It’s the attic leaking, not the plumbing stack
QUESTION: I was recently in my attic, which is a place I had never ventured before. I discovered a fair amount of frost build-up in the attic. This is something I was told was not unusual for a house built in the sixties. I searched for leaks into the attic and fixed an obvious leak problem but was rather surprised to find a second potential problem. Over the bathroom there is a thicker frost build-up, which gets thicker over the shower and progressively thicker the closer you get to the plumbing stack.
I removed the insulation around the plumbing stack, which is blown-in fibreglass that was upgraded only a few years ago. I then discovered that the first two inches of the plumbing stack closest to the attic floor was dry but the remaining part of the plumbing stack in the attic, about six to eight inches, was covered with a thick sheet of ice. This ice was right up to where the stack goes through to the roof.
Although I scraped off some of the frost from the wood on the underside of roof, I didn’t attempt to hack off the ice for fear of damaging the stack. Although I had a hard time seeing through the ice, I think I could see a bolt or screw and there was something that looked like putty or tar on the stack.
Is this normal to see this much ice on a plumbing stack inside the attic space? Could the plumbing stack be broken or leaking and contributing to the frost build-up? There is no visible ice when you look inside the plumbing stack from on top of the roof. I once asked a plumbing company to extend the plumbing stack higher, as I worried about snow and leaves getting stuck in it. They refused, saying that you should never mess with a plumbing stack, so I didn’t pursue it. I should also mention one other thing. I could not see a vapour barrier in the attic, though it does have a full wood floor, so could the vapour barrier be between the drywall and ceiling floor? Could one add vapour barrier to an attic or just in the attic above the bathroom? I appreciate any advice you can give me.
Thanks. Jenna Watersky, Winnipeg
ANSWER: The first problem with your attic has been properly identified by your removal of insulation from around the stack on the floor of your attic. This may be one of several areas where warm, moist air from your home can leak into the attic, which is the major source of the ice. When this warm air hits the freezing cold metal surface of the stack in the attic, as it cools by traveling through the insulation, it will immediately condense and freeze on the exposed pipe. This will be compounded by other moisture in the attic from other points of air leakage which may be causing the extrathick ice.
It’s no surprise that the area above your bathroom is the area of thickest frost build-up, because the bathroom is the area in your home that produces the largest amount of moisture in the form of water vapour. After a shower or bath, the relative humidity in this room will be at or above 100 per cent, even in the warm bathroom. If this moist air makes its way into the attic through openings in the building envelope, it will take only a small drop in temperature to reach the dew point.
A proper air/vapour barrier may not be installed in your home and attempts to add one now is not the best use of your energy. Sealing up the area around the stack where it penetrates the ceiling/attic floor with caulking or polyurethane foam will prevent excessive air leakage. It’s doubtful that you have seen damage or a fastener that has compromised the integrity of the pipe, more likely some solder or roofing tar. Nevertheless, it may be a good idea to insulate the pipe with moisture-resistant pipe wrap or more foam to keep it warmer and prevent condensation.
Also, checking any bathroom or other exhaust fans and ducting for leaks or missing insulation may provide more answers. Ensuring that your attic access hatch is properly insulated and weatherstripped is also a must. Finally, finding and sealing any other protrusions in the bathroom ceiling like electrical junction boxes or wall top plates will minimize moisture intrusion into the attic.