How to Install a Soffit Vent and Ductwork for a Bathroom Vent Fan

I’ve finish mounting and wiring the new Panasonic WhisperCeiling ventilation fan model FV-05VQ5 in the 2nd floor bathroom. Now I’ll install a new 4 inch soffit vent and insulated vent duct. This project is continued from Panasonic WhisperCeiling Bathroom Fan – Wiring and Mounting.

How to Install a Soffit Vent and Ductwork for a Bathroom Vent Fan

The contractor who installed the bathroom vent fan when the home was built was lazy and did a terrible job as explained here. The contractor apparently didn’t have a 24 foot extension ladder to install the soffit vent where it should be near the 2nd floor bathroom, so he put it where he could reach out the bedroom window:

Install a Soffit Vent for a Bathroom Vent Fan

Install a Soffit Vent for a Bathroom Vent Fan

Uninsulated Bathroom Fan Vent Duct

Placing the original 3 inch soffit vent above the bedroom window resulted in an much longer than necessary duct length which increases air flow resistance and reduces the bathroom fan efficiency. Uninsulated vent duct will cause condensation of the moist air inside the vent duct which can pool, promote mold growth and might leak on the attic insulation and drywall ceiling. That long run of 3 inch duct is illustrates the problem:

Old Bathroom Vent Fan Uninsulated Duct in Attic

Old Bathroom Vent Fan Uninsulated Duct in Attic

The new Panasonic WhisperCeiling bathroom fan requires a 4 inch soffit vent and 4 inch duct. I’ll install a new 4 inch soffit vent and insulated flex duct close to the 2nd floor bathroom at the top of the ladder in the first photo above.

Speedi-Products and Master Flow 4 inch Soffit Vents

There are many soffit vent products on the market – also known as “soffit exhaust vent” and “under eave vent” – with a wide range of features and prices. After reading the reviews I purchased the Speedi-Products 4 in. Round Soffit Vent Model # SM-RSV 4 for about $2 from Home Depot. It’s only available online with free ship-to-store and arrived in about 1 week.

After I received the Speedi-Products soffit vent, I decided that while the ~2 inch long neck could be taped to the vent duct I preferred a longer neck adapter. I rummaged around the Home Depot store and also bought the Master Flow 4 inch Eave Vent model # BFEV4 only to get the metal duct pipe connector. You’ll also need two 4 inch worm gear band clamps and a roll of 2 inch wide metal foil HVAC tape.

Bathroom Vent Fan - 4 inch Soffit Vent Parts by Speedi-Products and Master Flow

Bathroom Vent Fan – 4 inch Soffit Vent Parts by Speedi-Products and Master Flow

The 4 inch duct pipe connector isn’t fastened to the Master Flow eave vent and simply slides off the vent cap. The duct pipe connector is slightly narrower at the one end to fit the soffit vent. It also fit perfectly into the Speedi-Products SM-RSV 4 vent. The reasons for using parts from two different manufacturers are:

  • The Speedi-Products SM-RSV 4 vent:
    • Has a wide flange to conceal the mounting hole sawn in the soffit.
      This is important if you don’t have a hole saw for your drill to cut a perfect circle.
    • Is easy to fasten to the soffit plywood with three pan-head wood screws.
  • By comparison, the Master Flow eave vent:
    • Has a very narrow flange that just barely conceals a 4-1/4 inch diameter mounting hole drilled with a hole saw and only if it’s centered in the hole.
    • The narrow flange can’t be fastened to the soffit plywood with wood screws.
      It seems caulking is the narrow flange is the only option, however it’s nearly impossible to install because the bathroom vent duct will pull the vent sideways and out of the soffit mounting hole. You’ll end up with a smeared mess of caulk and a loose vent.
Bathroom Vent Fan: Speedi-Products 4 inch Soffit Vent Model # SM-RSV 4

Bathroom Vent Fan: Speedi-Products 4 inch Soffit Vent Model # SM-RSV 4

In retrospect, I think the Speedi-Products EX-EVML 04 4 inch eave vent would be a better choice because it combines the wide mounting flange with a long aluminum tail for the duct connection. Note the review on Amazon.com indicates it requires a 5 inch diameter mounting hole:

Install the Soffit Vent for the Bathroom Fan

The new soffit vent will positioned between the roof rafters and lookouts. I took measurements inside the attic using the rectangular soffit vent in the adjacent rafter bay as my reference. Working on the ladder, I measured and marked the center for the new vent relative to rectangular soffit vent. I then cut the 4-1/4 inch hole for the vent with a hole saw on a my cordless drill/driver:

Bathroom Vent Fan Installation - Drill Hole for Soffit Vent

Bathroom Vent Fan Installation – Drill Hole for Soffit Vent

Attic view of the 4-1/4 inch mounting hole in the soffit between the roof rafters:

Bathroom Fan Soffit Vent Mounting Hole: Attic View

Bathroom Fan Soffit Vent Mounting Hole: Attic View

The 4-1/4 inch hole saw attachment for my cordless drill/driver cut a perfect circle in the soffit plywood. Hole saws are available at most home improvement stores.

