The musty days of summer: Combating carpet smells
Some home ownership challenges are easy to understand and others are mysterious. Summertime mustiness is one of the mysterious kind. I was reminded of this when I got a message from a puzzled man growing weary of battling mustiness rising from his carpet. “If I don’t run the dehumidifier all the time during hot weather,” he explained, “the carpet on the concrete main floor of our home smells strongly of mould. What can I do?”
The first step to solving any problem is understanding it, and this is especially true with musty carpet syndrome. Imagine that you’ve got carpet on a cool concrete floor somewhere, and the outdoor air outside your home is 30 C with a 90 per cent relative humidity. That’ s your typical hot, humid day. The thing to understand about relative humidity is that it’s relative to temperature. The cooler the air is, the less moisture air can hold. So as this warm air makes its way into your house through open windows and natural air infiltration, some air will eventually enter the pile of the carpet, sneaking down toward that concrete floor.
As the temperature of this humid air declines in contact with the cool concrete, relative humidity skyrockets because the air is less able to hold moisture.
If this cooling continues far enough, relative humidity of the air at the interface between carpet and the concrete will rise to 100 per cent. Deep within the pile of the carpet or the underlay, right along the concrete surface, small droplets of liquid moisture will appear in the form of condensation. You won’t see it or feel it, but droplets of moisture will appear, much to the joy of the kind of microbes that live everywhere on earth. Moisture is all they need to grow, and when they do, they trigger mouldy smells and mustiness.
It’s at this moment that many people commit an understandable mistake. What do you feel like doing when there’s a bad smell in your house? Open a window and increase ventilation, right? In this case that’s exactly the opposite thing you should do. The source of mould-triggering moisture during summer is the outdoor air itself, so admitting more outdoor air is only going to makethingsworse.(Gotobaileylineroad.com/humidity-mustiness/ for a video animation).
There are only two true fixes for musty carpet syndrome. One is quick, the other only makes sense while you’re renovating.
Reducing summertime humidity levels in your house can make the air dry enough so it doesn’t condense even when that air comes in contact with cool concrete. Air conditioning reduces humidity with less electricity than a dehumidifier and without warming your already-hot house.
Another option is to get rid of the carpet or install it on some kind of subfloor system that separates the carpet and underlay from the concrete. Those ready-made subfloor tiles with dimpled plastic underlay on the bottom do a good job eliminating the dynamic that causes mouldy, musty carpets. Some people will tell you high quality foam underlay offers the same benefits. That’s not true because underlay still allows air to travel through it, permitting warm humid air from cooling down near the concrete and possibly condensing.
Musty carpets is more than just an annoying smell. It’s a sign that indoor air quality in your home isn’t all it should be. Understanding the dynamic of summertime condensation on cool, carpeted floors is an important step toward more healthful breathing.