RALEIGH - THAT’S A WRAP!
Raleigh, North Carolina. It’s A Cover-up And Everyone Knows It! Wherever you go the plastic housewrap used to cover homes is flapping in the breeze. If you take note of it at all, most of it seems to say Tyvek. I venture to say most folks don’t know what it is or, for the most part, what it does. The outside, or shell, of a home protects those within its walls. But whatever that shell is (brick, wood, stucco, etc.), it has to function in harmoney with other components of the home to protect the home against moisture penetration. Whatever siding is used, it is porous. There are many laps, joints, and connections in it and back of it that water and air can get through. Water that seeps through can cause wood to rot, mold to grow and lowers insulating values. Less energy efficiency means higher fuel and air conditioning costs. That’s where housewrap comes in.
Housewrap comes in various types and it serve three purposes: (1) - It creates a second weather barrier behind the siding. It keeps rain and other moisture from getting to and penetrating the sheathing (2) - It is a vapor-permeabler membrane that provides the reverse function of allowing moisture that is in insulation and framing lumber to get out (3) - It is an air barrier that helps prevent air infiltration. The bottom line is that it is used to protect the wood framing and reduce energy costs.
On a windy, rainy day water adheres to windward areas. Porous materials, such as masonry veneers and stained wood clapboards, absorb water. Materials such as vinyl and aluminum and newly painted wood do not. None-the-less, the water film adheres to all kinds of siding. Water moves up, down and sideways according to the speed and direction of the wind. It moves from high pressure areas to low pressure areas. The area behind a wind-blown wall surface has a lower pressure than the exterior area. The pressure difference sucks water inward through any crack or hole it can find. If there is no building wrap the wood sheathing will wick the water up an - voila - wood rot. However, just using housewrap is no guarantee of total protection. It must be integrated with the flashing, properly layered, overlaped and taped to get a complete and effective system accomplished. Caulking around windows and joints is supposed to solve the moisture penetration problem but it doesn’t. For example, the shrinking and swelling of wood corner board will eventually fail at bonded connections. Vinyl moves, caulk (in time) fails, and so caulking is not the answer. An expert in this area of keeping water and air out, Paul Fisette, says that of the three types of weather-barrier systems (Sealed-face method, rain-screen approach and redundant-barrier system) the redundant-barrier approach is the best and most cost effective. Quoting directly from an article he did in 2001 he said, “The redundant-barrier works well for the vast majority of homes built today. And this system has the advantage of being familiar to builders. Basically, putting tar paper or approved housewrap on the exterior walls before siding is installed is the first tep to build an effective redundant-barrier system. Proper installation is required to make this system work. You must design a drainage plane that keeps water out! When water penetrates the siding, it must have a clear path to follow downward. Water must remain outside of the protective wrap. Be sure that tops of windows, doors and penetrations are flashed properly. All water must be directed outward. Also, we must choose materials that are capable of providing the protection we expect and need. The barrier should be resistant to liquid water and air infiltrationl while being permeable to water vapor.”
Determining the product to use and installing it properly is the job of the builder and isn’t necessarily an easy one. There are many housewraps available and they don’t all provide the same function. While the American Society for Testing and Materials is in the process of trying to standardize tests for these materials they have not yet finalized them. However, Permeance ratings determine a material’s ability to transfer water vapor. The higher the permeance number is the more permeable the material is. Building codes specify a weather-resistive barrier to equal or exceed grade-D building paper. That paper has a permeance rating of approximately 5.0.
According to independent tests conducted by Mr. Fisette the housewraps that best resisted water infiltration and permitted water vapor to evaporate were Tyvek Home Wrap, R-Wrap by Covalence Coated Products, Typar and the traditional old standby #15 felt paper. While the #15 felt paper scores well and costs less, most builder prefer the plastic housewraps because they are available in a number of sizes which makes installation faster. Other factors come into play as well. For example, the type of siding used. With vinyl siding a smooth-faced housewrap is recommended. With tightly fastened board siding a furrowed rain-screen housewrap is recommended. With stucco a housewrap specifically designed for it is recommended.
Industry experts say that housewrap that is poorly installed causes more problems than it solves. They also say that properly selected and installed insulation is even more important than housewrap and seeing to that comes before the selection and installation of the housewrap. Another very important factor is properly used housewrap seam tape. It helps stop air infiltration and preserves the integrity of the housewrap during construction.
Mr. Fisette says that he has Felt paper on his own home and if he could choose between felt and housewrap he would still go with felt because he the feels that, under certain circumstances, it outperforms housewrap. Mr. Fisette’s recommendations are: * Always use housewrap * Determine if climate requires a vented rain screen or redundant barrier system * For redundant barriers I would choose Tyvek, R-Wrap or 15-pound felt * Tape all seams in barrier * Protect all flashings with overlapping wrap * Avoid use of caulking, concentrate on developing an effective drainage plane. * Protect all penetrations with appropriate detailing. * Prime all surfaces of wood siding (back-priming before applying top coats.
You might well say, “Whether I buy or build I expect the builder to know these things and I am stuck with what he does. Since I am not an expert in these areas and don’t have the knowledge or time to determine whether he is doing the right things or not this is information is of little value to me.” For most of you this may well be true. However, the more information you have about matters such as this the more useful your conversations with the builder and/or his subs can be. Yes, in this day and age we become more and more specialized and seem to have less and less knowledge about the things of everyday life we should have some knowledge of. Thus, for those of you interested in widening your horizons, this is for you.
That’s “a wrap” for 2007. I would like to wish each and every one of you all good things for 2008.
John Fish (919) 696-3474 Email: Marvmax@mindspring.com Website: www.JohnFish.com