Bathroom Vent Fan: 4-1/4 inch Hole Saw for Soffit Vent

Bathroom Vent Fan: 4-1/4 inch Hole Saw for Soffit Vent

Connect the Insulated Flexible Duct to the Soffit Vent

I’m now working in the attic to connect a new run of 4 inch insulated duct to the soffit vent duct pipe connector. I purchased a 25 foot box of R-8 insulated flexible duct at Home Depot. This flexible duct can be used for running air conditioning ducts or bathroom exhaust vents:

4 inch Insulated Flexible Duct for Bathroom Vent Fan

4 inch Insulated Flexible Duct for Bathroom Vent Fan

Workshop scissors, wire cutter and a tape measure are needed to install the insulated flexible duct. Gloves are recommended if your skin is sensitive to fiberglass insulation. Use the scissors to cut the silver jacket, pink fiberglass insulation and inner duct liner, then cut the duct liner wire with the wire cutters:

Install Bathroom Vent Fan Flexible Duct: Wire Cutter and Scissors

Install Bathroom Vent Fan Flexible Duct: Wire Cutter and Scissors

I pushed the 4 inch flex duct into the soffit where I drilled the mounting hole for the new vent. Take care not to tear the flex duct on the nails poking through the roof deck. BTW – the new bathroom fan is located beneath the 2×4 boards at the lower left corner of the photo:

Install Insulated Flex Duct for Bathroom Fan Soffit Vent

Install Insulated Flex Duct for Bathroom Fan Soffit Vent

Back outside on the ladder, soffit exhaust vent installation steps are:

  1. Reach into the soffit vent mounting hole and gently pull the flex duct inner line out about 6 inches.
  2. Slip the flex duct inner liner about 2 inches over the wider end of the metal duct pipe connector.
    Remember, the slightly narrower end of the duct pipe connector fits into the soffit vent cap.
  3. Seal the inner flex duct to duct pipe connector with two layers of HVAC metal foil tape.
  4. Slide a 4 inch metal worm gear band clamp over the taped joint and tighten the screw until snug.
    While HVAC tape is usually sufficient to fasten the duct pipe to the duct liner, the band clamp provides extra mechanical support.
  5. Slide the Speedi-Products SM-RSV 4 vent soffit vent over the narrow end of the duct pipe connector.
    The soffit exhaust vent must be pointed so it blows away from the house as shown.
    Why? If the vent were to blow moist air against the house it would condense on the wall, promote mold and rot and freeze in the winter.
  6. Seal the soffit vent plastic neck to duct pipe connector with two layers of HVAC metal foil tape.
    Smooth all wrinkles for an air tight seal.
  7. Push the duct pipe connector up into the soffit.

At this point the soffit vent is hanging loose on the duct pipe connector and flex duct:

Install a Soffit Vent for Bathroom Vent Fan - Duct Connection

Install a Soffit Vent for Bathroom Vent Fan – Duct Connection

The Speedi-Products soffit vent has three molded pockets equally spaced around the back of flange for screws. Mark the screw positions on the face of the vent because it’s nearly impossible to see these when the vent is against the soffit. Fasten the vent to the plywood soffit with three Simpson Strong-Tie SD8 #8 1-1/2 wood screws or equivalent pan head wood screws:

Install a New Soffit Vent for Bathroom Vent Fan

Install a New Soffit Vent for Bathroom Vent Fan

Connect the Flexible Duct to the Bathroom Vent Fan

Working in the attic, I measured and cut the 4 inch diameter insulated flexible duct to the needed length from shipping carton. The flexible duct is connected to the FV-05VQ5 fan by these steps:

  1. Remove the shipping tape from the fan exhaust backdraft damper if you haven’t already done so.
  2. Slip a 4 inch worm drive metal band clamp over the inner duct liner.
  3. Slide the flex duct inner liner over the fan duct adapter until it’s fully seated.
  4. Seal the inner duct liner to the duct adapter with two wraps of metal foil HVAC tape, overlapping the 2 inch wide tape equally on the duct liner and fan duct adapter. Press and smooth all tape wrinkles for an air tight connection.
  5. Draw the band clamp over the neck of the duct adapter and tighten until snug.
Panasonic WhisperCeiling Vent Fan Installation - Flex Duct Connection

Panasonic WhisperCeiling Vent Fan Installation – Flex Duct Connection

I then pulled the flex duct insulated jacket over the fan duct adapter against the fan body:

Panasonic WhisperCeiling FV-05VQ5 Bathroom Fan - Old Work Install: Insulated Flex Duct

Panasonic WhisperCeiling FV-05VQ5 Bathroom Fan – Old Work Install: Insulated Flex Duct

Here’s an overview of the new WhisperCeiling bathroom fan and flexible duct to the soffit vent. Keep the bends in the flexible vent duct gentle and wide; try to avoid sharp 90 degree turns and kinks. This new vent duct is at least 8 feet shorter than the original 3 inch duct installed by that goober contractor.

