ASHRAE Journal - February 2013 - 57

Intent of Sealant is to Limit this
Lateral Flow of Water Between
Sheathing and Building Wrap

Flashing Tape
Sealant
“Bedding” Joint

Building Wrap
“Wrapped” Into
Opening

Figure 3: Control of Water at Window Jamb. A heavy bead of
sealant is installed behind the flange of a window and it acts
like a “gasket” smushing the building wrap against the sheathing,
limiting the water that can run sideways into the opening. The
“red arrow” shows the “smush” direction created by screwing the
window flange into the sheathing and frame opening.

Photo 1: Gravity Lap Part I. The flashing strips are layered shingle
fashion. Horizontal bottom sill first. Vertical side jambs next. Horizontal head flashing last. Note the sealant used to provide water
control layer continuity between the window unit and the flashing
strips. Photo 2: Gravity Lap Part II. The rolls of building paper
are installed horizontally shingle fashion. The vertical edge of
the building paper is sealed to the edge of the window unit with
more sealant. Any water that enters at that joint is expected to
be controlled by running down the vertical “side flashing” or jamb
flashing and out at the bottom. The width of the jamb flashing is
problematic. That is why the flashing tape and housewrap detail
at the window jamb flange shown in Figure 2 (and in the sidebar
Installing Windows in Housewrap Openings) is such an improvement.

have a long history of successful performance. But understand
that in order for them to work they rely on tapes and sealants.
And, by the way, they leak, but not enough to get your knickers in a knot. All in all they provide a very robust system that
is hard to beat cost wise and performance wise. Having said
that, there are limitations to the technology. The systems are
not good for high rises because of wind loading. The wraps
flutter and tear and can’t take the wind load. And, with complex geometries at “punched openings,”
trades need to learn “origami” to wrap
openings. However, wherever there are
limitations, there are opportunities.
More on this later.
So what is wrong with just relying
on a “gravity lap” (aka, layering things
shingle fashion)? Gravity works, it is
reliable, never needs maintenance, and
the politicians haven’t figured out how
to tax it…yet. Well, without some kind
of sealant at vertical joints at “punched
openings,” gravity laps alone just do not
work. Check out what we have to do at
a window opening to try to make the Photo 3: Formable Flashing. Life got better when flashing systems became formable
gravity lap work (Photos 1 and 2). How and flexible. This photo dates back a decade to when I got my first sample of flexwide do you have to make the vertical ible flashing to play with in my backyard. I thought it was the greatest invention since
flashing at the jambs (“sides”) to handle the transistor. Ok, so I am exaggerating, but only a little. Look at how beautifully a
the flow where the building paper butts seamless under window flashing system is created. Photo 4: No Origami Here. Today’s
up to the window frame? This has never liquid applied systems easily handle complex geometries seamlessly.
worked where it rains. So it is used where
it doesn’t rain, like Las Vegas and Phoenix. With the advent of
In commercial and institutional buildings we have come
building wraps and housewraps that can be taped these issues to rely on fully adhered membranes and liquid applied memdisappeared. It got even better when flashing systems became branes (Photos 5 and 6). By adhering directly to the sheathing
formable and flexible (Photo 3). And better still when liquid the wind load issues become moot. And with no gap between
applied systems were developed (Photo 4).
the water control layer and sheathing we don’t have to worry
February 2013

ASHRAE Journal

ASHRAE Journal - February 2013

Table of Contents for the Digital Edition of ASHRAE Journal - February 2013