Dear James: My house has wood siding, which I painted last year. Most of it looks good, but it blistered in some spots and is getting worse. What is the best way to repair it? —Dwight R.
Dear Dwight: Blistering can look very bad and generally will get much worse and more widespread over time. Blistering is not just a symptom of improper application techniques; it often indicates some more serious problems with the siding or the wall behind it. You should definitely repair it soon.
Persistent dampness in the wood siding itself is the most common cause of blistering paint. The paint film can hold itself together at first even if it does not adhere well to the wood, but over time and weather changes, it will bubble up. You may actually find some rotted wood under those spots.
There are also some other application errors that can result in blistering. If the wall was painted on a very hot day in the sun, it may not have adhered well. If it rains not long after the paint was applied, the paint may blister.
A good method to determine the cause is to scrape off some of the blistered spots. You will have to do this anyway to repair the spots. If the blistering does not go all the way down to the wood, the cause is probably poor application procedures. If the blistering does go all the way down to the wood, it is probably caused by persistent moisture from below.
When there is dampness in the wood siding, the insulation below it is also often damp. Dampness in insulation dramatically lowers its insulating characteristics. When the insulation value is reduced, there is a greater chance of moisture condensation inside the wall, which takes a long time to dry out completely, if it ever does.
This dampness may come from inside the home through the walls or from the outdoors from leaky gutters, poor caulking, etc. If the blistering is most prevalent on the exterior walls of the kitchen, laundry room, and bathroom areas, it is likely coming from indoors. Just typical activities, such as bathing, laundry, cooking, and dishwashing for a family of four, emit three gallons of water vapor per day into the indoor air.
If your walls have an adequate vapor barrier film under the drywall, excessive indoor moisture should not get through and cause problems. Unfortunately, many homes were not built with good vapor barriers, or they were not sealed well inside the walls. Low-permeability indoor wall paints are available to slow the moisture migration into the walls. Also, reducing the indoor humidity level and using kitchen and bathroom vent fans can help.
Check the plumbing for leaks. These often occur from overflowing sinks and poor seals where the fixtures mount on the countertops. Check the underside of the roof for indications of leaks. Water always drips from the lowest point no matter where it enters the roof. The leak may be near the peak of the roof, yet it may not drip until it is all the way down over the exterior walls.
The best advice for avoiding blistering problems in the future is to precisely follow the preparation and application instructions on the can of paint. Different paints have different instructions, so always read them.