Panasonic WhisperCeiling Bathroom Vent Fan and Insulated Flexible Duct to Soffit Vent

Panasonic WhisperCeiling Bathroom Vent Fan and Insulated Flexible Duct to Soffit Vent

I pushed the loose fill fiberglass insulation back in place to complete the bathroom vent fan installation:

Bathroom Vent Fan Installation: Replace the Attic Insulation

Bathroom Vent Fan Installation: Replace the Attic Insulation

The bathroom exhaust fan is rated at 50 CFM (cubic feet per minute) which is correctly sized (see the Panasonic Choose Your Fan calculator) for this small bathroom which has only a shower and toilet because it’s part of a Jack and Jill bathroom. I placed a section of toilet tissue over the grille to demonstrate the fan operation:

Panasonic WhisperCeiling Bathroom Vent Fan: Finished Installation

Panasonic WhisperCeiling Bathroom Vent Fan: Finished Installation

The old and noisy contractor-grade bathroom fan always left the bathroom steamy, which makes more work for the air conditioner during the summer. My teenagers say the bathroom is no longer steamy with new exhaust fan. It’s been a major improvement!

Hope you found this helpful,

Bob Jackson

Copyright © 2016 HandymanHowTo.com   Reproduction strictly prohibited.

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15 Responses to How to Install a Soffit Vent and Ductwork for a Bathroom Vent Fan

  1. Lutz Kockel July 5, 2014 at 5:13 pm #

    Excellent and detailed description! The images alone are worth the time.

    I got the time spent reading through the article back x10. My old ventilation unit finally retired, after apparently transforming all the energy it consumed into noise.

    A quick note for the exhaust vent, that I had to mount on an vertical wall. I used the Alcoa Home Exteriors EXVENT EQ Louvered Exhaust Vent. Nice flange, 10 in long vent tube, 4 in diameter, bug shield, and louvered, as a second backdraft block.

    • Bob Jackson July 5, 2014 at 5:50 pm #

      I’m happy you found my project helpful and the wall exhaust vent recommendation.

  2. Joe S March 26, 2015 at 9:25 am #

    Extremely useful, and well written. Helped me solve problem I’ve been trying to figure out for some time now (wrt properly venting through soffit, which builder had not done correctly). A big thank you to the author for publishing this article!

  3. Dan June 20, 2015 at 12:39 pm #

    Thanks! great write-up. Will be doing something similar. I bought the Speedi-Products 4 in. Round Soffit Vent Model # SM-RSV 4 on Amazon as a Prime add-on with free shipping. HD wants the same price but charges too much to ship these parts to store – they should ship to store for free.
    Nonetheless, your article answered all my questions :)

  4. Michael McCarthy October 25, 2015 at 11:09 am #

    This was very helpful. Thank you for the detailed photos and descriptive step-by-step. I spent quite a while trying to find out the exact order of the steps to connect the insulated duct hose to the bathroom fan. Your pictures and verbal descriptions are perfect. I too installed a Panasonic whisper quiet fan, the largest model, and man is that thing quiet!

  5. Tim November 5, 2015 at 9:04 am #

    I have 3 Panasonic fans in my bathrooms and agree they are awesome. Stumbled across your great project documentation when doing some reading up on vapor barriers and if a boot was needed over the fan housing.

    I noticed you don’t seem to have any vapor barrier at all behind your drywall ceiling? Seemed strange to me.

    • Bob Jackson November 5, 2015 at 8:00 pm #

      That’s because the home builder didn’t install a vapor barrier between the drywall ceiling and blown-in attic insulation. The home was built in 2000 and the 2000 International Residential Code (IRC) in SECTION R322 MOISTURE VAPOR RETARDERS requires “a vapor retarder shall be installed on the warm–in–winter side of the insulation” [edit update] unless the attic qualifies for R322.1 Exception (2):

      “Exceptions: … 2. Where the framed cavity or space is ventilated to allow moisture to escape.”

      Personally I don’t like blown-in insulation because it’s messy and easily trampled/compressed. Kraft-faced fiberglass rolls or batts are my preference.

