TL;DR

1. The culprit: Cold air was leaking through a gap in the insulation (Isopink) at the corner where the outer wall meets the frame, causing condensation.
2. The fix: Filled the gap deep with urethane foam to cut off the cold air path.
3. The finish: Trimmed the cured foam, then leveled the wall with mesh tape and filler.

📚 Series Overview

• [DIY Fix for Winter Wall Condensation #1] Dew on the Wall? My First Encounter with the Cold Draft (Current Post)
• [DIY Fix for Winter Wall Condensation #2] It Ain’t Over Till It’s Over: Round 2 vs. Condensation
• [DIY Fix for Winter Wall Condensation #3] Repair Complete: Finishing with Daiso Wallpaper

I woke up one morning and the wallpaper was wet. I touched it — definitely damp. At first I thought I’d spilled something the night before, but looking closer, there were water droplets beading right on the surface. Condensation.

My place backs onto an exterior wall, so my first thought was that something was structurally wrong with the building. I called the management office right away. Their answer: “That’s an internal insulation issue. You’ll need to handle it yourself.” I hung up and just stared at the wall for a minute. My mind went blank.

I found out later that the right first move here is to check with the management office whether the spot falls under common-area repair or unit-owner repair, and to bring in a professional if the situation calls for it. I didn’t have the patience to sort that out at the time. I just didn’t want mold taking over the wall. I hesitated about cutting it open, but figured nothing gets fixed if I don’t — so I looked up how to do it and opened the wall myself.

Hunting Down the ‘Cold Draft’

After stripping back the wallpaper and gypsum board to reveal the pink insulation (Isopink) underneath, a few things became clear.

Found it. Cold air was pushing straight through the gap next to the Isopink.

1. The entry point: By hand, I could feel cold air pushing through the gap between the insulation and the concrete frame, and where the insulation panels met each other. That cold air hitting warm indoor air was what formed the condensation.
2. Not mold, just cement: The black marks on the Isopink looked like mold at first glance, but turned out to be leftover cement residue from construction. No deep mold infestation, thankfully.

What I Used (Under $10)

All materials were things I could get nearby without any special tools.

• Urethane Foam (2 Cans, 7,440 KRW): Bought as a 2-pack on Coupang. Used one full can for this repair.
• Heavy-Duty Cutter Knife: For cutting out the gypsum board and later trimming the cured foam.
• Daiso Filler (Ogon Bond, 2,000 KRW): A cheap filler from Daiso, used over the mesh tape to level the surface.
• Everything else: Cutter knife, mesh tape — stuff I already had lying around. Total cost stayed under 10,000 KRW.

Doesn't look like much, but it was enough.

[!NOTE] This post contains affiliate links from Coupang Partners, and I may earn a small commission from qualifying purchases at no additional cost to you.

My DIY Repair Diary

Step 1: Injecting the Urethane Foam

I pushed the nozzle deep into the gap where the draft was strongest and discharged the foam.

[!TIP] Pro Tips for Using Urethane Foam

1. Spray upside down: To get liquid instead of just gas, you need to hold the can completely upside down.
2. It’s single-use: Once you start, foam hardens inside the nozzle fast. Leave it a few minutes and it clogs for good. Plan your whole section before you start so you can finish in one go.

[!CAUTION] Urethane foam expands way more than you’d expect. Fill only about 50-70% of the gap, or the expansion will push the surrounding insulation out of place.

Step 2: Re-fixing the Gypsum Board

The foam went in and, right on cue, I felt the draft respond. Then came the part I didn’t expect: the foam expanded far more than I thought it would, and the gypsum board I’d put back started pushing outward. That caught me off guard.

1. Right after injecting the foam, I put the cut gypsum board piece back in place.
2. I could feel the expanding foam pushing against it. Pressing the board down slightly deeper than the surrounding wall was the key move here.
3. That intentional recess absorbs the expansion pressure and also leaves enough depth for the filler later, so the final surface ends up flush.

Step 3: Curing and Trimming

After letting the foam cure fully (at least several hours), I trimmed the excess that had oozed out of the seams with the heavy-duty cutter. That sealed off the cold air path for good.

Step 4: Reinforcing with Mesh Tape and Filler

To keep the seam from cracking later from vibration or humidity, I reinforced it with mesh tape. Instead of buying a dedicated roll, I reused some leftover window-screen tape I had at home — it held better than I expected. Then I applied several thin coats of Daiso tube filler over it to level things out with the rest of the wall.

Sealed back up, and it looks clean. The mesh tape did its job.

After Sealing the Draft

Once I blocked the path the cold air was coming through, the wall temperature came up noticeably and the water drops disappeared. Condensation isn’t just a humidity problem — you have to block the actual path the cold air is taking. That’s the thing I actually learned here.

If insulation is bugging you, the first move is finding that cold-air tunnel behind the wallpaper. But this wasn’t the end of it.

3 Days Later: New Drops Appeared (Update)

I checked the repaired spot every day to make sure it had actually worked. On day three, new water drops showed up right above where I’d sealed it.

I thought this was done. New drops formed just above the sealed spot.

The original entry point was sealed off completely, but it looked like the cold air that used to come straight through had found another tiny gap and rerouted itself upward. I learned the hard way that insulation isn’t done when you plug one hole — you have to leave zero gaps anywhere nearby.

Turns out the fight against condensation wasn’t a one-round match. I treated it as a process of ruling out one cause at a time, and started prepping for the next round.