Mold Behind Drywall in Older Homes: What to Look For and What to Do

Mold grows behind drywall when moisture reaches the wall cavity and has no way to dry out. In many 1950s Peninsula homes, that moisture comes from small, long-running leaks, condensation on cold walls, and bathrooms without exhaust fans. Because the back of the drywall is out of sight, mold there can grow for years before anyone notices.

The U.S. Environmental Protection Agency puts it simply: the key to mold control is moisture control. Find and fix the water, and mold stops growing. Leave the water, and mold comes back even after cleanup.

Why older homes are especially prone

Homes built in the 1950s were not designed to manage moisture the way homes are today. Four conditions are common:

• Little or no wall insulation. Uninsulated exterior walls stay cold in winter, so warm, humid indoor air condenses on them, often behind furniture or inside the wall.
• No vapor or water-resistive barriers. Older walls let moisture move through freely and rarely let it dry back out.
• Aging plumbing. Slow leaks from galvanized pipes and old fittings can wet a wall cavity for years without showing on the surface.
• Weak ventilation. Bathrooms and kitchens without exhaust fans push steam into walls and ceilings.

Signs that mold may be hiding

Hidden mold is hard to confirm without opening a wall, but these signs are worth taking seriously:

1. A musty smell in one room or closet that doesn’t go away.
2. Stains, bubbling paint or peeling wallpaper on an exterior or bathroom wall.
3. Condensation on windows or walls on cold mornings.
4. A past leak, such as a roof, window or plumbing leak, that was patched but never fully dried.
5. Allergy-type symptoms that ease when you’re away from home.

The EPA notes that hidden mold is often found on the back side of drywall, wallpaper and paneling, inside walls around leaking or condensing pipes, and behind furniture where condensation forms.

Can mold affect health?

According to the EPA, mold can cause allergic reactions and can trigger asthma attacks in people with asthma who are allergic to mold. It can also irritate the eyes, skin, nose, throat and lungs. If you have health concerns, talk to your doctor; this article is about the house, not medical advice.

What to do if you suspect mold

1. Find the water source first. A leak, condensation or poor ventilation is almost always the cause.
2. Don’t open the wall yourself if the area is large. The EPA recommends professional help when mold covers more than about 10 square feet, because disturbing it can release spores.
3. Remove, don’t just cover. Moldy drywall and insulation are usually cut out and replaced. Painting over mold does not fix it.
4. Fix the cause before closing the wall. Otherwise the mold returns.

How a remodel or rebuild solves it for good

A major remodel is the one time the walls are open, which makes it the best moment to deal with moisture properly. Greenberg’s approach:

• Look before you close. When walls come down, the cavities and framing are inspected, and any damaged material is removed. The moisture source is fixed before anything is closed up.
• Build a high-performance envelope. Continuous insulation and a sealed air barrier keep interior wall surfaces warm, so condensation has nowhere to form. Related: What a high-performance envelope is.
• Ventilate on purpose. Bathroom and kitchen exhaust plus an ERV keep indoor humidity in a healthy range. The EPA suggests keeping indoor humidity below 60%, ideally 30–50%.
• Replace old plumbing. New supply lines remove one of the most common slow-leak sources.

We know this one firsthand. When a Greenberg client’s 1957 San Carlos home was taken down for a rebuild, the crew found mold across the back of the drywall behind his son’s bed.

FAQ

How do I know if there is mold behind my drywall?
A lasting musty smell, stains or bubbling paint, and a history of leaks are the most common signs. A professional inspection can confirm it without opening every wall.

Is mold behind drywall dangerous?
The EPA says mold can cause allergic reactions and trigger asthma attacks in people who are allergic. Talk to a doctor about health concerns.

Can I just paint over it?
No. The EPA advises against painting over mold. Moldy drywall usually needs to be removed and the moisture source fixed.

Will a remodel fix a mold problem?
Only if the cause is fixed too. Insulation, air sealing, ventilation and new plumbing stop the moisture that feeds mold.

Planning a remodel or rebuild? Ask what’s behind your walls. Start the conversation

Part 1 of 7 in Healthy by Design, our Wellness series in the Journal. Next: What Is a High-Performance Envelope? See how Greenberg builds for wellness.

Sources: EPA, A Brief Guide to Mold, Moisture and Your Home