      Vapor barriers started as a cold climate solution. I live in Atlanta, GA area which has mild winters (most days reach at least 50F/10C) so moisture & condensation has never been a problem. Also see You Don’t Need a Vapor Barrier (Probably) by Energy Vanguard is a good read.

      Thanks,
      Bob

  6. helga November 30, 2015 at 6:55 am #

    We are considering venting our shower room extractor fan through the soffit as well. It all looks fairly straight forward. If I am correct, however, I see in photo 5 that your bathroom vent is located directly next to the soffit vent which is supposed to vent your attic? When hot steamy air gets vented out the extractor vent, the steamy air will rise and go right in the soffit vents and into your attic, causing mold over time???

    • Bob Jackson November 30, 2015 at 9:19 pm #

      Hi,
      There are websites that advise against exhausting bathroom air through a soffit vent claiming the warm moist air can create icicles and get drawn back up into the attic through other soffit vents. If you find a photo of icicles on a bathroom exhaust vent let me know because I can’t. While we’re on the subject, icicles on soffit vents is usually caused by ice dams due to inadequate attic ventilation.

      Back to the question at hand…

      The International Code Council (IRC) section M1501.1 Outdoor discharge states:

      “The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1506.2. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.”

      The “not be exhausted into an attic, soffit…” means the end of the bathroom exhaust duct can’t simply be poked into the soffit and hope for the best. Recall the IRC is the basis of the Building Codes.

      > When hot steamy air gets vented out the extractor vent, the steamy
      > air will rise and go right in the soffit vents and into your attic,
      > causing mold over time???
      Not if the vent is installed per my instructions with a directional soffit vent:

      Step #5: Slide the Speedi-Products SM-RSV 4 vent soffit vent over the narrow end of the duct pipe connector.
      The soffit exhaust vent must be pointed so it blows away from the house as shown.
      Why? If the vent were to blow moist air against the house it would condense on the wall, promote mold and rot and freeze in the winter.”

      The Panasonic WhisperCeiling fan blows strongly and I can feel the air stream several feet away from the vent. Little if any exhaust air will make it’s way back to the attic soffit vent.

      Thanks,
      Bob

  7. Ben January 10, 2016 at 10:28 am #

    So I have these installed in my house but it seems the hot air is just rising back into the attic via my accuvents. Causing condensation and eventually mold. What do you suggest?

    • Bob Jackson January 11, 2016 at 6:54 pm #

      Which “these” do you have installed on your house? The directional soffit vent with louvers?

      Is the soffit vent oriented so the air blows away from the house? How strong is the airflow from the vent fan? (I can feel the air blowing on my hand from 4 feet away.) Too long, undersized or loose duct connections can degrade the bathroom fan performance such that the air weakly wafts out the exhaust vent allowing it to get pulled into the attic vents instead of blowing well away from the house.

      I’m also wondering how the installer routed the exhaust fan duct through the AccuVent panels. Wouldn’t be surprised if the duct is constricted or knocked loose.

      You can send photos to bob[at]handymanhowto.com replace the [at] with the @ symbol.

      Thanks,
      Bob

  8. ron zahner February 28, 2016 at 3:28 pm #

    I can see from the picture the insulated 4 inch insulated flex duct is flattened out were it passes from the attic into the soffit ! Will that impede air flow ?

    • Bob Jackson February 28, 2016 at 3:54 pm #

      I was concerned about that during installation and there’s no other way to get to the soffit. The fluffy insulation jacket compresses well and the spiral wire inner duct still felt circular or maybe a little oval shaped. The real test was feeling the airflow outside – it was brisk and easily felt on my hand 4 feet from the vent.

  9. Tom July 10, 2016 at 10:46 am #

    I have a soffit ventilated bathroom exhaust fan that doesn’t seem to be venting the moist air out of the bathroom. I am having problems with paint peeling in the bathroom. The distance from the fan to where the vent pipe attaches to the soffit is about 12 feet. The sofit has holes in it, so I do not have a separate round soffit vent. Is it possible that the vent pipe is plugged? Or is the distance it is pushing air to the vent too long? I have the same set up in another bathroom, but it is closer to the soffit (about 6-7 ft) and there isn’t any problem with paint peeling.

    Thanks for your input.

    • Bob Jackson July 10, 2016 at 2:03 pm #

      12 feet of bathroom fan duct isn’t excessive under normal conditions, e.g. the correctly sized duct, gentle turns with no kinks and pinches.

      > The soffit has holes in it, so I do not have a separate round soffit vent.
      If you have ventilated soffit panels (the type with lot’s of small holes) and the end of the fan duct is taped, stapled or screwed to the inside of the soffit then that’s the problem. The small holes in vented soffit are mostly blocking the airflow. You should install a proper soffit vent for the fan.

      Also check the condition and layout of the exhaust duct in the attic for any problems.

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