A Common Problem With Roof Caps For Bath Fans

I start every home inspection by giving myself a quick tour of the inside of the house to get an idea of what I need to be looking for while inspecting the exterior.  While doing my brief initial walk-thru of the interior, I also turn on everything that removes air from the house, such as bath fans, kitchen exhaust fans, and dryers.  I do this to create negative pressure in the house so I can later make sure that all of the fuel burning appliances still draft properly, and to make sure that all of the fans exhaust the way they're supposed to.

As I turn the fans on in the home, I make a mental note of how many fans I've turned on, and I account for each exhaust while inspecting the exterior.  I've caught a ridiculous amount of bathroom exhaust fans there were vented in to attics or finished floor / ceiling spaces by doing this.  This is also a way for me to check to see that the fans are actually exhausting air, not just making a lot of noise.

I check the operation of bath fans by just putting my hand at the exhaust and making sure air is blowing out - after first checking for wasp nests, of course.  About one-third of the fans that are supposed to be exhausting through the roof are exhausting to a cap with a backdraft damper that is stuck closed, which causes a good portion of the warm, moist air to exhaust in to the attic space (pictured above).  Most of the dampers that I see are made by Broan, so I'll be talking about their specific roof cap.

Most roof caps for bath fan exhausts have a light aluminum backdraft damper which rests on a sealant strip, which is basically a foam piece of weatherstripping.  This sealant strip is installed to keep cold air from coming in, and according to a Broan customer service rep, it also "helps to prevent unbearable metal on metal chatter."  I'd have to agree!  Unfortunately, when the sealant strip gets hot from the sun beating on the cap all day, it gets sticky.  Eventually, the damper gets completely stuck to the sealant strip, which prevents the damper from opening.  This is what causes the black staining in the attic that you see pictured above.

When I find this defect, I usually take my awl and pry the damper open, which allows the damper to open and close freely again... until the next hot day.  Broan received so many complaints about these dampers getting stuck that they changed the material that was used for the sealant strip in 2006.  Now the sealant strips are made of polyester, and they don't get stuck any more.  If you own a house with older roof caps, you can order a replacement damper and sealant strip from Broan by calling 800-558-1711, part # s7017696.  Click here for damper replacement instructions - they're listed under the Maintenance section.  I'll be recommending this to a lot of my customers.

Reuben Saltzman, Structure Tech Home Inspections - Email - Minnesota Home Inspector


Special thanks to Judi Weber at Broan for contributing information to help write this.

Frost In Attics

This Minnesota winter has been an especially problematic one for frost in attics, and it seems that most people don’t understand why it happens or how to fix it.  There is a lot of mis-information floating around about why frost gets in to the attic and how to fix it, but I’m going to give you the real story.

This winter we experienced an unusually long cold spell in Minnesota, with temperatures never climbing above freezing for about a month.  This means cold attics.  When moist household air is allowed to escape from the house to the attic, it condenses on the roof boards in the form of frost.  When this happens for a long period of time, a lot of frost can accumulate.  When the frost melts, it soaks the insulation in the attic, stains the ceiling, and can even leak out through windows (I talked to someone last week who was having this problem).

I’ve heard several homeowners tell me they called their roofer out because they thought the stains on their ceiling were caused by a leaking roof, and of course it wasn’t a roof problem.  Nevertheless, the roofers typically suggest adding more ventilation to the attic space.  This will not fix the problem.  To suggest adding more ventilation to a roof to fix frost in the attic is like telling someone with a leaking water pipe to install a dehumidifier.  Ventilation is not the way to fix frost in the attic.


The fix for frost in the attic is sealing attic bypasses – these are passageways from the conditioned household space to the attic.  Bypasses are what allow moisture in the attic to begin with, and this is what needs repair.  While bypasses are certainly the most common cause of frost, there are other ways it can happen, and these are much more obvious.  Disconnected bath fans can be particularly problematic, and disconnected furnace flues can be catastrophic.  The photo below is one of my favorites – when the roof was replaced, the furnace flue came loose in the attic, which quickly turned the attic in to a winter wonderland.


If you’ve had frost in your attic, wet insulation in your attic, or stains on your ceiling, find your attic bypasses and seal them.  If you're not sure how, read my related posts about attic bypasses or give me call.

Reuben Saltzman, Structure Tech Home Inspections - Email - Minneapolis Home Inspections