Scenario#2: The Dry-But-Sometimes-Humid Basement

Scenario#2: The Dry-But-Sometimes-Humid Basement

This meter measures airborne humidity and temperature. You need some kind of device to help you know for sure what basement humidity you’ve got.

This is probably the most common basement scenario. Your basement never leaks visible water that you’ve seen, but it’s at least a little on the humid side, especially during hot weather. Your basement might even be damp and musty smelling sometimes, too. Your challenge with a situation like this is to find out where the humidity is coming from, then see how you can stop it effectively. This exercise is a little tricky, and even many building professionals don’t understand the dynamic behind the situation. There are two possible sources of non-liquid moisture that makes a seemingly leak-free basement damp or humid:

1. Moisture entering the basement from the outside air

2. Moisture vapour migration through masonry walls and floor.

Step#1: Dry Out the Basement Air

Naturally, you’ll need to try drying out your basement when it’s damp or humid, and the first thing to try is a dehumidifier (if you don’t have air conditioning). Close all the basement windows (and keep them closed), then set up a humidity meter to see what you’re starting with. Spaces generally start to feel damp when relative humidity rises above about 75%. Your basement could be quite a bit more humid than this. It takes a big dehumidifier to make a basement dry, so don’t skimp on the model you use. It also takes at least several days to make a significant difference to basement humidity. Also, an oscillating fan really helps to broadcast the dry air coming from the dehumidifier, spreading it out to dry the nooks and crannies of your basement. A dehumidifier will be roughly twice as effective when used along with a fan. You’ll need a dehumidifier that’s rated to remove at least 70 pints of water from the air per day to make a difference in a musty basement.

Does the humidity level drop and the freshness of the air improve after running the dehumidifier for a few days? If it does (and it probably will), then the regular use of some kind of dehumidification will almost certainly be part of the ongoing management of your finished basement. The tendency to be too moist will probably decrease after the walls and floor are covered properly, but you still need to be prepared to deal with too much airborne moisture. A dehumidifier is one option (the option I use in my own basement), but air conditioning is even better. If your basement improves with the dehumidifier, consider installing central A/C or ductless mini-split AC as part of your finished basement plan. You’ll get more effective humidification with less electricity and quieter operation. Whatever you choose, be sure to monitor basement humidity levels in an ongoing way.

Step#2: Eliminate Musty Odours

Drying out a humid basement reduces odours, but it probably won’t eliminate them completely. For this you’ll need to do some kind of odour elimination procedure going. An ozone generator is the most effective way I’ve seen for getting rid of odours in all kinds of situations. I own and use a professional-grade ozone generator in my own basement from time to time. I also find it helps a lot in vehicles and musty above-ground rooms.

TECH TIP: Ozone for Odour Control

This professional grade ozone generator is very effective at eliminating odours in many situations. It uses ultraviolet light to generate ozone from oxygen in the air.

Everyone has heard about the “ozone layer” in the upper atmosphere and how it filters out some harmful UV rays. What’s less well known is how effective ozone gas is at eliminating odours of all kinds, including the odours of a musty basement. Ozone generators are a regular part of how restoration professionals eliminate the smell of smoke in buildings that caught fire, for instance, and ozone works really well for basements, too.Ozone is nothing more than a particular kind of oxygen molecule. Instead of being O2 (like normal oxygen), an ozone molecule is O3. This third oxygen atom that’s part of ozone is not firmly bound in place, so ozone always results in the presence of free oxygen molecules in the air that are eager to bond to something in an oxidization reaction. It’s these reactions that make ozone such an effective way to get rid of smells

Ozone generators are boxes that you plug into an electrical outlet. An internal UV lamp causes the formation of O3 from the O2 naturally found in the air.  Although ozone can be irritating to the lungs in high enough quantities, it smells like the air after a lightning storm. Ozone is, in fact, what you’re smelling at times like that. Lightning creates O3 from the O2 in the atmosphere. If you’ve got a musty basement, you’d be wise to rent or buy an ozone generator as part of your campaign to dry your basement and keep it fresh smelling. The main thing is simple. If you want your finished basement to smell nice and fresh, you need to get to the point where the unfinished basement smells fresh all the time with some help and management from you. Click here for a unit that’s large enough for basement use and very reasonably priced.

Step#3: Test for Moisture Migration

While it’s true that outside air can bring moisture into your basement when outdoor temperatures are warmer than basement temperatures, but invisible moisture can also come in by water vapour passing through masonry walls and floors. If your basement is damp and musty, you must test for moisture migration but only after you’ve dried out the space using a dehumidifier. Use the same kind of plastic sheet testing or calcium chloride test kit I covered earlier in the course. Do some masonry surfaces in your basement have a kind of white, fluffy deposit on them? This might look like mold, but it’s probably efflorescence. This is a definite sign of inward migration of invisible moisture even if the area doesn’t seem damp.

QUICK DEFINITION: “Efflorescence”

Fluffy white powder on masonry is rarely mold. It’s usually efflorescence, a fibre-like mineral deposit that indicates small, persistent moisture inflow.

The fluffy, white buildup you see here is made of minerals deposited by a slow, inward migration of tiny amounts of invisible, mineral-laden moisture. This is efflorescence. As I explained earlier in the course, this moisture moves in through the masonry invisibly, it dries in contact with the air on the inside of your basement walls and floor, leaving minerals behind high and dry. The longer this process of migration and drying continues, the larger the fluffy deposits become. Ellforescence looks alarmingly like mold, but thankfully it’s not. Creating a moisture-proof barrier with waterproof paint is the key to preventing efflorescence after it has been removed. Are you having trouble determining if what you’ve got is mold or efflorescence? Take some close-up photos of your situation and mail them to me at [email protected]. I’ll take a look and we’ll figure it out together.

Humid Basement Bottom Line

If you’ve got a basement that’s sometimes humid you’ll definitely need to coat the inside of walls and floor with either Drylok or Xypex if you’ve detected significant inward migration of moisture. Also, you’ll need to close basement windows during humid weather, monitoring humidity levels with a moisture meter and reducing relative humidity with either a dehumidifier or air conditioning. In my own basement, windows are only opened when outside temperatures are lower than indoor temperatures. The windows stay closed all summer, every summer, and a dehumidifier reduces relative humidity levels to about 70%. This approach is highly effective and I learned it after a number or years of trial and error. Only proceed with finishing when you’ve proven you can keep humidity levels below 70%. Lower is better.