HI-85-22 NO. 3

Instrumentation for Measuring Moisture in Building Envelopes A. TenWolde G.E. Courville, Ph.D.

ASHRAE Member

ABSTRACT

This paper discusses moisture measurement instrumentation available or required for buildingresearch, together with some recent developments. The paper focuses on measurement of in situ moisture content (MC) and humidity in buildings and building components. Electric resistance measurements have been most often used for this purpose. This method has the advantage of being relatively cheap, but many interfering factors limit the accuracy obtain­able and the method is inherently intrusive. An alternative method, using capacitance measurements, is nonintrusive, provided that the area of interest is close to the surface. It is potentially less sensitive to interference from some extraneous factors, but at present it is difficult to use quantitatively. Both electrical methods face major problemsbecause they assume a homogeneous distribution of moisture and because conducting material layers can greatly interfere. A promising technique is nuclear magnetic resonance, but currently it does not yield reliable quantitative results and involves complex and expensive apparatus.

Relatively little is known about actual performance of instrumentation in buildings or building components. A survey of building researchers and others who could document their experience with various moisture measurement techniques would be very valuable. Developmentof a common evaluation procedure would also be helpful.

INTRODUCTION

Moisture is a leading cause of building deterioration. High moisture levels can cause decay, warping, or corrosion of materials. High humidity may result in mold, mildew, or staining, and paint may peel or blister as a result of too much moisture. Moisture problemshave been reported in basements, crawlspaces, attics, in and on walls, ceilings, and roofs and may occur in just about any other part of the building. Although moisture problems are so pervasive and detrimental, we have only a limited understanding of the causes of and con­ditions leading to the various problems. To provide practical information and advance our understanding of the complex movement of moisture in buildings, measurement is required, but the lack of suitable moisture-measuring equipment has impeded progress. This paper discusses moisture-measuring equipment needs and currently available equipment with its advantages and limitations in building moisture research. The focus is on instrumentation for in situ measurement of MC of building materials. Several recent developments and potential measuringtechniques are also discussed.

Anton TenWolde, Physicist, USDA, Forest Service, Forest Products Laboratory,Madison, Wisconsin; George Courville, Group Leader, Oak Ridge National Laboratory,Oak Ridge, Tennessee.

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AVAILABLE TECHNIQUES AND APPLICATIONS

By moisture measurements in buildings, we usually try to address two basic questions: How much moisture is present at a particular location and how does it get there? Water vapor permeance tests (ASTM E96, E398) are designed to characterize the movement of moisture through building materials. However, test results may not match actual performance because moisture transfer by air movement and hygroscopic and capillary flows are not considered in the test. However, no alternative has yet been accepted.

Measurement of the amount of moisture falls into two broad categories: air humidity measurements and MC measurement. Whiting (1984) discusses air humidity measurements in detail. Most such fall outside the scope of this paper. However, a small humidity sensor may be used to measure MC in building materials indirectly, with the assumption that the material, or at least the surface of the material, is in moisture equilibrium with the air (Whiting 1984).

A number of different physical properties are used to measure MC of materials. These include weight change, electric properties, dimensional change, thermal properties, and nuclear scatter and resonance properties. The gravimetric methods are based on weightchanges and are the most simple, widely used, and most accurate. They vary in nature of moisture extraction and are discussed in more detail in a previous paper (Courville and TenWolde 1984). They require samples to be taken from the material of interest and are therefore destructive. They are, however, essential for laboratory measurement and cali­bration of moisture meters.

Electric moisture meters have been used for many years in process and quality control,for MC determination of lumber, grains, and soils as well as in building moisture research. With these meters MC is determined by measuring electric resistance, capacitance, or admittance (impedance). A handheld electric moisture meter measures MC of a material directly by inserting electrodes (for resistance measurements) or placing electrodes in contact (for capacitance measurements). Alternatively, MC can be measured indirectly with a small sensor in moisture equilibrium with the material of interest. Electric moisture-measuring equipment is affordable, widely available, and therefore more commonly used in building moisture research than any other method. Several electric methods are well suited for in situ measurements and will be discussed in more detail.

