degrade within a decade.

Why is Expanding Foam Tape so important in EWI systems?

Preventing water ingress

• Full-Surface Contact: When applied, expanding foam tape fills gaps and joints entirely by expanding to conform to irregular surfaces, creating a seamless, continuous barrier. This eliminates any potential paths for water to penetrate, even in areas with uneven textures or small crevices that are hard to seal with traditional methods.
• Compression-Based Seal: Foam tape is installed in a compressed state. As it expands, it pushes tightly against adjacent surfaces, forming a barrier that resists contaminants.
• Long-Term Moisture Resistance: Unlike other sealants, expanding foam tape is typically made from closed-cell foam, which has a water-resistant structure that naturally repels water. This closed-cell composition prevents the tape from absorbing moisture, which could otherwise compromise the seal.
• Adaptability to Movement: Expanding foam tape can flex and move with the building’s natural expansion and contraction due to temperature and environmental changes. This flexibility ensures the seal remains intact over time, reducing the risk of cracks or gaps that water could infiltrate. In contrast, rigid sealants like silicone can crack as the building moves, creating vulnerabilities for water ingress.
• UV and Weather Resistance: Many types of expanding foam tape are designed to withstand exposure to UV rays and extreme weather. This resistance prevents degradation and preserves the integrity of the seal, ensuring that it remains effective against water ingress even in exposed locations like window sills, rooflines, and around exterior insulation panels.

Creates a seal

• Continuous Airtight Seal: Expanding foam tape expands to create a continuous, airtight seal around joints, insulation board edges, and between various EWI components. This prevents air leaks, which are a primary cause of heat loss and condensation within the building envelope. Without this airtight seal, an EWI system can suffer from drafts, cold spots, and reduced insulation effectiveness.
• Protection Against Thermal Bridging: Thermal bridging occurs when gaps or areas of poorly insulated material allow heat to escape. By sealing all joints and potential gaps between insulation boards, expanding foam tape blocks these thermal bridges, maintaining the insulation’s uniform performance and preventing cold spots on interior walls. This consistent thermal barrier is essential for optimising the insulation’s energy efficiency and lowering heating costs.
• Barrier Against Pollutants and Moisture: The airtight seal created by expanding foam tape protects against external pollutants, moisture, and dust. By keeping these elements out, the tape prevents issues like condensation build-up and mould, which can degrade the EWI system and lead to indoor air quality problems. This protective seal keeps the system and the internal environment healthier over time.
• Stability for Structural Components: The seal created by expanding foam tape helps stabilise the components within the EWI system, securing insulation boards and preventing movement due to wind or vibrations. This stability is especially important for high-rise buildings or areas exposed to extreme weather, as it prevents wear on system components.
• Support for Expansion and Contraction: Buildings naturally expand and contract due to temperature changes and environmental conditions. Expanding foam tape is uniquely suited to accommodate this movement without losing its seal. While rigid sealants like silicone may crack under these conditions, foam tape remains flexible, preserving the system’s integrity over decades. This adaptability ensures that the EWI system stays sealed without developing gaps or weaknesses over time.

Prevents thermal bridging

• Seals Joints and Gaps Between Insulation Boards: In EWI systems, insulation boards must be fitted tightly to form a consistent thermal layer. However, small gaps may still appear between boards even with precise installation. Expanding foam tape fills these gaps completely, expanding to conform to the edges of each board. This prevents heat from bypassing the insulation through these small spaces, blocking any potential path for thermal bridging.
• Creates an Unbroken Thermal Layer: Thermal bridging often occurs where different materials meet or break in the insulation layer, such as around windows, doors, and other penetrations. Expanding foam tape bridges these transition points, creating a continuous layer across the building envelope. By removing potential breaks in the thermal barrier, foam tape ensures that heat loss is kept to an absolute minimum.
• Maintains Insulation Performance in High-Risk Areas: Due to construction details, certain areas, like window frames, roofline junctions, and the damp-proof course (DPC) area, are particularly prone to thermal bridging. Expanding foam tape fills and seals these areas, adapting to surface irregularities. This reduces the likelihood of cold spots forming near these junctions, improving the comfort and thermal performance inside the building.
• Reduces Internal Condensation and Mould Growth: Cold spots caused by thermal bridging can lead to condensation inside the building as warm, moist indoor air meets cooler surfaces. Over time, this condensation can contribute to mould growth, affecting air quality and building integrity. By preventing thermal bridges, expanding foam tape reduces cold spots, lowers the risk of condensation, and helps maintain a dry, healthy interior.
• Enhances Thermal Performance Year-Round: Expanding foam tape keeps warm air inside the building in winter, reducing heating requirements and maintaining comfortable indoor temperatures. In summer, it helps keep cool air inside, improving the efficiency of air conditioning.

