BASEMENT INSULATION 101: The Real Way to Prevent Musty Basement Smells

How a basement is finished has a big effect on indoor air quality throughout your home. That’s because more than half the rec room finishing jobs I see are done badly enough to cause potential health issues with indoor air quality because they encourage mold growth. This widespread problem comes down to how basements are finished, and here’s a true story that illustrates the point . . .

There’s a finished basement I know that’s both typical and dangerous. It’s in a small town recreational building near where I live in rural Canada, and this basement hosts a number of different groups and functions year round. Other than the very slight musty smell of the place (not unusual in basements), I didn’t think much about indoor air quality in this basement until something happened to one of my sons.

Within minutes of walking downstairs for a church service one Sunday, his eyes began running, he became congested, his throat itched and so did his ears. No amount of kleenex could keep up. I’d always been fine in this place, as I was that day, but as word got around in the crowd about my son’s sudden allergy symptoms, half a dozen people piped up.

“That always happens when I come down here.”

“Me too. My throat tightens every time I walk down those stairs.”

“Coming into this basement always makes my hay fever worse.”

To all appearances, this basement has always been visibly dry, it’s bright and it looks good. The only outward sign that something is wrong is that very slightly musty odour that gets worse each summer. But basement mustiness is always just the tip of the iceberg. It invariably means there’s some kind of hidden mold growth somewhere – enough to send signals to your nose – and it’s happening in many finished residential basements because of the way they’re built. The crazy thing is, basements are still being finished today in ways that virtually guarantee hidden mold growth and poor indoor air quality over time. But finish your basement following the real-world mold-busting strategies you’ll find here, and you’ll be able to enjoy a cozy finished basement along with good health, too.

Tolerate No Moisture

The dark streaks on this block wall is mold growth, and this is the kind of thing that can happen big-time behind old-school finished basement walls if even a small amount of water leakage occurs.

No matter how well you follow the other four strategies, mold will always win in the presence of sufficient moisture. That’s why you must never, ever finish a basement space that’s anything less than 100% dry, 100% of the time. There are no exceptions to this rule. In fact, it’s not just liquid moisture that will ruin your basement and air quality, but invisible moisture vapour, too. That’s right. You can have a basement that’s looked dry for decades, yet you can still have a moisture problem. How do you know? Polyethylene vapour barrier plastic is your best detection tool.

Cut 24” x 24” pieces of clear polyethylene vapour barrier, then tape them to the walls and concrete floor in several places. Leave them there for a few days, and look for moisture building up on the inside face. What you see under the plastic is what will happen inside your walls after they go up. If you do see droplets of moisture (and it’s not unusual), remove the plastic, paint the bare masonry with a waterproof paint such as Drylok or Xypex, then test again. Only when your basement passes all moisture tests can you safely finish the space. Don’t let your enthusiasm get the better of you in this matter. It’s surprising how many people think “Oh, well. A little moisture. That shouldn’t be a problem”, then go ahead and finish the basement space. Big mistake.

Eliminate Organics

When it comes to basements (and many other areas of life), designing things with multiple layers of safety is always the best idea. If your basement really was 100% dry, 100% of the time, then there’d be no danger in using wood or paper-based drywall for finishing. But what if 100% dry actually turns out to be only 98% of the time? A little bit of moisture can trigger a lot of mold, but it has a harder time growing without organic matter to feed on. That’s why it makes sense to keep wood and paper building materials out of the basement, or at least well away from potentially moist masonry surfaces. This is where the still-common practice of using wooden wall studs and batt insulation in basements is especially foolish. Better to use metal studs or no studs at all.

rec room panels
Rigid foam-and-drywall panels like these are a much more mold-resistant way to create finished basement walls than the usual wood frame-and-insulation-batt option.

A growing number of foam-based, stud-free building materials for basements go a long way to eliminating organics in hidden places. Just be prepared for push-back from contractors who have “always used wood studs”. I have the greatest respect for good builders, but be warned that there are a whole lot of builders out there who confuse experience with knowledge. Just because a contractor has a 30-year legacy of creating finished basements that generate bad air does not make it right. And besides, what contractor regularly inspects the mold resistance of their work as the years unfold? It never happens, so the “always done it this way” argument doesn’t hold water.

Use Impervious Insulation

Moisture can invade a basement from two directions. It can travel in through masonry walls and floors in the form of liquid or vapour, but it can also come from humid air inside the basement itself. If this air is allowed to make it’s way into wall cavities, it can cause droplets of water to appear out of nowhere within the wall. Hidden mold is then guaranteed. What does this have to do with basement insulation? Quite a bit.

There are two kinds of insulations in the world: insulation that air can move through such as batts and loose fill fibers; and insulation that air can’t move through such as low-permeability foam. The best basement insulation doesn’t allow air to move through it at all, nor does it absorb moisture in the event of a liquid leak. This is the best kind for basements, the best kind by far. Click here for a video on one way to use impervious insulation as part of a basement finishing project.

