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Are my windows causing my mold allergy?

If you viewed the results of my home’s mold plate testing, you probably saw the high count of mold colonies in my sunroom–which doesn’t even have air conditioning vents in it.   What the heck?  I was scratching my head until I remembered that I had placed the plate on a table only a few feet away from the exterior wall of windows.  There are 12 exterior windows in that room!

Here are some of the ways that windows can increase mold counts in your home:

• Direct water leaks:  if the seals or caulking fail on your window(s), they could allow water to run into the wall, where mold can grow.

• Drafty windows allow air to pass from the outside in, or inside to outside, where the temperature and humidity difference can surpass the dewpoint and cause condensation.  Condensation can occur on the windowsill or anywhere around the window that is not properly sealed.

• Outdoor mold can grow on the screens, because they retain dust and moisture.  When you open windows, air blowing through the screens will blow mold spores right into the room.

• Heavy window treatments create a micro-climate between the room and the window.  Although they are great at insulating the room from heat or cold, fabric curtains retain humidity/moisture, and also create a cold pocket of air without air circulation.  With drafty windows, air between long curtains and the wall can allow condensation, and offer the ideal darkness for mold to grow.    

In my case, I believe it was the drafty windows that allowed air to blow around the frames and carry mold into the room.

Here are some ways to get that mold count down:

• Check for leaks in your windows.  Here are some warning signs that a window is leaking and how to determine where it is leaking (video):

• Peeling paint on an interior window sill 

• Peeling paint on an interior wall near a window

• Rotting wood on the exterior window frame or sill

If you have any of these symptoms, it’s best to remove the outer siding materials and find out where the water is getting in, because cosmetic repairs will not fix the leak. 

• Decide whether to replace or reseal your windows.  It’s rarely an easy decision because replacing your windows is not likely to save you money, considering the cost of the windows and installation.  However, there are some ways to know that it’s time to replace them:

• Replacement: If your windows are extremely damaged by water infiltration, then it’s a good case for replacement. (ecohome.net)  There are many options for energy-efficient windows, available in wood, vinyl or composite (fiberglass or a combination of materials).  Like many products, the installation of the windows is just as important as the quality of the windows themselves. Quality installation is critical for an airtight fit and a continuous water barrier to prevent drafts, water damage and condensation.(efficientwindows.org)  To select windows, the National Fenestration Rating Council (nfrc.org) is a non-profit organization that gives consumers energy performance ratings and other useful information about windows, doors, and skylights.  You’ll also want to consider that new exterior windows that meet Energy Star standards fall under “qualified energy efficiency improvements” which can generate tax credits.  For windows purchased in 2022, you can claim the Residential Renewable Energy Tax Credit for 30% of the total cost, up to a maximum of $600 for exterior windows and skylights. (filemytaxesonline.org) In addition, check with your local power supplier to see if they have more incentives for replacing windows. If you do decide to replace, remember that higher-cost wood windows (which are clad in aluminum or vinyl for weather protection) are only as good as the seals and techniques of cladding them; if water penetrates the cladding, the windows can rot in just a few years.  In addition, the vinyl or aluminum skins on these windows do not permit passage of vapor from the interior to the exterior, so that condensation forms on the inside of the skin.  Condensation = rot! (video)

• Reseal:  If the windows and frames are in good shape, but you have airtightness problems, it’s most cost-effective to apply some sealing around them. Caulk, weatherstripping and caulking cord are all products made for these purposes, and this video shows how to apply them.  In order to know where the air is coming in, you can do the following (houselogic.com):

• Seal the house by locking all doors, windows, and skylights.

• Close all dampers and vents.

• Turn on all kitchen and bath exhaust fans.

• Pass a burning incense stick along all openings -- windows, doors, fireplaces, outlets -- to pinpoint air rushing in from the outside. Smoke Pencil Pro ($44) is non-toxic smoke pencil for this purpose.

