Renovation & Design

Keep the pipes above zero, especially in winter

Water lines installed in unheated spaces often a problem in our climate

Question: I have an alcove in the kitchen that overhangs the outside of the back wall. Ever since I replaced the flow-form countertop with a quartz countertop, I’ve had a problem in the winter with the dishwasher water-supply line freezing. I was wondering, would putting styrofoam insulation under the overhang solve the problem? Thanks.

Claude Lambkin

Answer: The freezing of water lines installed in unheated spaces, like your cantilevered floor, is often a problem in our climate. Reinsulating the area is the likely solution, but your proposed method may not prevent a reoccurrence. Opening up the area will be required to diagnose the issue and allow proper installation of insulation and adequate warm airflow.

Water-supply piping, especially copper pipes like yours, are quite susceptible to freezing in cold areas of our homes. This is often seen in basements, where water pipes may be buried inside insulated walls built up against foundation walls. This can be an issue when basement walls are framed and insulated after possession of the home. Many homeowners don’t know they should not insulate over top of the pipes. Doing so can prevent warm air from the home from warming the cold pipes, while the insulation wrapped around can actually keep the pipes colder for longer periods. This can also be a problem when water pipes are installed inside cantilevered floors, like in your kitchen-overhanging alcove.

Because the cavity in between the floor joists underneath your kitchen cabinets overhangs the foundation, they are exposed to colder winter temperatures from outside the home. To prevent this area from being too cold, it must be insulated to prevent heat loss. Unfortunately, insulating this area is tricky, because of the limited space. To properly protect this cavity, it must have sufficient insulation at the bottom, sides and end of the cavity, while maintaining the continuous air/vapour barrier from the basement insulated walls below and the exterior wall above.

The only practical way to achieve this is by using rigid extruded polystyrene insulation and sealing the corners and joints, or installing sprayed-on high-density polyurethane foam insulation. Because your home is older, the cavity may be partially stuffed with fibreglass batts, which does not maintain the air/vapour barrier, but improperly fills the cavity top to bottom. Because of this, some warm air may infiltrate this cavity, but it will lose its heat energy as it makes its way into the fibreglass insulation. If it is really cold outside, this can cause condensation and ice formation. If it is relatively well air-sealed, condensation and moisture will not be an issue, but not enough warm air will penetrate the overhanging area to prevent frozen water supply pipes from occurring.

Installing a sheet of extruded polystyrene on the underside of the overhang may not be a bad idea, but will not be a complete solution. While this is one proper area for this good product, it will not provide total insulation coverage, or improve the air-sealing issue. More importantly, it will not help warm the cavity, but may only serve to prevent very minor heat loss. To properly achieve this, the cavity will have to be opened. This could be done from the exterior, by removing the covering and sheathing under the cantilever, or from the basement below. It may even be possible to evaluate and remediate the problem by partial removal of the kitchen cabinet bottoms, but that may be more difficult to fix afterward.

Once the area is opened up, the location of the problematic water pipes can be seen and any or all of the existing insulation removed. At that point, it can be determined whether rigid foam or spray-on foam is the most practical solution, or if the pipes may need modifications to allow for its installation. The inside perimeter of the cavity can then be properly insulated and air-sealed and the pipes installed in the open space between. It may be best to do this work from the basement, as the existing ceiling adjacent to the area will have to be opened anyway to allow warm air from the basement to infiltrate the newly insulated cavity, to prevent further frozen pipes.

You should know energy flows from an area of high energy to one of lower energy, so heat is escaping the floor structure to the colder exterior, not cold entering from outside. So, rather than trying to fix the problem from the exterior with a single sheet of extruded polystyrene, the focus should be on a better repair, allowing heat from the home in to warm the cantilevered area. Maintaining good airflow and a temperature at or slightly below the room temperature should ensure the pipes never get cold enough to freeze, even on the coldest days. This can only be accomplished by insulating and air-sealing with foam dense enough to prevent heat loss, while ensuring air leakage is at a minimum, to prevent condensation.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past 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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