Trained Eye


Insulating Floor Joist Pockets


My house is 8 years old and I'm finishing my basement. The pockets between the joists were not insulated. I filled the pockets with insulation, 2 pieces of fibreglass to total 5.5 inches thick, then sealed the pockets with a piece of polyethylene. On one side of my house, where the joist runs parallel to the outside wall, I insulated and put poly part way only but could not seal it because of a variety of obstructions like the heating vents.

I happened to take a look at that side of the house where the joist runs parallel and discovered mould on the outside wall. I am now concerned about mould in the other areas like the pockets that are sealed. If the insulation is touching the outside wall can this cause moisture to develop thus mould to develop? I did open up a couple of pockets and found that there was a little moisture between the insulation and the outside wall but no mould.

I need to know if what I did was correct. If I was wrong, I would like to know the proper way to insulate these areas.

 I am very concerned about this mould.  Any help would be greatly appreciated.



The small areas between the main floor joists, near the exterior foundation walls is a very tricky area to insulate and seal properly, as you have discovered. This is one of the main areas of concern I see during inspections of relatively new homes. The upper floors are usually very well insulated and vapour barriered, the basements fairly well sealed, but these pockets in between the two insulated wall areas is rarely done well. Part of the problem with this small area is that it is located in a problematic area to begin with, above the newly insulated basement walls. I will try to explain the reason moisture is building up in these areas and offer suggestions for the best way to insulated and seal them.

In new homes, the basement walls built inside the foundation are often insulated soon after pouring the basement floor slab, which is normally one of the last items completed before possession of the home. This new concrete, as well as the concrete foundation walls, drywall compound, paint, and wood framing in the home are giving off large amounts of moisture as they dry and cure. Some of this moisture can easily be trapped in and behind the fibreglass insulation along the foundation walls. As the house components dry and the basement is ventilated by opening the windows or dehumidified with air conditioning, some of this moisture will escape the wall cavity where it is partially trapped. A small amount may dribble out the bottom plate of the wall, but the majority should escape as water vapour through the top of the wall. This is due to natural convective forces and a phenomenon known as “the stack effect”. This is unfortunately the area where the pockets between the ends of the floor joists are located.

Because these areas are poorly sealed, difficult to insulate, and on top of the cold foundation wall, condensation and frost build up in the heating season is very likely. When the frost melts in the spring, the moisture may wet the floor joists, rim joist, and floor sheathing above, which is an ideal area for mould growth. Also, some moisture from the house air may leak into these poorly sealed cavities, creating the same situation. The solution is to properly insulate and air-seal these areas to minimize air intrusion and prevent access to the cool foundation walls.

Mould requires two main components to thrive; water and a cellulose-based food source. As previously illustrated in the above scenario, the moisture is present and the food source is the wood joists and floor sheathing. If the wood in these areas remains wet for an extended period of time, without allowances for proper drying, mould and rot will grow. During the long warm summer months, this area may dry enough to prevent major mould growth, but the cycle may begin again as soon as the weather gets colder and the foundation temperature drops.

The solution is to fully insulate these pockets with insulation that will not easily trap air and moisture, as is common with fibreglass batts. Also, insulation that is moisture and rot resistant often can provide a good air-vapour barrier as well. The two main choices are rigid sheets of extruded polystyrene, often referred to by one popular brand name Styrofoam SM, or blown-in polymer foam insulation. If the proper rigid foam sheets are used, they may be cut to the exact size of the joist space cavities and caulked in place and along the floor joists to provide a good air-vapour barrier. This may be time consuming although fairly easy to install at the ends of the joists, but may be difficult to accomplish in areas where there are obstructions or where the joists run parallel over the foundation walls, as in your situation.

The second solution, which is becoming more and more popular with conscientious builders, is to fill these cavities with blown-in foam insulation. There are many types available, but most commercial applications are a two-part polymer that sets very rapidly after injection. Some of these foams are closed-cell materials that provide an excellent air-vapour barrier as well as high quality insulation, but others are open-celled and have to be coated with a special paint to provide a good air-vapour barrier. Make sure you get specific details about the material before contracting out this installation. Either way, the true benefit of these blown-in materials is that they can completely fill any void and will prevent air leakage and mould growth in these problematic areas.