External Wall Insulation (EWI) offers both thermal efficiency and aesthetic upgrades to buildings. However, its effectiveness hinges on understanding the curing and drying times of the materials used.
Curing and Drying explained
At its core, curing is the process of allowing a material to achieve its optimal strength through controlled conditions. It is not merely drying but involves maintaining adequate moisture, temperature, and time to ensure material, often cement-based, sets and hardens with maximum durability. Many construction materials, like concrete, undergo a chemical reaction called hydration when mixed with water. Curing ensures this reaction continues at an optimal pace, producing a solid matrix that imparts strength to the material.
- Surface Strength: Proper curing enhances surface resistance against wear and tear.
- Reduced Shrinkage: Curing can minimise shrinkage cracks, maintaining the material’s structural integrity.
- Temperature Regulation: Curing often moderates the temperature of materials, preventing rapid temperature changes that might induce cracking.
Drying refers to the process of moisture removal from materials. It’s critical to ensure adhesion, prevent mould growth, and optimise the material’s performance. Moisture in materials typically moves to the surface through capillary action and then evaporates into the atmosphere. Factors like ambient temperature, humidity, and airflow can influence this process.
Consequences of Improper Drying:
- Mould Growth: Persistent moisture can facilitate mould growth, which poses health risks and degrades the material.
- Weak Adhesion: Materials that haven’t dried correctly can lead to poor bonding with subsequent layers or finishes, compromising the structure’s integrity.
- Discolouration: In finishes like paints and renders, uneven or improper drying can lead to blotchy or discoloured appearances.
Curing and Drying times of materials
As synthetic or mineral-based products, these boards don’t go through a traditional “curing” process like cementitious materials. Once manufactured and stabilised, they’re ready for use. While they might absorb ambient moisture, especially if stored improperly, these boards generally need to be kept dry. If exposed to water during installation, they should be dried (often taking 24 hours under optimal conditions) before proceeding with subsequent layers.
Basecoat Layers or Adhesives
These are usually cement or polymer-based, initiating a hydration process upon mixing. Most basecoats will require a 24 to 48-hour window to achieve a substantial cure, though full strength might be realised over several days. The drying phase often overlaps with curing. Ambient conditions, such as humidity and temperature, can sway these timelines. A thicker application might also extend drying times.
Acrylic and silicone renders contain polymer emulsions that coalesce as they dry, which usually takes between 24-48 hours. Mineral renders, due to their cementitious content, require hydration to cure, which could span up to 72 hours in standard conditions. For acrylics and silicones, drying and curing happen concurrently. However, mineral renders might feel dry to the touch before they’ve fully cured, so it’s vital to give them their full curing period before subjecting them to potential stressors.
Decorative and Paint Finishes
Paints, especially those that are solvent or water-based, undergo a process of solvent evaporation and resin coalescence. While they might feel dry in a matter of hours, full curing, where the paint achieves its maximum hardness and adhesion, can take up to 30 days depending on the product. Touch-dry times vary widely based on the type of paint (e.g., oil-based vs. latex), but many are touch-dry within a few hours. However, re-coat or handling should often wait for at least 12-24 hours.
Factors that affect curing and drying times
Temperature can affect the rate of chemical reactions occurring during the curing process. In drying, it can influence the rate of evaporation of moisture. In colder conditions, curing can be slowed down or even halted, risking the structural integrity of materials. Conversely, high temperatures can accelerate drying, potentially leading to uneven drying or premature curing, which can cause cracking or weakened bonds.
Humidity dictates the amount of moisture in the air. High humidity can impede the evaporation process crucial for drying and can affect hydration reactions in curing. Prolonged drying times due to high humidity can leave materials vulnerable to contaminants or mechanical damage. For curing, especially in cementitious products, high humidity can sometimes be beneficial, ensuring ample moisture for hydration. However, overly humid conditions can lead to problems like efflorescence.
Airflow can expedite the removal of moisture from the material’s surface. In confined spaces with limited ventilation, drying can be considerably delayed. However, excessive or uneven airflow, especially in outdoor conditions, can lead to rapid surface drying, potentially causing surface cracks or affecting the material’s overall integrity.
The thickness of an applied material dictates the volume of moisture present and the distance it must travel to the surface. Porosity affects the material’s ability to retain or release moisture. Thicker applications generally require more extended curing and drying periods. Highly porous materials can release moisture more quickly, potentially accelerating drying but also affecting the curing process if the moisture departs too rapidly.
The substrate’s nature, whether it’s absorbent like wood or non-absorbent like metal, can affect the moisture dynamics. Absorbent substrates might pull moisture away from the applied material, affecting its drying and curing. This can be beneficial for drying but detrimental for materials that need retained moisture for proper curing.
Additives and admixtures
Some materials have additives to modify drying or curing rates. For instance, accelerators can speed up curing, while retarders slow it down. The presence of these additives can significantly alter expected timelines and performance characteristics. Professionals must be aware of these additives and adjust their practices accordingly.
Ultraviolet (UV) radiation, especially from direct sunlight, can accelerate the drying process and can affect some materials at a molecular level. Extended UV exposure can lead to uneven or rapid drying and can degrade certain materials, affecting their colour, integrity, and lifespan.
How a material is applied, whether troweled, sprayed, or rolled, can influence its drying and curing characteristics. Sprayed applications might dry faster due to their typically thinner and more even layers. On the other hand, hand-troweled applications might be uneven and take longer.