By Steve Heinje
Understanding the limitations of using roof coatings in the fall and winter can fine-tune the way Canadian contractors treat the options and variables involved. Some product types are much more robust in inclement weather, and there are various options available for project teams.
It is well-known an acrylic elastomeric coating cannot be used in the rain or during freezing conditions. White acrylic coatings tend to be favoured in warmer climates and are mostly applied in drier months. However, experienced contractors can extend the season substantially, allowing the use of these coatings in challenging northern locations.
There are also other possible chemistries, such as asphalt cut backs, silicone, and polyurethanes. These options bring different trade-offs, making them important alternatives when a coating is needed during or closer to winter. In Canada, the typical window for applying water-based coatings is April to mid-October, with temperatures staying above 10 C (50 F). However, with knowledge and experience, this can be extended from early March into late November, above 4 C (40 F).
Good roofing practice always limits the use of any type of work in the winter. Cold weather slows down curing not only for coatings, but adhesives as well. The cold also limits penetration and interferes with asphalt flow and workability, and even affects thermoplastic seaming operations. Freezing weather is a recipe for trapping frozen water that can lead to blisters; cool wet weather increases the likelihood trapped water will not be detected or dry out and surface moisture will hurt the adhesion of plies and adhesives. Coatings are unique in that they are both formed (i.e. cured) and bonded in these challenging conditions—the entire cure period needs to be taken into account.
Why are water-based coatings so different?
Water freezes but ‘easy water-clean up’ is not good news in many roofing situations. The differences for water-based coatings go well beyond that. Water-based materials face two key problems:
- Water is present in the environment as humidity.
- Water itself responds more strongly to the temperature than any other carrier.
Humidity and cure
Whether methyl ethyl keton (MEK), xylene, or mineral spirits, when a solvent-based coating dries, the release of the solvent never competes, or is hindered by, the same solvent already present in the air. As a result, evaporation is more or less directly related solely to temperature. It follows the general rule that for every 8 to 14 C (15 to 25 F),
drying is either twice as slow, or twice as fast. For example, a cut-back that is ‘rain-ready’ on an overcast day in one hour at 29 C (85 F) might not be ready for four hours at 8 C (47 F). For water-based coatings, the situation is the opposite—water as vapour, typically expressed as relative humidity (RH) is often nicely below 50 per cent during an application, but typically hits 100 per cent (i.e. the dewpoint) as temperatures drop
For the water-based coating to become rain-ready, it must form a strong film before the evening. Thereafter, water, in the form of dew, will be entering the coating from the atmosphere. This dynamic is unique to water-based coatings. Drying is largely a function of humidity. In dry climates, these coatings behave much more like solvent-based products, but even in more arid locales, wet conditions often prevail in the fall.
Evaporation rate and temperature
Water is a dynamic material, and it changes as temperature changes. While it dries faster than many solvent-based coatings in the summer, it will be slower to evaporate when cold, including when conditions are dry. This adds to the variation in drying rate even more; in the end, a solvent-based product may dry over eight times faster in the summer than the fall, but a water-based product might see a 24-fold difference.
VOCs, cool roofing, and reactivity
Environmental regulations lie just below the surface of this discussion about application. In the last decade, strict volatile organic compound (VOC) rules have pushed water-based coatings into markets and applications consistently avoided in earlier years. VOC limitations not only favour water-based chemistry, but also trend towards slower curing grades of solvent-based products as well.
Fortunately, in roofing, the acrylic resins used in water-based applications have robust low temperature film formation, high water vapour permeance, and do not rely on volatile additives. Formulators are regularly innovating to improve dry and cure. The market opportunity for latex technology is huge, making it the target of most product research.
Today’s conventional coatings are often higher in solids, and need to use slower solvents. They still follow the simple, reliable, temperature-driven cure rate trend of earlier conventional coatings, but they show slower cures and often a much longer ‘tail.’ One strategy used to remedy this is employing exempt solvents, such as tertiary butyl acetate (TBAC), acetone, dimethyl carbonate (DMC), and parachlorobenzotrifluoride (PCBTF). Products employing these exempt solvents will likely be more expensive and accompanied by a stronger smell, but these products offer some of the best cold-weather performance available. Styrene ethylene butadiene styrene (SEBS) coatings using PCBTF are a good example of this.