• Clean your windows and screens!  On windows where screens are installed, a bi-annual cleaning will greatly reduce the amount of mold present.  In general it’s not better to wash the outside or the inside first, as long as both are done!  However, since outsides usually get dirtier than the insides, it may be easier to see inside dirt if you wash the outside first. You can use a bucket of warm water with a few drops of dish detergent on the outside with a long-handled brush and squeegee, and TotalClean or a mixture of one part white vinegar to two parts water in a spray bottle for the inside. 

• Get your drapes in order!  If drapes are causing condensation to form on or around your windows, this is not a good situation and it will lead to mold if left unchecked.  Here are some suggestions:

• Open the drapes during the warm part of the day so that temperatures between the room and window can equalize and dry out moisture. 

• Leave the ceiling fan running in the room to promote circulation. 

• True thermal curtains are made of 3-4 layers of material, including a vapor retarder that allows vapor to escape through the curtain.   Problems can occur if the curtains block vapor.  

If you still have a high mold count near your windows, it’s best to check with a qualified, reputable mold inspector to find out the source and be able to enjoy the sun and scenery through your windows instead of the mold!

Insulating drafty windows makes a difference in air quality, not just energy savings!

When you’re trying to make a whole house less drafty, you should go for the low-hanging fruit first.  That means big ol’ holes in the walls, floor or ceiling (like this disconnected register) come first.  If you don’t have any big holes, you can start on the smaller ones–and sometimes the smaller ones can add up.  That was the case in my sunroom. 

My sunroom is west-facing, which means in the summertime it gets brutally hot from sunlight, and in the wintertime it’s brutally cold from westerly winds.  On top of that, it’s fairly dusty and showed some of the highest counts of mold colonies in my home.  When I saw another cold front coming later in the week in December, I finally “made” the time to insulate the windows where I felt the drafts coming in.  I saw that the lower windows didn’t seal at the bottom when closed, AND the previous owner had cut the corners on the lift at the bottom for some reason (they rubbed on the trim?).  Due to these leaks, the room stayed quite “fresh”--meaning the CO2 didn’t really budge from outdoor levels unless I lit a fire in the fireplace on the other side of the room.  If you don’t have a CO2 monitor, I highly suggest getting one: here’s a portable version.

Therefore, the cons of my leaky windows easily outweighed this one “pro”: fixing them was a no-brainer!

Here are the tools I used: 

• A spray bottle of TotalClean and several rags for cleaning the windowsills
• Adhesive-backed weatherstripping in a suitable color to match your windows (low-profile like this one, ¼”, is good unless your windows are very misaligned) 
• Scissors
• Optional: A CO2 monitor is helpful–to see the effect on the room
• Optional: Window locks in case the windows don’t lock afterwards (see below)

If you do have a CO2 monitor, leave the windows closed and take a CO2 measurement before doing the insulation work. You might want to do it while there are a number of people in the room, or you have a propane or natural gas stove burning (I know there are a lot of gas stoves out there!).  Extra people and gas appliances do two things–they consume oxygen and they give off CO2.  These should cause the CO2 to be higher than when the room is empty or no gas appliances are lit.

Next, just get down to it: use the TotalClean and rags to clean off the window sill where the lower half of the window seats.  If you have vertical sliding windows, find the best place to attach the insulation in the vertical track and clean that.  When the track is clean and dry, start peeling the adhesive backing off and apply the weatherstripping a little at a time, cutting it when you get to the end of the track. Close and lock the window to make sure it’s placed correctly.

If you find your windows don’t lock because of the extra height of the insulation (this was the case for my windows) there are a number of window locks available that can be easily removed if you have to open the window for ventilation.  I ended up getting a thumb-screw version that can be moved up a few inches if I decide to ventilate, while still keeping the window secure.  

That’s it!  When you have all of the insulation installed, close the window and repeat the CO2 reading, with or without activity (people or gas-burning appliances).  The CO2 levels should go and stay higher because there is less fresh air coming in.  Fresh air ventilation is needed in a home, but it’s best to do it in a controlled way, not just letting the air come in wherever there’s a small gap or crack.   For more ideas on how to air seal leaky windows, check out this video.