EMF shielding by building materials
Attenuation of microwave
band electromagnetic fields
by common building materials
The U.S. National
Institute of Standards and Technology (NIST) and the University of the German
Federal Armed Forces have separately done extensive testing of how well various
common building materials can shield (dampen) electromagnetic fields. A wide range of materials and
thicknesses were tested, such as bricks, concrete, lumber, drywall, plywood,
glass and rebar.
Keywords: microwave,
radio frequency, RF, shielding, building, materials, concrete, bricks, window,
shielding fabric, shielding paint
The need for shielding
The ambient levels of microwave radiation have risen exponentially since the mid-1990s and have reached levels where independent scientists are concerned about the public health. There are also people who are hypersensitive to this type of radiation and have acute symptoms at levels common in cities.
Building houses of shielding materials, or shielding existing houses, may be a solution to this public health issue. The problem is to get sufficient shielding, since a reduction (attenuation) of the radiation will often need to be 20 dB (hundredfold) or more.
Microwave frequencies
The tests were conducted
for frequencies that cover emissions from cell phone towers, 3G, 4G, LTE,
Wi-Fi, DECT, cordless phones, digital television, GPS, wireless smart meters,
baby monitors and many other devices.
The frequency bands used
by broadcast radio (AM, FM, shortwave, etc.) and wireless power systems were
not covered. Since they are lower
frequencies, it is reasonable to expect less shielding of these types of signals
than demonstrated in the tests.
The new 5G systems will
use higher frequencies than used in these tests. The tested materials will probably shield the 5G signals
better than 3G/4G signals, though the higher frequencies are better at
penetrating small gaps (holes, slits) in the walls of a house, as well as the
holes in shielding fabrics and meshes.
Measuring shielding effectiveness
The effectiveness of a shielding material is how much the radiation is reduced while passing through the material compared to when there is unrestricted passage. This is called attenuation. The result is given in decibels.
factor |
reduction |
decibel |
10 |
90% |
10 |
100 |
99% |
20 |
1000 |
99.9% |
30 |
10,000 |
99.99% |
40 |
100,000 |
99.999% |
50 |
For shielding of humans
against EMF, these are the magnitudes that are relevant.
The tests in these
reports were all done by measuring the power density (W/m). When comparing with other sources, be
aware that if the measurements are using the field strength (V/m) a reduction
of 90% (factor 10) is 20 decibels and a 99% reduction (factor 100) is 40
decibels.
The NIST measurements
The National Institute of
Standards and Technology in the United States published their results in a 1997
report titled Electromagnetic Signal Attenuation in Construction Materials (report
NISTIR 6055). This report is
freely available on the web.
The reason NIST did these
extensive tests was to prepare for future generations of wireless control
systems at construction sites, as well as for tools to measure the thickness of
walls. It was not related to
protecting the public health against EMF.
In general, these tests
show that standard building materials provide poor shielding. Materials such as gypsum drywall, glass
and lumber are almost fully transparent to microwave radiation. Even bricks are not a very effective
shielding material.
The shielding values do
vary with the frequency, with the materials mostly performing better at higher
frequencies. However, that is not
always the case.
Concrete
NIST tested eight different concrete mixes, each at three different thicknesses. The concrete was solid and without any reinforcement. The key findings were:
á
the attenuation increases with higher
frequencies
á the attenuation varied little between the eight mixes
á the attenuation depends directly on the thickness of the concrete
If a concrete wall that shields (attenuates) by 15 dB is doubled in thickness, the wall will then attenuate by 2 x 15 dB = 30 dB.
The measurements also found that thick concrete could provide good shielding, with typically 25 dB attenuation for eight inches (203 mm) at 1 GHz. However, these measurements turned out to be misleading. The samples NIST used were cured for 28 days, but later measurements by Pauli & Moldan in Germany showed that concrete loses some of its ability to absorb microwaves over time. In their experiments they found that 16 cm (6½ inch) concrete attenuates 5 dB less between curing for one month and nine months.
Reinforced concrete
Concrete reinforced with a mesh of steel rebar is not really better than plain concrete. NIST tested two standard mesh sizes (70 mm and 140 mm between rebars) and compared with a concrete wall without rebar. 19 mm (3/4Ó) thick rebar was used on a 203 mm (8Ó) thick concrete wall.
Concrete |
|
|
|
|
|
without |
23 |
27 |
35 |
55 |
73 |
with rebar |
23 |
27 |
31 |
53 |
68 |
with rebar |
26 |
30 |
37 |
56 |
71 |
The above table shows the attenuation in decibel (dB) for concrete that has cured for 28 days. The attenuation will be less when fully cured.
Concrete blocks
Concrete blocks with hollow cavities inside were tested for walls one, two and three blocks thick. The study did not test the shielding value of the blocks if filled with concrete. It would probably be slightly less than a solid concrete wall of the same thickness.
