Volume 8, Issue 1 - January 2007

The "W" of Weatherstripping
The What, When, Where and Why of This Material?

by Don Merrill

“No code is written that you can’t satisfy,” says Ray McGowan, technical service manager for the National Fenestration Rating Council (NFRC).

That includes manufacturing codes for doors, windows and the weatherstrip that seals them.
The topic of weatherstripping has grown over the past few decades. It is a material that has changed as doors and windows continue to evolve and as manufacturers of these products aim to meet the stricter building codes.

Then and Now
The introduction, first of double-glazed windows in the 70s, followed by low-E glass and improvements in U-factors and solar heat gain coefficients (SHGC), helped technology advance rapidly. Today, improvements in insulating quality, lightness, strength, durability and aesthetics promise a nearly 3.7 percent growth in the door and window industry through 2009, according to a March 2005 study by the Freedonia Group on the door and window industry.

But what about weatherstripping, which is the lynchpin to the structural and energy efficiency of doors and windows?
Manufacturers navigate more than 50 different sets of building codes in the United States. They’ve adapted to that variety by building the components for doors and windows to the toughest requirements. Angela Dickson, a spokesperson for the American Architectural Manufacturers Association (AAMA), says the group assists manufacturers in reaching those standards, “... by addressing weatherability for vinyl profiles, coatings and finishes.”

What Has Stayed the Same?
Manufacturers gather the components of doors and windows from a variety of specialized industries to combine those parts into a system. Frame and door material, glass and mechanisms for individual doors and windows are generally viewed as a whole, rather than parts of a whole. But the parts can fail. “If you do have a lot of freeze and thaw, or extreme temperatures, some materials like PVC or flexible PVC have a greater degree of shrinkage in those type of areas,” says John St. John, director of sales and marketing for Amesbury Door Hardware of Sioux Falls, S.D. “But, that’s the only thing that really comes to mind.”

And that’s probably true today. But in the 1980s, manufacturers experimented with new methods and compounds, leading to more comprehensive warranties as ideas were tried and discarded for new ones.
“A lot of times, it does take being out there in the real world and over a certain amount of years to find out that maybe there was a breakdown in a certain weatherstripping or what not,” says Chris Lunt, sales manager for Jeld-Wen of Klamath Falls, Ore.

Lunt says expanded warranties covered those early problems. Likewise, poor installation or improper care after the purchase also contributes to problems with functionality of key components.
According to the Department of Energy (DOE), 21 percent of home heating and cooling is lost through its doors and windows. And the average, well insulated homes contains cracks and gaps between building materials that add up to a hole about 14 square inches. After adequately sealing framing members and caulking smaller openings, proper weatherstripping is critical. A fitting that is too tight can prevent doors or windows from working properly, while a loose fitting doesn’t slow infiltration.
Types of Weatherstrip
Weatherstrip has two basic applications; compression and sliding. A double-hung window with a sliding sash generates friction in the window jambs. There, as in up to 95 percent of all sliding applications, exists a polypropylene pile. It used to be wool pile, but new age polymer filaments handle the heat much better.
The other type of weatherstrip, designed to handle compression, falls into the low-, medium- and high-quality categories. Low-quality includes single diameter PVC bulbs. They’re the least expensive type of weatherstrip and are known to crack under extreme heat and cold, as well as being subject to compression sets. A compression set is what happens when, after pressed against a window for a long period, the weatherstrip stays flat and fails to rebound when the window is finally opened.

Medium-quality weatherstrip is the same bulb in design but consist of an engineered plastic made from santoprene or TPE. This type offers less shrinkage and greater compression set recovery than those made of lower quality plastic.

And high-quality weatherstrip consists of a foamed weather seal. This type of weatherstrip has even less shrinkage and even better compression set recovery. At this level, some foam seals for doors and windows hold their shape, unaffected by weather or compression, for years.
St. John says weatherstrip isn’t really needed if nothing opens or closes. A fixed picture window or a circle on top of a double-hung window with silicone sealant doesn’t need weatherstripping. The other thing is that weatherstrip must be replaceable. “That’s a key word,” he says. “Replaceable.” 

According to the University of Minnesota, there are more than half a dozen specific types of weatherstripping which also vary according to whether they are tape or nail-on, as well as in their type of material, service life and function. These types fall in the small percentage that are not pile or PVC compression types, and include:

Metal Strip: This nail-on weatherstripping seals by spring tension and is nearly invisible with proper installation. It has a 20- to 30-year service life, is very effective, is moderately easy to install and can be used on most wood window channels as well as on the tops and sides of door frames.

Rigid Metal Strip Tube Gasket: Also nail-on, this weatherstripping consists of metal strips with adjustable slots rather than holes for fasteners and lasts between five and ten years.
Vinyl/Plastic/Mylar Tape: Similar to metal weatherstripping, this type is installed flat and then bent to a “V” shape, with the open end of the “V” facing the outside of door which opens inward. It is very effective and lasts between two and 20 years, depending on quality.
Felt Strips: These work best on narrow, even gaps, although it separates where sliding occurs. It can’t be painted and is ineffective when wet. Felt strips last one to two years and are considered by some to be the least effective form of weatherstripping.

Don’t Forget the Doors
“Factory applied weatherstripping on doors, as opposed to windows, is a relatively recent development,” says J. Robert Cusick of the Department of Environmental Design at the University of Missouri. That means millions of exterior doors throughout the country have little or no weatherstripping. But there are several after-market types available to prevent infiltration. They include:

Door Sweeps: This type of weatherstripping consists of a flap of latex rubber attached to a metal strip which is attached to the bottom edge of a door. The rubber flap contacts the floor and blocks infiltration.

Door Thresholds: A metal threshold is affixed to the floor directly beneath the closed door with a vinyl tube running its length. When closed, the tube pushes against the bottom of the door, sealing against outside air.

Dickson says that although sealants and flashing represent a very small part of the cost of a wall system, they play a major role in its performance. AAMA, NFRC, the DOE and a host of other entities are working to remind consumers that although the door and window system is important, the whole is not necessarily more important that its parts.

the author
Don Merrill of Don Merrill & Associates (www.donmerrillassociates.com) is a freelance writer based in Salt Lake City. 


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