Detection of Moisture Accumulationin Wall Assemblies UsingUltra-Wideband Radio Signals

William M. Healy, Ph.D. Eric van Doorn, Ph.D.Member ASHRAE

ABSTRACT

This paper discusses the use of ultra-wideband radio signals near 5 GHz to detect moisture problems within building assem-blies. Ultra-wideband signals are made up of a large spectrum of electromagnetic waves, and hardware is available to temporallyresolve the signals to a time scale on the order of picoseconds. By emitting these signals toward a wall assembly and analyzingthe reflected signal, information can be obtained on the wall. Most notably, because water reflects these waves more significantlythan dry building materials, the magnitude of the reflections can be correlated to moisture levels. This principle has been incor-porated into a synthetic aperture imaging technique to create images of the moisture state of building assemblies. The picturescan help identify locations of unwanted moisture intrusion in a nondestructive manner. Advantages of using ultra-widebandcompared to conventional radio frequency techniques include the improved spatial resolution and the ability to penetrate a widerange of materials.

INTRODUCTION

As moisture issues in buildings continue to draw signifi-cant attention, tools for diagnosing excess moisture within thebuilding envelope are needed by practitioners and researchers.Ideally, such a tool would be nondestructive, though anecdotalevidence seems to indicate that one of the most common meth-ods used to find moisture problems involves tearing apartwalls. Better alternatives are available, though, and should beused in place of more primitive techniques. Instruments suchas electrical resistance pin probes, capacitance meters, andrelative humidity sensors are frequently used to estimate themoisture content of building materials. Each of these instru-ments has positives and negatives, as described by severalauthors (Ten Wolde and Courville 1985; Derome et al. 2001;Healy 2003). One of the drawbacks of probes, such as a pinprobe, is the fact that contact is needed with the moist speci-men to diagnose excess moisture. Tools that do not requirecontact to locate moisture problems in walls would greatly aidin finding problems in a nondestructive fashion. Furthermore,techniques that can scan larger areas of the building envelope

would assist in isolating areas of excessive moisture comparedto the more laborious task of examining individual areas withlocalized meters and sensors.

A number of different methods proposed could comple-ment the instruments previously mentioned in isolating mois-ture problems in buildings. Infrared cameras have been used tolocate moisture because of the lag in temperature changes ofmaterials laden with water compared with dry materials (Tobi-asson et al. 1977; Korhonen and Tobiasson 1978). Suchtemperature differences can be observed with infraredcameras and can provide a simple-to-use tool for investigators.One drawback of such a method is the fact that the infraredemission is a surface phenomenon that can be affected by suchsurface properties as emissivity and roughness. Other draw-backs include the fact that temperature gradients from featuresother than moisture can create false identification of wet areasand that information regarding the condition inside the wallmay not be attainable using this surface-dependent technique.Other methods that may provide noncontact determination ofthe moisture level within walls include gamma-ray attenuation

©2004 ASHRAE.

William M. Healy is a mechanical engineer in the Building and Fire Research Laboratory, National Institute of Standards and Technology,Gaithersburg, Md. Eric van Doorn is the director of sensor technology at Intelligent Automation Inc., Rockville, Md.

and neutron scattering (Knab et al. 1981). Such devices canaccurately determine the presence of water within a wall, yetthe transportation of the radioactive sources used in suchdevices causes concern.

An emerging technique for nonintrusively detecting themoisture condition within a wall is the use of ultra-wideband(UWB) radio waves. Healy and van Doorn (2004) recentlydiscussed preliminary work that shows the potential of thistechnique. In this work, ultra-wideband radar was first used todetermine the moisture content of gypsum board, pine, andoriented strand board. The technique was then used on a simu-lated wall to independently determine the moisture content ofthe individual layers. Results were then shown in which athree-dimensional map of the wall located a moist patchwithin the wall. The technique has the ability of locating notonly the location on the face of the wall but also the depth ofthe water within the wall.

