When EIFS is attached using mechanical fasteners, one of the things that sometimes happens is that the fastener's location can be detected by simply looking at the wall. Usually, this takes the form of a dark spot at each fastener location. Building owners and designers can get pretty riled up about this and I am sometimes asked what can be done about it, both to avoid it in the first place, and to stop in from happening if it occurs. This month's column will explain what this phenomenon-called thermal bridging-is about and what can be done about it.
The reason that thermal bridging occurs is that the mechanical fastener, both the metal screw and the plastic washer, are more thermally conductive than the surrounding EIFS insulation. EIFS is unusual in that the insulation layer is continuous and unbroken. This causes the lamina to be at a very uniform temperature. The fact that the lamina is thin and therefore has little heat storage capacity also means that its temperature goes up and down quickly. This is in contrast to say, a concrete wall, which holds a lot of heat energy due to its heavy mass and takes a lot longer to heat up and cool off.
The bridgeThe fact that there is an occasional thermally conductive "bridge" through the insulation layer at each fastener causes the temperature of the lamina to be different at the fastener than in areas where there are no fasteners. This, in turn, causes the moisture pickup of the lamina to be different at the fasteners than in non-fastener areas. Because one area is damper than the other, the wall appears to be a different color and hence gives the appearance of a "spot." This effect tends to be more pronounced on metal stud walls than on wood studs walls, as wood is less conductive than metal.
Thermal bridging tends to be transient in nature and often shows up early in the day when the wall is damp from moisture that had accumulated overnight. Later in the day, when the wall dries out and the lamina comes to a constant temperature, often the spots disappear. Thermal bridging tends to be more common on buildings where there is large difference between indoor and outdoor temperatures and also in areas where there is, or is not, a normal amount of moisture in the air; it can happens in deserts, as well as the artic.
Sometimes, the increased moisture content of the finish at the fastener location causes dirt to be absorbed into the finish. This sometimes leads to permanent spots. This dirt is nearly impossible to remove; so one way to get rid of it is to paint over them.
Thermal bridging rarely causes any physical damage to the EIFS, although some poorly designed mechanical fasteners have limited holding power and hence can cause other sorts of problems that are structural, rather than thermal, in nature. Thus, thermal bridging effects are mostly an irritating aesthetic issue.
So, suppose you are aware of this phenomenon and want to avoid it on a new building. What can be done? There are many things.
SolutionsThe most obvious one is to use adhesives instead of mechanical fasteners to attach the EIFS insulation to the substrate. Sometimes, this is simply not possible. An example would be an existing painted masonry wall and one does not wish to take the risk of an adhesive not bonding well to the paint. Another example might be a building with corrugated metal siding and one should mechanically attach the EIFS directly to the siding.
A bulletproof approach would be to use mechanical fasteners to install the first, inner layer of foam and then to adhesively attach a second, outer layer of foam. The outer layer effectively shields the fasteners from the lamina but this approach is expensive and time consuming.
A way of dealing with thermal bridging that I have seen in Europe is to first drill a flat-bottomed hole part way through the EIFS insulation. The hole is slightly larger in diameter than the mechanical fastener washer. The fastener is then driven into the pilot hole and seats itself at the bottom of the hole. A round foam cap is then friction-fitted into the hole, thereby covering the washer. The foam cap is then rasped to make it flush with the surface of the foam board. Thermal bridging does not occur with the technique, but it is expensive and time consuming. It also requires using thicker foam to achieve the same wind pull-off resistance, as the foam is weakened by drilling the pilot hole.
One thing to avoid but which sometimes is suggested, is to auger the mechanical fastener washer into the foam and to cover it with a thick layer of basecoat adhesive. Clearly, it is good practice to "spot" fastener washers to a thin, flush dab of basecoat adhesive prior to applying the basecoat but burying the fastener head in a thick layer of adhesive can cause cracking of the basecoat at the fastener.
A thicker lamina is helpful, as it tends to dissipate the temperature difference within a more massive layer of coatings. This is one of the reasons why stucco, which is much thicker and denser than the EIFS lamina, does not often display this phenomenon, although sometimes one can "see" the studs behind stucco for similar thermal reasons. Direct-applied finish systems are notorious for displaying thermal bridging spots. The reason is simple: There's no insulation to isolate the studs from the sheathing and hence the studs show through like an X-ray.
It helps if to use mechanical fasteners that are specifically designed for use with EIFS. Many of the leading brands, such as those by Wind-Lock, have a pocket in the washer that places the screw head well below the surface of the foam. This small amount of isolation reduces the bridging problem considerably. Some "generic" mechanical fasteners that are not designed for EIFS have the screw head flush with the outside of the foam. This is a bad design from a thermal standpoint and can also sometimes cause rust stains, as the protective coating on the screw's head gets worn away slightly by the action screw gun's drill tip.
Choose woodIf you have a choice of sheathing types, such a wood-based vs. gypsum based, wood works better because you can screw into the wood (which is less conductive), rather than into metal studs as is required when using gypsum sheathing (the gypsum is not structural enough to allow fastening into it).
You can isolate the fastener's screw from the interior temperature by using a spacer that is not thermally conductive. This approach is similar to the plastic "thermal break" found in aluminum-framed windows. For example, putting a wood strip spacer over the outside face of a metal stud helps reduce the heat flow through the screw, provided the screw does not go into the metal stud.
There are all-plastic mechanical fasteners available. The plastic is less conductive then the metal screws and hence perform better from a thermal bridging standpoint. However, this type of fastener is normally used only on solid substrates like masonry or concrete, and requires pre-drilling a pilot hole.
If dealing with an existing building that has thermal bridging spots, what can be done about it? Not much.
One technique that helps a little is to seal the surface of the EIFS finish. This reduces the natural tendency of the finish to change its moisture content in response to ambient levels of humidity in the air or from rain. If trying this, be careful, as by doing so may induce condensation within the wall assembly by reducing the permeability of the EIFS lamina. Any EIFS supplier can tell what type of coating, if any, might work and also can advise if there a chance that it results in condensation issues.
In extreme cases, where the building only simply cannot live with the spots, I have heard about people attaching metal lath through the EIFS and then covering the EIFS with stucco. I've even heard of adhesively attaching a whole new EIFS on top of the existing EIFS finish.
The thermal bridging issue tends to surface now and again, and can lead to some really irate building owners who had no idea that it would occur. It's a shame that they become aware of this phenomenon after the building is done. In the end, this is basically a marketing and design problem wherein this aspect of the way mechanically-attached EIFS walls behaves, needs to be discussed to building designers and owners prior to installing the EIFS. Clearly, it might affect their decision on how to attach the EIFS, at least on their next project.