Choosing Elastomeric Coatings for Stucco: When They Help and When They Trap Moisture

Elastomeric coatings form a flexible film over stucco that stretches to bridge hairline cracks and accommodate substrate movement. This elastic behavior helps hide small defects and keeps water from ponding at the surface.

These coatings can act like a liquid-applied barrier but still vary in vapor permeability; some shed bulk water while allowing water vapor to pass, others are much less vapor-permeable. Check product technical data sheets or labels for terms like “vapor permeability,” “permeance,” or “breathability” so you know which performance tradeoffs apply.

What ‘Elastomeric’ Means in Practice

Elastomeric coatings are called that for a reason. They’re stretchy, like rubber bands.

When we say ‘elastic film formation’, it’s about how these coatings can move with your stucco as it expands and contracts. Stucco breathes, you see? It moves with temperature changes and moisture. An elastomeric coating needs to bend without cracking.

This elasticity is crucial. Hairline cracks in your stucco are normal. An elastomeric coating should bridge these, staying put even as the stucco shifts around it.

Vapor Permeability Versus Liquid Water Resistance

Elastomeric coatings are great at keeping out liquid water. That’s their main job, after all. But they’re not all the same when it comes to letting moisture vapor pass through.

Moisture vapor transmission is like your stucco breathing. It needs to let excess moisture escape. If a coating traps this vapor, it can lead to problems later on.

So, you want an elastomeric coating that’s water-resistant but also breathable. Check the product’s permeability rating. It should be high enough to let your stucco breathe, but not so high that it lets in liquid water.

Product Composition Overview and Where to Check Specifics

Elastomeric coatings are made up of binders, pigments, and additives. Binders give the coating its stickiness and elasticity. Pigments give it color. Additives can do all sorts of things, like improve flexibility or add mildew resistance.

The exact mix of these components affects how well a coating performs. That’s why you should always check the product data sheet.

Look for metrics like permeability rating and elongation at break. These tell you how much the coating can stretch before it breaks, and how well it lets moisture vapor pass through.

Back to top ↑

Elastomeric coatings are useful where UV degradation, surface chalking, or cosmetic cracking are the main problems because they renew appearance and protect against sunlight. They also perform well on mechanically sound stucco where cracks are limited and movement is expected.

These coatings work best when the wall assembly can dry to the exterior and there are no active leaks or trapped moisture behind the stucco. Verify substrate dryness and compatibility by checking the manufacturer instructions and the product data sheet before proceeding.

Ideal stucco substrates and assemblies

Elastomeric coatings work best on certain types of stucco. They’re ideal for traditional three-coat systems: a base coat, a brown coat, and a finish coat.

Ensure your stucco has proper drainage: Elastomerics need a dry substrate to perform well. Check that your stucco has weep screeds at the bottom and adequate flashing around windows, doors, and other openings.

Avoid applying on thin or weak stucco: Elastomeric coatings can’t fix poor workmanship. If your stucco is less than 3/4″ thick or feels soft, consider having it repaired before applying an elastomeric finish.

Performance benefits and expected outcomes

When applied correctly, elastomeric coatings can extend the life of your stucco’s appearance. They bridge hairline cracks and prevent them from growing wider.

UV protection: Elastomerics contain pigments that reflect UV rays, slowing down the sun’s damaging effects on your stucco.

Weather resistance: These coatings form a flexible barrier that resists water intrusion. They allow moisture to escape but prevent liquid water from entering, protecting your stucco from further damage.

One common failure is sealing moisture that’s already in the wall, which prevents drying and can lead to staining, mold growth, and failure of adhesion. Poor drying capacity becomes critical when there are impermeable layers behind the stucco or when water intrusion continues from leaks.

Interactions with existing impermeable coatings, poorly tied-in flashings, or blocked drainage planes increase the risk of trapped moisture and accelerated deterioration. Inspect assemblies and check manufacturer guidance on maximum allowable substrate moisture and compatibility with existing coatings.

Common ways coatings trap moisture

Elastomeric coatings can trap moisture if applied incorrectly or over unsuitable surfaces. Here’s how:

Wet Stucco: Applying the coating before the stucco has dried out completely seals in moisture, leading to problems later.