This paper describes techniques that have been developedto create images of the moisture state of a wall. New hardwarewill be described that complies with recent regulations passedby the Federal Communications Commission (FCC) and thateases the acquisition of moisture images of a wall. A briefdiscussion will be made concerning issues that are currentlybeing investigated that will complicate acquisition of imagesin actual buildings. The next section will describe UWB radiotechnology in more detail and will discuss some of the advan-tages and disadvantages of the method. Experiments that wereperformed on a sample wall section will then be described todemonstrate the performance of the equipment and software.

ULTRA-WIDEBAND RADIO OVERVIEW

Ultra-wideband radio signals are electromagnetic trans-missions that comprise a broad range of frequencies. By defi-nition, a signal is considered to be ultra-wideband if its relativebandwidth, η, is greater than 0.25. The relative bandwidth isdefined as

(1)

where

fh = highest frequency contained in signal, and

fl = lowest frequency contained in signal.

One way to achieve a signal with such a large frequencyspectrum is to emit pulses that are inherently composed ofwaves of various frequencies. For example, by taking theFourier transform of an impulse, one can observe that such anideal pulse consists of an infinite frequency bandwidth. On theother hand, the Fourier transform of a sinusoidal signal showsthat the signal is composed of a single frequency. Such a signalis characteristic of conventional radio or radar. A good over-view of ultra-wideband technology is given by Taylor (1995).

The hardware used in the present study is designed to emitpulses having the idealized shape of a Gaussian monocycle, asshown in Figure 1. The Fourier transform of this pulse (Figure1) displays the broad range of frequencies that make up thissignal. The center frequency for this particular design is4.7 GHz and the bandwidth is 3.2 GHz. This large bandwidthallows one to term the signal an ultra-wideband signal. Onemight imagine that such a signal would interfere with manydifferent devices. One of the characteristics of UWB systems,however, is that the signal is emitted at a very low power. Thehardware used in the current study emits pulses at an effectiveisotropically radiated power of 0.33 mW (–11 dBm). Thispower level is considered to be below the electromagneticnoise floor in most situations. The question is then logicallyraised as to how an antenna picks up the signal if it is below thenoise floor. The key to this problem is the fact that the hard-ware is capable of generating and detecting pulses at anextremely rapid rate (up to 9.6 million pulses per second). Byemitting many identical pulses and averaging the received

Figure 1 A Gaussian monocycle pulse and its spectrum.

ηfh

fl

–

fh

fl

+------------- ,=

2 Buildings IX

pulses, the presumably random noise is averaged to zero andthe underlying signal is recovered. This series of pulses istermed a pulse train. The spacing between pulses is approxi-mately 100 ns, while each pulse width is on the order of 1 ns.

Uses of Ultra-Wideband Signals

One application of UWB signals lies in wireless commu-nications (Taylor 1995). Once a transmitter and receiver set upcontact, binary digits can be transmitted by altering the timingof the pulse. For example, a pulse that is sent 10 ns before anexpected time can be considered as a 0, while a pulse sent10 ns after a prescribed time can be interpreted as a 1. Anexample of the bandwidth capability can be obtained whenconsidering the hardware used in the present study. Since thehardware is capable of transmitting up to 9.6 million pulses persecond, it is conceivable that 9.6 megabits can be transmittedper second. Maxing out in such a way would not allow for anyerror correction but, in principle, such large communicationbandwidth could be attained. One advantage for using UWBsignals for such communications is the fact that the signals arevery low power. This characteristic will minimize interferencewith other components in buildings that operate on radiofrequencies. The fact that the signal is composed of a largefrequency range increases the robustness of the transmissionsince it is likely that some frequency component will be ableto travel from the transmitter to the receiver without beingattenuated. Entry of UWB technology into the communica-tions market was aided in 2002 by a decision from the FCCthat permitted its use in a wide array of applications (FCC2002). Hopes are high that such FCC authorization will speedthe technology to market and decrease the cost of the equip-ment needed to generate and receive UWB signals. The hard-ware used in this study complies with these regulations.

