Dan Morrison | May 02, 2016

Details // HVAC & Mechanicals, Insulation & Air Sealing, Kitchens & Baths

Sealing Holes in Bath Fan Ventilation Duct System (Energy Star 5.1.4)


A tight path to the outdoors keeps the indoors fresh and you can call it a day because you are one step closer to the Energy Star.


The Energy Star checklist has a lot of items. Some of them relate to bathrooms.

Most bathrooms have bath fans, and this one has a light, too, and most bath fans have ductwork that directs moist air outside.

Bathrooms also have ceilings, and that is a place for air leaks to occur: between the edge of the fan and the hole cut for the ceiling.

Even after the hole is sealed, though air can escape the fan housing: if holes are not sealed. Use foil tape, foam, or caulk to seal these holes

For retrofit work, you can build a box over the fan—using drywall or rigid foam— with all the seams sealed. The hole for the ductwork can be plugged with low-expansion foam.

As long as you are sealing holes in the system, seal the connections and seams in metal ductwork using dust mastic—not duct tape. Because duct tape works for everything except ducts.

Outside is where the vent cap comes into play. That begins at the beginning—with house wrap on the wall. Cut a hole in the house wrap and then cut a hole in the wall sheathing. Cut a flap at the top of the hole, to overlap the vent cap flange, and make the vent cap appear.


Apply a bead of caulk to the back side of the damper flange, to bed the damper into the house wrap and insert the damper into the hole. Tape the side flanges with construction tape and fold the flap down. It’s best if the tape extends up to the top of the flap. Tape the sides of the flange to seal the corners.

A tight path to the outdoors keeps the indoors fresh and you can call it a day because you are one step closer to the Energy Star.



Download the thermal bypass checklist:


I enjoyed the video of the bath fan ventilation duct system, although I think something was overlooked - insulation. Sealing and enclosing the fan to the ceiling combined with a properly sealed damper works well to prevent air infiltration into the house, but what about insulating the duct? There could easily be a condensation problem in colder climate areas when a warm exhaust duct passes through un-condition space, such as the attic or crawl. As a home inspector I often see the adverse affects this condition; mold and water stains.  

Good video, David brought up a point concerning insulating the pipe in cold climates, which is essential. 

I would like to add a few methods that we have implemented for cold climates.

  1. Use rigid metal pipe, typically 4", longer lengths are best, locally we can get 10' pieces of warm air ducting (WA).
  2. Run the ducting as low as possible to the outside, usually just over the top of the bottom chord of the truss, run slightly up hill to the fan, in this way any moisture that accumulates in the ducting will run ro the exterior rather than back to the fan. 
  3. Keeping a low exit makes it easier to bury the pipe in the attic insulation, or mound insulation over the pipe, sometimes we use the next size bigger insulated jacket over the duct. Keeping the duct warm will help prevent condensation for freezing in the pipe.
  4. Exit the building on a gable end, if you exit at a eave the moisture laden air can go into the attic through the soffit vent. 
  5. Support the pipe so it straight and no low spots to accumulate moisture. Long lengths help in this regard. 
  6. Wall caps can freeze up, on some models we have drilled a 3/8" relief hole so some air can move and warm the cap up enough so it will open, the theory is if the cap is frozen shut and the fan is deadheading the flap will not open. 


Add new comment

Filtered HTML

  • Web page addresses and e-mail addresses turn into links automatically.
  • Allowed HTML tags: <a> <em> <strong> <cite> <blockquote> <code> <ul> <ol> <li> <dl> <dt> <dd> <img>
  • Lines and paragraphs break automatically.

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
By submitting this form, you accept the Mollom privacy policy.