No Carpet Directly on Concrete

Rec room carpet illo
This cross section of carpet on a concrete basement floor shows how trouble happens. When indoor air is allowed to enter the carpet pile and extend down to the concrete, this air can cool and deposit tiny moisture droplets of condensation within the pile and underlay. This is the cause of most basement mustiness.

Nobody likes to walk on a hard concrete basement floor, and that’s why installing carpet and underlay are such popular basement strategies. Trouble is, they can also be very powerful mold breeding grounds, especially during hot, humid summer weather. If warm, outside air is allowed to make it’s way into the basement, it will settle on the floor and sneak into the carpet and underlay. Since the concrete floor underneath is always cool, it will cool the air to the point where it can’t hold all the water it used to at higher temperatures. The result is tiny droplets of condensed water forming within the pile of the carpet and the underlay. Mold loves to grow in conditions like this, and in time your carpet will smell like a wet dog. There’s nothing wrong with installing carpet and underlay on a basement floor, but only do it on top of a raised subfloor. This eliminates any chance that warm, humid air will reach the cool concrete.  A non-organic subfloor under your carpet is essential. Click here to watch a video animation of the carpet condensation dynamic.

Ventilate Intelligently

If you think that ventilation is the best cure for basement dampness and moisture problems, you’d only be partially correct. Ventilation can actually be a source of damaging levels of basement moisture. When it’s warm and humid outside, outdoor air transports moisture inside. As this outdoor air cools in the basement, it can’t hold as much moisture as it once did. It’s not unusual for outdoor air at, say, 75% relative humidity outdoors to skyrocket to 90% or even 100% relative humidity when it cools in a basement. Go ahead and open basement windows when outdoor temperatures are cooler than basement temperatures. That’s a good idea and will lower basement humidity levels in most cases. Just never open basement windows when it’s warmer outside than it is inside. The only safe way to lower basement humidity levels under these conditions is with a dehumidifier or air conditioner.

rec room dehumidifier
This is the basement dehumidification set up I have at my place. I only use it in summer when outdoor temperatures are higher than basement temperatures. The blue box on top of the dehumidifier is a timer that lets the unit run for 45 minutes, then shut off for 15 to allow ice on the coils to melt. The oscillating fan in the foreground boosts the effectiveness of the dehumidifier by blowing the dried air around and by speeding the defrosting of the coils.

If you look at newly-finished basements these days, most violate some or all the mold-busting strategies you’ve learned about here. So don’t let popularity or the “experience” of a contractor guide you in the details of how you finish your basement. What’s commonly done today will be considered completely unacceptable tomorrow. After all, people get tired of grabbing a box of kleenex every time they go downstairs.

Seal Rim Joists

Impervious insulation is always an excellent idea for basement walls, but it’s absolutely essential where the floor joists of the level above meets outside walls along the edges of a basement ceiling. Most homes have fibre-based insulation stuffed into this place, partially covered by a feeble and pointless layer of polyethylene vapour barrier. But this barrier can never work because it can’t be properly sealed around all those joists. This is why the only mold-resistant way to seal and insulate the rim joist area in your basement is with closed cell spray foam. You need at least 3” to prevent hidden condensation behind the foam. In 9 homes out of 10 that I inspect that have fiber insulation in the rim joist area there’s black mold growing on the back face of the insulation.

Six Rec Room Precautions

Got an old, previously-finished rec room you’re refinishing? It’s virtually guaranteed that mold will be revealed when you tear into those old walls and floors. Follow these six mold-safe demolition features and you’ll stay safe and eliminate mold spores that could otherwise grow back:

  1. Seal off the basement from the rest of the house before demolition begins.
  2. Buy and wear a HEPA-rated respirator before you begin demolition work. You don’t know when you’ll encounter mold before you find it. Dust masks offer no protection against mold.
  3. Remove all building material back to the foundation walls and move gutted material out from your basement to the outdoors as soon as it comes off.
  4. Have a dumpster or construction site trash bag waiting.
  5. Allow two weeks for the bare basement to dry out before you rebuild walls or floors. Ventilate well if outdoor temperatures are lower than basement temperature. Use a dehumidifier if it’s warmer outside than it is inside.
  6. Use basement drying time to inspect for existing mold. Kill it with a registered, non-toxic fungicide. Use a fogger or spray equipment to get mold control products into all infested areas.

Basement Finishing Course

If you’re planning to finish your basement (or have it finished by a professional), there are many more things to learn beyond just mold prevention. Soundproofing, heating, layout, plumbing and more are all part of the job. My online course FINISH YOUR BASEMENT THE RIGHT WAY explains everything you need to know, plus you get access to me to answer questions as you progress. Money-back guarantee, though I don’t recall anyone asking for a refund yet. Use discount code 20%off to save some cash.

Did you find this article useful? I hope so. Please consider helping me cover the cost of producing this content and publishing it online. Click the “buy me a coffee” button below and it’s easy, safe and fast to make a contribution. And it will be much appreciated, too! Thanks to everyone who helps to keep this website up and running.