How I finally stopped that gross black stuff from creeping up my window sills

I keep seeing borax recommended for this, but I’m not totally sold on it—at least not as a long-term solution. I tried it last winter, and while it definitely slowed things down, the black stuff still came back once we hit that first week of constant rain. Maybe I didn’t use enough, or maybe my sills are just too far gone? Hard to say.

One thing I did notice: after I sealed up every little gap with caulk (not just foam strips), the moisture problem dropped off a lot. It’s a pain since nothing in this house is straight either, but I went around the window frames with a flexible caulk and just smoothed it in with my finger. After that, even when the outside was soaked, the inside stayed way drier. No more puddles on the sill.

Dehumidifiers are helpful, but if there’s still air sneaking in, it feels like fighting a losing battle. For me, sealing first made a bigger difference than what cleaner I used. Anyone else try that route?

Yeah, I’ve seen borax suggested everywhere too, but honestly, it’s more of a band-aid than a fix. Once that moisture finds a path in, you’re just cleaning up after the fact. I’m with you—sealing up every oddball gap around those old frames made the real difference for me. The hardest part was getting the caulk to stick to my ancient, slightly warped sills, but the payoff was fewer drafts and less condensation. Cleaners are fine for the surface, but unless you stop the leaks, you’re just chasing your tail. Glad I’m not the only one who found sealing to be the real game-changer.

I hear you on the caulk not wanting to stick—my windows are from the 50s, and nothing is square anymore. I ended up having to use a flexible exterior-grade sealant instead of regular caulk, just because of all the shifting and settling over the years. It was messy at first, but it’s held up better than anything else I tried.

I’m curious, did you have to pull off any trim or just work around what was there? I debated taking off the old quarter-round to get in deeper, but honestly, I chickened out since I wasn’t sure I’d get it back on without splitting the wood. Ended up just running a bead along the edge and smoothing it with a damp finger. Not perfect, but it made a difference.

Also, how did you deal with the paint after sealing? My sills needed a fresh coat, but I wasn’t sure how long to wait for the caulk to cure before painting over it. I’ve heard some folks say 24 hours is enough, but others swear by waiting a full week. I don’t have that much patience, especially in winter when everything dries slower.

One more thing—did you notice any change in your heating bills after sealing? Mine dropped a bit, but I’m not sure if it was just the windows or if the weatherstripping on the doors helped too. Always hard to tell with these old houses where every little fix adds up...

Yeah, I hear you on the quarter-round—I've snapped more than a few old trim pieces just trying to pry them off. Sometimes I just leave well enough alone and work around it, especially when it feels like the wood might just crumble in my hands. As for painting, I usually wait at least 48 hours, but once I got impatient and painted after a day... ended up with some weird bubbling, so now I err on the side of caution. My heating bill did drop a bit, but honestly, I think the biggest change came after I tackled the attic insulation. Ever try air sealing up there? It’s a pain, but made a bigger dent than I expected.

Ever try air sealing up there? It’s a pain, but made a bigger dent than I expected.

Yeah, air sealing the attic was way more effective than I thought too—crawling around with a caulk gun and a flashlight felt like spelunking. For the quarter-round, I started scoring the paint first with a utility knife, then prying super slowly. Still snapped a few, but fewer splinters in my hands. And totally agree on waiting for paint to cure... learned that lesson after peeling up blue tape and half the paint came with it.

For the quarter-round, I started scoring the paint first with a utility knife, then prying super slowly. Still snapped a few, but fewer splinters in my hands.

I’ve gotta say, I tried the utility knife trick too and didn’t have much better luck. Maybe it’s just the age of my trim (original to the house—1920s), but even with careful scoring and slow prying, I ended up with a few pieces that just crumbled. At some point, I started wondering if it was worth all the effort or if replacing with new quarter-round would actually save time and headaches. The price of wood these days isn’t great, but neither is patching up splintered bits.

On air sealing—yeah, crawling around up there definitely isn’t fun, but I’m still not convinced it’s always the magic bullet for window issues. In my case, after sealing every gap I could find in the attic and rim joists, I still had that black stuff creeping up around the windowsills every winter. Turned out it was mostly condensation from poor insulation right around the window frames themselves. Ended up having to pull off some casing and stuff extra insulation in those gaps before things finally improved.

