black mold/mildew on windowsills in rental - our fault

We are six months thru our one year lease here in this house (and we plan to move out when the lease is over). we installed blackout curtains, with the landlords permission, in our childs bedroom. since winter, ive noticed a lot of condensation on the windows when i open them in the mornings. i didn't think about it (ugh!). recently i saw that there is a lot of black looking mold/mildew all around the sills. clearly from all the wetness due to the heavy black out drapes that we have closed all night and for several hours during the day (and sometimes my husband doesnt even open after taking the baby out of the room, them so they are closed 24 hours straight).

there are many posts on mold/mildew so

A) i am scared about our babies/our health (we sleep in the room too). should we pay to get it tested so we know if we are at risk? what test/where do we get this?

B) i know i need to tell the landlord asap but i am scared! will we need to pay for all new windows in that room - so expensive! do we clean as best we are able, and continue to clean regularly, and make a much bigger effort to crack the windows and open the drapes whenever they are not in use, etc. ?  What is the worst case scenario - could we be liable?

Any advice on how to present the issue to protect ourselves - i.e. clean it first and then invite the landlord over to inspect...?

I didnt inspect before we moved in , so perhaps there was a little bit there already, but there were no blinds , so it was definitely not to the extent it is now.

Thank you so much - pretty freaked out for HEALTH most of all but also the financial implications.

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Don't panic! This is an extremely common issue in the Bay Area in the winter if your windows aren't air-tight (and most aren't). You can address it pretty easily by wiping your windowsills down with a 3:1 bleach solution and monitoring it regularly. (Yes, it will come right back unless your landlord opts to replace the windows, so do clean often.) A dehumidifier may also help if the room is very moist. The blackout shades are likely contributing to its growth by cutting off the airflow that was preventing it before you installed them, but the underlying issue is just that the windows are older and no longer airtight. (Do watch the shades as well, as mold can grow there too--there are instructions for washing shades that vary a bit by brand, so just follow your manufacturer's guidance.)

Hi! We just went through a mold situation, good news is it takes a lot more mold than i thought to impact air quality. That being said I'd get an air quality test ASAP.  I'd also take photos/video of the mold first and then wipe it away with a recommendation you can google (no bleach for a kid's room). We used Mold Busters for the air quality testing: and Ido Taylor for the mold removal: Ido was AMAZING. Feel free to reach out with any questions.

Quick question, what kind of windows are they? We still have the original windows in our house (single pane metal frames) and condensation has always been an issue and we don't have blackout curtains, so it may not just the blackout curtains that are causing the problem. It's poor insulation around the windows & lack of airflow. You can either clean them yourself or call Mr. Sparkle, or another reputable window cleaner to have them cleaned. They should be able to tell you how to prevent future infestations. 

Healthwise here is what the CDC has to say

Hi - 24 hours without airing doesn’t seem like long enough time to elicit mold growth.  What actually may be happening is the windows and insulation may be failing causing the sweating.  

from what I understand, if windows and wall insulation are working correctly, windows should not sweat, especially if they are double pane windows.  

I would get it tested. It’s not cheap - around $600 dollars.  These experts can also tell you if it’s due to condensation or due to water intrusion because of failing insulation.  They can test the window sill and walls for water content.  Normal condensation with only show up to a certain water content in the wood in the window sill.  

try contacting Ricardo Medina.  He’s great - we had him take a look at apartment which we were renting which had massive mold issues the landlord had been hiding (previous tenant most likely left because of it....her handy man just painted over the mold on the window sill to cover up the issue).

Curtains cannot cause black mold or mildew. This is being caused by moisture coming in from outside. The windows and/or moulding must not be sealed.  

I would start by getting an inexpensive humidity reader as well as a dehumidifier (these unfortunately cost a few hundred dollars but they are worth it, you will be amazed at how much water it pulls out of the air).  Put the dehumidifier near the curtains area and run it frequently. The goal is to keep the room below 55 or 60 percent humidity. Mold cannot grow unless there is a moisture source (condensation in your case) so you can rest easy that the mold will not be growing once you're maintaining a lower humidity environment.  You might also want to get a small fan that is aimed at the curtains. Keeping the air moving means any moisture that does condense will not be trapped behind the curtain.  And then if you can sufficiently clean the surface mold on the paint or the sill, that might be the end of the issue?   

If you decide to get the mold tested, avoid air tests which are really unreliable in my experience. You would want someone to take a physical sample of the mold on a piece of tape in order to test that at a lab.  

I don't think you need to worry. I know there are some toxic kinds of mold out there, but my understanding is that the common black household mold that often forms on windowsills is not particularly harmful. I had this problem once when I lived in a particularly damp SF studio apartment on the ground floor. It's likely your landlord already knows the apartment is susceptible to it. Just clean it well (with bleach! Clorox! Now is not the time for vinegar). I would also suggest getting a dehumidifier for the room - that's what really ended up making the difference in my old apartment - just search for a small mobile unit on Amazon that you can plug in. I was SHOCKED to see how much water it would pull out of the air in 24 hours - gallons! Best of luck! 

