Back in April I wrote about the dangers of finishing your basement - the wrong way. Moisture, or water, was the culprit I said, and you needed to ensure you had bulk water management systems in place before doing anything. Evaluate before you renovate, was and still is my message.
I also mentioned there were solutions out there and all you had to do was find the right person to ask. Well, since I've written that article I've been asked what the right way to do things is. That means I'm the go-to guy for this type of information, so here is what you need to do to create a healthy basement.
Insulated Wood Frame Walls Need Two Sides to Dry Out
A typical method used to insulate basement walls is to build a wood frame wall, leave a 1 inch air space between the wood studs and the concrete wall, fill the stud bays with fiberglass batt type insulation, and then cover the interior side of the assembly with a sheet of 6 mil polyethylene vapour barrier. With the plastic sheet installed on the warm side of the wall, this limits the wall to only being able to dry out towards the exterior.
Since basement walls are unable to dry out towards the exterior because the ground is damp, and with the tight film of plastic preventing walls from drying out towards the interior, the result is the small amount of water that does make its way into the foundation walls moves into the framed wall and stays there, trapped.
The Better Way to Finish
The better way to finish your basement is to create a wall and/or floor assembly that enables the walls and floor to breath. Using permeable materials allows moisture to travel through them and this helps both the foundation walls and/or floor slab to dry out. The excess water vapour is managed by using a dehumidifier. To construct wall and floor assemblies that help damp concrete to dry out will require you to use vapour-permeable materials like EPS for insulation, latex for paint, and cork for flooring, for example.
All of these finishes allow water to move through them. Although it’s impossible to keep your foundation and floor slab from getting wet because they are in contact with the ground, using a wall and floor assembly that allows the walls and/or floor to dry out is the better solution.
A Final Word Of Warning
A more common water issue impacting homeowners in developed areas in both the US and Canada is urban flooding. Paul Kovacs, executive director at the insurance industry's Institute for Catastrophic Loss Reduction, points out that, according to his research, basement flooding has emerged as one of the fastest growing causes of losses and extreme damage in Canada, costing $2 billion just in direct insurance payments annually. You can read the full study here in Urban Flooding in Canada.
The Center For Neighbourhood Technology in the United States has published a case study on the Prevalence and Cost of Urban Flooding in Cook County, IL, and say “the economic and social consequences can be considerable: experts estimate that wet basements decrease property values by 10 - 25 percent.” For a home with an appraised value of $300,000 that’s a monetary loss of $30K to $75K.
So before you decide to finish your basement make sure your bulk water management systems in place work, understand the risks associated with flooding, and when making your finish material selections, chose permeable finishes that breath so things dry out when they get moist.