Question -- I have a question that hopefully you can answer, or at the very least, offer up some recommendations. I have a 1960s bungalow and recently had the bathroom renovated, which included the installation of a new bathroom fan. The fan that was installed is a high-end model and has more than adequate air flow for the size of the bathroom. The old fan was vented through the roof. The new fan is vented through the attic space and exits out the gable end and has approximately a 20-foot run. The fan is initially connected to solid metal venting and is then connected to flexible tubing for the remainder of the run. The venting goes up a short distance from where it connects to the fan, turns 90 degrees and is sloped downward towards the vent cap. Both the fan and vent cap have flappers to prevent back drafts. The venting is wrapped in insulation.
Please note that the fan was installed just prior to the winter. Also please note that the fan has not been used. The problem is that moisture came down the outside of the fan housing and damaged the drywall ceiling. The moisture does not appear to be coming from within the fan. Upon investigation, we discovered that the outside of the metal venting and fan housing is covered with a thin layer of ice, which is melting near the fan and running back into the ceiling. There is also a buildup of ice inside the venting, but I assume that this is due to the use of flexible venting.
Given that the fan has not been used, because we are not using the bath or shower, this is not the cause of the moisture. In layman's terms, it appears that the moisture that is naturally in the attic space is condensing on the outside of the fan housing and venting, and then following gravity back down the outside of the housing and into drywall ceiling. I have done a little research and have learned that the use of flexible metal venting is not recommended. I have also seen it suggested that one can use 4-inch plastic sewer pipe as it is smooth and not subject to condensing and icing as is metal venting.
What do you think about using plastic tubing instead of metal? What is the correct type of insulation and insulating wrap to use given our Winnipeg climate? What is the proper way to insulate around the fan housing and the venting that runs up from where it connects to the fan and before it turns 90 degrees and then slopes down to the where it exits the gable?
Any suggestions will be much appreciated.
--Bob Bouchard, email
Answer -- As the recent, bitter cold snap appears to be behind us and the temperature approaches the freezing point, I have been receiving numerous phone calls and e-mail regarding water leakage into homes, similar to yours. Consequently, I thought that this warm spell would be the ideal time to deal with this common issue, often not experienced until the Spring thaw.
With several weeks of constant, bitter cold weather, nearing -30C every night, we often experience an invisible issue in our homes. Excessive frost build-up can develop in attics, particularly in older homes, on the underside of the roof sheathing and gable ends. With normal winter temperatures, sunny days, and adequate attic ventilation, this frost may slowly melt, the water evaporate and be eliminated from the attic through the vents. With an extended period of unusually cold weather, like that just behind us, the frost may not melt and will continue to thicken until a heavy layer covers all the wooden components in the attic. While the temperatures remain in the sub-arctic range this causes little problems and will not usually be noticed until there is a sudden warm spell, when the frost may melt very quickly and saturate the insulation in the attic. As we have just seen, this can cause leakage into the ceilings and walls in the home.
The same thing can occur even in a well insulated, air sealed and vented home, through the one large opening in the attic floor, the exhaust fan. While there may be little area for warm, moist house air to infiltrate your attic in most of the ceilings, this is quite possible though a poorly sealed bathroom exhaust fan. The small damper in the fan is not often well sealed enough to prevent warm air intrusion into the duct from the bathroom. While this may cause some condensation and leakage, it may only be a portion of the problem in your bathroom.
The housing for most bathroom exhaust fans is made of metal, has several small holes and will conduct heat quite well. Considering that the underside of this fan housing is at room temperature and the top side exposed to the freezing attic, it is highly prone to condensation. Not operating the fan may exacerbate the situation, as warm air may sit in the fan housing and not be blown harmlessly out through the normal ducting and vent hood. A lack of high-volume air movement may contribute to frost building up in and around the fan ducting, due to a lack of fresh, warm air to melt the existing frost. I would operate the fan, periodically, even if you are not using the shower, to prevent this.
The ultimate solution is to ensure that the ducting and housing in the attic is well sealed and insulated. This can typically be accomplished with a combination of several small cans of blown-in polyurethane foam insulation and conventional duct pipe wraps, or pre-insulated ducting. The foam should be blown to a thickness of five centimetres or more, around the entire fan housing and the solid metal ducting, and then covered with conventional insulation to match the rest of the attic floor. If the flexible ducting connected at this point is not insulated, it should be discarded and replaced with pre-insulated flexible ducting or solid metal, which can be then wrapped with several layers of pipe wrap insulation. While solid piping is a better choice than flexible ducting to prevent loss of air volume, rarely is plastic piping used in this situation. I don't see any reason not to use ABS pipes, as you have suggested, other than the difficulty working with long lengths of this rigid piping in the confined space of your attic and the added cost, but it will still have to be insulated to prevent freezing.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba (www.cahpi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.
