Having rebuilt a number of rotten bathroom walls, I've come
to believe that windows should not be placed inside the shower
enclosure. But there are times when there's no other
When I expanded and remodeled the 5-foot-by-12-foot bathroom
shown here, for example, there was only one logical place to
put the tub: along the end wall. Since that was the sole
exterior wall, it was the only possible place for a
If I can't avoid placing a window above a tub, I use glass
block, so that the window and all the trimmings can be made
with rotproof materials. In this particular project, I used
concrete and granite for the sill, ceramic tile for the window
surround, and glass block laid in mortar for the window
Glass block offers privacy without vinyl curtains or blinds,
which tend to harbor mold; also, the construction of a
glass-block window is seamless, which means that keeping water
out of the wall is much easier than it would be with an
Framing a Waterproof Rough Opening
Stone, concrete, tile, glass, and mortar are great finishes to
use in bathrooms because they're unaffected by moisture. The
underlying wooden framing, however, is a different story.
Untreated wood rots and attracts termites and carpenter ants
when it gets wet. And any wood — treated or not —
absorbs water and swells, then shrinks as it dries, causing all
sorts of mischief. In addition, wet wood promotes the growth of
mold (see Figure 1).
Figure 1. In many
bathrooms, the only available location for a window is over the
tub. But when the window isn't designed to shed water on both
the exterior and interior, moisture can penetrate the walls and
cause rot, mold, and other damage.
So my goal in this project was twofold: to keep water from
getting into the framing in the first place, and to provide a
way for the framing to dry out if moisture still manages to
For starters, I used treated framing lumber, inexpensive
insurance against termites, carpenter ants, fungi, and mold.
Built with 2x6s, the wall had plenty of depth for insulation
and for supporting the window's wide granite sill and
4-inch-thick blocks (Figure 2).
Figure 2. Glass block
can't be cut, so the rough opening must be sized to fit the
dimensions of the block. With one vertical and one horizontal
mortar joint, a standard 7 3/4-by-7 3/4-inch block measures a
convenient 8 by 8 inches. A 54 3/4-inch-wide by 37-inch-high
rough opening accommodates a glass-block panel measuring six
blocks (plus five mortar joints) wide by four blocks (plus four
mortar joints) high, with two 2x6 trimmers on each side and
1/2-inch expansion gaps at the sides and top.
Also, I sheathed the inner face of the framing with 3/4-inch
Structurwood Edge Gold sheathing (Weyerhaeuser, 800/525-5440,
www.structurwood.com). Designed for floors,
these T&G OSB panels hold up extremely well in the presence
of water; they would provide solid backing for the tile
backerboard and for future grab bars.
Although the sheathing did add 3/4 inch to the total thickness
of the wall, it also allowed the final, finished depth of the
sill inside the bathroom to be 4 inches, a perfect shelf width
for soap dishes, shampoo bottles, and other showering
In addition to placing polyethylene between the OSB and
backerboard, I covered the bottom of the window opening with a
peel-and-stick elastomeric membrane, carefully detailing the
corners of the opening where the sides and bottom meet.
I wrapped the membrane up the first 5 inches of the
rough-opening sides and folded it down a few inches over both
sides of the wall. The membrane separates the concrete sill
from the wood framing and serves as a waterproofing membrane if
the concrete gets damp.
On the outside, I sheathed the wall in fiberboard, a
vapor-permeable sheathing with a proven track record here in
North Carolina's hot and humid climate. The fiberboard should
allow any water that makes it into the wall cavity to dry to
Thermal Performance of Glass
Glass blocks are made by melting
together two pressed glass halves. This process
creates a partial vacuum inside, giving the blocks
an R-value of 1.96 and a U-value of 0.51 —
better than most insulated glass.
Although some thermal bridging occurs around the
edges of the blocks and in the mortar joints, a
glass-block panel as a whole performs well because
it's virtually airtight. Setting the blocks in
beads of silicone caulk instead of mortar can
reduce the bridging and maximize thermal
Cast Concrete Sill Is Key
To manage water flowing down the vertical glass-block panel
onto the horizontal sill, I designed a 41/2-inch-thick
cast-in-place concrete sill (Figure 3).
Figure 3. The
cast-in-place concrete sill provides a level, stable foundation
for the glass-block window (top). It was cast with a drip
channel that prevents water from seeping along the underside
into the building envelope (bottom).
