Counter Flashing Windows in a Straw Bale House
flashing window straw bale wall
Windows are one of the most risk prone and failure prone areas of a house, conventional or straw bale. In fact, any penetration in the exterior wall is at risk of water infiltration and thus failure. Most contractors are aware of the risks associated with this area of construction yet they do not fully understand how to minimize that risk.
I have seen countless articles in trade magazines like Fine Home Building and the Journal of Light Construction showing professionals how to properly flash a window. Inevitably, contractors wrap the house wrap on the wrong side of the flashing or improperly flash the window in general.
This is speaking of conventional construction, of course, as neither of those two publications deals much with straw bale house construction yet. Notice I say "yet."
Here are some common mistakes:
1. Flashing installed from the top down, allowing water to get behind the layers of flashing.
2. No counter flashing installed. This is the flashing placed before the window is installed.
3. Wrapping the layers of waterproofing in the wrong order which allows water in.
4. Nail thought the flashing close to the window, thus penetrating the seal.
5. Nailing through the flashing as described above and then pulling the nail out to reposition it.
6. (SB specific) Insetting the windows from the exterior face of the wall.
The most common mistake is not counter-flashing windows at all. The belief is that today's flashing is so good that we no longer need to counter-flash our openings. I disagree. For the 5 minutes per window it takes and the $5 in materials per window, I believe it is worthy insurance.
Without it, you are relying 100% on the single layer of flashing to hold up without any wiggle room for leaks or failure. Know this: water has an uncanny knack for finding its way into houses. Do whatever you can to protect your investment well.
A quick discussion on window placement. Many folks love the adobe look created by setting windows roughly 6" back into the bale wall. This exterior sill gives the home facade some texture and depth; however, it increases the risk of failure.
Although I like the look of this architectural detail, I do not build it that way. I place my windows flush to the exterior of the wall. Here's my reason. If the flashing fails in the inset window, any water will leak directly onto the top of the bales below the opening.
This means the water will soak into the sponge-like bale for months, if not years, before you know you have a leak. The section of wall beneath and adjacent to that leaky window will be ruined by the time you know there is a problem.
If the flashing fails in a flush-mounted window, the leak will drain directly into the plaster. You will start to notice staining below the window in the plaster or even a constantly wet section of plaster. This will be an early clue that something is wrong and will allow you to address the problem before it is too late.
The following steps to applying proper counter flashing are better learned in person. I will attempt to detail it for you here:
1. Use adhesive bich-a-theen (I have no idea how to spell that properly so I went phonetically!) This material is black and fully adhesive. It works best when it is warm and pliable.
2. Cut squares off the roll for each corner. You will need 2 pieces per corner.
3. Start at the bottom and place one square in the corner. Cut one side of it diagonally from the corner to the joint of the sill/trimmer stud. Stick it down.
4. Use another square and cut from the opposite corner of the square to the same joint. Stick it down. Make sure no wood shows in the corner. By placing two squares cut in opposite directions, you should have completely seal the corner.
5. Do the other bottom corner the same way.6. Cut a piece to fit the bottom sill. It will wrap into the window opening a bit and overhang the trimmer stud on each side by about 6".
I know that is a really hard way to learn counter-flashing. Sorry. I teach this in my workshops, and you can certainly visit a contractor on site and ask for a quick lesson or just watch what they do. Make sure your contractor/teacher knows what they are doing. Take this blog entry as notes to ensure they are tip-top.
-Andrew Morrison