Lacking Drainage Plane: If your wall assembly doesn’t have a proper drainage plane, water can get trapped behind the stucco and coating, causing issues.

Impermeable Undercoats: Applying elastomeric coatings over impermeable undercoats or primers can also trap moisture, as these don’t allow water vapor to escape.

Hidden conditions that increase risk

Some issues aren’t visible at first glance but can lead to moisture problems with elastomeric coatings:

Blocked Weep Screeds: If the weep screeds at the base of your walls are blocked, water can’t escape and will build up behind the coating.

Damaged Flashings: Roof or window flashings in poor condition can let water in where it shouldn’t be, leading to trapped moisture.

Roof or Window Leaks: Even small leaks from above can cause problems. Water entering at the top of a wall has nowhere to go but down, potentially getting trapped behind the coating.

Substrate Delamination: If your stucco is delaminating (peeling away from the substrate), it’s not a solid surface for the coating and can trap moisture.

Symptoms and timeline of moisture-related failures

Moisture problems with elastomeric coatings don’t always show up immediately. Here’s what to look for:

Bubbling: You might start seeing bubbling or blistering on the surface, indicating trapped moisture underneath.

Staining and Efflorescence: Water stains or efflorescence (white, powdery deposits) can appear on the surface. These are signs that water is trying to escape but can’t.

Musty Odor: A musty smell often indicates mold growth due to trapped moisture.

These symptoms may not show up for months or even years after application, depending on how severe the underlying issues are. Regular inspections can help catch problems early.

Back to top ↑

Start with a systematic inspection: look for active leaks, soft or delaminated areas, efflorescence, and visible mold or staining, and tap the surface to find hollow spots. Remove a few samples or open flashings where practical to verify the condition of the backup wall and drainage plane.

Test substrate moisture with appropriate meters and confirm compatibility with existing finishes by consulting product labels or data sheets. If you find trapped moisture, structural damage, or uncertain assemblies, consider repairs or a specialist assessment before coating.

Tools and tests for moisture assessment

To ensure your walls are dry before applying an elastomeric coating, use these tools to check for moisture. Remember, each tool has its limitations, so use them together for better accuracy.

• Visual inspection: Your eyes can spot obvious signs of moisture like stains or efflorescence (white powdery deposits).
• Probe test: Using a screwdriver or awl, probe the stucco to check for soft spots or crumbling. This indicates potential moisture damage.
• Moisture meter: Rent one from your local hardware store. Set it to ‘wood’ mode and insert probes into the stucco. Readings above 15-17% indicate high moisture content.
• Infrared thermometer (optional): Around $200-$300, this tool shows temperature differences that could indicate moisture. It’s not a standalone test but can complement others.
• Plastic sheet test (DIY): Tape a clear plastic sheet to the wall on a sunny day. If water droplets form on the inner side, there’s moisture behind the stucco.

When to call a professional for further investigation

While it’s great to be hands-on, some situations require expert help. Here are signs that you should call a professional:

Repeated elevated moisture readings: If your moisture meter keeps showing high readings even after addressing visible issues, there might be hidden problems like damaged drainage planes or missing vapor barriers.

Extensive cracking: While minor cracks can be patched and painted over, extensive cracking could indicate structural issues that need professional assessment.

Structural concerns: If you notice sagging walls, bulging stucco, or other signs of structural compromise, stop work immediately and call a professional. These are serious safety hazards.

Suspected concealed water intrusion: If you can’t find the source of moisture but suspect it’s coming from behind the stucco, it’s time to bring in an expert with specialized tools and knowledge to locate and fix the problem.

Back to top ↑

Select a product with proven adhesion to cement-based finishes and an appropriate balance of elasticity and vapor permeability for your assembly. Look at the technical data sheet for adhesion test methods, listed substrates, recommended primers, and stated vapor-permeance or perm ratings.

Also verify UV resistance, recommended film thickness, and any required primer or surface prep steps on the label and data sheet. When in doubt about compatibility with paints, waterproofing layers, or existing coatings, contact the manufacturer or choose a product with explicit approval for stucco.

Material specs and standards explained

Choosing the right elastomeric product starts with understanding its material specifications. The right choice ensures a durable, long-lasting finish.