A second large application area for UWB is in sensing.Most sensing applications rely on the fact that electromagneticpulses are reflected differently from objects depending upontheir physical makeup. These reflections can be analyzed todetermine the location of objects and possibly the compositionof the product. The excellent temporal resolution of the equip-ment can be used to make precise estimations of the locationsof objects that reflect the pulses. The added feature that UWBbrings to remote sensing applications compared to singlefrequency signals is the likelihood that some component of thesignal will be capable of penetrating most materials so thatinformation can be obtained regarding objects behind anobstruction. With narrowband signals, the chances that onecan find a signal that passes through obstructions yet is sensi-tive to the constituent of interest are much less. This propertyof ultra-wideband has led to sensing applications related toobject location for military purposes and identification ofpeople within buildings or rubble.

Sensing Technique

The basis for the sensitivity of radio waves to moisturelies in the large difference between the dielectric constant ofwater (εr = 81) and that of porous building materials such aswood (εr ≈ 5). The dielectric constant affects the propagationof radio waves in several ways. First, the speed of propagationof electromagnetic waves, c, through a solid is dependent uponthe dielectric constant, as described by Equation 2.

(2)

whereµo

= the magnetic permeability of free space

µr

= the relative magnetic permeability of the medium to

that of free space

εo

= the permittivity of free space

εr

= the dielectric constant of the medium

The second way in which moisture affects the propaga-tion of radio waves through a solid is through reflection at theinterface between the surface of the solid and the adjacentmaterial. The reflection at the interface between materials 1and 2 is governed by the reflection coefficient, Γ12.

(3)

where c1 and c2 are the speed of light in materials 1 and 2,respectively, as described by Equation 2. This numberprovides the ratio of the amplitude of the reflected waves tothat of the incoming waves. Since the speeds of light, c1 andc2, depend upon the dielectric constants, the reflection coeffi-cient also depends upon the dielectric constants. The dielec-tric constant affects attenuation of the electromagnetic wavesas well, though this trait will not be used in this sensing appli-cation. It should also be noted that the dielectric constant is afunction of frequency and temperature. Adjustments may beneeded to any measurements made at temperatures differinggreatly from that at which a material is calibrated. The presentstudies were carried out in a laboratory in which the temper-ature was held at approximately 20°C ± 2°C, so no effect oftemperature has been investigated here. The fact that thedielectric constant varies with frequency may actually be anadvantage in the use of ultra-wideband signals considering thelarge frequency spectrum contained in the pulses.

SYNTHETIC APERTURE IMAGING

The UWB signals have been used in two related methodsfor investigating the moisture state of building materials. Theaim of the first method was to obtain quantitative estimates ofthe in-situ moisture content of materials that are below thesaturation level. Preliminary results are discussed in Healy andvan Doorn (2004), and work is ongoing to better determine therelationship between the reflected signals and the moisturecontent of building materials such as oriented strand board(OSB), plywood, gypsum board, and fiberglass insulation.This investigation is not the focus of this paper.

c µ0µr

ε0εr

( )⁄=

Γ12

c1

c2

–

c1

c2

+----------------=

Buildings IX 3

The second method of using UWB signals for detectingmoisture in buildings is aimed at locating bulk water intrusionand materials that are close to saturation. To make this tasksimpler, UWB can be used to create three-dimensional imagesof the moisture state of the wall. Because the incident pulsesshould theoretically be able to penetrate several layers of awall, water intrusion that is not visible should be detectable bythe technique. Such images are obtained by taking scans of thereflected radio waves at different locations in front of the wall.These techniques are the focus of this paper.

