HOME DESIGN : Moisture Is the Cause of Paint Blistering

One of the major pains of home ownership is blistering house paint. Nearly every home will suffer from at least one or two cases. In really bad cases, a house gets them all over, year after year.

Your first reaction when you see the ugly swelling appear may be to blame the paint. But the root cause of all blistering is moisture. It gets behind the paint and, when heated by the sun, turns to vapor and tries to escape. The only way it can get out is to literally push the paint right off your house.

Get rid of the moisture and you’ll get rid of the blisters. It’s as simple as that.

To do that, first of all, never paint your house in full, direct sun. If you do, the heat of the sun can cause the paint film to skin over before the solvents in the paint have a chance to evaporate. When that happens, the solvents will force their way out, forming small blisters in a matter of minutes or hours.

If you get blisters almost as soon as you put the paint on, it’s probably for this reason.

With that special case out of the way, let’s get down to more common, and more serious problems. Your first step is to locate the sources of the moisture that’s feeding your blisters. Where does water come from? One of two places. It either comes from inside the house, or it comes from outside and sneaks in.

Inside Moisture

This water comes from such sources as cooking, showering, unvented clothes dryers and so on. It permeates the air in your home and migrates out through the walls until it reaches the film of paint on the outside. You already know what happens then.

To solve the problem of indoor moisture, follow a two-pronged plan of attack.

First, try to eliminate the source. If you think pasta boiling on the stove is a major source, install a vented range hood. If your clothes dryer is not vented outdoors, buy a venting kit and rectify the problem.

If your family takes lots of long, hot showers, make sure each bathroom has a powered vent to send all that steamy air outside before it takes a shortcut through your paint job. If necessary, use a dehumidifier to lower the overall humidity level inside your home.

Your second prong of attack on indoor moisture: Make sure your home has an effective vapor barrier on all exterior walls. Ideally, this vapor barrier will be built into the wall, just behind the Sheetrock or whatever the interior face of the wall is.

In new homes the barrier may come in the form of a foil facing on the insulation, or it may be a separate plastic film.

If your walls don’t have vapor barriers, the easiest way to get them is to paint the interior faces of those walls with a vapor barrier paint. Any good paint store should have these paints. Apply this (and cut off moisture at the sources as noted above) and your troubles with indoor moisture should be over.

Outside Moisture

This is the water that leaks into your home. It can get in through bad caulking and unsealed cracks around window and door frames. It can seep in behind cracked, warped or rotted siding. It can get in through any unsealed joint in the siding, where boards meet end to end, or where they butt into corner trim. It can also get in through leaks in the roof, then seep down into the walls.

The solution to all these problems is to locate the source of the leaks and seal up or repair the entry points. In most cases, the location of the entry points should be obvious. Blisters are rarely far from the source of moisture that nourishes them.

Even after controlling the source of moisture, your walls may be so damp that they take weeks (or even longer) to dry out fully. For this reason, don’t repaint immediately after making your fixes. Give the house time to dry out.

You can speed this process along--and also gain a little insurance against future blistering--by installing plastic mini-vents in your siding. These are little plastic vents that fit holes drilled up under the edges of your siding. They give water an easy way out so it doesn’t have to bull its way through your paint.

If you have regular beveled siding, the holes for the vents go through the siding only. If you have lapped siding, drill through siding and sheathing. The special drills, guides and vents you need to do this kind of work are available at good building supply outlets and hardware stores. The kits come with instructions for proper spacing and so on. Once your moisture problem is solved, you still have to touch up the blisters. To do this, strip down to bare wood and sand the surrounding paint to a feather edge. Check the label on your house paint to see what kind of primer is recommended. Prime the bare wood with this primer, let dry, then follow up with two coats of house paint.