Cool roofing has had a more subtle and often overlooked impact on cold-weather applications. Modern coatings are rarely sold as grey or tan, and rather are very white. This means they stay cool even if the sun is out. Often, a light gray or tan coating works better in the off-season because the sun’s heat often raises the temperature of the coating from 11 to 28 C (20 to 50 F). At least while the sun is out, colour can provide a favourable cure temperature. Tan is often the best alternative to white being more reflective than aluminum or grey, easy on the eyes, and still faster at curing.
One final strategy is reactive curing. Recently, silicone coatings as described by ASTM D6694, Standard Specification for Liquid-applied Silicone Coating Used in Spray Polyurethane Foam Roofing Systems, have become an important alternative product to asphaltic and acrylic options. Polyurethanes still find use most notably as a type of aliphatic coating described by ASTM D7311, Standard Specification for Liquid-applied, Single-pack, Moisture-triggered, Aliphatic Polyurethane Roofing Membrane. A new chemistry often called silicone polyether urethane rubber (SPUR), based in silicone chemistry but with much greater tolerance of moisture in field applications, has long been used in building sealants and is now being used as a high-end cool roof solution in difficult weather.
Coatings set, dry, and cure
This article defines ‘cure’ as a coating or membrane that has achieved its ultimate properties. ‘Dry’ indicates when most of the solvent or water has evaporated and it has bonded. In water-based coatings, the material can still be a bit soft with maybe two per cent water remaining, will resist light frost, and can be considered ‘dry.’ In solvent-based applications, tail solvents can be present for weeks, but in both these cases, the material can resist heavy rain, light traffic, and is a candidate for receiving more coats.
‘Set’ is defined as when a coating is dry to touch, or is a film that can resist rain or dew. In water-based cool roof coatings, special chemistry can provide rapid and somewhat humidity-proof set. In solvent-based materials, especially silicone and conventional asphalt, coatings can provide early set simply because these materials resist water effectively, even as liquids. ‘Cure’ is important with regard to the risk of freezing. In water-based coatings (often, white coatings), the product can set and resist rainfall, but still be vulnerable to freezing.
The contactor may be tempted to apply more coating if there is a break in the weather, perhaps if there are two good days of sunny, dry, cold weather ahead. The coating will set and dry, but may not cure. However, it cannot be allowed to freeze until the water underneath the skin, at the substrate, has dried. If it freezes, the bond is lost and the coating fails. Typically, this period is about three days for white coatings during the fall months, but if the coating is over-applied it might be as long as a week. A freeze-damaged water-based coating makes for an unpleasant repair.
Rain resistance and different coatings types
The rules by which coatings are used derive from their chemistry; when the chemistry changes, the rules change. As mentioned above, water-based indicates rain can wash the coating off because water is the ‘solvent’ for the coating. As such, these coatings are at risk even from early dew such as the typical coastal dew.
For some time, faster-skinning versions, and even darker-coloured quick-setting grades, are available to provide good dew-resistance. Since these technologies do not change the physics of water, they will often fail to withstand a heavy rain since the coating is still wet underneath. For a host of reasons, a water-based reflective coating may be specified for use under these poor conditions. With care, these faster-setting water-based coatings can be made to work, but there are fundamentally different alternatives to consider.
One such option is conventional asphalt. Anyone who has ever used an asphalt wet patch can appreciate why asphalt remains an important roofing material. Asphalt benefits from being black, and if conditions are cold but mostly sunny, the asphalt is able to form a strong skin. This material has a low risk of wash-off due to dew or even a drizzle. If not pushed, it is unlikely to suffer from a wash-off even in a heavy downpour. Conventional aluminized asphalt coatings are a great wet-weather choice as well.
Silicone and SPUR coatings provide Energy Star-compliant materials that almost seem to ‘like’ bad weather. The moisture will promote cure so these materials can handle dew exceptionally well and, by early in the night, they can also resist a downpour. The silicone’s cure is chemically driven, so this material cures independent of the sun, making it an especially good choice in typical overcast winter weather or a threatening afternoon storm.
Polyurethanes are a mixed bag for winter conditions. Many one component moisture-cure urethanes as per ASTM D6947, Standard Specification for Liquid-applied Moisture Cured Polyurethane Coating Used in Spray Polyurethane Foam Roofing System, do not cure well in cold-wet conditions. Moisture triggered aliphatics (as per ASTM D7311) perform better and this may explain their popularity in Canada, Europe, and the northeastern United States. The ultrafast polyureas may cure well in the cold, but one must be aware of substrate surface moisture when applying them, since they have no solvent to help bond to a cold damp substrate.
