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Bath exhaust vent fan installation into a cathedral ceiling over a shower (C) Daniel FriedmanBath Exhaust Fan Through Soffit

Route bath fan out through eaves or soffit

Bathroom exhaust fan routing and materials when venting down through a soffit.

We choose materials, routing, slope, and insulation of the exhaust duct on a bathroom exhaust fan system to prevent condensation and leaks into the building ceiling.

This article series explains how to install bathroom exhaust fans or vents, the vent ducting, the vent termination at the wall, soffit or roof, vent fan wiring, bath vent duct insulation, bath vent lengths, clearances, routing, and we answer just about any other bathroom ventilation design or installation question you may have.

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Bathroom Vent Fan Soffit Installation

Bath exhaust vent fan installation into a cathedral ceiling over a shower (C) Daniel FriedmanHere we describe and illustrate the materials, routing, positioning and final installation of a bathroom exhaust vent fan/light combination.

The fan and light will be installed over a bathroom shower.

[Click to enlarge any image]

The exhaust duct, using solid metal (steel in this case) is routed through the ceiling, out over the wall top, down through a soffit or roof eaves overhang, and then enclosed against pest entry.

Wiring for the exhaust vent fan/light is routed to wall switches that in this case will turn on the fan if the light over the shower is also switched on.

The bathroom will have a separately-switched light over the sink so that the room can be lit without forcing use of the fan.

In this installation we are routing the exhaust duct through a ceiling cavity that is also the under-roof space: a cathedral style ceiling covers this building.

Because this is a hot roof (un-vented roof cavity) design, we're going to use spray foam insulation in the ceiling and around the exhaust duct.

Watch out: in any bathroom exhaust vent design routing duct through a ceiling or attic above, take care that the duct slopes to drain any condensate out of the duct opening, not back into the ceiling or into the fan / light fixture.

Details are at BATHROOM VENT DUCT SLOPE

I prefer solid metal ductwork for exhaust fans. The less-costly flexible spiral fan duct material is less durable, obstructs air flow, and risks sagging, collecting moisture that can encourage mold growth in the duct.

In addition, because we're using spray foam to insulate the ceiling/roof space, I want the ductwork to be as durable as possible: it would be messy as all get-out if ten years later we need to tear down the ceiling to replace damaged or leaky thin plastic spiral flexduct in the same space.

Bath exhaust vent fan installation into a cathedral ceiling over a shower (C) Daniel Friedman

Above you can see the exhaust duct is a straight run from the fan housing (upper right in the photo) to the wall top penetration to the outdoors.

Advantages of Venting Bath Exhaust Out & Down Through a Soffit or Eaves

  1. This design usually permits the shortest duct route length, maximizing fan and vent efficiency
  2. This down and out design avoids cutting a penetration in a roof surface: up-vented ducts drip back into the building and through-roof vents add another potential point for roof leaks
  3. This down and out through soffit vent design similarly avoids cutting a vent opening in a vertical wall like a gable-end wall: this avoids the risk of wind-blown back-drafting when the fan is in operation
  4. A down-through-soffit exhaust vent design, by leaving warm air in the exhaust duct when the fan is off, creates a heat trap that reduces heat loss out of the bathroom through the exhaust fan duct when the fan is off, in comparison with up-routed vents or even horizontal vents through a gable end wall.

Watch out: however take care to use a frost-free exhaust vent termination cover in freezing climates, and take care to use moisture-resistant soffit covering or paints - which you should be doing at any outdoor surface anyway.

Below are details about the penetration through I-truss blocking at the wall top.

Bath exhaust vent fan details as it passes over the wall top into the eaves / soffit (C) Daniel Friedman at InspectApedia.com

Above: the bath exhaust vent duct is routed through the ceiling, between I-trusses used to support the roof, over the wall top, to the building exterior, into the soffit cavity.

The metal visible below the exhaust duct was put in place to reinforce the supporting web between the I-trusses. The yellow electrical wire is routed through the same ceiling to a nearby light fixture, passing above the ductwork.

Watch out: take care that the total equivalent length of ductwork, including factors for bends or elbows, does not exceed the bathroom exhaust vent fan manufacturer's specifications for your fan. Details are

at BATHROOM VENT DUCT LENGTHS.

Metal bath exhaust fan duct installation in new construction (C) Daniel Friedman

Above we illustrate bath exhaust vent ducting before and after it is covered by a spray foam insulation job in a cathedral ceiling.