Various other experimental methods have been employed to measure MC. Some of these, such as nuclear magnetic resonance, are very expensive and often restricted to laboratory use. Other methods, such as infrared thermography or neutron thermalization meters, are appropriate for the detection of the presence of moisture during moisture surveys but not reliable for determination of the exact amount of moisture in materials.

CRITERIA FOR IN SITU MEASUREMENT

Criteria for instrumentation obviously vary with the objective of the measurements. Infrared thermography, for instance, has been successfully used for locating wet areas in roofs (Tobiasson and Korhonen 1978) but cannot be used to quantify the exact MC of the insulation. In moisture research, in the laboratory as well as the field, MC of materials needs to be determined with varying degrees of accuracy. Often the materials of interest are located within a building component and not easily accessible without damaging the component or changing the conditions. For instance, gravimetric methods require sampling and, therefore,alter the component as well as the humidity and temperature conditions and can, therefore, be used to determine the moisture conditions only at the end of a test.

Handheld electric moisture meters may be used if the interest is limited to surface or near-surface materials and if no great accuracy is required. Humidity sensors are needed when moisture conditions within the building component are of primary interest. These sensors should be small enough to fit in various locations in building components without unduly disturbing temperature and humidity conditions. Several types of electric resistance or capacitance sensors are available that meet this requirement. However, greater accuracythan these sensors have is needed for some applications. Range can be a limitation with some of these sensors, notably, the wood electric resistance sensor.

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Accuracy and range are especially crucial when investigating moisture movement. There is no moisture equivalent of the heat flow meter, i.e., we cannot measure moisture flow directly. The only alternative is to measure humidity throughout the building component, even though this does not strictly measure flow. The usually steep and changing moisture and temperature gradients across the components require sensors of sufficient accuracy over a wide humidity range. The sensors should also be able to withstand condensation without shifts in response.

A time of response criterion is difficult to specify. Whiting (1983) suggested that the response time should be less than one minute for 95% response. This is a stringentrequirement and quite a few currently available sensors are not able to meet it. In most applications, a longer response time could be tolerated. Moisture storage strongly affects the rate of change in humidity in most building components. This means that rapid changesin indoor and outdoor conditions generally result in much slower changes in moisture within building components.

Thus response time requirements depend on the placement of the sensor in the component.If no moisture storage is available close to the sensor, more rapid changes in moisture may occur. The effect of location is well illustrated in Figure 1, which shows the difference in humidity as measured by Duff (1971) in an insulated wall during summer with one wood moisture sensor on the polyethylene vapor retarder and one sensor on the plywood sheathing.The fiberglass insulation and polyethylene offer little moisture storage, but plywood is a good storage medium. Figure 1 shows that the sensor on the plywood did not record anychange in humidity, even though the plywood experienced wide swings in temperature. At the same time, the sensor on the interior surface (vapor retarder) recorded rapid changes in humidity, primarily because no storage was available at that location. Even so, that sensor did not record changes faster than 2% to 3% MC per hour. This corresponds with a change of roughly 10% to 15% relative humidity per hour. Such changes are well within the responsecapabilities of the wood sensor. Other measurements of moisture in building components have generally shown lower rates of change. Thus, for many applications, currently available sensors, including the wood electric resistance sensor, seem rapid enough. Response time becomes an issue, however, when extreme changes are expected and no modifying moisture storage medium is present.

Some applications require many sensors. An example is the measurement of moisture movement in wall or roof cavities where moisture gradients can be large because of steep temperature gradients. In such cases cost of equipment becomes an important issue. This is especially true in field studies when sensors are imbedded in the building components at the time of construction and cannot be retrieved. For most sensors the current cost is prohibi­tively high for such application. Wood or paper electric resistance sensors are a notable exception.

Conditions in buildings are often more demanding than in the laboratory. For instance, mirror dew point hygrometers are the most accurate humidity instruments currently available,but there have been problems operating them in dusty attics. We often need instruments able to operate in a dusty environment.

The objective of most moisture surveys is to determine the occurrence and assess the severity of moisture accumulation in roofs and walls. Tsongas (1980) used a dielectric typemoisture meter to locate the areas of highest MC in residential walls and then used electric resistance meters for more accurate in-place determination of MC, together with gravimetrictechniques on selected samples. Infrared imaging devices are used routinely for nonintrusive location of moisture in roof systems, as are neutron moderation techniques and devices monitoring electrical capacitance. Although possible in principle, reliable quantitative measurement is not now technically feasible with these devices. The principal value of these techniques is that they are nonintrusive and provide quick results. Detailed reviews of moisture survey devices are available (e.g., Tobiasson 1978; Jenkins 1981).

