Wintertime humidity in homes

No, you may not take a break from thinking about humidity just because it’s wintertime.  The first few cold days always kicks off the season when I get calls from people experiencing window condensation.  This week, I’ll share with the blog community some of the insights that I usually end up sharing with them.

Even more than usual, the following caveats apply:  (1) I need to see and test your house before I can give you advice specific to your situation.  So please consider these insights, but not actual personal advice.  (2) What I’m about to say is climate specific.  If you’re not in Western NC, my thoughts are only valid to the extent that your climate and local construction practices are similar to ours.

The problem shows up as window condensation on unusually cold mornings.  It’s typically worst around the edges of the frame.  Why does this happen?  Windows are the least insulated part of your home’s exterior (I hope!), and the frame is actually worse than the glass, so the edges of the window are the “weak link” in your home’s exterior envelope.  Warm air can hold more moisture than cold air can, so your cold window works just like a glass of iced tea on a hot summer day.  If the air indoors is warm and moist enough, you get condensation.  And here’s the rub:  if the window is cold enough, it doesn’t have to be that warm or that moist indoors for this to happen.

The good news is that there are a limited number of places the humidity can come from.  It’s NOT coming from your over-sized air conditioner (for real, I had someone call me and tell me they’d actually paid an auditor who told them this).  If your air conditioner hasn’t run for 4 months, you can’t blame it.  Your basement or crawlspace isn’t going to cause this either (that would be one horrifying crawlspace).  The outside air in the winter is as dry as a bone.  It’s coming from you and the things you do indoors, primarily showering and cooking.

The trend toward open showers is one of the biggest problems here.  It is very difficult to remove moisture from an open shower, even with a bath fan that you are using and that actually pulls as much air as it’s supposed to.  High flow rain heads are also a problem – more water, more humidity.  It helps to close the bathroom door during and after your shower and to have the bath fan on a timer delay or continuous run.

Bath and kitchen fans that people don’t use or that don’t work are another frequent cause.  If your house was certified Energy Star after July 1, 2012, the bath and kitchen fans have been tested and should be pulling the correct amount of air.  If it wasn’t, my experience has been that it’s unlikely for the bath and kitchen fans to all be working correctly.  I would need to measure it to be sure, but if a fan is very loud and won’t hold up a couple of pieces of toilet tissue, that’s a pretty good sign that you need to do some fan troubleshooting.  Also a fan only works if you use it.

Humidifiers are another problem.  I moved to Asheville from Omaha, where I learned the finer points of humidifier operation.  With the cold temperatures regularly experienced in the upper midwest, all kinds of havoc would ensue if people operated humidifiers like Southerners do.  You won’t die if the humidity goes below 50 percent.  Or 40 percent.  Or even 30 percent.  As it gets colder outside, you actually have to turn your humidifier down.  Here’s a great online resource from Minnesota (where they know something about cold weather).  According to them, when it’s 10-20 degrees outside you want your indoor humidity below 35%.  If that seems too dry to you, look at what their chart says for minus 20 and be thankful you live in North Carolina.  Keep in mind these are guidelines that will depend on the actual windows you have.

The windows that you have are part of this equation.  Unfortunately they are a part of the equation that most people are unlikely to change.  Windows are expensive and the weather in Asheville means that it’s difficult to financially justify a really well insulated window based on energy use.  This is also a problem that I see occurring frequently in passive solar homes, where the South-facing glass often doesn’t have a U-value quite as low as other windows.  In retrospect, I think I would have gone with a fiberglass window for my own house, simply based on durability.  Hindsight is 20/20.

I hesitate to say that tight houses also contribute to the problem.  It’s true, but building a leaky house is NEVER an appropriate approach to moisture control.  Building a leaky house brings in summer humidity, costs  you lots of money for energy year-round, and brings in air that you probably don’t want from your garage and crawlspace.  Build a tight house and ventilate it for indoor air quality.  And understand that you may need to do a little wintertime moisture management.

This is a good place to mention again a pet peeve of mine.  YOUR HEAT DOES NOT DRY OUR YOUR AIR.  Your heating system raises the temperature of your indoor air and if you want to get technical about the psychrometric chart this DOES decrease the percent relative humidity, compared to what it would be if you didn’t use heat.  However, the low absolute humidity that we experience in winter is because the cold outside air is very dry.  The more outside air you have infiltrating into the house, the drier it will be inside, and the colder it gets outside the more infiltration you’ll have.  It gets dry when your heat runs more because cold outside air causes both things to happen.  Correlation, not causation.

But we’re talking about high humidity.  In tight houses.  Because in a very leaky farm house you’d have a hard time generating enough indoor humidity to cause a problem.  The tighter your house is the harder you’re going to have to work to get shower steam and cooking moisture out of the house.  That’s why Energy Star has these pesky new requirements for bath fans.

Which brings us to ventilation. It has a role to play, but it’s a supporting role.  It’s always better to remove moisture at the source (bath and kitchen fans, turn off the humidifier) than it is to try diluting it with dry outside air.  Dilution is expensive so we save it for pollutants that we can’t control and can’t isolate.  With that said, your choice of ventilation system does have a minor impact on the overall indoor humidity picture.

Supply and exhaust-only systems will introduce outside air and will be the most drying in the winter.  An HRV system will recover some heat but virtually no moisture and will be similarly drying.  An ERV system will recover heat and moisture and will end up drying things out only about half as much as these other methods.  Before you decide ERVs are bad – the situation is reversed in the summer.  An ERV will add only a little humidity in summer and all the other methods will make your house more humid.  I often base my advice on ERV vs. HRV on whether I think a house is more at-risk for summer or winter humidity issues.  If the house is very tight, passive solar, and no basement or crawlspace I usually recommend HRV.  Otherwise, I go with ERV.  It’s important to remember that your ventilation system is not a dehumidifier.  So you can’t expect it to solve problems.  All you can do is select it to contribute less to the problem that you expect to be most likely to have.

If you have window condensation it’s hard for me to say how many steps you’ll need to take to reduce it to a manageable level.  I always start with by making sure the bath and kitchen fans actually work and that they are always used.  The next step would be to verify that you have a low-flow showerhead and are closing the bathroom door.  After that I’d consider a shower door.  Leaving shades up so that it doesn’t get even colder at the window helps. I’d consider ventilation and possibly increasing the flow rate of the ventilation.  In a few cases, I’ve recommended switching to an HRV or to exhaust based ventilation for the winter.  A whole house dehumidifier is an option, but due to their energy use, I prefer to try all of the above strategies first.

The question of how much condensation is OK is individual and depends on your specific windows.  Zero condensation is ideal.  But assuming that you have a wall section that can accommodate a reasonable indoor humidity, what you’re seeing on the window edge is probably the worst of it.  If you get a little condensation in the morning but it dries every day by lunchtime and it’s not damaging or water-marking the wood window frame you might decide that this is manageable.  If it’s not drying or is damaging the wood you need to keep working on indoor humidity.  Be a knowledgeable homeowner and monitor the situation so it doesn’t get out of control.

Copyright 2013.  Amy Musser.