Bubbling of exterior paint coating is usually caused by moisture in the substrate (wood siding, stucco, etc.). Moisture bubbles, unlike heat blisters, will go through all coats of paint down to the substrate.

Under normal conditions, water will want to evaporate. If water is in the substrate and it doesn’t have any coating on it, it isn’t a problem. It will just evaporate—dry out.

However, if the paint coating gets in the way of water trying to evaporate, the water vapor, unable to penetrate it, will put enough pressure on this coating to force it away from the substrate, creating a bubble.

To stop moisture bubbles, you must find and eliminate the source of moisture intrusion.

Common Sources of Moisture Intrusion

1. Roof and Gutter Leaks
– Check for leaks that might be allowing water to enter the substrate from the roof or gutters.

2. Leaky Windows and Lack of Proper Flashing
– Ensure that windows are properly sealed and that flashing is installed correctly to prevent water from seeping in.

3. Improper Construction Techniques
– Unsealed wood end-grain, uncaulked vertical joints, and seams around windows and doors can all allow rainwater to enter the substrate and cause paint to bubble.

4. Internal Moisture Sources
– Moisture moving through a wall, from inside the home to the exterior paint, can come from plumbing leaks, sinks, and tub overflows. These shouldn’t be ruled out as possible culprits.

5. Environmental Moisture
– Moisture vapor from hot showers, cooking, damp basements, and the use of humidifiers can all be the cause of excessive moisture in the substrate.

If there isn’t any vapor barrier, or if the barrier is incorrectly installed, the vapor can easily move through the exterior wall and cause paint bubbling.

Suspect this if you find bubbling on the outside of bathrooms, laundry rooms, and kitchens. Improve ventilation (install fans, open windows, etc.) and consider strategic use of dehumidifiers.

6. Bedrooms
– Paint bubbling outside of bedrooms often puzzles homeowners and professional painters alike. On average, a person loses about 28 ounces of water due to respiration and perspiration per 24-hour period.

This breaks down to about 9.5 oz. per night (8 hrs.) per person. As the sun comes up in the morning and the outside temperature rises, this moisture will want to evaporate, and some of it will find its way to the back of your exterior paint coating.

The hotter the surface temperature, the stronger the push to evaporate, and the more likelihood of bubbling.

Tips to Prevent Paint Bubbling

# Use Lighter Paint Colors

The lighter the surface color, the more sunlight energy is reflected from it. The darker the color, the more sunlight energy is absorbed by the surface and the hotter it gets. Choosing a light exterior color can help with the paint bubbling problem and will generally extend the lifespan of a paint job.

# Ensure Substrate Breathability

Paint substrate has to be able to “breathe,” meaning it must be capable of releasing moisture it has absorbed. Proper venting and free-to-move expansion joints (as in the case of wood siding) are all very important for this reason.

However, vents are often missing altogether (or are plugged by caked-on paint) and expansion joints are often caulked shut by over-zealous homeowners and amateur painters. This can result in moisture being trapped in the substrate.

# Manage Paint Coating Thickness

Paint coating itself is generally breathable, allowing moisture vapors to pass through it. However, after a home has been repainted five or ten times, it can have ten or twenty coats of paint on it.

This can bring the breathability of paint, and the substrate it is coating, to near zero. This is why you can often solve a persistent paint bubbling problem by stripping all of the old coating from a substrate before repainting.

In a situation where the substrate has lost most of its ability to breathe, even a very small amount of moisture entering it can build up and result in paint bubbling.

Keep in mind that perfectly dry and suitable-for-painting wood will have between 6 and 15 percent moisture content.

Even if the surface has never bubbled before, just by decreasing its breathability (through adding coats to already heavy paint buildup) and/or raising surface temperature even by just a few degrees (due to a darker color paint used) you can cause paint to start bubbling.

Recap: Dealing with Moisture-Caused Bubbles

1. Detect and Eliminate Sources of Water Intrusion
– Perform thorough leak detection and address issues such as roof and gutter leaks, leaky windows, and improper construction techniques.

2. Handle Excess Moisture Generated Inside the Home
– Increase ventilation and use dehumidifiers to manage internal moisture sources such as hot showers, cooking, damp basements, and the use of humidifiers.

3. Increase Substrate Breathability
– Improve exterior venting and strip old paint to ensure that the substrate can properly breathe and release absorbed moisture.

4. Reduce Surface Temperature
– Use lighter paint colors to reflect more sunlight and reduce surface temperature, minimizing the likelihood of bubbling.