ELECTRICMETHODS

Electric properties of porous materials generally change drastically with MC. It is therefore possible to calibrate this change and determine MC by measuring the electric property.Electric measurements fall into three broad categories: Resistance, capacitance or capacitive

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admittance, and power loss measurements. Of these the resistance measurement has been most widely used in building moisture research.

Electrical Resistance Measurements

Electric resistivity of most porous materials decreases with increasing MC. By measuringelectric resistance, therefore, it is possible indirectly to measure the MC of materials. A nearly linear relationship usually exists between the logarithm of the resistance and the MC. Figure 2 shows this relationship for several materials.

Because a dry porous material generally has a high resistivity, most of the conduction takes place through the moisture contained in the pores. The range in resistivity from low to high MC can therefore be very wide. Resistivity of Douglas fir at 7% MC is 50 thousand times higher that at 25% MC (James 1975). Other materials generally show a narrower range.Plaster of paris, for instance, varies in resistivity by a factor of 350 with MC varyingfrom traces to saturation (Bouyoucos 1965) and concrete only by a factor of ten (Whittington et al. 1981).

Temperature has an effect on resistivity. James (1975) reports that resistivity of wood is roughly halved for each 10°C increase in temperature. Concrete also exhibits a decrease in resistivity with higher temperatures (Whittington et al. 1981).

Resistivity of hygroscopic materials is not only influenced by MC and temperature but also by the presence of ions in the material (Pande 1975). In concrete the ions primarilyinvolved are calcium, potassium, sodium, and sulfate ions. In wood they may be naturallyoccurring electrolytes or salts which have been added to preserve the wood. Murphy (1929)theorized that the presence of these ions also causes the electrical conductivity of cotton to increase with frequency. For example, the conductivity of cotton for an alternating current (1000 Hz) is higher than for a direct current at humidities below about 80% (Figure 3). A similar effect occurs in other porous materials at low MCs (Pande 1975). The greater conductance at higher frequencies is due to the fact that at higher frequencies the partially bound charge carriers (ions) transfer the same charge per unit of time with smaller displacements and therefore with less resistance. When a current is maintained over a period of time, apparent conductivity also decreases. This effect is most likely caused by polarization. It is therefore recommended that DC resistance readings be taken as quickly as possible.

Moisture content can be determined by measuring electric resistance of the material itself. This method has been used for many years to determine MC in wood (James 1975).Portable battery-operated resistance moisture meters are readily available. These instruments generally use a direct current. A two-pin or four-pin electrode is driven into the wood and resistance between the pins is translated into MC. If the meter has insulated electrodes, the MC at different depths can be determined approximately. The meter is generally cali­brated for one species and one temperature. Manufacturers provide information on correction for temperature and wood species. Of course, handheld meters cannot be used for moisture measurements within wall or roof cavities without damage or alteration to the system. To obtain such measurements, we may install the electrodes permanently during construction; for example, Luft (1983) and others have used nails as electrodes to measure the MC of wall studs, although experience has shown that long-time stability of permanently installed electrodes may be poor.

For continuous resistance moisture measurements in paper or textiles, the electrode consists of two rollers (Pande 1975). Alternative electrode designs have been used for measurement of MC of a variety of materials.

Several factors influence the accuracy of electric resistance moisture readings. Highconcentrations of extraneous ions may cause readings that are too high. Accuracy requiresgood contact between the electrode and the material. A discussion of some possible errors of measurement can be found in ASTM Designation D 2016, Standard Test Methods for Moisture Content of Wood. If a drift toward lower MC occurs, it is important that the reading be taken as quickly as possible. If properly calibrated, resistance moisture meters can have an accuracy of 0.5% MC (Pande 1975; Ballard 1973). In practice, the standard deviation of readings is more likely to be in the range of 0.5% to 1.5% MC for wood specimens of equal MC (James, private communication). In general the possible error will be greater for materials with greater variability in composition.

