Finish Line: Mechanical Fasteners: Part 1
This is the first of a two-part article on mechanical fasteners. This method of attaching the EIFS insulation has an interesting history. It is also different from the normal way of installing EIFS from a contracting standpoint, namely, screw guns and washers instead of plastering tools and adhesives. By understanding the background of this attachment method and how it works, you can create a better wall and get access to additional projects, such as buildings where adhesive fastening just doesn’t work.
The modern Type PB EIFS-the most common type in North America that uses EPS insulation and thin resinous coatings-originated in Europe after World War II. In Europe, EIFS is applied mostly over masonry, using an adhesive. The combination of a cement-based adhesive and solid substrate makes for a strong, durable attachment system. However, occasionally in Europe-and much more often in North America-there were solid and sheathing substrates that adhesives simply would not stick to. This includes common substrates like painted masonry and waterproofed concrete. In such cases, a mechanical method of attachment is needed. Fast forward to America in the 1960s.
EIFS, when first used in North America, gravitated toward lightweight stud cavity wall construction. Such substrates were much more common here than in Europe. The question immediately arose regarding how to attach the EIFS insulation to these various types of sheathing substrates. This included some very common types, such as gypsum sheathing, plywood, OSB, and various cement boards. As it turned out, exterior grade gypsum sheathing was compatible with cementitious EIFS attachment adhesives, and untold thousand of projects were done this way-including many large commercial buildings. The cementitious adhesives, however, did not work well on wood-based sheathings. This resulted in the development of noncementitious adhesives for this type of construction-mainly for wood frame residential and light commercial buildings.
However, the problem still remained about what to do when going over unbondable surfaces like painted masonry or waterproofed concrete. So, the search was on for mechanical fasteners that would work well on lightweight North American types of walls and “problem” solid substrate surfaces, such as paint. In addition, there was also the thorny problem of the moisture sensitivity of certain common sheathing substrates. Some building designers felt that certain types of sheathings were insufficiently durable for use with adhesives alone and thus opted for mechanical anchors in lieu of adhesives.
What evolved from this search for proper mechanical fasteners was a number of unique mechanical fastener designs that incorporated features intended to work specifically with EIFS.
HOW THEY WORK
Adhesives work by developing a chemical bonding action between the substrate, and the adhesive and insulation. Mechanical fasteners work via a clamping action, similar to a nut and bolt. The types of EIFS attachment adhesives used with EIFS are thick film adhesives, and the thickness of the adhesive layer (usually 1/16 to 1/8 inch) allows for some adjustability between the inside surface of the insulation and the surface of the substrate. This is an important feature, as it allows precision alignment of the outside face of the rigid insulation boards from one board to the next. This is a practical and necessary feature when thin coatings, such as basecoats and finishes, are used as the outside surface.
Mechanical fasteners do not have this “adjustableness” capability and must be cinched down tight to get a good grip on the wall. If the insulation is not tight against the substrate, the foam will be loose and flex, and cracks would form at the insulation board joints. Also, the fastener washer needs to be flush with the outside face of the insulation, lest a discernible bump be created in the thin EIFS lamina. Conversely, augering the fastener into the face of the insulation results in a dished-out area that gets filled with basecoat adhesive. This causes a hard spot in the coatings that can cause cracking at the fastener. A special fastener was needed.
There are all sorts of washers that are used with screws to attach various soft materials like insulation. For instance, faced blanket insulation can be attached upside down to the underside of a ceiling to insulate a roof. It turned out that what was needed for EIFS was a special washer with certain features that worked with EIFS, such as:
Low thermal conductivity, so the fastener washer does not show through the EIFS lamina.
A recessed socket for the screw head, with protective “flaps” to keep basecoat adhesive out of the screw socket, but still allowing the screw gun tip to get at the screw head.
A perforated surface, so the basecoat adhesive can key into and get a good grip on the fastener.
A large diameter, so the washer can get good bearing onto the soft EPS insulation.
The ability to use various types and lengths of screws to accommodate various insulation thicknesses and substrate materials.
There are many types of existing washers that conceivably might be used as an EIFS fastener but few that work. This is because not many have the required low thermal conductivity, are the wrong shape or are too small. A thermally conductive fastener washer will show up as a dark spot on the wall. Also, unless the washer is made of a non-corroding material, like plastic, the washer would rust eventually. Even plated steel, for instance, would eventually rust.
The screws must be non-corroding also. This means they must be heavily plated, since stainless steel self-drilling screws are really expensive and nearly impossible to obtain. If inexpensive, lightly plated screws, like drywall screws, are used, the twisting action of the screw gun tip will scrape off the coating, allowing corrosion to begin. This can result in visible rust spots on the EIFS lamina.
The screws need to be recessed and thus not in contact with the underside of the EIFS lamina, lest they rust through the coatings. They need to be down within the insulation layer, not at the surface. Many of the plastic washers that were originally thought to be suitable were found to be unusable due to rusting problems due to the fastener head being on the surface.
