Stucco Plastering in Cold Weather
How you interpret the title of this article may depend largely on where you live. In Alaska, cold weather may bring memories of the Taku winds with its super-frigid air coming off the mountains that can instantly freeze any and all objects. In Southern California, cold weather just means you need a jacket.
A myth to dispel is that cement stucco is only for the southwest and that cold weather and stucco do not mix. While plastering in winter can be challenging for the applicators living in cold climates, a cement stucco cladding provides a service life in these cold climates. I learned from spending time in Alaska as a technical plastering consultant, that cement stucco and EIFS do better than most other claddings in the cold climates. The Anchorage School District has many school buildings with stucco that are more than 50 years old and while these older schools need constant roof repairs, a properly applied stucco only needs occasional re-painting. This fact did not go unnoticed by the local architects, as they often preferred stucco for the schools and hospitals for appearance and long life with minimal maintenance.
Some designers are concerned that the freeze-thaw effect on fully cured cement plaster will cause plaster to explode or spall. While this is possible, it is very uncommon and will not happen to properly designed, mixed and installed cement plaster stucco. The only reason for cement plaster to explode or spall is if the cement membrane is “fully” saturated with water and freezes suddenly. This does not happen to cement plaster that is properly mixed, applied and densified.
Cement plaster may absorb some moisture but will not allow enough moisture into the membrane to become fully saturated. To become fully saturated, it has to be essentially in standing water with no drainage. Even cement plaster exposed to severe wind-driven rain will drain water sufficiently to prevent freeze-thaw problems. Applying plaster in these climates can be challenging; even marginally cold climates have to pay attention to cold weather when applying plaster.
The International Building Code now covers the entire country and it covers cement plastering (stucco) in cold weather in section 2512.4 in Chapter 25.
“Plaster coats shall be protected from freezing for a period of not less than 24 hours after set has occurred. Plaster shall be applied when the ambient air temperature is higher than 40 degrees Fahrenheit (4 degrees Celsius) unless provisions are made to keep the cement plaster above 40 degrees Fahrenheit (4 degrees Celsius) during and after application for 48 hours.”
To interpret what this really means, let’s look at the three components, in a simplified interpretation.
Protect plaster from freezing.
Do not plaster in less than 40 degrees Fahrenheit.
Allow the plaster to set.
Items two and three are meant to ensure that the most important item, number one, is achieved. Cement plaster requires hydration to set and to do so you have to understand how plaster is applied, sets, and cures to fully understand the intent of all three code references.
The 40 degree fahrenheit rule
Cement plaster does not freeze when applied at temperatures just above freezing, so why does the code state not to apply at less than 40 degrees Fahrenheit? It has more to do with setting than freezing. We understand that sufficient moisture is required to properly hydrate the cement paste. When water and cement are brought into contact with each other in the mixer, a complex chemical reaction starts. A series of new chemical compounds bind together the grains of sand within the mixture. Water is a critical factor in this reaction, as it actually combines with the cement. This reaction is designated as “hydration.” If hydration does not take place, no value will have been gained from mixing Portland cement with sand and water.
The tarps protecting this stucco lifted and the sub-zero degree cold weather froze the cement stucco finish coat (white areas). The tarp was secured the next morning; the plaster was slighted re-hydrated (fogged) and regained its strength.
At temperatures below 40 degrees Fahrenheit, the chemical process begins to slow and stops when the water evaporates or the water freezes within the mix. The slowed chemical process results in delaying the initial set of the cement plaster. The 40-degree rule is meant to provide a cushion from freezing and keep optimum set times for the cement plaster, but this does not mean that cement plaster exposed to temperatures below 40 degrees Fahrenheit during the initial set will fail or have less of the desirable properties.
Locals know how
Plasterers in cold regions know that cement plaster can be applied in some pretty cold weather. The real fear is below freezing and for a longer period of time. Cement plastering is often done at temperatures near the freezing mark (32 degrees Fahrenheit). While this is not recommended as best practices for optimum initial setting, it also does not mean that the cement plaster will fail to set or perform as intended. Plasterers know that cement plaster gives off some heat as it begins its initial cure and can withstand pretty cold temperatures. Again, it is relative to how cold and for how long.
In short, before building owners panic about fresh plaster being exposed to temperatures below 40 degrees Fahrenheit, they should talk to the plasterer and look at the weather closely. If winter is setting in and temperatures are expected to be freezing and stay there, supplemental heat is definitely needed. On the other hand, if the temperature is expected to possibly hit freezing in the middle of the night and then return to the mid ’40s the next day, the concern should be discussed, the plaster monitored, observed and checked for sufficient and adequate set the next day.