SEBS coatings behave like conventional asphalt coatings and can be Energy Star-compliant as well, making them a preferred choice for contractors used to conventional cut-backs. SEBS coatings have low permeance, so extra care must be taken to avoid surface moisture and trapped water to avoid blisters. In these cases, chemistry cannot trump good roofing practice and coating basics like surface preparation and achieving a good bond.
Seasons and working-day length
Coating during the fall months is emphasized because this time of year generally poses more risk than the spring. During fall, the roof is moving into less-favourable conditions, while in spring the roof is moving into more favourable conditions (this generalization depends on local weather patterns, since autumn is drier than spring in some locations). This makes repairs more challenging in the fall, and resolving problems can become more critical because damage to the building from potential leaks or trapped water is more likely. Spring, leading into warm weather, means the weather that over-taxed a coating or liquid-applied roofing system is likely to work much better in a few weeks, and conditions will be getting drier.
‘Working-day length’ refers to when coating operations can start and stop, as opposed to ‘day length itself.’ Unfortunately, working-day length can be a big stumbling block during the fall. This means on a typical day, work can only start after the dew burns off, often around 9 or 10 a.m.
The other limit is ‘set;’ the coating must set before the dew returns or it washes off. Generally, this means application stops in early afternoon for water-based materials. It is only safe to apply water-based coatings during a rising temperature, because humidity will spike as soon as the temperature starts falling. Some products use various quick-set, fast-set, or even instant-set approaches that gel the coating to prevent a wash-off and extend the working day. It is important to remember solvent-based products have the advantage of curing at night, so they can stretch the working day. In fact, silicones, SPUR, and some other coatings are reactively moisture-cured, so they can cure even under a layer of dew.
The impact of colour
White coatings can prove challenging in cold-weather conditions and tan is often the best alternative. The substrate and its colour also have an impact. Metal often gets cool at night but hot in the day. The special problem of metal is when it is coated, its emissivity rises to that of the coating, so it lets its heat out faster than before it was coated, which can fool an applicator. Coating a thick, black built-up roof (BUR) is favourable; as it will typically be at least 17 C (30 F) above the ambient temperature, with its mass able to hold temperature into the evening and drive cure for the first coat.
When a white coating is applied in cold weather, often as a final top coat, it slows the cure of the entire coating system beneath it. It is recommended to coat a basecoat only once it is dry or mostly cured, especially when using a lighter coloured coating on top—that is, until a film has formed and bonded, through to the substrate. If not, the basecoat could freeze underneath. There are several potential pitfalls here:
- a white coating can allow a basecoat to freeze that would have otherwise properly cured;
- a frozen basecoat will often appear walkable in the morning and one should make sure it has bonded and dried through to the substrate; and
- if the basecoat includes fabric as part of a detail step, it will be walkable in cold weather, but still wet underneath so it should not be top coated yet. The darker colour of the basecoat should be allowed to dry out the first application. Often, these detailed areas have thicker applications that need more time to dry.
Pushing the project in poor weather is the natural response to the pressure of these scenarios. From a roofing practice perspective, using less coating that is cured is better than a thicker coating that is not yet waterproof. When adding more product per coat, it is important to remember the coating dries slower and the cure time is extended. Coating and detailing a roof in darker colours and then applying the reflective coating topcoat later would be an ideal way to work within the limitations of water-based coatings.
Energy mandates, VOC regulations, and other environmental regulations and project delays mean most commercial roofing installers will face the challenge of applying coatings in less than ideal conditions. Technology, both old and new, provides the tools needed to meet these challenges, even in the fall and winter months. Sometimes weather may require a change in specification, application, or coating type. The tools to meet these coating challenges come in the form of not just products, but also product knowledge, chemistry, and better weather information. Manufactures have a combination of water-based, conventional, and high-solids coatings to extend the application season to meet the expanding needs of their customers.
Steven Heinje is the technical service manager for Quest Construction Products, in Phoenix, Ariz. For most of his 30 years as a coatings scientist, he led the research and development lab in Spokane, Wash. Heinje is the vice-president and board of directors’ member of the Roof Coatings Manufacturers Association (RCMA). He contributes extensively to ASTM D08, as well as being active in Reflective Roof Coatings Institute (RRCI), American Society for Quality (ASQ), American Chemical Society (ACS), and Federation of Societies for Coatings Technology/National Paint and Coatings Association (FSCT/NPCA). Heinje can be contacted by e-mail at email@example.com.