Sunday, January 1, 2012

Bathroom Building Science

Weird title on this one, huh?  Well, this idea developed organically after taking a shower one morning.

In September, we re-painted our bathroom and changed the color.  Once the weather started to get a little bit colder, I started to notice that, after every shower, we had visible streaks of water droplets on two sections of the wall, opposite the shower.  I am sure that this was happening previously, I had just never seen it until we changed the color of the paint on the wall.

I didn't really pay much attention to it.  I'm groggy in the morning, saw the streaks on the wall, and never really thought much about it.  Then one day....EUREKA!  Building science in action.....

The water streak marks are happening at only two places: on the interior wall directly behind the water closet, and on the exterior wall at the ceiling/wall connection.  Why are these the only two places that we are getting this condition?  Once I finally thought about it for a moment, it was pretty clear: the two areas where the streaks appear are condensing surfaces.  After a shower, the ambient air in the bathroom, with extremely high relative humidity, comes in contact with these surfaces.  Water vapor touches the cold wall and condenses into liquid water, which then runs down the wall and leaves the streaks.  Pretty simple.  But why was it happening only at these two locations?

The exterior ceiling/wall connection is fairly obvious.  This area is a pretty typical thermal weakness.  A infrared scan showed that the attic insulation is pulled back directly above this location.  Both the exterior top plate and the drywall are completely exposed to the attic over an area of about 1 sq ft; exposure to the unconditioned attic space drives surface temperatures to below the dewpoint of the moisture laden post-shower air and you get condensation.

The interior wall is a little trickier.  What we have here is a plumbing penetration for the vent stack rising into the attic within the partition wall stud cavity.  This attic penetration is not air sealed.  Unconditioned attic air is falling down the wall, driving temperatures down and creating the condensing surface.  Cool!

Contributing to this whole scenario is the fact that I don't have an exhaust fan.  Code is satisfied by the presence of an operable double hung window, but who's going to open that when its cold out?

The solutions to these problems would be to air seal and insulate the attic properly, and to install a properly vented exhaust fan.  And, probably, to re-paint.....

Dark spot behind the toilet is cold air that has entered the wall cavity through an un-sealed plumbing penetration in the attic, creating a condensing surface.

Poorly installed attic insulation creates a cold spot and the condensing surface.
The cause and solution to our interior wall condensation problem.

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