Renovation & Design

ASK THE INSPECTOR: Cool floor by bay window needs extra insulation

QUESTION: Our house was built in 2009 and has a bay window which juts out from the foundation in the living room. We've noticed the floor near the bay window is very cold even though there is a heating vent in the floor directly below the window. The window faces west.

Is there anything we can do to improve the amount of heat we get in this area of the living room even though there is no foundation beneath it? Any advice you could give us would be most appreciated.

Thank you, Elaine Lafleche

ANSWER: The problem of having cool floors under bay or bow windows and alcoves is a very common one. Because it is difficult to properly insulate and air seal these areas without filling the cavity, getting sufficient heated air in to warm these areas during the winter months can be tricky.

Because of attempts to build modern homes with various cantilevered floor-joist areas, alcoves for china cabinets and bay or bow windows, a cooler floor in these areas relative to the rest of the home is an undesirable consequence. New homes require elevated levels of thermal protection in the walls and basements in the form of insulation, so anything that strays from a simple format can be tricky to deal with. Cantilevers, which are areas where the floor extends beyond the exterior of the foundation walls, fall into this category. The reason for this actually begins in the basement at the junction of the foundation and the floor.

In a straight-forward home design, where the ends of the floor joists sit near the outside face of the foundation walls, it can be quite simple to extend the insulation from the perimeter basement walls straight up to the underside of the floor sheathing. This is often done by cutting and stuffing in smaller pieces of the same fibreglass batts used to insulate the walls between the floor joists. Six-mil polyethylene sheathing, used for an air/vapour barrier over the walls, is extended up between the joists and normally caulked and stapled to the insides of the joists and underside of the floor sheathing.

This method creates a fairly uniform, continuous layer of insulation from the basement floor slab right up to the highest ceilings. When it's done properly, there are should not be many gaps for cold air intrusion from outside, or large areas where heat can be lost due to a phenomenon called "thermal bridging."

However, when a section of the floor is extended beyond the foundation wall, the uniformity of the insulation at the building envelope goes out the window. To properly insulate and seal this extension, the air/vapour-barrier installation becomes much more difficult. Because the air seal has to be installed on the warmer side of the insulation, the transition of the poly sheathing from the basement walls to the cantilevered floor is tricky.

The solution is often to simply stuff the entire cavity extending beyond the foundation with insulation after the air/vapour barrier and sheathing have been put in place above. This solves the dilemma with the poly, but it leaves little room for warm air to infiltrate the heavily-insulated floor cavity.

So, while it will prevent excessive heat loss through the cantilevered floor, it will also prevent heated air from the home entering the cavity. Combined with thermal bridging through the floor joists, which are almost directly exposed to the cold outside temperatures, it will make the floor surface feel cooler than the surrounding floor.

Because your problem is created by a lack of room for warm, heated air to circulate between the floor joists, we have to find a way to allow that to happen while maintaining a proper level of insulation. The two possible solutions both require removal of the existing batt insulation and replacement with better choices.

The first possibility is to insulate the cantilevered area from the outside, rather than inside. This may be easily done on the underside by simply removing any stucco or parging to expose the sheathing. Fastening and caulking a couple of layers of rigid extruded polystyrene sheathing in this area should do the trick. It may not be possible to extend this to the ends and sides of the joist areas from the exterior, so more sheathing could be installed on the interior faces in those two areas.

While this does not leave a perfectly continuous air/vapour barrier, if it's well-sealed by caulking, it may prevent enough air movement to suffice. Otherwise, the sheathing underneath the cantilevered area could be removed and the foam board installed all from the underside, which would make a tighter seal.

The other method, probably less labour-intensive, but requiring more costly insulation, is to blow a layer of high-density foam on the inside of the bottom cantilever sheathing and inside the perimeter joists. The benefit of this method is it may not require removal of anything on the exterior. The downside is it may leave less of a cavity for warm-air intrusion than exterior insulation.

The ultimate remediation could actually include a combination of rigid foam sheathing on the underside of the problem area and blown-in material on the inside faces of the perimeter joists. If done properly, that method could ensure excellent air/vapour barrier contiguity, with a maximum warm-air cavity underneath the floor sheathing.

While all this may seem complicated, the simple underlying principle to remember is you need sufficient thermal insulation to prevent excessive heat loss around the perimeter of the cantilever, while maintaining enough open air space under the floor to let warm air circulate.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the 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 or check out his website at



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