Basement renovations: how to prevent mold when retrofitting basements

Why are basements moldy? Because we build walls so they can't dry. Simple changes in wall design will result in mold-free durable basements.

Spray polyurethane foam on basement walls prevents mold and midlew
Spray polyurethane foam on basement walls prevents mold and midlew © Bala Structures

Basements don't have to be moldy, that is the first thing to realize. It's easy to build them better but we have resigned ourselves to the fact that they are humid and dank and we don't realize how simple the solutions are.  

Depending on the age of your home, you may have some interior basement insulation or maybe none. You may have an exterior damp proofing spray on foundation walls but likely not a proper drainage mat. Take a deep breath and don't worry, it will work out. 

Outside upgrades are ideal given the way basement walls were left exposed in the past; however, unless you have serious structural problems or plan to excavate for other reasons, the cost problaby isn't worth it, and you can deal with most issues inside much more affordably. The basement insulation solutions proposed here are for those that have no exterior protection.

Related topics:

How insulation works...and why it sometimes doesn't. Step by step guide for slab-on-grade home construction A slab-on-grade can be a cheaper and  better alternative to a basement.
Finished basements can lead to mold and mildew Find out why basements are moldy, and why it would be so easy to prevent. The core of an HRV Healthy home air depends on proper ventilation - HRVs and ERVs.

Where the moisture really  is, and where it wants to go:

The key to building better basements, is to understand that there is a completely different set of challenges facing walls built below grade. 

The greatest source of moisture to contend with above grade is the warm, humid air generated by cooking, washing and simply breathing. Below grade, it is that big porous sponge called concrete that is sucking up water from the wet ground.

A different problem demands a different solution - wearing a raincoat will keep you dry when standing in a cold rain, but it won't keep you dry when jogging under a hot sun. And unfortunately the way we currently build basements is about as logical as jogging in a raincoat. 

Above grade, walls are designed to dry to the exterior. That is impossible below grade, but for some reason we still build as if it were. Despite the fact that interior vapour barriers in below grade wall assemblies are commonplace, frustrated building scientists insist that is the worst thing you could possibly do down there.

As one of the foremost authorities on basement construction, Professor John Straube of the University of Waterloo dubbed this type of wall assembly a "mold incubation chamber". For further convincing on the topic, read 'Built wrong from the start' by Joseph Lstiburek of Building Science Corporation.

The key to better basement construction is to acknowledge where the source of moisture is, and decide to either keep it out entirely, or let it pass safely through. Below are suggestions for both those solutions; note that neither involves a polyethylene vapour barrier, but rely on latex paint only to slow the movement of water vapour.

Vapour barriers in basements cause condensation mold and mildew

A polyethylene vapour barrier on the interior of basement walls will not prevent  condensation and mold, it will only cause it.

Where condensation will occur may vary by season - sometimes on the concrete, sometimes on the poly, but always on the inside of the wall.

It is important to avoid installing polyethylene, foil, or any other vapour impermeable material on the interior of basement wall assemblies.


Better basement insulation techniques

The materials and design you choose are very important and should be appropriate for the environment they are destined to inhabit. Be very careful how and where you install wood and fiberglass insulation in basements, and be sure they won't see prolonged exposure to moisture.

Spray polyurethane foam (SPUF)

When SPUF works out, it can be one of the better performing products in this application. Being sprayed directly onto concrete walls, it ensures an even and total protection. SPUF has a high R value per inch, acts as an air barrier and a vapour barrier. However, it has been known to shrink after application, leaving air gaps and a lack of thermal protection. 

Another downside to SPUF is that blowing agents have a high global warming potential (GWP). For this reason, we like to explore less harmful alternatives whenever possible.

And probably the most worrisome problem (as reported by a SPUF installer) is that there is a great temptation to tinker with the chemical mix a bit, giving it more volume with less material, which is more profitable for the contractor. There are not a lot of reported cases of this, but when it goes wrong it goes very wrong. Personally, I wouldn't let someone spray foam my house without some pretty good references. 

All that said, here's how if you plan to do it:

Spray foam on basement walls
Spray foam on basement walls © Bala Structures

  • Frame a 2x4 stud wall at 24" centres leaving a gap of at least one inch between studs and concrete.
  • Shim up bottom plates to allow for water to pass under in cases of mild flooding.
  • Have foam sprayed against the concrete wall. The space left between concrete and studs is crucial to allow a seamless blanket of insulation to create an air and vapour barrier. Unprotected wood touching  concrete will wick moisture and eventually rot. One inch of SPUF will solve your problems; three or four inches will save you money in the long run.
  • Have insulation blown onto rim joists if it isn't already done.
  • Foam must be covered by drywall for fire protection.
  •  Don't install an additional vapour barrier behind drywall; latex paint will act in this capacity.
Basement spray foam insulation

There have been a few nightmare cases reported where a SPUF installation went very wrong and apparently made homes uninhabitable. Given the fact that application involves a wet spray that hardens on surfaces, it is very difficult to remove.

These reported cases are rare, but if you are at all concerned, there are alternatives to spray foam listed below  that work well.



EPS (Expanded Polystyrene), XPS (Extruded Polystyrene), Mineral wool:

SPUF is quite expensive; rigid insulation panels and a 2x4 wall with batt insulation can be a much more affordable option.

Rigid insulation board against concrete adds R value, breaks the thermal bridge and raises the temperature of the stud wall, which helps prevent moist air from condensing. The stud wall allows for the installation of wiring, additional insulation and drywall.

Drywall should be attached directly to studs (and directly over batts). No polyethylene vapour barrier should be included; latex paint will act as a vapour retarder slowing any outward moisture migration while also allowing walls to dry inwards.

You may hear that EPS is somewhat moisture permeable; this is true, but only slightly so and not enough to be a problem. According to Professor Straube, two inches of EPS has a perm rating in the range of 60-75 ng, or 1-1.25 US perms, which is right on the cusp of what is defined by building code as a suitable vapour barrier (60ng and under).

At that level of moisture permeability, an almost immeasurably small amount of moisture will be able to migrate into the wall, but with no interior vapour barrier it will pass through harmlessly. EPS, XPS and mineral wool are unharmed by moisture.

Protecting basements from moisture damage and thermal bridges
Protecting basements from moisture damage and thermal bridges

  • Install two inches of rigid insulation board directly against concrete. It is easiest to attach it to the wall with a couple of concrete nails or even some adhesive while you frame walls.
  • Frame an interior 2x4 stud wall at 24 inch centres, pressed tightly against foam panels.
  • Install mineral wool batts in the cavities. In the event of minor flooding, mineral wool is unharmed by moisture and will retain its shape and R value when it dries; fiberglass is not so resilient.
  • Drywall should be attached directly to studs, and directly against batts - with no poly!
  • Latex paint as vapour retarder.

This technique of rigid and batt insulation may require a few more steps, but it can offer you a greater R value per dollar invested than SPUF and it allows you to do a lot more (or all) of the work yourself.

For reasons other than performance, we prefer mineral wool or EPS over XPS. XPS has a slightly higher R value per inch, but like SPUF, the blowing agents are exponentially more  harmful, about 200 times more so than EPS.

Further reading: