Please note: This older article by our former faculty member remains available on our site for archival purposes. Some information contained in it may be outdated.
Results from the most recent Building Materials Laboratory study show some housewraps may do little to block rain intrusion.
Liquid water is without question the most destructive force acting on light frame structures. As builders, we are mesmerized by vapor diffusion and vapor barriers. But it is still good old fashion water leakage that rots walls, sills and other building elements. There is no shortage of pathways leading water inward. Any discontinuity in a wall surface will do nicely: seams, windows, butt joints, knots, and siding overlaps are thirsty conduits. The trick is to build energy-tight walls that drain water leakage outward. Installation of high-tech housewraps is the most common attempt to have it all.
I find myself inspecting a growing number of failures related to water intrusion and remain convinced the leading cause of water damage is poor design. Proper flashing and the integration of flashing with an able housewrap are perhaps the most critical details enabling walls to function as weather barriers (see http://www.umass.edu/bmatwt/walls.html). However, I have found that even properly flashed walls sometimes leak. Field investigations and conversations with other building scientists nudged me to question the viability of housewraps as weather barriers. It is widely known that water leaks through virtually every type of siding. But are all housewraps equally able to shunt water once it gets past siding?
One of the primary ways that rain is driven into walls is by air pressure differentials. In a word – wind. During the fall of 1998 I brought samples of 15 pound felt, Tyvek, Rwrap, Typar, Amowrap, Pinkwrap, and Barricade into the University’s Building Materials Laboratory to see how these popular housewraps stand up to pressure-driven water. The housewraps were subjected to stress imposed by columns of water and the results were reported in the November 1998 issue of JLC (see http://bct.eco.umass.edu/publications/by-title/housewraps-felt-paper-and-weather-penetration-barriers/). Non perforated wraps (Tyvek and Rwrap) and 15-pound felt paper won that round by a knockout. Perforated wraps leaked like sieves. I also did some exploratory work to determine the effect of soaps (from powerwashing) and water soluble extractives (like those found in cedar and redwood) on water permeability of housewraps and determined that soaps and to a lesser degree extractives can cause wraps to leak. As promised in the November 1998 issue of JLC, I have extended the study to look at the ability of housewraps to block the capillary flow of water.
Siding provides discontinuous coverage. True, overlaps and seams are slender ports, but narrow breaks are large enough to pass significant amounts of water. Some of the most common wall soakings result from splashback, inadequate overhangs, blocked gutters, roof-wall intersections; and exposure to prevailing wind. Capillary suction is a strong force. Even under conditions of light or no wind pressure, water can be sucked through seams, cracks, joints and upward behind the overlaps of horizontal siding. Add to that fact, most wood siding is not back primed, so its backside sucks water up like a sponge. The face of wood siding is usually painted. When painted wood absorbs water, it takes a long time to dry. Paint holds water in. As a result, wet siding is held tightly against housewrapped sheathing in most homes. The question we tried to answer is: Do housewraps suck?
I am leery of extending lab results into predictable field performance. However, lab testing can help clarify our understanding of field-use potentials. The typical wall system considered for our experimental design is built this way (starting from the outside): wood siding (unprimed and unpainted), housewrap, and sheathing. We imitated this lay-up in our experiment.
First we cut samples of 1/2-inch bevel cedar siding into 2-inch squares and soaked them in water. Saturating the siding was an attempt to mimic what happens to untreated wood siding or wood siding that has not been backprimed when it is exposed to a period of rain. Next we placed a square of saturated siding into each of 5 petrie dishes that were half-filled with water. The top half of our siding samples remained well above the water line. Water in the dishes served to charge our samples, maintaining saturation throughout the test period. Next, we placed a 4-inch square of housewrap over each piece of saturated siding so that its edges extended well beyond the perimeter of the dish. On top each square of housewrap we positioned a 2-inch diameter disk of blotter paper. The blotter served as a visual indicator to tell us if any water leaked through the housewrap. Finally, a 2-inch square piece of 1/2-inch plywood was placed on top of the blotter paper. The entire sandwich was held together firmly with an elastic band. The sandwiches were unbanded and checked every 2-hours for 2 days (9:00 am – 5:00 pm — no checking at night). This test was performed for Tyvek, Rwrap, 15-pound felt, Amowrap, Pinkwrap and Barricade.
