That leaky basement

Posted Wednesday, November 30, 2011 in Sustainable Maine

That leaky basement

by Paul Kando with Topher Belknap, illustration by Topher Belknap

Basements leak, especially when the heating system is running. The leaks are inbound because the air heated by the furnace, boiler or stove rises and literally sucks in replacement air through every opening available. Here is where a lot of cold air enters into most homes. Likely places for air leaks in a basement or cellar include cracks where wood meets stone, concrete, or masonry; around the rim joist, the beam that sits on the foundation around the perimeter of the house; around basement windows; cracks in the foundation (especially between granite blocks in old foundations); around utility entry/exit points (pipes, conduits, wires, including gas lines and the pipes connected to the oil tank); exhaust vents (like the exhaust duct of a clothes dryer or gas appliance); holes and cracks in the foundation or the rim joist; and doors (including pet doors) to the outside. Winter is a good time to deal with most of these problems, since the work required is done indoors. In upcoming columns we will address them all.

There is one exception: the biggest basement leaker of all, the bulkhead. Bulkhead (also known as Bilco) doors are notorious for leaking air, water, and heat. If at all possible, the best strategy is not to use such a door during the cold months, and seal it well.

Topher Belknap describes how to seal and insulate a bulkhead door:

“The first remedy is to caulk all the cracks on the outside of the door. Use a good long-life caulk (I recommend silicone). Then weather-stripping should be added to the doors themselves. Match the type of weather-strip to the gaps. I used EPDM 3/8-inch D-shape for the outer edges; a piece of half-inch backer rod (normally used for caulking large gaps) for the center gap; and half-inch pipe insulation for the large bottom gaps.

"Weather-stripping will work only if the door closes tightly against it. I used a couple of hooks attached to the sides of the enclosure, and a rope from them to the door-locking mechanism to accomplish this. A couple of heavy-duty bungee cords would work as well.

“Next insulate the door. Extend the top stair step with boards along the walls, and a cross-piece across the opening (Diagram 1), making sure that, come spring, the cross-piece is easy to remove. Foam board insulation is placed over the framework next. If your basement is unheated, 2 inches of foam should suffice; for heated basements 4 inches is better. It is easier to use two pieces of tongue-and-groove insulation than trying to jam in a single piece, and a nice tight fit can be achieved (Diagram 2).

“The remaining opening is filled with a couple of pieces of similar insulation (Diagram 3). Seal any gaps; duct tape works well for this. Try to get a complete air seal. If your basement is heated, another layer of insulation on the outer surface will help reduce heat loss and air leakage further (Diagram 4).

“This will fix what is often the worst problem in basements, reducing heat loss, and the likelihood of freezing pipes.”

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