Lots of people want the look of adobe homes when they build with bales. The biggest risk to that is in the inset window details. If this detail is not properly constructed, there’s a good chance, not just a small chance, but a good chance, that you’ll end up with water damage beneath your windows. I’ve posted a sketch of a quality flashing and construction detail for this application that will help you ensure water tight seals around your windows.
The detail is for a house that has plaster finished tight to the windows. If you wanted to add a sill of some other material, that is absolutely an option. The key is to use supported bituthene under the windows and over the bales. This way, it’s not going to get poked by the straw over time and end up with holes in it. The outer framing and inner framing help keep the entire unit in place and solid, again, protecting the longevity of the flashing. Be sure to counter flash (adhesive flashing installed on the jambs and sill, before the window is installed. Use the absolute minimum number of nails or screws to fasten the bottom of the window. Some windows don’t allow you to nail the bottom flange at all, so read the manufacturer’s recommendations.
Once the window is installed, lay a large sheet of adhesive flashing (bituthene) over the window flange and out over the edge of the plywood base. Turn the edge of it down over the outer frame so that any water will drain directly to the plaster, not into the bales. Install the side adhesive flashing and then the window head flashing in that order. I prefer adhesive flashing all the way around. Finally, install a J-metal or custom metal flashing under (and around) the window to create a clean stop for the plaster. Use the bare minimum fasteners for this flashing. If possible, only nail it in two places, outside the window frame by extending the legs of the J-metal wider than the window but while keeping the J channel in line with the rest of the flashing around the window. In other words, the j channel should miter in the corners with the vertical pieces yet the nail plate of the bottom piece can extend out beyond the window frame. Sorry, that’s hard to describe and much easier to show in person at a workshop!
Although the image shows the J-metal out in front of the window flange and frame, it will actually be installed under the window so that water cannot drip in behind it. I had to “explode out” the view so it would make more sense. You can place a bead of clear caulk in the joint between the window and the flashing for extra insurance. Hope that all makes sense. As I said, it’s easier to show you in person than on a blog.