Masonry
block |
|
|
|
|
|
203 mm (8Ó) |
8 |
12 |
11 |
15 |
18 |
406 mm (16Ó) |
13 |
17 |
18 |
27 |
30 |
609 mm (24Ó) |
26 |
28 |
30 |
39 |
39 |
Lumber
Regular lumber in thickness up to six inches was tested. The wood was either spruce, pine or fir, which are the typical sorts used for construction in North America. Heavier types of wood, such as oak, may have a better shielding effect.
It was found that fresh (moist) lumber provides more shielding than lumber that has aged. This must be because of the water content. The table below is for dry lumber.
Dry lumber |
500 MHz |
1 GHz |
2 GHz |
5 GHz |
8 GHz |
38 mm (1.5Ó) |
2 |
3 |
3.3 |
4 |
4 |
76 mm (3Ó) |
1.5 |
3 |
4.7 |
8 |
9 |
152 mm (6Ó) |
4.5 |
6 |
8.5 |
20 |
25 |
Bricks
Brick walls consisting of one, two or three bricks were tested. Even three courses of bricks do not provide much useful shielding, except at the highest frequencies.
Brick |
500 MHz |
1 GHz |
2 GHz |
5 GHz |
8 GHz |
89 mm (1 brick) |
0 |
3.5 |
5.5 |
15 |
16 |
178 mm (2 bricks) |
3.5 |
5.5 |
7.5 |
32 |
14 |
267 mm (3 bricks) |
4 |
7 |
10.5 |
32 |
27 |
Glass panels
Glass window panels with regular clear glass were tested and found to have very little shielding effect. NIST did not test windows with a metallic coating (low-E), which do provide very significant shielding (see later).
Glass panels |
500 MHz |
1 GHz |
2 GHz |
5 GHz |
8 GHz |
6 mm (1/4Ó) |
0 |
0.8 |
1.4 |
1 |
1.5 |
13 mm (1/2Ó) |
1.2 |
2.2 |
3.4 |
0 |
1.6 |
Drywall
Gypsum drywall consists of 85-95% gypsum. The rest is mainly paper and various additives. Drywall has no shielding effect.
Drywall |
500 MHz |
1 GHz |
2 GHz |
5 GHz |
8 GHz |
6 mm (1/4Ó) |
0.1 |
0.3 |
0.6 |
0 |
0.4 |
13 mm (1/2Ó) |
0.1 |
0.3 |
0.6 |
0 |
0.4 |
The German measurements
Much more extensive tests of building materials and special shielding materials were done at the University of the German Federal Armed Forces by Peter Pauli and Dietrich Moldan. Their first report was published in 2000 in both English and German. The English title is Reduction and Shielding of RF and Microwaves: Construction Materials, Screens, Wainscots and Tissues. This report can be purchased from Dr. Dietrich Moldan, Am Henkelsee 13, D-97346, Iphofen, Germany.
The same authors have since continued their tests and published expanded versions of the original report. The most recent is from 2015. However, these later versions are available in German only. The data is displayed in large colorful charts that can be interpreted with the help of a German dictionary.
The 2015 edition is titled Reduzierung hochfrequenter Strahlung im Bauwesen: Baustoffe und Abschirmmaterialien, and is also available from Dr. Moldan.
The English version displays data for about 80 different materials while the 2015 German edition lists 150 materials. The materials were tested at frequencies ranging from 100 MHz to 10 GHz using standard test methods (including NATO MILSTD 285 and IEEE 299). The results are largely consistent with the NIST results; however, since these reports are copyrighted we canÕt display the results.
As mentioned earlier, a major discovery is that concrete provides less shielding once it has cured for nine months, compared to after just one month. (The English version displays data for one-month-old concrete only.) This raises the question of how much humidity influences porous materials. Will they perform as well in dry Phoenix, Arizona as they do in humid Munich, Germany?
Other important results are how well regular sheet metal, aluminum foil and metal-coated (low-E) window glass can shield microwaves — as opposed to bricks, concrete, roof tiles and wood. Much can be done with inexpensive shielding materials.
The reports also cover several materials specifically developed for shielding, including special plasters, gypsum boards, copper-coated wallpaper, shielding fabrics and rigid meshes (netting). Most of these materials were developed by European firms and may not be available elsewhere. The 2015 edition also has a section about shielding paints. The shielding values of these materials vary greatly, with some providing less than 10 dB (tenfold) attenuation while others exceed 50 dB (hundred thousand fold). In general, the materials using metals (reflective shielding) perform much better than the non-metallic materials (absorptive shielding). The exception is some materials using a type of clay (ÒlehmÓ). The clay tests are not in the English version.
Also not in the English version is documentation of how the width of a slit affects the effectiveness of a shield.
The many graphs can be understood well by someone without a good knowledge of German, though a dictionary will be helpful.
The 2015 edition also includes extensive information on how to shield houses. Even though this part has many illustrations, it requires a proficiency in the German language to read.
Other EMF shielding documents
See www.eiwellspring.org/shielding.html
2012 (updated 2017)