Bath Fan Insulation Blow Back

Attached is a graphic showing insulation blow back from a bath fan with condensation stain.


Nice graphic but the proportions are a little off. Typical roof truss is 2x4s, blown in insulation covers the bottom chord and the fan itself is usually a flat square box with the exhaust on the side.

Well I used 2x6 rafters this time, for no particular reason. That made the insulation deep enough to cover the fan, so I pulled the insulation back enough to see the fan box.


Thanks, Randy. :slight_smile:

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Is this meant to cover the case of an attached vent pipe not shown,
or bath fan venting to the open attic?

This is a case where the vent pipe is not attached.

Excellent graphics Randy!


Nice graphic that would illustrates the point you’re trying to get across.


How about a transparent vent pipe in the picture, to get the point across that the vent pipe itself reduced the insulation thickness, leading to the harm shown…

Here in the Denver metro area, the condensation is always on the underside of the roof decking because the floor of the attic is warm from heat inside the house. And then the mold begins…


I did not see where it said it was a truss built house. I hardly ever see trusses. 99% of houses here are stick built on site.

But the fan looks like it is installed sideways.

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I did not see where it said that the entire fan was exposed.

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Nice job. Gets the point across. New guys eat this stuff up.

Nice Graphic. That’s exactly what we found last week on a fan buried in insulation.

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In your case, was there a duct and external venting: or did the bath fan just dump into the insulation? Got to love that electrical wire also…

No duct. That fan (metal box in lower corner of picture) was just blowing against a pile of insulation scraps. Years of moist dust developed a layer of mystery growth. Yum.

This was on a vacant flip-house a week or so ago. Beautiful inside, but the attic and crawl are where my students got to witness the good stuff. (L-R: Brian from Gatlinburg, Ryan from Nashville, Terry from Atlanta)

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I see this fairly often.


The more creative people make some attempt to direct the exhaust to an attic vent.


This guy couldn’t be bothered and just decided to humidify the basement utility room.