On the exterior, the sill extends 3 inches beyond the existing
siding, providing a generous overhang that will project over
the new rain-screen detail and fiber-cement siding planned for
From the point where the outside face of the glass block meets
the sill, the sill slopes a steep 20 degrees to direct
rainwater flowing down the glass panel to the outer edge of the
On the underside, a mild 5-degree slope with a drip groove
keeps water flowing off the sill rather than moving back toward
the house through surface tension, which can cause water to
cling to the underside of horizontal (or nearly horizontal)
The concrete sill is 6 inches wider than the glass-block
panel, allowing it to extend to the outside of the 4-inch-wide
exterior window trim. Because the wood casing terminates over
the sill instead of running past it on either side, water
running down the casing will flow over the top of the sill and
drop onto the ground rather than toward the building
Forming the sill. To make the sill, I assembled L-shaped
brackets out of some scrap 2-by material and attached them to
the exterior of the wall (Figure 4). I then carefully built the
formwork, making sure that the sill's dimensions would be
accurate and that concrete did not leak out during the
Figure 4. After
building the form as shown, the author poured the sill using a
smooth, aggregate-free concrete mix. Two lengths of #3 rebar
placed during the pour reinforce the sill.
I formed the drip channel with a strip of wood ripped to about
1/2 inch square. Beveling the two sides slightly so that the
wood strip was trapezoidal in shape and applying paraffin to
the surface before tacking it in the form made it easier to
remove the strip without damaging the new concrete.
On top of the form, toward the inside, I used a piece of 2x6
ripped to 4 1/2 inches wide to form a 1 1/2-inch-deep rabbet in
the concrete sill. This rabbet accommodates the finished
Placing and finishing the concrete. Before pouring the
concrete, I placed a piece of fiberboard at each end of the
form to allow vapor diffusion at the ends of the cast-in-place
concrete near the outside wall.
I made the concrete for the sill out of a 3-to-1 mixture of
white concrete sand and white portland cement. After allowing
the concrete to cure for a couple of days, I carefully removed
the forms and finished the surface of the sill with a rubstone,
using a slurry made of equal parts white portland cement and
white sand mixed with water to the consistency of grits.
I brushed the slurry on, then rubbed the surface with the
stone, a process that grinds off high spots and fills in
surface voids at the same time.
After finishing the surface of the concrete sill, I installed
the 6-inch by 52 3/4-inch by 1 1/4-inch-thick granite slab
(Figure 5), which had been cut and polished by a local
Figure 5. Installed on
a bed of Type S masonry cement, the granite slab sill angles
down slightly — toward the tub and away from the
glass-block panel — for drainage.
The vertical joint between the granite sill and the concrete
is centered within the 4-inch thickness of the glass-block
panel. With the panel straddling this joint, and the interior
and exterior sills sloped away, water running down both faces
of the panel will flow away from the joint.
My final step was to install two treated trimmer studs on each
side of the window frame, thus decreasing the width of the
framed opening by 3 inches on each side. I left half-inch gaps
under these trimmers, which are wrapped with peel-and-stick
membrane, and filled each gap with sealant.
Later, after installing the backerboard and tile, I gave each
gap another half inch of sealant.
Glass Block Finishes The
Glass block is unforgiving, so it's important to start on as
straight and level a surface as you can possibly create.
On this job, I checked and rechecked my form to make sure that
the concrete sill was straight and level. And during
installation I aligned and spaced the blocks in the first
course very carefully, because my work there would affect all
of the courses above.
When building a glass-block window, the general idea is to
build a panel that is supported by — but not built hard
against — the opening. (For more information on building
with glass block, visit Pittsburgh Corning's informative Web
site, www.pittsburghcorning.com; or see the JLC
articles "Glass Block in the Bathroom," 11/92, and "Glass Block
Tips & Tricks," 11/00.)
I built this glass-block panel with a 1/2-inch clearance
between the top and sides of the panel and the opening.
To hold the panel in position, I used Pittsburgh Corning's
panel anchors, screwing them into the sides of the opening and
setting them in the mortar joints (Figure 6).
Figure 6. Accessories
like panel anchors (top of top picture), polyethylene expansion
strips (bottom of top picture) and spacers (bottom) make
setting block easier.
During installation, polyethylene expansion strips placed
between the panel and the rough opening helped hold the blocks
in place. Later, these expansion strips would serve as a backer
for the caulk I'd use to seal the interior and exterior
perimeters of the window.
Because glass doesn't absorb water from mortar, glass block
tends to slide all over the place if you make the mortar too
wet or work with a heavy hand. I made things easier by using
Pittsburgh Corning's specially formulated glass-block mortar
with a stiff mix, which helped reduce the tendency of the glass
units to "float" on the surface of the wet mortar.
I also used the company's VeriTru plastic spacers, which are
designed to hold the blocks in place as the mortar dries.
Along with the panel anchors, the finishing materials —
backerboard and tile on the inside, wood window casings on the
outside — help hold the finished panel in place. Instead
of installing these materials tight against the panel, I
maintained a small joint, which I later filled with
Besides accommodating differential movement between the
glass-block panel and the framing, this caulked sealant joint
prevents air and water from moving through the window.John Carroll is a mason and builder in
Durham, N.C., and the author of Measuring, Marking &