Allows for building movement

• Flexible Structure: Expanding foam tape is made from a highly compressible and elastic material, such as polyurethane foam, which can stretch and contract without losing its sealing effectiveness. This flexibility allows it to adjust to small shifts in the building structure, preserving the seal across critical joints and interfaces within the EWI system. The tape acts like a buffer that absorbs movement without tearing or cracking.
• Expands and Contracts with Temperature Changes: Buildings expand in warm weather and contract when temperatures drop, which can lead to subtle but consistent shifts in building materials. Expanding foam tape is designed to handle this thermal expansion and contraction by maintaining a constant pressure within the joint. As it is applied in a compressed state, it can contract when temperatures rise and expand to fill gaps as temperatures drop, keeping the seal intact throughout seasonal cycles.
• Effective in High-Movement Areas: Expanding foam tape is highly effective in these high-movement areas because it conforms tightly to surfaces, even when materials shift. It prevents gaps that could otherwise form with rigid sealants, reducing the risk of water ingress, thermal bridging, and air leakage.
• Adaptable to Wind and Vibrational Loads: Wind loads can create pressure on a building’s exterior, especially in tall structures or in regions exposed to high winds. Expanding foam tape has a resilient structure that absorbs and dissipates vibrational energy, meaning it won’t crack or detach under minor shifts caused by wind.
• Preventing Cracks and Damage in the EWI System: Rigid sealants can crack or debond over time as the building moves, leading to water ingress, thermal bridging, and energy loss. Expanding foam tape, on the other hand, is designed to maintain its position and seal despite movement, preventing the formation of cracks that can compromise the EWI system’s integrity.

Where should you use Expanding Foam Tape?

The easiest answer to this question is…EVERYWHERE! Expanding foam tape is essential and should be abundant throughout an EWI system. The point of an external wall insulation system is to create a sealed barrier around the whole property. Any small gaps will compromise that. Therefore, those gaps should be filled with expanding foam tape when applicable.

• Roofline Closure Systems: To create a weatherproof seal between the EWI system and the roofline, ensuring water doesn’t penetrate the system.
• Between the DPC and Insulation: Expanding foam tape seals the junction between the damp-proof course (DPC) and the insulation, preventing moisture from rising into the system and compromising insulation performance.
• Around Window Head Beads and Stop Beads: Foam tape can fill gaps around window head beads and stop beads, helping to seal and stabilise the system, especially as these areas are prone to slight movement over time.
• Around Weep Vents in Window Sills (Especially with Extended Oversills): Adding expanding foam tape here is essential, especially when oversills are used to extend existing window sills, as it prevents water ingress through these critical points.
• Between Insulation Boards in Certain Instances: In situations where insulation boards do not fit perfectly, expanding foam tape can fill gaps, minimising the risk of thermal bridging and air leakage.

How do you install it?

1. Full Compression: The tape needs to be fully compressed to create an effective seal. This ensures that no air or moisture can pass through. A practical test for full compression is to try and blow through the tape; if air passes, it needs further compression.
2. Positioning the Tape: Place the tape carefully, ensuring it fully covers the gap you’re sealing. Take care to align it with the surfaces you’re joining, whether around a bead, sill, or between insulation boards.
3. Avoid Overlapping: Ensure the foam tape is cut to fit each area precisely, avoiding overlaps. Overlapping can create gaps and reduce the tape’s effectiveness.
4. Check for Expansion and Coverage: After installation, allow the tape time to expand to its full potential, filling any uneven areas or gaps that may be present in the substrate or insulation boards.

Why is Expanding Foam Tape a better solution than silicone sealants?

Although silicone is commonly used as a sealant, it isn’t the most durable option in an EWI system. Over 5 to 10 years, silicone can degrade, becoming brittle and losing its sealing capabilities. This degradation exposes the system to water ingress and potential thermal inefficiencies.

In contrast, expanding foam tape and beading have a design life of 30 years, offering a more sustainable and cost-effective solution in the long run. Foam tape can withstand the building’s natural expansion and contraction without breaking down or detaching from surfaces. Paired with oversills and beading, it offers superior protection against the elements compared to silicone, ensuring the system remains sealed and effective for decades.