Overview of Synthetic Aperture Imaging

In a stationary system in which one antenna transmits asignal and a second antenna receives the reflections of thatsignal from various objects, the typical output scan shows thevoltage received at the receiving antenna as a function of time.Reflections will be coming from many different surfaces, soreflected signals will be returning to the receiving antennaover a large time frame. To create an image from these multi-ple scans, a technique proposed by Lorenz et al. (1991) hasbeen implemented in the present work. By noting that thewaves travel at the speed of light, the path length of thereflected wave can be determined, and an educated guess canbe made regarding the surface from which it has reflected. Fora given time of flight, the exact point of reflection is not knownexactly, but it is known that the reflection will have occurredon a hyperboloid, as shown in Figure 2 (all points on thesurface have the same combined distance to the two foci of theshape). This estimate assumes that all responses received atthe receiving antenna are from single reflections and that thespeed of light changes little in the different materials throughwhich the waves pass. A longer time of flight corresponds toa larger hyperbola, so information is gained about thesurroundings at each set of antennae positions. There is stilluncertainty, however, as to where a particular amplitude in thesignal obtained at the receiver originates. To resolve thisuncertainty, triangulation is used between multiple antennapositions to identify the source of the reflection source. Whileother radio frequency techniques could be used to locate mois-ture, the time resolution of the UWB hardware and the abilityof the UWB waves to penetrate various forms of constructionmake it an attractive choice for carrying out such analyses.

To create an image of a building assembly, scans are takenat different locations in front of a wall, and the assembly is thendiscretized into an array of pixels. For each scan, the amplitudeof the received signal is examined as a function of time afterinitial transmission of signal. At each time of flight, we add theamplitude of the signal to all pixels in the discretized wallsection that may have contributed to that signal. The antennaeare moved to different locations and the process is repeated.Pixels that represent an actual location of moisture within thewall will continue to be identified by all scans and will there-fore get a higher amplitude tally as more scans are added to themap. A few examples will show this technique in action.

EXPERIMENTS

Test Samples

To test the scheme, two different test walls wereconstructed. The first wall section was composed of 7.6 cm (3in.) of R-19 fiberglass insulation sandwiched between a 1.27cm (0.5 in.) sheet of gypsum board and a 1.27 cm (0.5 in.) layerof oriented strand board. The insulation had kraft paper thatfaced the gypsum board. Panels of these materials measuring0.61 m (2 ft) wide by 0.91 m (3 ft) tall were placed within awooden frame constructed of 2-by-4s of sugar pine. The rigwas constructed so that panels of varying moisture contentcould be interchanged to obtain a wide range of moistureconditions. To simulate water intrusion, a wet cloth wrappedin plastic was temporarily attached to different places of thewall. The dry cloth and plastic bags were also tested to ensurethat they had no independent effect on the reflected radiosignals.

The second test wall was larger and included threeconventional stud cavities. It was constructed after the conceptwas proven with the smaller wall section. The width and heightof the second wall were both 1.2 m (4 ft), while the sheathinglayer and interior wall layer were separated by an 8.9 cm (3.5in.) cavity made by four 2-by-4s spaced 40.6 cm (16 in.) apart(center-to-center). The main purpose of the second test rig wasto develop techniques to handle the presence of nonparallelfeatures such as studs and wires in the imaging techniques. Asin the previous case, moisture intrusion was simulated by plac-ing a wet cloth inside a plastic bag and attaching it in variouspositions within the wall section. A layer of OSB sheathingwas clamped to the outer plane, while a panel of gypsum boardwas attached to the interior.

Figure 2 Hyperbolic times of flight. With knowledge of onlythe time between the transmission of a signal andthe reception at a second antenna, one can onlypinpoint the location of the reflector on ahyperbolic shape. Inclusion of different antennapositions allows one to use triangulation topinpoint the location of the reflector.

4 Buildings IX

Movable Antenna

To test the concept of synthetic aperture imaging to detectunwanted water in walls, the antenna was set up in front of thewall with the gypsum board facing the antenna to simulate theinstrument being used on the interior side of the wall. For thefirst set of experiments, a single antenna was used that servedas both the transmitter and receiver with a high-speed switchchanging between modes. This antenna was then attached toa robotic arm that automated the task of moving the antenna infront of the wall to provide the multiple scans that are neededto develop a synthetic aperture image. The antenna was main-tained at a distance of 1.4 m from the gypsum board in themodel wall. This distance was somewhat arbitrarily selected;studies are ongoing to determine an optimal setting for thisdistance. The antenna position was modified along a squaregrid having 11 × 11 points and a resolution of 70 mm betweenadjacent grid lines. These 121 measurement locations createda square synthetic aperture having a length of 700 mm on eachside.