Bidding: Plastering contractors who are based in areas where very cold winters are common know that preparing their plaster bids must take into account the cost of tenting, or hoarding as it is known in Canada, and supplemental heat required. More often than not, they will have language that either limits the length of time for plastering or places the cost of tenting and heating as the financial responsibility of the owner or general contractor. After all, the plasterer has no control over the weather, the project site or other subcontractors. He can wait until weather improves to above freezing, however it is typically the owner who wants and benefits from the project being completed by a particular date and he/she should bare the brunt for the costs of heating.
In addition, tenting and heating requires the cooperation of all persons on the project, not just the plasterer. Only the general contractor and/or owner can direct other subcontractors, delivery persons or others not to alter the tenting procedures (i.e. keeping doors closed, tents secured and/or leaving the heating elements on and in place).
Best Practices and Responsibilities: A general contractor is responsible for the entire project and can direct all on site, but the plastering contractor is responsible for his plaster and deemed the expert on initial setting and curing. A conscientious plastering contractor is concerned about his plaster freezing and is ultimately liable for choosing to plaster in conditions that can possibly effect the setting of his plaster. While this seems to contradict what was stated in “Bidding,” it does not.
“Tenting in” a large project requires substantial cost and is common in parts of the country where cold winter weather is anticipated.
The plastering contractor knows about plastering, how cold weather effects the application and must notify the owner if he is directed to continue or meet a schedule that has potential problems. He also understands that the 40 degrees Fahrenheit application rule is good, sound advice, but it is the freezing of plaster issue that concerns him most. While he cannot control the weather, the other subcontractors, or the overall project schedule, he is responsible for taking on self initiated actions or allowing himself to be bullied.
Basic rules to cold weather stucco work
So now that we are all really confused, let’s try to make some sense of this. Here are some basic rules to follow for plasterers in cold weather:
Bids and contracts should anticipate cold weather.
Plaster should never be applied to frozen surfaces.
Plan ahead, watch weather reports, avoid plastering if the ambient air temperature is anticipated to get to and stay at freezing levels, especially within 24 hours after plastering.
Water can be heated to accelerate the set and provide added protection of the cement paste. It is recommended not to heat the plaster mixture to more than 120 degrees Fahrenheit.
Acrylic finish coats dry to a set and require air movement as well as heat.
The use of additives (antifreeze) to cement plaster should only be used under strict manufacturer’s recommendations.
Do not over use or over mix air entrained cement.
What if the plaster does freeze? How do you know? Does it have to be removed? Cement plaster can freeze and then be salvaged, depending on how long it was frozen and how quickly the cement paste can be re-hydrated. For example, it is not uncommon that an overnight freezing wind or sudden unexpected drop in temperature can cause plaster to freeze. The plaster will typically have a white chalky look and be soft. If the plaster is returned to a temperature to above 40 degrees Fahrenheit within 24 hours of application, and then re-wetted (hydrated) slightly, it will typically set and cure to a hardened state with no long-term defects.
Heat is often provided by the use of what is commonly called a “salamander” and propane tank. The salamanders are heaters and most use propane as a fuel. Salamanders can be directed in a specific direction, usually at an angle along the base of the wall. While the salamanders are not overly expensive to rent, they can go through a 100-pound tank of fuel every 14 to 15 hours. In cold/frigid regions, this can cost some serious money to keep the tented temperatures at or above 40 degrees Fahrenheit.
Propane heaters also pump moisture into the air, and while this is not a problem for cement plaster, it can create problems for an acrylic finish coat. Plasterers tend to refer to acrylic finish coats as “drying” and the added moisture can slow the drying process. Acrylics typically dry from the outside in. This means that even when the acrylic finish coat feels set on the surface, it can be soft at the base. Most experienced plasterers in cold regions that use acrylic finish coats know that air circulation can be even more important than heat. Premature removal of tenting before the acrylic finish has fully and completely set can cause the finish to blister or wash off in the first rain.
Depending on where you live will determine how much attention to cold weather plastering should be a concern. For example, in Minneapolis cold winters are a way of life and contacting the Minnesota Lath and Plaster Bureau is highly recommended for best practices in that region. In Seattle, the Northwest Wall and Ceiling Bureau would probably advise contractors to be cautious in winter, but since that region reaches freezing temperatures on an occasional basis, freezing cement plaster is rarely a problem. In the southwest, the Technical Services Information Bureau would probably be shocked to hear of frozen plaster. Even those rare nights in the low ‘30s do not typically produce frozen plaster in the morning. In short, areas like southern California and Florida do not really have cold weather plastering. But looking at the rest of North America, this topic is certainly relevant. Let’s warm up and think Spring. W&C