Bubbles in the exterior paint coating are 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 the normal conditions, water will want to evaporate. If water is in a substrate and it does not have any coating on it, it isn't a problem. It will just dry out. However, if the paint coating gets in the way of the water which is trying to evaporate, the water vapor, unable to penetrate the coating, will put enough pressure on the paint to force it away from the substrate, creating a bubble.
To stop moisture bubbles, you must find and eliminate the source of the moisture in the substrate.
Roof and gutter leaks, leaky windows and lack of proper flashing are all usual suspects in leak detection. Improper construction techniques, unsealed wood end-grain, uncaulked vertical joints and seams around widows and doors can all allow the rain water to enter the substrate and cause paint coatings to bubble.
Likewise, moisture moving through a wall to the exterior paint can come from inside plumbing leaks, sink and tub overflows. These should be suspected and ruled out as possible culprits.
Moisture vapors from hot showers, cooking or using a humidifier can all be a cause of excessive moisture in a substrate. If there is not a vapor barrier or if the barrier is incorrectly installed, the vapor can easily move through exterior walls and cause paint bubbling. Suspect this if you find bubbling on the outside of bathrooms, laundry rooms and kitchens. Improve ventilation (fens, open windows, etc.) in those areas and consider the strategic use of dehumidifiers.
Paint bubbling outside bedrooms often puzzles home owners and professional painters alike. This is because there seems to be no obvious source of moisture. Well, let’s take a closer look. On average a person loses about 28 ounces of water due to respiration and perspiration per 24 hour period. This brakes down to about 9.5 oz. per night (8hrs) per person.
What happens with all of this moisture? 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 the exterior paint coating. The hotter the surface temperature, the stronger the push to evaporate, the more likelihood of bubbling.
Here is a paint tip: use lighter paint colors whenever possible. The lighter the surface color the more of sun light energy is reflected from it. The darker the color, the more sunlight energy is absorbed by the surface and the hotter it gets. This is why choosing a lighter exterior color can help with the paint bubbling problem and will generally extend the life span of a paint job.
Paint substrate has to be able to "breathe", that is, it has to be capable of releasing moisture it has absorbed. Proper venting and free to move expansion joints (as in the case of wood siding) are very important for this reason. But vents are often missing all together or are plugged by caked-on paint and the expansion joints are often caulked shut by over-zealous home owners and amateur painters. This can result in moisture being trapped in the substrate.
Paint coating itself is generally breathable. That is to say, it is capable of allowing moisture vapors to pass through it. However, after a home is 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 bubbling problem by stripping all of the old coatings 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 a substrate can build up and result in paint bubbling.
Please also keep in mind that a perfectly dry and suitable for painting wood will have between 6 and 15 percent moisture content. So, even if the surface has never bubbled before, just by decreasing its breathability (through adding coats to an already too havy of a paint built up) and/or rasing the surface temperature by just a few degrees (due to darker color paint used) you can couse paint bubbling.
So, let’s recap. To deal with moisture caused bubbles one has to:
1) Detect and eliminate sources of water intrusion (leak detection)
2) Handle excess moisture generated inside a home (increase of ventilation and use of dehumidifies)
3) Increase the substrate breathability (improve exterior venting and/or old paint stripping)
4) Reduce the substrate surface temperature (use lighter paint colors)
I hope this helps.