QUESTION: I appreciate your articles and as a senior with minimal building skills, I read with intent to monitor potential problems. Your previous article about leaking in the attic helped because I experienced something similar. However, a neighbour assisted me by going into our attic and noted a few things.
There was a water stain on the inside of the boards where the stack meets the underside of the roof. The stack is a metal pipe. At the stack and ceiling intersection, the stack is connected by a ribbed connector, which is where there is a small gap of air escaping. It seems the ribbed connector is to accommodate the stack join at the ceiling, because the stack is at a slight angle from the bathroom. While running the bathroom fan with moisture from the hot shower running, it revealed that at this area there was a bit of air leakage.
Should the stack be replaced with PVC pipe or is there a sealing solution that can be applied to the metal pipe?
Thanks for your consideration in this matter. -- Tom Breen
ANSWER: Unless there is a problem with excessive corrosion, cracks or leakage from an older metal stack, it is normally unnecessary to replace it with a newer ABS drain pipe. It is true that poorly insulated and sealed cast iron or copper stacks can create a moisture issue in the attic, but it is not the fault of the pipe. You should be able to fix this problem and the difficulty or ease will depend on the accessibility through your attic.
It appears you have a good grasp on the cause of the moisture intrusion into your bathroom ceiling. I am unsure of the nature of the ribbed connector at this area, but any leakage at this critical junction can cause the problems you are experiencing.
You are correct that warm, moist air leaking into the attic or insulation above the ceiling can condense on the cold metal stack. This condensation will likely freeze in the winter and can build up to produce a thick layer of frost or ice over the entire heating season. When the frost melts it wets the insulation below and often causes damage to the ceiling or leaks into the bathroom. This type of problem will normally become worse every year. While the metal pipe may be contributing to the condensation, this issue can also happen on ABS plastic vents. The real culprit is the gap at the bathroom ceiling.
Sealing the junction between the living space and the attic around protrusions like your plumbing stack can often be a challenge. While it may seem simple to plug this gap to prevent excessive air leakage, access can often be difficult. If you have a low-pitch roof or the plumbing stack is located near the eaves it may be next to impossible to crawl far enough into the attic above this area to address the problem.
In your case, this does not appear to be too difficult, otherwise your helpful neighbour would not have been able to easily remove the insulation and locate the "ribbed connector" you have noted. Small gaps may be sealed with quality caulking before covering with a sufficient amount of insulation. Better yet, any gaps can be filled with blown-in insulation available at home centres in small cans. This same insulation can be used to coat the outside of the metal stack to prevent further condensation, but may be hard to gauge when enough is applied.
This repair will only be possible if the bathroom ceiling is not badly damaged or mouldy, which would require a larger area to be fixed. If that is your situation, cutting out the area of damaged drywall or plaster is necessary.
After replacement with new, water-resistant drywall and taping, the air sealing repairs can begin. This should include blown-in foam installation at any openings around the pipes and sealing directly to the existing polyethylene air/vapour barrier if present. Covering the pipe with a box made from rigid extruded polystyrene insulation may be a more economical method than coating it with several cans of foam. If that is done, all seams and any small gaps in the foam sheathing can be coated with blown foam to form a tight air seal. This should be done as close to the underside of the roof sheathing as possible to provide the maximum protection from condensation.
One last area to address in this repair is in the basement of your home. Check the area where the main stack penetrates the main floor below the sheathing. If there is a gap around this area, which is typical, shine a flashlight up to see how extensive the space around the pipe is. You may be surprised at this large gap, which can allow considerable warm air leakage into the attic around the pipe due to the "stack effect." Sealing the bottom of this area with blown foam, between the floor joists and wall studs, will substantially restrict air leakage and condensation. Closing up all of these gaps to prevent warm, moist air leakage into the attic, and properly insulating the metal stack, will prevent the need for replacement.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors -- Manitoba (www.cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.
trainedeye@iname.com
