Poor adhesion of extruded insulation boards can directly affect the safety and stability of the exterior wall insulation system, potentially leading to issues such as board detachment and insulation failure. The causes can be analyzed from four key dimensions: substrate preparation, material quality, construction practices, and environmental factors, as detailed below:
The substrate serves as the “foundation” for insulation board adhesion. If it is not prepared according to specifications, it will directly compromise the adhesion strength of the bonding interface, making it one of the core causes of poor adhesion:
1. Uneven or defective substrate surface
- Substrate walls with protrusions, depressions, holes, or cracks (such as pitted surfaces on concrete walls or uneven mortar joints on masonry walls) prevent insulation boards from tightly adhering to the substrate. The adhesive only makes partial contact, leading to uneven stress distribution and potential issues like hollow spaces or detachment. For example, if depressions in the substrate are not leveled with mortar, the insulation board may become partially suspended after bonding. Over time, external forces or temperature changes can cause it to gradually loosen.
2. Contaminants on the substrate surface
- Residual dust, oil stains, release agents, old paint layers, or loose mortar on the substrate can form a “barrier layer” between the adhesive and the substrate, impairing the adhesive's wetting properties and bonding strength. For example, if release agents are not removed from concrete walls, the adhesive cannot effectively bond with the substrate and instead adheres only to the release agent surface, making it prone to peeling later.
3. Excessive moisture content in the substrate
- Excessively high moisture content in the substrate wall (e.g., newly constructed masonry walls or exterior walls with unresolved leaks) can affect the curing process of the adhesive: water dilutes the binding components in the adhesive, reducing curing strength; if moisture evaporates later, it may form bubbles at the bonding interface, leading to hollows or delamination.
II. Substandard material quality (core issue)
The quality of extruded insulation boards and adhesives directly determines the bonding effectiveness. Substandard materials or mismatched materials fundamentally cause poor bonding:
- Excessively smooth/dense surface: XPS boards have a high closed-cell rate and a dense surface. If “surface treatment” (such as applying a bonding agent or roughening the surface) is not performed during production, the surface adhesion with the adhesive will significantly decrease—the adhesive cannot penetrate into the board to form “mechanical interlocking,” and reliance solely on surface adhesion is prone to peeling.
- Low board density: Some low-quality XPS boards reduce costs by having a density below the specified requirements (typically, exterior wall XPS boards should have a density ≥30 kg/m³). The boards lack sufficient strength, and during installation, they may deform under stress, causing the adhesive interface to crack. Over time, this can lead to loosening.
- Unstable panel dimensions: Substandard XPS panels have excessive thermal shrinkage rates (specifications require ≤2%). After installation, temperature changes (such as high summer temperatures or low winter temperatures) can cause the panels to shrink or expand, disrupting the bond interface with the adhesive and leading to delamination.
2. Adhesive quality issues or incorrect mixing ratios
- Substandard adhesive itself: The selected adhesive (typically polymer cement mortar) has a cement, polymer powder, and sand ratio that does not meet specifications, or the polymer powder content is too low, resulting in insufficient bonding strength and flexibility of the adhesive. This prevents it from withstanding the weight of the panels or temperature stress, leading to cracking and delamination.
- On-site mixing errors: Adhesives are typically mixed on-site as “dry powder + water.” Excessive water addition (reducing strength), uneven mixing (resulting in insufficient polymer powder concentration in localized areas), or prolonged standing time after mixing (exceeding the initial setting time, rendering the adhesive ineffective) can significantly reduce the adhesive's effective bonding strength.
- Incompatible materials: Failure to use a specialized adhesive compatible with XPS panels (e.g., using ordinary cement mortar to bond XPS panels) results in ordinary mortar being unable to form an effective bond with the XPS panel surface, leading to “hollow drumming-peeling” issues.
3. Non-compliant construction operations (key contributing factors)
Even if the substrate and materials are qualified, improper construction operations can directly lead to poor adhesion. Common issues include:
1. Insufficient adhesive application method/area
- Failure to apply adhesive using the “dot-and-frame method” or “strip bonding method” as specified: For example, in the dot-and-frame method, the width of adhesive around the board edges is insufficient (specification requires 50-70mm), excessively large spacing between intermediate adhesive points (exceeding 300mm), or adhesive application area below specification requirements (exterior wall XPS board adhesive area ≥40%), resulting in insufficient effective adhesive area between the panel and substrate to support the panel's weight, leading to potential detachment over time.
- Uneven adhesive application: Localized missed areas, uneven thickness, or failure to apply panels promptly after adhesive application, causing the adhesive surface to skin over and reduce bonding strength.
2. Improper installation of anchorage components (failure of auxiliary fixation)
- Anchorage components serve as “auxiliary fixation components” for XPS boards after installation (especially in high-rise buildings). If the number of anchorage components is insufficient (e.g., fewer than 4-6 per square meter as per design requirements), insufficient drilling depth (failing to penetrate the insulation board into the base wall by ≥50mm, or only fixing within the insulation board), or improper installation angle (not perpendicular to the wall), the anchorage components cannot effectively distribute the panel weight. All loads are borne by the adhesive, which may fail when exceeding its load-bearing capacity.
- Anchors installed too early: Installing anchors before the adhesive has cured can displace the panels, damaging the adhesive interface and causing hollow areas.
3. Failure to promptly compact/adjust panels after bonding
- Failure to use a 2m straightedge to promptly flatten the panels during bonding, or failure to promptly adjust panels after they shift out of position, can result in gaps between the panels and the substrate, preventing the adhesive from fully filling the gaps and causing localized suspension; or insufficient compaction force can prevent the adhesive from fully contacting the substrate and panel surface, affecting the bonding effect.
- When panels are joined, they were not staggered (e.g., continuous joints), or corners were not interlocked, causing localized stress concentration. Over time, this can lead to failure of the bonding interface due to stress concentration.
4. Incorrect construction sequence
- Failure to follow the standard construction sequence of “from bottom to top, horizontal laying,” or failure to set horizontal reference lines on each layer, resulting in uneven force distribution during panel bonding (e.g., the weight of upper panels pressing on the uncured adhesive of lower panels), which may cause lower panels to shift or delaminate.
4. Environmental factors (external interference)
Environmental conditions such as temperature, humidity, and wind speed can affect the curing process of the adhesive, indirectly leading to poor adhesion:
1. Inappropriate temperature and humidity
- Low temperature: When the ambient temperature is below 5°C, the curing speed of the adhesive significantly slows down or even stops. At this point, the adhesive cannot form sufficient strength. If it is subjected to the weight of the panels or subsequent processes (such as installing anchor bolts) too early, it may lead to delamination.
- Excessively high temperature/strong wind: In high-temperature (＞35°C) or strong wind conditions, moisture in the adhesive evaporates rapidly, causing premature drying and cracking of the adhesive. This prevents effective bonding with the substrate and panels, resulting in “false adhesion” (where the surface appears securely bonded but internal cracking has occurred).
- High humidity/rainy weather construction: Rainy weather or standing water on the wall can dilute the adhesive, reducing curing strength; simultaneously, rainwater may penetrate the adhesive interface, compromising bonding strength.
2. Insufficient post-construction curing
- Adhesive curing requires a certain amount of time (typically initial setting time ≥ 2 hours, final setting time ≤ 24 hours). If proper curing measures are not taken after construction (e.g., insufficient water spraying for moisture retention in high temperatures or lack of insulation measures in low temperatures), the adhesive may not cure fully, resulting in insufficient strength. This can lead to loosening over time.