Tyvek, Rwrap and 15-pound felt showed no signs of liquid water leakage throughout the 2-day test period. However, the blotter paper on the Tyvek and Rwrap samples felt slightly damp to the touch by the end of the period, suggesting that there had been some transfer of water vapor by diffusion. Blotter paper disks on the 15-pound felt paper samples remained bone dry to the touch. Typar samples all leaked within the first 2-hours of the test. Two of the Typar samples leaked immediately – as soon as the blotter paper was placed onto the wrap. Blotter paper indicators for Barricade, Pink Wrap, and AmoWrap (all perforated wraps) became saturated before the layers of the test setups could be secured with an elastic band.
Before we packed up the testing gear, we decided to look at one more detail. What happens when you nail the sandwich together as you do on a real wall? We repeated the experiment only this time we drove a nail through the siding before setting it into the petrie dish. We forced the square of housewrap and disk of blotter paper over the nail. Then we drilled a tight-fitting hole (slightly larger than the nail) through the plywood so that we could slide the plywood on and off the nail to inspect the blotter paper for leakage. No surprise here: all wraps leaked around the nail within the first 2-hours of the test.
End Points – What it all means
As I said earlier, I am reluctant to predict field behavior from lab bench results. However, it is abundantly clear to me that perforated wraps, in their current state of development, can leak when placed in contact with water and/or wet wood. So why use them? You can choose a product that won’t.
Unprotected wood absorbs water. Most wood siding is not backprimed. Let’s stop this practice. Backprime and end coat wood siding before it is installed. A water repellent treatment is best. This one step alone will not only minimize rot, but will also prolong the service life of the exterior paint. Over the life of a house this represents a significant savings! If the budget permits, build a vented rain screen. Building an airspace directly behind the siding lifts the siding away from the drainage plane. This encourages free drainage, air-pressure equalization (reducing wind-driven leakage), and allows the back of the siding to dry more easily. It also reduces the likelihood that water will leak through nail holes that perforate the housewrap.
Use a housewrap as a secondary weather barrier under all siding. For my money, I’ll choose a housewrap that shows the most promise. Test results narrow my field of potential candidates to Tyvek, Rwrap and 15-pound felt. Felt paper leaked after 30 minutes in the column test, but still makes my list as a strong candidate.
I think that felt paper blocks the flow of liquid water under pressure (column) for a respectable length of time. When it leaks under pressure, it seams to leak very slowly. It blocks capillary flow for days. Felt is inexpensive. What is also appealing is that felt is forgiving. If water gets on the wrong side of a felt-wrapped wall, the felt can absorb the water and over time allow drying to the outside of the structure. Plastic housewraps don’t move water this way. Plastic wraps are non absorbent. They rely on vapor diffusion to move water that gets on the wrong side of the wrap. Diffusion is a slow and weak force. The perm rating of felt is much lower (less permeable) than the plastic housewraps when it is dry. But this does not mean that it traps water vapor in the wall cavity. As the relative humidity rises, the perm rating of felt rises too- eventually above that of Tyvek. Felt is dynamic. Having said that I also like Tyvek and Rwrap. They cost more than felt, but these products stand out to me as being very strong products. They roll out as large continuous sheets. They have shown an ability to reduce air exchange and energy consumption in some studies. And they have demonstrated to me that they can block the flow of liquid water, while allowing diffusion of vapor.
Thumbnail Photos, click on photo to select enlarged view:
Figures 1 – 4 depict the study described in the article found at http://www.umass.edu/bmatwt/weather_barriers.html
Figure 1. The column test apparatus.
Figure 2. Typar leaking under the force of a 3-1/2 hydro head.
Figure 3. Owens Corning PinkWrap leaking under the force of a 3-1/2 hydro head.
Figure 4. AmoWrap leaking under the force of a 3-1/2 hydro head.
Figure 5. Placing unpainted cedar siding specimen in water.
Figure 6. Covering saturated cedar square with wrap and then blotter paper.
Figure 7. Covering the blotter paper with a dry piece of plywood.
Figure 8. Note leakage of water moving upward, marking blotter paper by capillary suction.
Figures 9 – 12 show tests that explored the effect that nail insertion had on capillarity. All wraps leaked upward around nail.
Last updated: September 7, 2016