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Moisture content can also be determined indirectly by placing a material with known resistance-MC variation in equilibrium with the unknown medium and measuring its resistance. This allows measurement within wall and roof cavities if sensors are installed at time of construction. One advantage of this method is that the relationship between resistivity and MC of the sensor can be precisely calibrated. The disadvantage is that the relationshipbetween the MCs of the sensor and the medium at equilibrium is often not known precisely,and the assumption of equilibrium may not be true at all times, especially when conditions are changing rapidly.

Plaster of paris has been widely used as the calibrated material for measurement of the MC of soils. Bouyoucos (1965) claims that under well- controlled conditions, the error is less than 1% in terms of total soil moisture.

For moisture measurements in building components, perhaps the most widely used method involves a wood sensor (Duff 1966). These sensors, originally designed for continuous measurement of moisture in wood members, can be cheaply manufactured in various sizes, as small as 1/16 x 1/16 x 3/4 inch (Figure 4). They can measure moisture between 7% and 30% MC in wood. More recently wood sensors have also been used to monitor humidity conditions in wood-frame walls (Sherwood 1978, 1983). The humidity range of the sensor is approximately35% to 100% relative humidity. Measurements at the low end of this range (between 35% and 50% relative humidity) are difficult to perform because of extremely high resistance (in the order of 1 to 10 gigaohm). Conventional amplifier signal conditioning circuits used in conjunction with the sensor, as shown in Figure 5, have proved inadequate to cover that range of electric resistance. However, an electronic log-amplifier circuit can extend the range of the signal conditioning system.

Errors are usually in the order of 2% MC, but individual calibration of each probe can bring this down to approximately 1%. This corresponds with an error of 5% relative humidity.Temperature effects, drift, and hysteresis are the most prominent sources of error.

Response of the wood sensor is relatively slow especially under drying conditions. Response depends on the speed at which the sensor reaches equilibrium with its environment, and this process is apparently slow when the sensor is drying. The speed of moisture sorption greatly depends on the surface to volume ratio of the sensor. Thus, dimensions of the probe can greatly influence response.

There are several commercially produced resistance sensors. Some of these cover the full humidity range and are claimed to have errors as low as 1% relative humidity and fast response times. Usually no published data are available to verify this or other claims such as long-term stability. The sensors also tend to be rather costly.

Capacitance Measurements

Capacitance moisture meters measure the change in dielectric constant of porous materials with changing MC. Moisture content is determined from this calibrated relationship bymeasuring the capacitance of a capacitor with the porous material as dielectric medium. This capacitance also depends on the geometry of the capacitor plates. The simplest geometryof large flat parallel plates with the material in between is shown schematically in Figure 6. The capacitance of this configuration is

where K is the dielectric constant of the substance. The area and separation of the plates are respectively A and d, and is a constant ( = 8.9 x 10-l2 C2/N·m2).

For more complicated geometry, the calculation of capacitance is a difficult mathemati­cal exercise, and only in a few instances is a closed form solution such as Equation 1 possible. If, however, the material is homogeneously distributed, then it is possible to write

where Co is the capacitance calculated with air (Kair =1) in the volume element and K againis the delectric constant for the material. Thus: for arbitrary geometry it is still

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meaningful to use the parameter K to characterize different materials between the plates.Furthermore, if the medium is filled with two homogeneously mixed materials

(3)

where v and v2 are the specific volume fractions for the two materials with dielectric constants K1 and K and

m K is the effective dielectric constant for the mixture.

Recognizing that v2 1

= 1 - v2 and differentiating Equation 3 results in

(4)

in nearly all

is taken as a measure of the sensitivity of the circuit to changes in the mixture,significant to note from Equation 4 that K2 - K1 is anomalously large for water

building materials (for water K ~80 while most other materials have values of K between 1 and 5). This, of course, provides for good sensitivity of the measurement of MC with the capacitance probe.

Although illustrative of the effect, Equations 2, 3, and 4 are not strictly true for real systems, partially because the boundaries of the medium cannot be precisely defined and partly because its substance is not always homogeneous. More importantly, the capacitivebehavior of hygroscopic water is often greatly affected by the hygroscopic medium. The dielectric constants are, therefore, often not strictly additive. The dielectric constant of moist wood, for instance, is much larger than would be expected from Equation 3. This synergism is usually very dependent on frequency. For this reason, meters based on measures of dielectric constants must be calibrated for each geometry and material if they are to be used quantitatively.

