Why do I have window condensation in my new house?

Amy Musser

I want to celebrate the unusually cold weather we’re having in WNC by posting about a problem that comes up quite a bit this time of year: interior window condensation. It’s one of the few problems that I can usually diagnose over the phone. In this blog, I’m going to walk you through my thought process.

First, let’s look at why window condensation happens. Cold air can’t hold as much moisture as warm air can. That’s why you can see your breath outdoors in the winter. The warm, humid air that you exhale instantly cools and the moisture condenses out of it and forms a small cloud. It’s also why your lemonade glass gets condensation in the summer. The warm, humid air around the glass is cooled by the glass and condensation happens on the surface. The same thing happens with your windows. They’re the “weak link” in your building envelope, and the colder it gets outside the colder their surface gets. If it’s humid enough indoors, you’ll get condensation on the glass. It will usually start at the edge near the frame, but if it gets bad enough it could cover the whole window.

How concerned should you be? If it’s only happening in the morning on the few coldest days of the year and it’s drying by mid-day without causing any damage, it’s probably not something to be very worried about. If it’s happening every day and creating enough water that you have to wipe it up or staying around long enough to cause water marks, you need to lower your indoor humidity.

If the house has a humidifier, you need to turn it down. I get asked a lot what the ideal setting for a humidifier is, and I always answer “off”. New homes are tight enough that they tend to retain enough indoor-generated humidity that a humidifier is seldom needed. I see them causing a lot more problems than they solve. In the South, homeowner’s expectations tend to be unrealistic. You can’t keep your house at 50% relative humidity all year round. Building envelopes aren’t designed for that much winter humidity. I think it’s unlikely that the interior of a new home would get dry enough to cause serious comfort issues or problems for wood furniture in WNC. Here’s a good article that discusses these issues in more detail. If you’re not going to turn it off, then you need to pay attention to your window condensation and turn it down when the condensation happens. The colder it is outside, the lower your setting needs to be. And don’t freak out about the number on the dial – I’m not convinced that the sensors are as accurate as people think they are anyway. It could say 25% and you might actually be getting 30%.

If that doesn’t do it, my next step is to look at bath and kitchen fans. Bathrooms and kitchens are two of the biggest sources of indoor moisture, and often you will notice that a problem is worse in these rooms or on windows near these rooms. It should be obvious, but these fans only work if they are being used. Make sure you are using them every time you cook or take a shower. Leaving them on for 10-20 minutes afterward is also helpful.

If bath fans are being used but the bathroom still seems like it’s a problem, I next look at the effectiveness of the fan. If the house was Energy Star rated after 2012, the fans would have been required to be tested to be moving at least 50 cfm of air. If it was not certified, or was certified under an earlier version of Energy Star, they may not have been tested. It’s very common to find bath fans that move very little air, and there is an easy do-it-yourself test. With the bath fan on, you should be able to get it to hold up about 3 pieces of toilet tissue. If it doesn’t, you should have someone look at the fan to determine why it’s not working right. The other thing you want to look at is the location of the fan, and whether it can effectively remove moisture from your shower. Ideally the fan would be in the ceiling of the shower or very close to it. If the only fan in your bathroom is in a separate toilet compartment, it’s not going to do much to remove humidity. Open showers and rain-style showerheads are also a problem. With an open shower, you’ll need to keep the bathroom door closed during and after your shower, and run the fan longer after the shower is over. If the flow rate of your showerhead is more than 2.0 gpm, consider replacing it with a more water conscious model. You can also increase the size of your bath fan to help it work more effectively.

If you don’t have a whole house ventilation system, you should have one anyway. Whole-house ventilation is NOT a humidity control strategy. However, most ventilation systems will decrease the indoor humidity in winter a little bit. Some reduce it more than others, so if you have this problem, you should look at exhaust-only, supply-only, or HRV system. If you already have one, sometimes increasing the flow rate of these systems a little bit in the winter can help. There’s an energy cost associated with doing this, but if a modest increase can solve the problem, it’s worth it.

Homeowners should also consider their own activities and whether those could be adding an unusual amount of indoor humidity. Pools, spas, and saunas make me very nervous in this climate. They’re difficult to bring indoors in a safe way. Some people do a lot of cooking or boil water for tea all day. I’ve even heard of a moisture problem that was caused by homeowners line-drying multiple loads of laundry in their basement every day.

Another simple thing that can help a lot in some homes is to remove the window screens in the winter.  If you happen to live in a passive solar house, you should do this anyway to let more solar energy get into your house during the day.  It can also allow more air to circulate near the window surface, raising the interior temperature a little bit and making condensation slightly less likely to occur.

This is the process that I use to diagnose and solve winter humidity problems in new homes. Older homes rarely experience this problem because they experience so much infiltration with ultra-dry outdoor air that they have the opposite problem and may, in fact, need a humidifier. Sometimes you see an older home with very inefficient single pane windows that will develop this problem, and in that case you would want to consider window replacement (or the addition of bath fans if it’s only happening in the bathroom). This gives you some ideas to try. If you’re having persistent or unexplained problems that you can’t solve, be sure to enlist the help of a professional. Moisture problems shouldn’t be allowed to persist indefinitely.

Copyright 2015 Amy Musser.