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Tips to keep snow from blocking vents

Q. We have a generator and a small HVAC condenser outdoors that are mounted on a 2-inch concrete platform. Both have air vents (or air-intake grills) that I believe should be kept free of snow to allow for airflow. Last year, the vents and air intakes were covered with snow for most of the winter. Neither of the units was in use (we never lost power, so the generator didn’t turn on, except for its weekly test run of 10 minutes or so, and the heat/AC unit is for our newly finished room over the garage, which is not often in use). What should I do, if anything, about keeping the airways open? Neither manufacturer seems to provide a solution. By the way, we are in our 70s and should not be shoveling! Is there another answer? A tent of some sort? A table-like structure to elevate these units? Thanks for your advice.

MARY JANE McCOOL, Bridgewater


A. I worry about snowdrifts or a large snowfall blocking my furnace- and water heater-exhaust pipes. If they are blocked, the blower will stop working and can affect the safe operation of the appliance. In geographical areas with considerable snowfall, it is advisable to locate exhaust vents much higher than the minimum 12 inches above the ground to prevent blockage by snow. In order to keep snow away from sidewall exhaust vents, I made a simple A-frame out of ½-inch plywood to protect and deflect. Hire someone to build A-frames with hinges so they can be folded up and placed in storage.

Once constructed, I place the A-frame over the exhaust pipe and up against the house. The A-frame keeps snow buildup away from venting systems and from accumulating around my exhaust pipe. In your situation, the A-frame can simply be pushed against the generator to cover and protect the exhaust port.


Q. I am looking for advice on the best way to insulate the section of my chimney that passes through the unheated attic space before exiting the house. We supplement our home heating with wood. When we use the woodstove, the chimney gets quite warm, which is great for the house, but not so good for the unheated attic. The heat that is pouring into the attic heats up the roof, and when there is snow . . . well, you can guess the rest (ice dams). Is it possible to insulate the chimney in that area? If so, what is the best way to do that? The attic floor has two layers of R19-value fiberglass batts at 90 degrees to each other. There are soffit vents and a full ridge vent.


A. Sounds like the attic floor has a decent amount of insulation. That aside, I would take a second look at your attic insulation and air-sealing. I bet there are a lot of opportunities to improve air sealing around the base of the chimney and in and under your insulation, and you are probably losing a lot more heat via that route than what is being radiated from the chimney.

Chimneys are a major source of air leaks in attics. To seal around the chimney, jam fire-safe rock wool into the air gap around the base and floor framing. Push the rock wool down an inch or two below the top of the floor joists, and seal it completely with fireblock insulating foam sealant.


The proper (code-approved) way to insulate your chimney is to build a wall and enclose it with a minimum of 2 inches of clearance between the masonry and the stud material. Do not attach anything to the chimney itself. Rock wool would be my choice of insulation. Although not really needed, you could then sheath the walls with ¼-inch fiber-cement board.

State building code dictates that “a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches. . . . Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch. . . . The air space shall not be filled, except to provide fire blocking in accordance with 780 CMR 6001.16.”

Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com or tweet them to @globeaddress or @robertrobillard.