Hi there,

This happened to us in our last rental. I told my landlord immediately, and she asked us to get a mold test (which she paid for). You can buy the test kit online, or I think in a drugstore; swab a sample of the mold, and send it off to a lab. The results came back that it was just basic mold, not the toxic black mold, even though it looked black and scary.

Because it wasn’t toxic, I decided to clean it myself, using a vinegar solution (wearing a medical mask and gloves) and possibly Lysol. You can Google the most effective cleaning agents for nontoxic mold — I think that’s why I chose vinegar. 

I fully understand how scary this seems, but I think nontoxic mold/mildew is quite common here due to condensation on older windows. Hopefully yours turns out to be low-risk too. Good luck!

I wouldn't make any assumptions about what caused the mold. Were they single paned windows and up to code? So many questions, don't assume you caused the problem.

If you're worried about possible financial liability, I'd consult a lawyer experienced in representing tenants before you communicate anything to the landlord.

Black mold is usually bad news, especially for your baby. It may or may not be your fault. Landlords have been known to paint over mold, which does not go away, it grows again through the paint. So, it may have been there before you got the drapes. Make your landlord get a professional to clean it up ASAP! Spores are released when you clean so it must be done with the right protective equipment. Good luck.

I’m not a mold expert but I think you’re conflating 2 different mold issues. The scary mold, I believe, is inside walls and you can’t see it. It can be very extensive and cause serious health issues. If that kind of mold existed already, and the curtains made it show up, then that’s on your landlord. I think it’s worth exploring by hiring the right person - I’d do this without involving landlord at first. Just call companies and discuss the situation.

I suspect the mold your curtains have created is easily solvable and less scary. Bleach the hell out of everything and get mold killing stuff. Wipe down every week. Air that space out daily. I am certain that many people have experienced this issue and there’s good advice - easy advice - for minimizing it online. I’m thinking use a double curtain rod - inner curtains are thin absorbent cotton, outer are the blackout ones, hung several inches away from the wall, with airflow.

good luck!

Just want to know that we noticed the same thing.  We never opened the blackout curtain and noticed the black mold on the window seams.  We spread lots of bleach, and start to air out every day.  The mold never came back again.  We have lots of other windows.  None of them had mold problem except the one with the blackout curtain.  

I can't speak to your specific questions, but I did want to make a suggestion, as we lived in a rental that was very humid and produced mold in the bedroom and bathroom, and opening windows didn't help much. After you do clean everything, buy a large dehumidifier (not those tiny ones!); it costs about $150, but so worth it. You can run it during the day to reduce the condensation from building up and it will help prevent further growth. 

This is not a huge deal and not unusual.

A very similar thing happened with us. We had a skylight over the bed. My partner and young child both couldn't sleep. We couldn't install blackout shades due it being flush with the slanted ceiling where the lowest point was 12' and it went up higher from there, and there just wasn't any existing hardware for us to install shades. We did a DIY solution of aluminum foil around the skylight, installed loosely because we were also worried about mold and mildew. We never took the foil off for the year we were there, and when we moved out, there was also mold and mildew (and also leaking around the skylight that we didn't know about before). The owners charged us for cleaning/disinfecting and repainting and that was that, a couple of hundred dollars). Nothing was replaced. 

So to put your mind at ease, it's not a HUGE deal financially, and not all black colored mold or mildew is the kind that is going to make the occupants seriously ill.

And really no point in testing if you see the problem and will be addressing it to get rid of it. Whatever harm to your health is already done, there's no changing the fact that you've already breathed it. Going forward, just keep the mold/mildew under control. If you were in a situation where there's unmitigated mold not of your doing and owner is non-responsive, then you can get testing done to make a case that it's dangerous to your family and it needs to be addressed.

The mold and mildew can be cleaned off with diluted bleach and aired out. You can do it yourself now and just keep an eye on it over time, and then let the owner know when you move out that you had clean it up. You can also run a dehumidifier in the room to help with the moisture if you're not opening the windows.

Sorry about what is happening to you and your family-- with the mold in your family's bedroom.    Here's something for you to consider doing.   Because it's possible that what you see around the windows is the dreadful Black Mold, you need evidence--- so please don't clean it up.  If you have cleaned it, ok.  It will return.  

  I'm assuming you live in Berkeley-- please contact "Berkeley Rent Stabilizer" 510 - 981-7368.  Tell them something on the order of --------"I'm a renter, living at _______street.  I fear there's black mold around the windows in the room where my 2 year old, husband and I sleep. I'm afraid to tell the landlady/owner.  Can you please send someone out to take a look?" 

   All the best.