Bath exhaust fan in insulated ceiling after foam insulation installation (C) Daniel Friedman

Watch out: we do not recommend purchasing vent ducts with insulation on the duct interior.

Although fiberglass-lined HVAC ducts are widely used in heating and air conditioning systems as well as in heat recovery ventilator systems, for a standard bath exhaust fan vent duct, the combination of permeable fiberglass or other insulating material and very high moisture levels is asking for mold or other trouble inside the vent line.

Bath exhaust vent fan installation into a cathedral ceiling over a shower (C) Daniel Friedman

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Bath exahust vent temporary support during soffit finishing (C) Daniel Friedman at InspectApedia.com

Above: using foil tape, we temporarily secured the exhaust duct down-extension through the soffit, holding it in the proper position.

I wanted to assure that foam insulation to be sprayed into the soffit eaves would not move the duct out of its proper position (shown below).

Bath exhaust vent fan installation into a cathedral ceiling over a shower (C) Daniel Friedman

When the insulation was in place and cured we could trim the exhaust duct down-extension to its proper length to permit first, installation of the soffit covering (solid plywood in this case as the roof is an un-vented hot roof design), and then the installation of the exhaust vent terminating cover.

Bath exhaust vent fan installation into a cathedral ceiling over a shower (C) Daniel Friedman

Above: the finished exhaust vent duct termination is shown in the horizontal, down-facing surface of the soffit - under the roof eaves. The cover is connected to the metal ductwork using a suitable-diameter steel worm-gear clamp.

This Everbilt™ paintable polypropylene, frost-free exhaust vent cover was inexpensive (less than $15. U.S. at a Home Depot supplier), fits 4", 5" or 6" diameter exhaust ducts and includes an outer grill to keep birds or rodents out. Installation instructions for this vent cover are given below.

Below you can see condensation on the outside surface of the bathroom window just below the exhaust vent as the fan is operated during winter weather and while the shower is in use.

Condensation on window near soffit exahust for bathroom vent fan (C) Daniel Friedman at InspectApedia.com

Watch out: when selecting an exhaust vent terminator for a bathroom fan being installed in cold climates, select a vent terminator or damper that is "frost free" - that is, a self-closing exhaust termination cover that will not freeze shut in cold weather.

Watch out: also, some readers have worried that discharging a bath exhaust vent down through a soffit will permit moisture to rise back up into the attic or roof space IF the soffit and roof cavity are vented.

Certainly it is improper to terminate any exhaust vent directly into an attic or roof space.

Principally the claim that a properly-installed soffit exhaust vent will back-draft into the soffit and attic of a vented roof seems to be more OPINION than substantiated by scholarly research.

I think that the downwards velocity of the exhaust and the down-slope of the duct combine to vent both moist air and any condensate away from the home enough that this is not a common issue, but we continue to research this case.

For installers who want a soffit exhaust vent that directs air out horizontally away from the building see the Primex SV28 illustrated and cited below.

Readers with photos or field experience are invited to post those images or comments at the end of this page.

Detail of finished bath exahsut vent in ceiling (C) Daniel Friedman at InspectApedia.com

Above and below we show the finished-installation of the combination bathroom exhaust vent fan and light over the shower where it will be best situated to pick up and exhaust moisture when the shower is in use.

Finished bath exhaust vent fan in ceiling combines light and exhaust located to pick up shower steam. GFI protected. (C) Daniel Friedman at InspectApedia.com

Watch out: when a light or fan over a shower could be touched by someone who may be wet and near plumbing it is important that the circuit also be GFCI or ground-fault circuit interrupter protected.

Bathroom Vent Fan Choices & Installation Guides

Broan 100HL fan, heater, light combination - cited in detail at InspectApedia.com

Above: Broan's 100 HL combination light, exhaust fan and heater.

Lambro 4" under-eave vent at InspectApedia.com

Above: Lambro's plastic under-eave vent described in the instructions given just below.

Panasonic EZSoffit Vent Soffit Termination Cover cited in detail at InspectApedia.com

Illustration: Panasonic's EZSoffit Vent™ Termination System for Bathroom exhaust vent systems. Cited in detail below.

Primex SV28 Exhaust Vent directs air horizontally away from the building. Cited in detail at InspectApedia.com


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