Building Better Basements
By Doug Tarry
Builders and renovators are very aware of the problems that can result from finishing a basement too soon after construction, and in the past often suggested that homeowners wait at least 18 months before doing so, to give the walls time to "dry out." While this advice often worked, with today's building codes and the desire of our customers to move into a fully completed home, an 18-month wait is not a practical solution.
The problem is moisture condensing on the insulated side of the polyethylene to the point where the insulation becomes saturated, water runs out onto the basement floor and mould grows inside the wall. This leads to the secondary problem of having unhappy customers and complaints.
This situation does not happen with all basement walls, and the conditions that seem to be common where problem occur are:
‣ Full-height batt, or bag wrap insulation in a stand-off wall.
‣ Full-height polyethylene on the interior side of the stand-off wall.
‣ East, south or west facing walls (walls that are in the sun).
‣ Summertime (warmer outside than inside).
‣ New construction (0-18 months old).
‣ Poured concrete walls (not ICF).
The Building Science
What do we know?
‣ The construction is new, so the concrete and the wood studs have a lot of water in them.
‣ Concrete wicks water very well.
‣ Below grade, the wall cannot dry to the outside, due to damp-proofing, drainage membrane and soil.
‣ The wall also cannot dry to the inside, due to the poly layer.
‣ The only area where drying can occur is above grade and to the outside.
‣ Water vapour moves from areas of high concentration to low concentration.
What is happening?
Moisture moves from wet to dry, and heat moves from warm to cold, so outside to inside during the summer when the sun heats the concrete. When moisture is pushed out of the concrete as water vapour and migrates towards a cooler space of the basement interior it is trapped on a cool surface of the poly and condenses. This now-liquid water then runs down the polyethylene or it is absorbed by the batt insulation.
Doug Tarry points to the batt insulation used in the stand-off wall. Behind Doug, the rigid insulation installed full height against the concrete wall and behind the studs is visible prior to the batt insulation being installed.
While several solutions to this problem exist, not all are Code compliant despite a solid basis in building science. Our Code compliant solution is as follows:
‣ Start with damp-proofing and a mechanical drainage layer on the exterior of the foundation wall below grade to ensure no water is being held against the wall.
‣ Add a membrane between the footing and the wall to prevent water wicking.
‣ Use the concrete wall as the air barrier, rather than poly.
‣ Add full-height rigid insulation, tight to the concrete wall on the inside using a material which allows water vapour migration (in our case this is ROXUL ComfortBoard IS which is both vapour permeable and water repellent).
‣ Construct an insulated, full-height stand-off wall against the ridged insulation layer.
‣ Add a "smart" vapour barrier membrane and eliminate the polyethylene (we use Smart MemBrain to replace 6ml poly). This smart membrane is also an air barrier. This prevents the same moisture problem in reverse during the winter time when harder to spot because you don't see condensation on the poly. It only starts to show up when it leaks onto the floor.
The "smart" membrane has a CCMC Evaluation, making it acceptable for use under the prescribed conditions. The material changes its vapour permeance depending on the conditions. When the vapour pressure is high (summer), the material becomes more permeable, allowing the moisture to dry to the inside and into the basement. When the vapour pressure is low (winter) the material becomes less permeable, preventing moisture from passing through it.
The result has been dry basement walls, happy customers and the satisfactory solution of a problem that has become all too common since full-height basement insulation (with poly) was first used. Most importantly, we have not had a basement call back since we started using this system.
Editor's Note: Thanks to the ongoing work of Doug Tarry, George Brown College and the University of Waterloo on this technical issue, the CHBA has submitted five Code Change Requests to the NRC to make Doug's and other sound technical options available under the National Building Code. If adopted, these Code changes will provide builders with more robust options for providing full-height basement insulation with few moisture-related problems.
Doug Tarry is an innovative builder and owner of Doug Tarry Custom Homes Ltd. in St. Thomas, Ont., and well known in the industry for his keen interest in all things technical. Doug was awarded CHBA's prestigious William M. McCance Award for his contributions in the technical area.