Failure of House Paints from Moisture Part 1: Causes

Most wood problems can be traced to a moisture issue.  The same holds true for paint problems on a wood substrate.  When too much water gets into paint or into the wood under the paint, the paint may blister or peel.  Understanding the causes of these failures will help homeowners to diagnose the particular problem and take corrective measures.  This is a four-part blog series.  Part 2 will deal with controls to the problem.

Blistering

Moisture blisters should not be confused with temperature blisters.  Moisture blisters usually contain water when they form or soon afterward. Temperature blisters do not contain water and usually occur within 1 or 2 days after painting with dark-colored paint on cool days in the fall and usually only appear in the last coat of paint.

Moisture blistering usually includes all of the paint down to the wood surface and indicates that the wood behind the paint is excessively wet.  Moisture blistering usually occurs in early spring and will occur first on only specific areas in heated buildings.  These are areas that enclose rooms with a high relative humidity in the winter or areas wet because of gutter problems, etc.

Moisture blistering is often more common in new, thin coatings of oil-base paint containing zinc oxide pigment than in old, thick coats.  Older and thicker coatings are usually too rigid to swell enough to form blisters; instead, they are more prone to crack and peel after excessive wetting.

Peeling and Cracking

Peeling is a common type of water damage to paint and does not usually involve the formation of distinct blisters.  Failure occurs in both heated and unheated buildings on siding areas where rain and dew commonly wet the paint.  Such failures are associated with porous, flat, oil-alkyd, and latex paint systems which hold water on the surface and so provide ample time for water penetration into the layers of paint.  Peel can occur at the wood interface or at some weak bond between layers of paint.

Cracking failure, followed by peeling at the ends of boards and the lower portion of horizontal siding, also indicates the adverse effect of rain and dew penetrating through paint or through cracks in paint.  Such failures will occur on all sides of houses and also on unheated buildings.

Peeling failure at localized areas, such as gable ends of a heated building, indicate that the moisture is coming from within the building.

Sources of Problematic Moisture

Water that works its way behind or through the paint film to cause blistering and peeling can come from several sources.  Water might be getting in from the outside or going out from the inside of the house.

Outside Water Coming In

Rain and dew account for tremendous volumes of water which come into contact with painted surfaces.  If the outer layers of paint are flat, porous systems, they will absorb large quantities of water which can gradually penetrate down into the paint coating.  During weathering, cracks also gradually develop in paint, particularly over joints and at the edges and ends of boards on the side walls of a building.  Water can pass through these cracks, soak into the paint or wood, and produce paint peeling.  Leaks in the roof, or the formation of ice dams on a roof, also allow outside water (melted snow) to enter the wide walls and damage paint.

Inside Water Going Out

Water from inside of a building can attack paint on the outside by soaking through the wood.  This water can come from such faulty conditions as leaks in the plumbing, overflow of sinks and bathtubs, or shower spray on a bathroom wall that is not properly sealed. Also, it can simply result from high humidity.

Interior water vapor can penetrate through the ceiling to condense on the gable ends and roof boards.  If the attic space is not adequately vented, this moisture can cause paint failure on gable ends and other side walls when the water runs down the rafters and roof boards to the outer walls.

Excessive humidity inside a building facilitates the cold-weather condensation problem.  Frequent condensation on the windows during cold weather indicates that the humidity inside the building is too high. 

Most homes have many sources of water vapor:  bathing, laundering, clothes dryers, humidifiers, etc.  Crawl spaces, without vapor barriers, also contribute moisture that works up into the home.