Renovation & Design

There is more than one way to insulate between joists

QUESTION: I know the use of extruded foam insulation on interior basement walls must be covered, but what about the header areas? If you seal them with extruded foam, too, must those areas be covered? Doug

ANSWER: You have identified an area of considerable debate and controversy in new-home construction. The cavities between the floor joists over top of the foundation has been a difficult area to deal with in the building envelope since modern insulation and air/vapour has been used in basements. The technical answer to your question may differ from what is normally seen or done in many homes.

The area above insulated basement walls against foundation walls has always been a difficult area to deal with in relation to air sealing. This is due to the limited flexibility of typical 6MIL polyethylene sheathing used as an air/vapour barrier in this area. While it is easy to install and seal this thin sheathing on the wall studs, it is very difficult to cut and fit between the floor joists above this wall. Even with lots of acoustical sealant or plastic tape, it is near impossible to install without several gaps.

This is very important because any gaps in this membrane can allow warm, moist air from the home into this cavity. With conventional fibreglass batt insulation, warm air leaking into this cavity will likely cool, condense and can lead to frost and mould growth.

Because of possible convective air currents within the insulated wall cavity, condensation is most prevalent at the top of the insulated system, which is the troublesome area you have inquired about. This is compounded by the fact that this area is above grade and will be colder in the winter than much of the rest of the wall assembly, which is partially insulated by the soil and snow outside. These two factors often lead to frost forming behind the insulation if the wall is poorly sealed.

The good news is that modern insulation methods can be used that virtually eliminate this issue. Rigid extruded polystyrene has been used for several decades in this location to help minimize both the air-sealing issue and the chance of condensation. While this can be very labour-intensive to install, having to cut each piece exactly to fit between individual floor joists, it can easily be sealed at the bottom and perimeter with caulking or small cans of blown-in foam. With sufficient thickness and sealing, this material works very well to provide a combination of waterproof insulation and good air/vapour barrier.

An even easier, newer solution is to have high-density polyurethane foam blown into each cavity, which has similar properties but fills all gaps even better. The only drawback in using either of these materials is they are combustible and may produce toxic fumes during a fire. To minimize this risk, covering it with a fire-retardant sheathing, such as drywall, is required.

Your question is addressed by this passage on the City of Winnipeg website: "Can foamed plastics used as an insulation on a basement wall be left exposed? No! The Building Code expressly denies the use of foamed plastics for interior finishes. This is because the material is considered to be a fire hazard when left exposed. Where foamed plastic is used on interior walls, it must be covered by any of the approved interior finishes listed in the Building Code, i.e., drywall, plastering, plywood, hardboard, particle board, waferboard, strandboard, or wall tile (plastic or ceramic)."

I don't think this could be any clearer, but what is normally seen in many homes, especially newly built ones, differs considerably. Quite often, basement perimeter foundation walls are framed, insulated and covered with 6MIL poly to the underside of the floor joists and loosely stapled in place between these pockets. In better-insulated situations, these cavities are partially filled with high-density foam or extruded foam pieces. The poly is often sealed to the foam, creating a continuous air/vapour barrier from basement floor slab to the underside of the main floor sheathing.

Unfortunately, this is rarely covered with any type of sheathing. This is done partially to allow installation of electrical wiring and boxes in the perimeter walls and any other modifications before covering with drywall or other wall panels.

While this may seem a rather complicated route to answering your direct question, the underlying theme is simple. Any plastic foam insulation must be covered to meet existing building and fire codes. This does not mean you will be forced to immediately cover this insulation after installation if you are not completing your wall assembly. If you are planning on leaving these walls unfinished, with exposed polyethylene sheathing, this should be all right for the time being.

While this may seem like a contradiction, I see it in dozens of homes every year. In fact, many builders are installing white-coloured poly in an attempt to make a brighter, more attractive basement area. I can only surmise this is being done because it is assumed this area will remain uncovered for some time.

While this appears to be in direct contravention of the previous website notification, I don't know why this is so prevalent or allowed. Contacting your local building officials for clarification may answer your question more fully.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors - Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 and



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