Best methods for sealing plumbing stack
A: There may be several methods to properly seal a plumbing stack as it enters the attic through the ceiling. I will provide you with the merits and disadvantages of your suggestions and you can decide on which method suits you best. It is a very good idea to make an attempt to airseal anything that penetrates the attic floor from the home. Successfully accomplishing this task will help prevent warm-air intrusion into the attic, which can lead to frost and moisture issues in our brutal winters. You must remember sealing this area will only stop a portion of the air leakage into your attic, so it may be a good idea to address other areas at the same time. One of these areas I frequently see improperly insulated, and poorly sealed, is the attic access hatch. It is true quite a bit of air leakage could be attributed to the gap around your main stack, but the attic hatch is often the worst area for air leakage from the home into the attic. Since you have to remove the hatch to access the attic for stack-sealing repairs, it makes perfect sense to fix it then. Many homes like yours have attic hatches that may be constructed from plywood or some form of particleboard. Plywood hatches are ideal, as they are somewhat moisture-resistant and the large amount of adhesives provides a reasonable air/vapour barrier, especially when painted. Particleboard, on the other side of the scale, is a poor choice for this use, as it tends to swell and deteriorate if it gets wet, especially at the edges. This can even cause them to jam against the sides of the attic hatch, making removal quite difficult. If you have a hatch cover constructed of this material, or other sheathing affected by moisture such as drywall, replacing it with G1S plywood sheathing will be a major improvement. Once you have your nicely fitted plywood hatch cover, supported by secure wooden cleats at the bottom of the hatch, the next thing to address is the insulation. Most hatches I see, regardless of the age of home, are normally insulated with fibreglass insulation on top. This may vary from a batt or two of fibreglass fit roughly to the size of the attic opening, to loose, blown-in material contained in a box constructed as the hatch cover. The problem with those installations is fibreglass is good thermal insulation, but a lousy air barrier, so any air that does leak around or through the hatch cover will easily make its way into the attic. Worse yet, it may cool considerably as it travels through the fibreglass, reaching its dew point and condensing inside the insulation. That will lead to frost and water damage to the hatch, and sometimes even dripping from the hatch itself. The simplest solution to this problem is to install extruded polystyrene on top of the attic hatch cover, instead of fibreglass. This material comes in large sheets, which can be easily cut with a utility knife and secured to the hatch with special adhesive. It is very light, so laminating a few layers of this insulation on top of the hatch will provide good thermal protection and an excellent air/vapour barrier. The final step is to install a good-quality weatherstrip on the top or sides of the cleats holding the plywood in place to prevent air leakage around your nice new attic hatch cover. The same approach can be taken with air-sealing your ABS plumbing stack. While the cellulose fibre insulation may be a good air/vapour barrier if enough thickness is present, it is doubtful that will be the current situation around the stack. Securing polyethylene sheathing around the stack will be effective in minimizing air leakage, but it can be very difficult to accomplish a good seal with this semi-rigid material. Even with liberally applied acoustical sealant, it can be quite tricky to get a good seal between the tubular stack and the attic floor. For that reason, foam insulation may be a better bet. This can be most easily done with a can or two of blown-in polyurethane foam, but that material is only a moderately good air/vapour barrier, unless applied in several layers. One possible air-sealing method you have not mentioned is to cut a small piece of rigid extruded polystyrene slightly larger than the opening around the stack. This can then be cut in half, with two semicircles, fit to the size of the stack, carved to make a tight fit when joined together. This collar can then be fitted and sealed tight to the attic floor and caulked around the stack from above. It may also be possible to blow a small amount of foam underneath and above this rigid collar, to seal this area even better. There may be several ways to air-seal the attic floor around your plumbing stack, but there are other areas to consider improving at the same time. Using various types of foam insulation, rigid or blown in from a can, should give you the best job with the least amount of effort.