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Bathroom Vent Fans; Ducting or without Ducting?

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If you are wondering whether ducting of bathroom vent fans is necessary, everything depends on two things: the climate, as well as where the bathroom is situated in the house.

If the fan is vented into an attic that is used for storage, etc, then you need to have ducting expel the moist air outside, irrespective of where you happen to live.

If the fan is vented into an unused roof space or crawl space the climate becomes a factor. There are some countries where the days are so hot and dry that moisture from the bathroom vent fans dissipates so quickly that it may not be necessary to have ducting. The dry regions of Australia and Africa are examples.

However in most houses venting into the roof space is unacceptable. It does happen that people installing bathroom vent fans try to save time, effort and money by leaving the ducting out. The moisture can cause wood rot in the beams of the roof and mildew, as well as corrosion of any metals in the roof or attic. Accumulating moisture that condenses and drips can damage your ceilings, too. Dust mites flourish with a little moisture in their habitat, as well as many other insects.

Successful ducting

The secret of successful ducting is to have as few elbows and bends as possible. The effectiveness of bathroom vent fans depends to a large extent on the smoothness of the duct, the length of it and whether there are sharp turns to interfere with the airflow. So, when you go up into the roof space to make your initial inspection, see where the fan is situated and what is the shortest route to the nearest outside wall or the roof. Try and plan straight lines with no turns around obstacles.

If obstacles are unavoidable you will have to plan a slow turn with flexible aluminum ducting.

If your line is short and straight, you might like to consider PVC piping of the correct diameter instead of the usual aluminum ducting. The smoothness inside the PVC can improve your bathroom vent fanís performance considerably. You will have to have an elbow attached to your vent fanís outlet port. This should be as slow a turn as possible and should face the direction you want the ducting to go.

If you have an elbow with a ninety degree turn your fan will be noisy and far less effective. Seal all joins with foil duct tape. If you are using PVC you need to join it with PVC glue. The flexibility of aluminum ducting makes it far easier to install, but it has ridges that hamper the airflow to a certain extent. If you do use it, support the flexible ducting along its length so that it doesnít lift and sag as it crosses the wooden beams, and keep it in as straight a line as possible.

When you have selected the spot on the wall, in between the wall studs, where you are going to have your duct exit, drill a marker hole through so you will be able to locate it from the outside. You can push a long nail or piece of doweling through if that will help. Cut out the hole for the wall cap from the outside. Your diameters from the bathroom vent fanís outlet port, the ducting and the wall cap (or roof cap) should remain constant. Avoid reducing the diameters at any stage with adapters. Make sure you buy the correct diameter ducting and wall cap to start with. Seal the wall cap and washer, if you have one, with silicone and press it into place firmly. Use corrosion proof screws, such as brass, to fasten it in place. Inside the roof space you can attach the other end of the ducting to the protruding part of the wall or roof cap. Seal up with foil duct tape.

If you live in a cold climate, you can use insulation to prevent condensation in the ducting dripping back down into your bathroom through the bathroom vent fanís grill. You can either wrap the whole duct with insulation, such as fiberglass, or remove the insulation from below and place it above the ducting. In this way the warmth of the house will help to keep the ducting warm.

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