5.3 Shielding of the Building Facade
How to shield the building facade (outer skin)? What constitutes the basic material under the building skin or envelope, such as with vinyl siding? In USA, it is typically a plywood lath sheathing in the North and concrete blocks, stuccoed over, for residential in the South. For commercial it may be aluminum composite panels, copper or stainless steel sheets, weather-board, etc. These and other materials are dependent on whether or not the building already exists or has not yet been designed or built. Most specifics of this discussion are beyond the scope of this article and involve important architectural matters.
Another article spoke of BI-PV (third generation materials = building integrated, photovoltaics) and involve the simultaneous decision of combining solar BI-PV additive which also becomes the building facade. Of course, this is best done for a new building. However, BI-PV would have to be shielded.
If the facade of the existing building permits the direct
addition of an aluminum foil, then the household, 1 mil
(= 0.001 inches = 0.0254 mm) or a more ruggedized
version (e.g., 3 mils) provides all the shielding needed.
For example, our shielding effectiveness computer
program #330A provides the following shielding
performance for 1 mil aluminum foil shown in the
graphic to the right..
One mil of aluminum foil provides 96 dB of shielding.
From Chap 3, 80 dB was determined to be the required
amount of building skin shielding to protect against
a EMP incident for most applications. So there exists
sufficient shielding. But, how are sheets of foil to be
bonded to their mounting material siding and how are
they mated together at their edges?
Basically, an adhesive spray is made on the mounting material back and the foil is placed thereon. A squeegee may be used to smooth the mounting. However, the foil overlap should approximate one inch (2.5 cm), and a masking tape used to secure the overlap junction. Do not spray the foil adhesive in the 2.5 cm region as metal must be bonded to metal without any other material in between to ensure a high conductivity.
The foil must end at each window sill or outside door sill or frame as the window/door will receive its own shielding. An electrical bonding agent or gasket (described below) is used to electrically connect the building facade with each window and door periphery.
To demonstrate the need for meticulous workmanship, sup-pose that a hole in the shield foil as small as 0.1 inch (2.54 mm) resulted. What is the new shielding performance of the foil with the hole? Fig 5.5 shows the resulting performance of 58 dB at 64 MHz – the frequency of the second node in the HEMP time-domain pulse discussed earlier in Section 3.4. This hole can be developed in many ways. One way: suppose a workman secured the foil with a screw into a plywood siding (accidentally or intentionally); then he removed the screw for whatever reason! This explains why two layers of foil were used earlier and two layers of wire mesh screen are used on windows and solar panels.
One possible option to the above foil is to use copper or aluminum paint, applied by brush, roller or spray. One source (LessEMF.com) reveals a copper latex paint reported to produce less than 0.1 ohm/square (shielding effectiveness of >72 dB below 1 GHz) for 2 mil deposit. Five mil in two coats will produce the required 80 dB at a paint cost of roughly $2/sq ft ($21/sq.m). Exclusive of windows, doors and other building skin discontinuities, a 20,000 sq. ft building will cost $80k in conductive paint (Ed: possibly reduced to ≈ $25k for quantity of over 200 gallons).
A finishing non-conductive protection coat of latex paint is applied. One major benefit of the paint approach is the relatively easy application by spraying and the avoidance of foil overlaps, and electrical gaskets except at windows, doors, etc. described below.
Duralux Aluminum Marine Boat Paint Green, or equivalent may be a viable protection at a significantly lower cost per gallon ($85). In all cases the surfaces must be clean free of dust, dirt, oxides, etc. Sand blasting may be a necessary first step. Application to new buildings is less expensive than retrofits to existing buildings.
To Be Continued