To illustrate the potential of the synthetic aperture tech-nique for visualizing the moisture condition of a wall, we willfocus on one experiment in which the wet cloth was placed onthe interior side of the OSB panel in the model wall. If thislayer were wet, it would not be visible to an observer withoutmodification to the structure such as pulling away siding orinserting pin probes through holes drilled in the envelope.Figure 3 shows an image of the radar return at the plane corre-sponding to the OSB location. The red section indicates aregion of high radar reflection, and the arrow on the plot showsthe width and location of the wet cloth that was attached to thewall. One can see that the location and size of the wet spot have

been well identified by the technique. An apparent anomaly inthe results is that the region of high reflection appears circular,but the cloth that has been inserted to simulate the wet area wasrectangular. The reason for this behavior is the smearingnature of the algorithm. If better resolution of the sharp edgesof the towel were desired, the antenna spacing could bedecreased. Thus, one technique for finding moisture problemswould be to carry out an inspection with widely spacedantenna positions and then to use finer spacing in questionableareas. A three-dimensional image (Figure 4) can also becreated that provides another view of the wall to identifypotential moisture accumulation. Here, the planes correspond-ing to the OSB and gypsum board are plotted along withimages in the other two planes. Images in the planes of thebuilding panels show the location of the moisture patch, whilethe images in the other planes show that moisture is notdetected in the layers away from the OSB. This view clearlyshows that the synthetic aperture imaging technique yields animage that can be studied to detect the location and size ofpotential moisture problems within a wall. An additionalfeature that is not clearly evident in this view is the fact that drywalls do show some reflection. It is therefore expected that apriori knowledge of the wall dimensions may not be necessaryin using this technique. This theory will be tested further in thefuture on walls of different dimensions. The uncertainty in thedepth resolution is approximately 5 mm, based on the timingaccuracy of the units used in this study and the changes in thepropagation speed through the wall. Further studies can bemade to reduce the ambiguity of the moisture level at thegypsum board layer. The result, however, is a promisingmethod for creating contour plots of the moisture levels atdifferent layers within a wall.

Figure 3 Synthetic aperture image of moisture patch onOSB panel (dimensions in mm).

Figure 4 Three-dimensional synthetic aperture imageshowing moisture accumulation in planecorresponding to OSB panel location(dimensions in mm).

Buildings IX 5

Antenna Array

It should be noted that a robotic arm was used in the previ-ous study, but that device was simply used for the convenienceof carrying out the study. Any technique can be used for locat-ing the radar at appropriate points in the viewing field. As analternative to moving the antenna, it was surmised that astationary system would be easier to use and would decreasethe chance of user error in creating the images. An array ofantennae was built that allows for several combinations ofantenna positions without requiring motion of the antenna. Byswitching between different transmission and receiving anten-nae, one can create a synthetic aperture with no movement. Apicture of the antenna array is given in Figure 5 (the target inthis case is a military helmet instead of the wall envisioned forthis application). The array consists of four rows of four anten-nae in a square pattern for a total of 16 antennae. The top rowand the third row from the top consist of transmitting antennae,while the other two rows consist of receiving antennae. Tworadios are connected to the antenna array to generate the signaland to analyze the received signal. Switches on the arraychange the connection between the radios and the appropriateantenna.

