Construction Design-Windows in Shower Enclosures

As a homeowner, would you choose a design that included a window in a shower?  As a builder, are you comfortable with warrantying a window in this location?  What changes to the shower enclosure over a normal custom shower are you requiring sub-contractors to make?  As a building science nerd and a builder who has built his fair share of custom showers, I have my thoughts.  More on those in a bit.

Windows in showers have always been a concern to me.  Back when I was a general contractor, I never suggested or allowed a customer to choose this option, to me, a window is great in a bathroom, but why challenge both the window and building’s structure with an excessive potential moisture load.

Let’s look at how we install windows on the exterior.  (We are only going to discuss windows with a nailing flange.)  First, the rough opening is made more water resistant by sloping the sill or utilizing a back dam and also using tapes or some other type of water resistive product such as a fluid applied membrane (STPE sealants) to keep the sill from being damaged by any water that finds its way past the next steps.  We then place a bead of sealant around the top and two sides of the exterior rough opening and press the nailing flanges of the window into the sealant (the bottom is left open to allow any water that gets into the assembly a way to drain out).  Next, we install a tape over the side nailing flanges to connect the flange to the water resistive barrier (WRB).    Another piece of tape is installed over the top window nailing flange and sealed to the sheathing.  Z-flashing (usually metal) is installed over the window head with the top of the metal also taped to the exterior wall sheathing.  The WRB flap at the top of the window is folded down over the flange and a skip tape (when using a mechanically fastened WRB) keeps the shingle lapped WRB pressed against the bent Z-flashing head gasket.  Thats a lot of work to prevent most exterior bulk water from penetrating the window installation.  We still need to make the assembly airtight on the interior, tapes (my preferred method), caulk and backer rod or minimal expanding canned spray foam are all products we can choose from for connecting the window to the air control layer.

If the shower is used every day, the inside of the window will see more water than the outside.  Some of the water will be in liquid form and some will be in vapor form.  The window itself is constructed to be made water resistant on the exterior, from the inside, probably not.  What choices can we make during installation that can improve the longevity of both the window and installation?

  1. Choose a window that is made from materials that are less susceptible to moisture.  PVC or fiberglass would be two preferred options.  I would stay away from any windows containing wood.
  2. Choose a glass option for the window that is more condensation resistant.  Triple pane glass would be my first choice.
  3. Stay away from factory installed extension jambs.  Personally, I would build the top and side extension jambs from a product that doesn’t care if it gets wet.  PVC is one option or use a tile or some sort of solid surface material for the extension jamb, such as Corrian or Cambria.  These products can be sealed to the window and pitched correctly to prevent any standing water.
  4. Extend whatever water management system that is being used for the shower walls into the window rough opening.  Preferably all sides and the top will be treated.  (Remember, the window sill may be pitched to the exterior, you may need to add some sort of material to pitch the inside back into the shower.)  My choice of systems is Schluter Kerdi membrane with inside corners installed.
  5. Be generous with sealants, they will be a major part of keeping water out of places it shouldn’t be.
  6. Keep the window as high in the wall as possible.  Reducing the liquid water the window sees while the shower is in use is the best strategy.
  7. Make sure you’re dealing with water in vapor form that accumulates while showering.  Use a bath fan or balanced mechanical ventilation strategies to reduce humidity levels in the bathroom.  Ideally, they would be installed close to or in the shower encloser.
What a window can look like when exposed to higher indoor humidity levels during very cold outdoor temperatures.

My recommendation, keep windows out of shower enclosures, but if you need to install one, use the advice above.  Remember, the number one killer of buildings is water!





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