Waiting for paint to cure is one thing I wish someone had hammered into me earlier. Like you said, blue tape can be brutal if you’re impatient. Learned that lesson on my first window repaint—took off half the finish because I wanted to see results fast.

If anyone’s on the fence about tackling this stuff: sometimes it’s about picking your battles. Air sealing helps for sure, but if you’re still seeing mold or mildew by your windows, might be worth checking insulation or even humidity levels inside. Sometimes it’s not just drafts—it’s moisture hanging around where it shouldn’t be.

Not saying don’t air seal (it definitely helped with drafts upstairs), but for me, fixing those little cold spots around each window made more difference than anything else.

- Gotta push back a bit on the idea that air sealing isn’t a “magic bullet” for window issues. For me, it made a bigger difference than expected, even though my house is a leaky old 1940s build. I hear you on the insulation gaps—those are real—but if you skip air sealing, you’re basically letting all that cold air find its way in somewhere else.

- About the quarter-round:

“At some point, I started wondering if it was worth all the effort or if replacing with new quarter-round would actually save time and headaches.”

- On waiting for paint to cure... yeah, guilty of yanking tape too soon more than once. Now I just leave it for a full day, even if it means living with blue stripes for a bit.

- One thing I’d add: sometimes that black gunk is less about insulation or drafts and more about how humid your house gets in winter. If you’ve got condensation, maybe check your humidifier settings or run the bath fan longer. Just my two cents—sometimes it’s a combo of issues, not just one fix.

“If you skip air sealing, you’re basically letting all that cold air find its way in somewhere else.”

That’s been my experience too. I live in a drafty old place (1955, nothing’s square), and after years of battling mystery drafts I finally sealed up every weird gap I could find around the windows. Didn’t expect much, but it actually cut down on the black crud a ton—guess less cold air means less condensation, which means less of that moldy stuff creeping in.

About the quarter-round, I tried to save the old stuff at first. Ended up with more splinters in my hands than actual usable trim. Gave up and bought new... not cheap, but at least my blood pressure stayed normal.

One thing I noticed nobody talks about: if you have plants on your window sills, they can crank up the humidity right there and make the problem worse. Moved mine a foot away and it helped more than I thought it would. Sometimes it’s just a combo of little fixes that finally does the trick.

That’s interesting about the plants—I never would’ve guessed moving them a little could make that much difference, but it makes sense now that you mention it. I’m still figuring out the balance between sealing things up and not making the house too airtight (old place, so probably not a real risk). Also, totally agree on the trim... I tried to sand and reuse mine, but it looked rough and I got tired of picking tiny wood bits out of my fingers. Sometimes new is just worth it for the sanity.

I tried to sand and reuse mine, but it looked rough and I got tired of picking tiny wood bits out of my fingers. Sometimes new is just worth it for the sanity.

Couldn’t agree more on the trim. I’ve gone down that “maybe I’ll save some money” rabbit hole too many times. Last time, I spent a weekend sanding down old baseboards from the 50s—ended up with splinters, uneven edges, and honestly, they still looked tired even after two coats of paint. The amount of work just didn’t add up. These days, if something’s really shot or full of old water damage (which is usually why you get that black gunk on the sills in the first place), I just bite the bullet and replace.

On sealing things up, you’re probably right that an old house isn’t going to be too airtight—drafts find a way no matter what. But I did notice when I caulked every single gap around my windows (after scrubbing off that black stuff for the millionth time), it made a difference with condensation. Not perfect, but less water sitting around equals less mold creeping back.

And about moving plants—I was skeptical too. But my wife shifted her jungle off the window ledge last winter and suddenly the sills stayed dry. Less moisture trapped against cold glass, apparently. Now all the leafy stuff lives on stands across the room... looks weird but at least there’s no more black streaks.

Honestly, sometimes it feels like these fixes are just band-aids in old houses, but at least they buy you some peace of mind—and fewer Saturday afternoons spent scrubbing mold.