This array contains 64 different antenna combinationsthat can be used to construct a synthetic aperture. The resolu-tion that can be attained using this array is a function of thebandwidth of the signal, the spacing of the antennae, thedistance between the wall and the array, the antenna size, andspacing between antennae (the array is placed parallel to thesurface of the wall). With the array placed close to the wall ata distance of approximately 1 cm, it is estimated that featuresof the wall can be resolved to within 3 mm. The drawback ofbeing so close is that the field of view is limited to the approx-imate size of the array. By pulling the array farther away fromthe wall, one can obtain a larger field of view at the expense ofresolution. It is expected that fields of view on the order of asingle-story wall can be coarsely examined using this tech-nique. At this point, work is ongoing to investigate the effec-tiveness of this technique in making such measurements andto determine the resolution that is capable for locating mois-ture.

The Effect of Studs and Wires

The results from the smaller wall having no studs showsthe ability of this technique to detect moisture in a wall consist-ing of planar layers of building materials. The effect of non-parallel features, such as studs and wires, has not beenaddressed in these studies. It is expected that the syntheticaperture images will show the presence of those features veryprevalently, especially wires owing to the fact that metalreflects radio waves very well. The antenna array is currentlybeing used to determine if there is an easy way to differentiatebetween reflections due to nonplanar features and those due tomoisture.

SUMMARY AND DISCUSSION

The use of ultra-wideband signals holds promise for help-ing determine the presence of unwanted water within walls ina nondestructive manner. UWB signals have the advantageover other radio-frequency techniques in that the time resolu-tion is fine enough to pinpoint the location of moisture and thatthe signals penetrate a wide range of materials. While the tech-nique can be used to obtain quantitative numbers that indicatethe moisture level of different layers within a building assem-bly, qualitative detection of bulk water intrusion may prove tobe the most useful application for this type of instrument.Much work needs to be done, however. For example, the detec-tion of studs, nails, pipes, and other nonplanar features iscurrently being investigated to see if it affects the detection ofmoisture within a wall. Other building assemblies should alsobe examined, including the effect of siding layers and masonryproducts. Images that have been created have already shownthe potential for this product to serve its purpose as a nonde-structive technique for locating moisture intrusion within awall. Work is continuing to examine the effectiveness of anantenna array for real-time creation of synthetic apertureimages that indicate the hygrothermal state of a wall assembly.

REFERENCES

Derome, D., A. Teasdale-St-Hilaire, and P. Fazio. 2001.Methods for assessment of moisture content of envelopeassemblies. Thermal Performance of the Exterior Enve-lopes of Buildings VIII. Atlanta: ASHRAE.

FCC (Federal Communications Commission). 2002. OETestablishes waiver process to permit existing ultra wide-band devices to continue operation. FCC News, July 12,2002.

Healy, W.M. 2003. Moisture sensor technology—A sum-mary of techniques for measuring moisture levels inbuilding envelopes. ASHRAE Transactions 110:232-242.

Healy, W.M., and E. van Doorn. 2004. A preliminary investi-gation on the use of ultra-wideband radar for moisturedetection in building envelopes. To appear in ASHRAETransactions, Vol. 111.

Figure 5 Picture of antenna array used for syntheticaperture imaging.

6 Buildings IX

Knab, L., R. Mathey, and D. Jenkins 1981. Laboratory evalu-ation of nondestructive methods to measure moisture inbuilt-up roofing systems. NBS Building Science Series131, Gaithersburg, Md.

Korhonen, C., and W. Tobiasson. 1978. Detecting wet roofinsulation with a hand-held infrared camera. Proc. 4thBiennial Infrared Information Exchange.

Lorenz, M., L.F. van der Wal, and A.J. Berkhout. 1991. Non-destructive characterization of defects in steel compo-nents, nondestr. Test. Eval., Vol. 6, pp. 149.

Taylor, J.D. 1995. Introduction to Ultra-Wideband RadarSystems. CRC Press: Boca Raton, Fla.

TenWolde, A., and G.E. Courville. 1985. Instrumentation formeasuring moisture in building envelopes. ASHRAETransactions 91:1101-1115.

Tobiasson, W.N., C.J. Korhonen, and T. Dudley. 1977. Roofmoisture survey—Ten State of New Hampshire Build-ings. CRREL Report 77-31, Hanover, N.H.

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