Myths & Facts: Wall and Window Flashing

Contributed by ​Roy F. Schauffele

I opened up my business in Texas on July 4, 1987 and have been in continuous operation since then. When I came to Texas about 65 to 70% of the leak issues I dealt with were roof related, now the vast majority of building envelope leaks that I handle are related to the walls and windows. There are a variety of reasons for these problems:
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1. A very thin workforce and not always well-trained installation crew; most flashing installs are unintentionally done incorrectly.
2. Folks not realizing the absolute long-term cost of improper product selection (chemical incompatibility) or installation.
3. The failure of all involved to not recognize the changing chemical mixtures in today’s wall design.

We all need to realize that today’s walls are being designed for a 40+ year life span and we must adjust our thinking accordingly. Product warranties could be a good indicator of how long the product is anticipated to last. We must also realize that the total installed cost of a flashing system should be taken into serious consideration, and not just the cost of materials. The Brick Institute Association (BIA) has excellent Tech Notes 7 & 7A (Click for link) on this topic at www.gobrick.com/Technical-Notes. This is one of the reference resources I use.

Please remember that for through wall flashing to work properly, the flashing material/assembly must extend beyond the face of the mortar.

Today’s cavity walls are a chemical soup of ingredients:

1. Continuous Insulations: extruded and expanded polystyrene, foil faced polyisocyanurates, spray polyurethane foams and mineral wool.  Some of these materials require a wider cavity wall space to achieve the design effective R-value.
2. Air Barriers: acrylic, silicone, neoprene, polymer sheet goods, polymer modified asphalts and several others.
3. Windows: metal, PVC-coated metal, Kynar-coated (Wikipedia definition: Polyvinylidene fluoride, or polyvinylidene difluoride, (PVDF) is a highly non-reactive thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride), and others
4. Exterior Sheathing materials: plywood, OSB, foil faced board, gypsum boards
5. The chart immediately below is indicative of some of the chemical compatibility problems you could run into. The data is pulled from a variety of manufacturers web sites.
6. The question you need to ask of your suppliers is, “Is your recommended flashing compatible with XYZ cavity wall component?” and get a letter to that effect from each manufacturer that touches another? This is for the protection of both the architect and installer.

Many a time in our industry, the myth becomes reality. Here are a couple of the most egregious myths and the manufacturer’s position:

1. Self-adhered: It is so sticky that you don’t need a primer.  Synthetic, man-made polymeric sheets, including “peel & stick” membranes require the use of a primer, and you need to obtain a letter from the manufacturer that the primer is chemically compatible with your substrate.  
2. Self-adhered (peel & stick): it just sticks so well, no need to roll.
3. All peel and stick membranes must be hard-rolled onto the substrate to ensure full & complete adhesion, not just hand pressed.
4. Thermoplastic Flashings: These synthetic membranes are deformable by heat and require a physical support across the cavity wall air space or else they will sag down, forming a trough to collect water, leading to further deformation and sagging, and ultimately a failure. This deformability could cause some problems with end dam integrity in hotter southern climates. See BIA Tech Note 7A.
5.  Lap Joints: You don’t need to be a “rocket surgeon” to realize that the more lap joints per 100 lineal feet, the greater the potential for joint failure and water leaks. Many synthetics require/recommend a lap joint every 6’ for 16/17 lap joints per 100’; metallic flashing-like copper or stainless steel require 2 or 3 per 100’ 
6. UV Exposure: Many of today’s flashing materials are not meant for more than 60 days exposure to sunlight, after which time they begin to rapidly lose their desirable physical properties. Be very careful with this exposure issue.
7. True Through Wall Flashing: many of the product technologies that have a UV sensitivity concern advise the “holding back” from the face of the mortar to prevent UV exposure, but to ensure proper water drainage, a drip edge must be installed to allow complete water evacuation through the face of the mortar. See BIA Tech Note 7A. 
8. Metallic Flashing: They are too expensive. Sorry: Generally, copper or stainless steel are not subjected to the limitations and assemblies listed above. They are of a different nature and chemistry compared to the synthetic or man-made materials. Various manufacturers data show that the installed cost of copper or stainless steel is less expensive when compared to the installed according to the manufacturers/industry specifications, synthetic materials.

As a wrap up to this article, through wall and window flashings cost about 0.15% of a project’s cost and comprise way too many of today’s leak complaints. All decisions should be based on physical properties (like puncture resistance, number of lap joints, chart below), methods of proper installation, chemical compatibility, with a good look at the product warranties which could be indicative of anticipated wall-life expectancy.