Condensation Prevention in Metal –Walled Livestock Buildings
Metal-walled (or metal sheathed walls) are commonly used for livestock construction. The metal sheathing is attached to the exterior of the building frame and it provides the first barrier between outside and inside conditions.
The metal is fastened with either vertical or horizontal seams, and it can perform as an effective rainscreen, i.e. a barrier to prevent wind-driven rain from penetrating to the interior. Metal wall sheathing is cost-effective and durable, it is available in a variety of colors, and it is readily cut to desired lengths. Metal wall sheathing is the predominant sheathing found on modern agricultural and livestock buildings. It can work well for structures with limited interior moisture production and works well with insulation and vapor barriers used on the warm (interior) side of the wall in northern US climates.
Buildings used for livestock and poultry can have significant interior moisture production from animals. This moisture comes from manure and directly from the animals. If the metal interior temperature is below the dew-point temperature of the interior air, condensation occurs on the wall. Insulating the interior side of the metal sheathing can prevent surface condensation but will not prevent condensation from occurring inside the insulation or between the insulation and the interior metal surface unless vapor barriers are properly utilized or closed-cell insulations (spray-on or rigid board) are included in the wall section design. The vapor barrier on the interior wall must be protected from penetrations, so an interior sheathing is typically added. This composite wall (metal exterior sheathing, insulation, vapor barrier, and interior sheathing) adds substantially to the cost compared with the metal sheathing alone.
It must be stressed that ventilation is the primary means of reducing condensation potential. These guidelines assume that appropriate ventilation is available and being used.
A few alternatives to conventional wall section construction are available, and their cost-effectiveness varies greatly. Different wall section construction methods are summarized below:
- Metal sheathing placed over rigid insulated foam sheets. The metal and the rigid insulation are both fastened to the building frame exterior. The metal sheathing forms the rainscreen, and it is critical to provide an air cavity behind it (between it and the rigid insulation) for any penetrating water to drain down and out of the wall, and to not further penetrate into the wall cavity. This sort of metal sheathing uses horizontal seams that provide continuous drainage lines for any penetrating water (technical term is a "pressure equalized exterior rainscreen"). The sheathing must be installed according to manufacturer specifications. The separate sheets of rigid insulation can be of any thickness needed to provide thermal comfort, energy savings, and condensation control on interior surfaces; it is impermeable so that water vapor from the interior cannot migrate into it. This is the approach utilized in modern commercial construction, and it has many advantages. It is also more expensive than traditional methods.
- Single-component (panelized) wall sections. Typically called Structural Insulated Panels (SIP), these are factory-assembled wall sections with seams designed to provide the same water drainage as in the previous example. They incorporate the exterior sheathing and the insulation into a single panel. Condensation cannot occur inside the wall if installed properly. The panels can be specified with a variety of sheathings, on both inside and outside surfaces, and are manufactured to order in 4' or 5' sheets in lengths up to 20'. Labor to install this system is generally less than the previous example since the wall panels arrive on site ready to fasten directly to the building frame using fasteners and methods recommended by the manufacturer.
- Metal exterior sheathing with a specially applied vapor barrier on the inner surface. The vapor barrier is a so-called "smart" material, which can absorb water vapor at high relative humidity, and release it back to the interior in lower relative humidity conditions. Using this material on the interior side of the exterior sheathing for applications with high interior moisture loads will still experience condensation in cold weather, and this condensation will create a water problem on the interior base of the wall. This is especially problematic if some sort of interior sheathing on the interior of the frame is used to protect the wall from livestock because this can trap any moisture that does condense and drain down. This is exactly the problem with the use of metal-walled exteriors without insulation, or with insufficient insulation or poor vapor barrier installation. A path for this draining condensation should be provided. We also encounter pooling water where poor wall to above-grade concrete stem wall sealing was made.
- Liquid membranes applied to the interior of metal sheathing. Similar to the previous example, except this liquid membrane is nearly impermeable, and it can be sprayed, brushed or rolled onto the surface. This technique will not prevent condensation from forming, but it can serve as a barrier to both water vapor and liquid water and slow the corrosion process. If used, attention to draining the condensate from the wall is critical. This system may be useful in some applications with relatively low interior moisture loads where insulation is not desired or needed. The metal sheathing acts as the rainscreen.