Renovation & Design

ASK THE INSPECTOR: Replacing flexible ducting can be exhausting

QUESTION: I live in a single-storey home that is about 55 years old. The roof is described as a gable roof with a two-foot overhang front and back and a one-foot overhang on each end. The attic is insulated with the original wood shavings and two layers of R20 fibreglass insulation on top.

A few years ago, as part of a bathroom renovation, I had an exhaust fan installed in the bathroom ceiling. The contractor said he wouldn't advise venting through the soffit, as the moisture could enter back into the attic. He also did not recommend venting straight up through the roof, a distance of about two feet, as condensation and moisture could drip back through the fan.

What he decided on was to run a special flexible tube, which he claimed was "code," up from the fan at about a 45-degree angle towards the peak of the roof. The distance was about three feet high and nine feet long and towards the gable end, with a gradual slope down and a vent hood on the north gable end of the house.

This past winter I had an unusual amount of moisture in my attic and it seemed most concentrated by the bathroom fan and venting. I had a roofer examine my roof for leaks. He checked in my attic and said he thought that I didn't have enough ventilation. I have a vent at each gable end, a whirlybird in the centre of the roof near the peak, and two plastic-type vents in the roof near the front and beck. There is very little if any soffit venting.

I am wondering if the moisture in my attic is due to the bathroom fan venting. I have contacted the contractor and he said he has seen problems with this flexible tubing material, with moisture being trapped in it. He will replace the venting and is planning on running a rigid tube -- I'm not sure if it's metal or some type of plastic -- then insulating it.

Is this the proper way to vent the bathroom fan? Also, have you any idea what else could cause the moisture in my attic?

Thanks, Fred.

ANSWER: Installation of properly insulated and sealed exhaust fan housings and ductwork is critical to prevent excessive moisture intrusion into the attic. But I doubt the flexible, insulated ducting in your attic has much to do with the moisture, unless it is damaged, loose or improperly sealed at the attic floor.

The metal fan housing must be covered and sealed with six-mil polyethylene and insulation, or a thick layer of high-density foam or extruded polystyrene, to prevent warm air leakage around this area. If the fan housing is not well-sealed, this could lead to excessive moisture leakage into the attic or condensation and dripping from the fan area itself.

If the flexible ducting becomes wet or filled with frost, it may not vent as well as it should, but will not normally cause moisture issues in the attic, unless it is damaged. Excessively long vents can also cause some leakage when frost inside the ducting melts, but this usually only occurs periodically and can be easily cleaned up. The true culprit may be the ducting itself, or the overall length and location.

It appears that you have found a good contractor to install your exhaust fan ducting and hood, because he has provided sound advice. You never want to vent moisture through the soffits, and sometimes short, straight runs of ducting connected to roof-mounted vent hoods can cause leakage due to frost or blowing snow.

Unfortunately, excessively long runs of this type of ducting can also cause the same issue. Because the flexible ducting is corrugated, the longer the length, the poorer the airflow. The corrugations slow air movement and can cause the warm, moist air to cool and condense inside the piping before it can reach the vent hood on the gable end.

Excess water inside the duct can cause the insulation to become damp and the entire duct to sag, leaving a small bend or trap which will make drying and escape of water very difficult. The length and slope may prevent the water from dripping down through the fan housing, but it can severely affect the ability of the warm air to exit the duct.

The likely solution is to replace the flexible, corrugated duct with a solid metal one, but care must be taken to tape the joints to avoid using screws that will further impede airflow. The smooth inner surface will promote better airflow but, because it is metal, the new duct will be more prone to frost buildup due to heat loss. Insulation should be increased all around, and the length should probably be shortened to further prevent cooling and condensation. This may require moving the vent hood to the roof rather than the gable end.

As far as excess moisture in your attic goes, there are several factors that can affect what you're seeing, including the time of year. In some years, when we have a very wet autumn and a quick drop in temperature outside, excessive moisture can be trapped inside an attic. This occurred a couple of years ago and was an unusual phenomenon. Most of this moisture will evaporate due to the dry outside air in the winter and heat from the sun, so looking into the attic in mid-winter or summer may show different results.

If there is no damage to your exhaust-fan ducting and the housing is well-sealed, there should be little chance of severe amounts of moisture entering the attic from that source. But installing some soffit venting, or more roof or gable venting, may help improve air movement in your attic and prevent further issues in the future.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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