Q: I am trying to figure out why paint is bubbling on a ceiling in my home. Could it be from sweating on the HVAC duct, which sits above the area where the paint is cracking and peeling? If so, what is the solution?
In your case, you have a good idea that the problem might be moisture, and you know where it might be coming from. To confirm your hunch — or as a first step when there is no obvious explanation — use a moisture meter, such as General Tools’ pin-type digital model ($36.97 at Home Depot). Or get a pinless meter or one that switches between pin and pinless, such as General Tools’ combo model ($54.97). (Pinless operation is easiest for checking a general area; pins excel at pinpointing a problem.)
First take a reading where the paint is intact; it will probably be between 5 percent and 15 percent, depending on the relative humidity in your home that day. Then check the drywall where the paint is peeling. If that reading is significantly higher, check surrounding areas to map out where the ceiling or wall is moist and where it is dry. You might discover a trail of moisture that leads to a plumbing leak, a roofing issue or even a drainage problem outside your house. Whatever the scenario, fix the moisture issue before you deal with the paint.
Condensation forms on heating and air-conditioning ducts when warm, moist air hits a cold surface. If the condensation is forming only in one place, perhaps there is a gap in insulation around the ducts or a leak between duct sections. If you feel comfortable crawling around your attic, you can try to locate the cause by looking for dirty insulation, which shows where mildew is growing because of the moisture. Replace any missing insulation.
If a duct seam is leaking, apply duct sealant ($8.67 for 10.5 ounces from Master Flow at Home Depot), which will work better in attic conditions than foil tape made for sealing ducts. Apply the mastic with a caulk gun, but smooth over the bead with a gloved hand, a brush or a putty knife to ensure the material fills the gap.
If attic DIY jobs aren’t your thing, hire an HVAC company to check and repair the ducts. Also ask the company to ensure the system is calibrated properly, without clogged vents or a dirty filter. A company that provides air-sealing and insulation services can address overall attic conditions that may be making condensation worse by plugging gaps where indoor air is leaking into the attic and where insulation is too thin to be effective.
If you rule out moisture as the reason your paint is peeling, there are numerous other possible explanations. Scrape off a bubble or two to see whether just the last layer of paint is peeling or whether the failure goes all the way to the beginning. You might even find a layer of drywall mud between two layers of paint, a sign of an earlier repair.
If only the most recent layer of paint is peeling, the surface might have been dirty, which kept the new paint from gripping well, or too hot, which caused it to dry too fast. Indoors, high temperatures are most likely to cause paint problems where the sun shines onto a wall; the overall room temperature might be okay, but the surface might be well above the 85- to 90-degree maximum listed on the paint label. Paint can also bubble when someone paints directly over new drywall, or a drywall patch, without priming first. The drywall absorbs so much of the first paint layer that the next coat can’t grip well.
Whatever the cause, preparing for new paint begins by scraping off the bubbles and wiping down the surface. If it might have been dirty before the last paint job, use soapy water, then clear water, to clean it. Greasy walls and ceilings can be huge issues in kitchens. Let the surface dry, then inspect it. If it isn’t smooth enough to repaint, apply a skim coat of lightweight drywall mud, allow it to dry, then smooth it with 100- or 120-grit sandpaper. Or, to avoid creating dust, you can use a damp, stiff sponge that you rinse frequently in a bucket of water and wring out well. Clean off all the sanding dust, or wait for the wall to dry. Then prime the surface, wait the time recommended on the label and repaint.
If moisture was the culprit and you’ve fixed that problem, follow the same procedure for prepping the surface for paint. But use a stain-blocking primer to make sure any stains from the previous leak don’t migrate into your new paint.
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