The screws also have to have the right thread design and tip for the material being screwed into; finer threads and a self-drilling tip for metal studs, and coarser threads for wood. Thus, the ability to use various types of screws is critical, and it’s clear that the cost effective way is to have the screws be able to be added-to, or removed-from, the washer. Nails don’t work as you have little control over how well seated they are and possibly can mash the EIFS insulation while pounding them in.
Of course the length of the screw needs to be right, so that it goes all the way through the insulation and into the sheathing or stud. When using foam shapes, the fastener screw length issue can be a problem, especially if you’re trying to get into a narrow stud face or have very thick foam. In the case of thick foam shapes, it’s easier to use a thin base layer of insulation over the whole wall and then glue the foam shape to the base layer.
Most of the current EIFS mechanical fastener washers (sometimes also called plates) look like the one shown in the photo (with the screw in place). The other, less common type is a one-piece all plastic fastener, which is used for masonry and concrete.
ATTACHMENT STRENGTH COMPARED TO ADHESIVES
In the development of mechanical fasteners, it was apparent that the issue of attachment strength needed to be addressed. It was obvious that using a small number of “point fasteners” (washers and screws) through a weak insulation material would probably not result in high attachment strength compared to adhesives.
Adhesives spread out the wind forces over a much larger area, which is desirable when bonding to a weak material like foam plastic insulation. The attachment strength of mechanical fasteners was confirmed by doing full-scale tests of mechanically attached EIFS on stud-and-sheathing wall mock-ups.
A substantial safety factor was applied to the tested failure load to come up with a working strength value. Various fastening patterns were developed-with the end result that EIFS fastened with mechanical fasteners-only has pull-off strength best suited for low-rise buildings and homes. To get maximum pull off strength, attachment using adhesives is needed, such as on tall buildings or in hurricane areas. (More on this subject of attachment strength and fasteners patterns in Part 2.)
WHY NOT ADHESIVES PLUS FASTENERS?
The question often arises about whether there are advantages to using adhesives and mechanical fasteners together at the same time. The answer is yes and no. One would think that adding mechanical fasteners to an adhesively attached system would markedly increase the pull off strength of the EIFS. It doesn’t. What really happens is the adhesive does all the work (since it is a more rigid form of attachment and doesn’t “give”), and when the adhesive finally lets go, the fasteners are immediately overwhelmed. Thus, there’s no big benefit strength-wise, under normal conditions.
The story is different if the substrate deteriorates or the adhesive lets go (for example, on a painted wall that the adhesive appeared to stick to properly). Often mechanical fasteners go directly into the stud framing, and thus the sheathing is redundant-it acts as a spacer and a flat backing, in terms of the EIFS’ need for it from a structural standpoint. In such a case, if the sheathing fails, at least there is something (the fasteners) holding the EIFS onto the wall. This situation can occur, for instance, if there are massive water leaks and the substrate is ruined.
I vividly saw this once on an OSB substrate job on a low-rise wood frame motel. The fasteners went into the studs all right but when I opened up the wall from the outside to check out the cavity, I was staring at the fiberglass insulation: the OSB had “dissolved” and was a pile of mush at the bottom of the cavity. This is the case where, if you feel that back-up protection against delamination of the entire EIFS is needed, a belt-and-suspenders approach-fasteners and adhesives-can keep the wall intact until the problem source is fixed and the wall rebuilt.
FASTENER SHOW-THROUGH AND THERMAL BRIDGING AND ENERGY EFFICIENCY
One of the reasons EIFS has such outstanding energy efficiency is that the insulation layer is continuous and unbroken. At least this is true with adhesively attached EIFS. This is not quite true when mechanical fasteners are used. The screws and washer create a small discontinuity through the insulation. This results in the lamina being slightly hotter or cooler than the surrounding EIFS lamina. This, in turn, results in a dark spot on the lamina (i.e., the fastener location is visible). This phenomenon is sometimes referred to as fastener show-through, and is the result of what is called thermal bridging.
Visible fasteners can be aesthetically objectionable, as the wall looks like it has measles. Luckily, the fastener show-through causes no physical damage, and goes away. By “goes away” I mean this phenomenon is transient and as the lamina comes to thermal equilibrium during the middle of the day, the spots disappear.
There is some loss on insulation efficiency when mechanical fasteners are used. A Swedish study calculated that the use of mechanical fasteners reduces the energy efficiently of the EIFS portion of a wall by less than 5 percent, which is almost nothing compared to other claddings with massive energy losses due to thermal bridging at their perimeter or through the studs, such as can occur with extruded aluminum and glass facades.
NOT AT THE FINISH LINE, YET
As you can see, there is a lot to consider when using mechanical fasteners-they are more than simply a washer and screw. Next month, Part 2 of this article will cover other aspects of mechanical fasteners, such as: the perimeter of the EIFS (backwrapping); EIFS with drainage; rasping the insulation; spotting of fastener heads; fastener patterns; cost and productivity issues; using mechanical fasteners through the basecoat; re-attaching loose EIFS by using mechanical fasteners; dealing with over-driven fasteners; and more. Stay tuned.
Thanks to Wind-Lock and Demand Products for permission to use photos of their products. W&C