• Vapor Transmission: Look for products with high vapor transmission rates (perms) to prevent moisture buildup behind the coating. Low perm ratings can trap moisture, leading to peeling and blistering.
• Adhesion Test Results: Check adhesion test results to ensure the product bonds well to stucco. Poor adhesion can result in cracking and delamination.
• Recommended Substrates: Verify that your specific stucco type is listed as a recommended substrate. Using an incompatible product can lead to weak bond strength and premature failure.
• Industry Standards: Consult applicable industry standards like ASTM D6083 for elastomeric coatings to ensure the product meets minimum performance requirements.
• Local Code Requirements: Check local building codes to ensure the chosen product complies with any specific regulations in your area.

Compatibility testing and site-specific selection

Before applying an elastomeric coating, it’s crucial to test its compatibility with your existing stucco finish. This step helps avoid costly mistakes.

For unfamiliar products or when coating over existing finishes, perform adhesion/pull-off tests or apply a small test patch. This allows you to assess the product’s bond strength and overall performance under real-world conditions.

If testing reveals any issues, consider using an approved primer or adjusting surface preparation methods to improve compatibility. Always follow manufacturer recommendations for best results.

Warranties and manufacturer support considerations

Manufacturer warranties provide valuable protection, but they also come with requirements. Understanding these can help you maintain your warranty’s validity.

Typically, warranties require proper surface preparation, approved primers, and adherence to application guidelines. Always follow these instructions to the letter.

Retain all documentation related to your purchase, including receipts, product data sheets, and any correspondence with the manufacturer. This information is crucial if you ever need to make a warranty claim.

Back to top ↑

Prepare the surface by removing loose material, cleaning contaminants, and repairing cracks and delaminations before coating; follow the product prep instructions to the letter. Apply in recommended conditions—temperature, humidity, and no rain—and use the specified wet film thickness and number of coats from the data sheet.

Keep a compact tools list on hand: pressure washer or stiff brushes, repair mortar, primer, rollers/airless sprayer, and wet-film gauges if required by the manufacturer. If you encounter conditions outside the product’s stated limits, stop and re-evaluate rather than improvising.

If the wall cannot dry adequately or there is a history of trapped moisture, consider breathable coatings, mineral or silicate-based finishes, or repairs that restore drainage and flashing performance. These alternatives trade some waterproofing for higher vapor permeability to reduce trapped moisture risk.

Mitigation can also include fixing leaks, adding or repairing flashings, installing drainage planes, or removing impermeable coatings before applying any finish. Verify recommended options with product documentation and, for complex envelope repairs, consult a building-envelope specialist.

Envelope repairs and drainage improvements

Before applying any coating, ensure your building envelope is in good shape to prevent moisture issues. This includes fixing leaks, improving drainage, and addressing any gaps or cracks.

Flashings should be intact and properly sealed to direct water away from walls. Check and repair metal flashings, sealant joints, and caulking around windows and doors.

Clear drainage planes and ensure weep screeds are unobstructed to allow moisture to escape. Improve gutter performance by cleaning them out, ensuring downspouts direct water away from the foundation, and addressing any leaks or gaps.

By addressing these envelope repairs and drainage improvements, you create a better environment for any coating, reducing the risk of moisture-related problems.

Using test patches and phased approaches

Before applying coatings to your entire wall surface, consider using a test patch approach. This allows you to monitor the drying behavior of your walls and identify any potential issues before committing to full-scale application.

Apply a small, monitored test area (around 10-20 sq ft) in an inconspicuous location. Keep an eye on it for signs of moisture buildup or other problems over several weeks or months. If the test patch performs well, you can proceed with phased coverage.

Phased application involves coating a section at a time (e.g., one wall or floor level), allowing each section to cure and dry before moving on to the next. This approach helps ensure that any moisture-related issues are identified and addressed promptly, preventing them from spreading across your entire wall surface.

Back to top ↑

After coating, build a simple inspection routine to watch for new staining, soft spots, blisters, or loss of adhesion and address issues early. Regular cleaning and prompt repair of sealants and flashings extend performance and reduce the chance problems worsen behind the coating.

If you find failures, document them with photos, dates, and ambient conditions and retain the product label and technical data sheet to support warranty or claim discussions. For persistent or safety-related failures, follow the manufacturer’s claim process and consider engaging a qualified inspector.