Weather has a significant effect on the application requirements and nuances of plaster/stucco. Contributing factors include temperature, humidity, wind and sun exposure.
These factors change with the transition from summer to fall, which means the requirements for properly applying plaster/stucco change as well.
Regardless of the weather conditions varying across the country due to the change in seasons, water still freezes at 32 degrees Fahrenheit (0 degrees Celsius) everywhere and that is the critical factor in the applying of plaster/stucco.
The cement used in plaster/stucco is naturally hydraulic, meaning the water mixed with the plaster is the catalyst for the chemical reaction that makes the material take on its initial set, get hard and then subsequently cure. As a result, plaster/stucco sets slower in the winter. This article will offer some helpful hints for the placement of plaster/stucco materials in cold conditions.
Don't Be Left in the Cold
Plastering when the temperature is at or near freezing creates conditions the contractor or installer needs to recognize. There are precautionary measures that can be taken to protect the project during cold, adverse weather conditions. Extra care should be observed if the temperature is not at least 40 degrees Fahrenheit (4.4 degrees Celsius) and rising, or if the temperature drops below 32 degrees Fahrenheit (0 degrees Celsius) after the wall has been plastered.
The International Building Code for cement plastering requires in Section 2512 that “plaster coats shall be protected from freezing for a period of no less than 24 hours after initial set has occurred. Plaster shall be applied when the ambient temperature is higher than 40 degrees Fahrenheit (4.4 degrees Celsius) unless provisions are made to keep cement plaster work above 40 degrees Fahrenheit (4.4 degrees Celsius) during application and 48 hours thereafter.”
Similarly, ACI 24, Guide to Cement Plastering, requires that, “Plaster installed when ambient temperatures are below 40 degrees Fahrenheit (4.4 degrees Celsius) must be maintained in a sheltered and heated environment with continued curing to assure cement hydration.”
At 32 degrees Fahrenheit (0 degrees Celsius) and below, plaster will freeze on the wall and cause weakening of the plaster. Damage to the plaster can be so severe that you can rub it off the wall using only your hand.
Cold, dry air on the surface of the wall can absorb warm moisture from the wall rapidly and can lead to plaster shrinkage cracks. Low temperatures can cause cement plaster to take longer than normal to set and gain strength, which delays floating, troweling and production, which raises labor costs unnecessarily. If a wall has been frozen, the resulting wall may have excessive porosity that can cause further deterioration during freeze-thaw cycles if a driving rain hits it and it freezes. Cold conditions, as such, can also make the wall more susceptible to efflorescence forming on the surface of the wall. To counteract these cold weather problems, several precautionary measures can be taken which may involve enclosing and heating the structure.
When the cooler weather approaches, a contractor should plan and budget for enclosing and heating the project if necessary. This will reduce the effect of cold weather on plastering/stucco projects and increase the likelihood of successful project completion in freezing weather.
When air temperatures start to fall towards the end of the year, special considerations or precautions need to be given to the installation/application of cement plaster/stucco. The application of cementitious materials is typically restricted to temperature of 40 degrees Fahrenheit (4.4 degrees Celsius) and rising. This minimum is critical to the proper curing and overall performance of the material. Application of materials in cold and freezing conditions can cause materials to crack, flake, soften and delaminate.
Things to Remember:
- Hydration of cement-based materials is slower at cooler temperatures.
- Set times of the material will be slower in cooler temperatures. They may freeze or not set up at all.
- Strength development of the material can be reduced.
- Thermal fluctuations will cause movement in the substrate, which can produce cracking.
Any crack in the substrate including joints between substrate components is subject to movement. Thermal cycling and maximum movement is at its highest frequency during the fall months. Cracks that appear narrow in the warmth of the afternoon (when substrates are expanded) may widen significantly during the night when the temperature falls. This thermal movement can cause surface cracking. (In repair/restoration situations, the cracking can be even more severe as the patching material that fills in and around existing material has not developed to full strength.)
- Do not apply plaster/stucco to frozen surfaces.
- Do not apply cementitious materials at temperatures below 40 degree Fahrenheit without monitoring weather conditions. Monitor the extended weather forecast during the installation and curing period for severe temperature drops.
- Keep materials stored at 45 degrees Fahrenheit (7.2 degrees Celsius) or higher if possible.
- Tenting and enclosures may need to be utilized for the proper application under cold conditions to avoid freezing. If they are used, pay close attention to enclose the entire application area to properly withstand wind and rain.
- To provide for proper air movement for curing and safety, adequate ventilation should be engineered into the enclosure. Avoid using gas, kerosene or other fossil fuel heaters because they can cause rapid carbonation of cementitious material during initial set and cure.
- The heated enclosed area also needs to have the heat source run long enough before and after the application for proper conditioning and cure times.
- The temperature of the substrate also needs to be taken into consideration. Heating the mixing water up to but not exceeding 120 degrees Fahrenheit can offset the lower temperatures of the substrate or wall surface and plaster materials themselves. Using a pre-blended plaster/stucco eliminates the need to heat the sand and frozen sand piles.
- Reduce the amount of mixing water to the practical minimum to minimize shrinkage cracking, install sufficient control and expansions joints as well as properly water cure and retain heat until plaster has reached sufficient strength.
It has been observed that efflorescence is usually a seasonal problem associated with cooler weather. Cooler days and nights seem to bring out salts that are not as evident during warmer periods. The cause behind cold weather efflorescence can be linked to seasonal variations in the evaporation of moisture. Under warmer or hot conditions, the rate of evaporation may be high so that the moisture evaporates within the cladding rather than on the surface. In colder weather however, evaporation may be slower, allowing moisture to move to the outer surface of the plaster/stucco before it evaporates leaving the salt deposits on the surface. Following the proper cold weather processes may reduce and/or eliminate some of the efflorescence experienced during the cooler months.