In contrast to most resistance meters, capacitive instruments use alternating current. Capacitance can be determined by making the specimen part of a capacitance controlled oscillator. The oscillator frequency changes with changes in dielectric constant. The much more common capacitive admittance type meter uses the specimen as a capacitive element in a resistance capacitance bridge circuit. The meter indicates the imbalance in the bridge in proportion to the dielectric constant and resistive losses in the sample (James 1975). Most of these operate on frequencies between 103 and 108 Hz.

Capacitance probes for moisture measurements are used in several industries. Examples are wood and paper and agriculture (James 1975; Fletcher 1965). Typically, the probes are of regular geometry (e.g., parallel plates, concentric cylinders, or parallel cylinders) and are immersed in the test medium (e.g., flour, wood, or earth). Application to buildings is more recent (Jenkins 1981) and has been restricted to roof moisture surveys. There is little detailed information available in the open literature on the properties and perform­ance of devices for in situ moisture measurements using capacitance techniques.

Power-Loss Measurements

Power-loss meters measure the energy absorbed by the specimen when subjected to an alter­nating electric field. This power loss depends on moisture content and can therefore be calibrated for any porous material. Readings are affected by temperature in a complex manner and need to be corrected for it (James 1975). Portable power-loss meters are commercially available and are used in the forest products industry.

Recent Developments

Oak Ridge National Laboratory (ORNL) is currently investigating two types of capacitanceprobes monitoring moisture content in roofs. The first is a variant of a nonintrusive device for roof moisture surveys. The original device, shown schematically in Figure 7, involves two plates lying in the plane of the roof surface, separated by a distance, D. Changes in the electric current in the external circuit depends upon the ac voltage applied to the circuit and upon the capacitance in the fringing electric field that penetrates the roof surface. Moisture in this region has a significant effect upon this capacitance and, hence, the measured electric current. These devices give a maximal signal for moisture near

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the surface and do not detect moisture beyond about 2D from the surface. This limits their usefulness for quantitative measurements.

The ORNL experiment involves five plates as shown in Figure 8. A 150 kHz signal is applied to circuits between electrode 1 and each of the other electrodes sequentially and the resultant currents are recorded. As for the two electrode devices, all circuits respondmaximally to water near the surface. However, pair 1-2 are not affected by moisture beyond2D, pair 1-3 by moisture beyond 4D, and so on. A mathematical model has been developed that synthesizes a set of readings for an arbitrary moisture distribution and predicts the causal moisture distribution. At the present time, this probe concept is beset by two difficulties;(1) the model requires a complex probe calibration for each application, and (2) the changein capacitance is very small when moisture concentration is changed at depths greater than 4 or 5 in. Additional work is required before this type of probe, or some variant, can be shown feasible.

The second concept under investigation at ORNL is a variant of the cotton bale device mentioned earlier and a followup on a dc technique used some years ago at the National Bureau of Standards (Powell). This is an intrusive device that utilizes thermocouplespreviously embedded in a roof system. In this case, an rf-signal is applied between adjacentpairs of thermocouples and the resultant signal is a measure of the capacitance between those portions of the two thermocouples that are not shielded. Preliminary experiments at ORNL indicate that this type of probe is quite sensitive to moisture concentration, readilypicking up diurnal vapor transport resulting from temperature gradients. Another advantageof the device is its potential low cost. Parts for an eight- channel circuit cost about $20. One disadvantage is that the device is intrusive; a more serious disadvantage, if the device is to have any quantitative capability, is the difficulty of calibration .

At the Forest Products Laboratory further development of a cheap electric resistance humidity sensor is underway. The sensor currently being tested consists of a rectangularpiece cut from the edge of a standard electric circuit board with a small piece of filter-paper glued to the two leads with conducting paint (Figure 9). An incision in the board is usually made between the two leads to expose the center of the paper to the air on both sides. The size of the sensor is approximately 3/16 x 5/8 in, comparable to the wood sensor, but it can be manufactured quicker and cheaper. This sensor also has a much faster response time. Tests are currently focusing on filterpaper doped with lithium chloride to extend the range down to about 15% relative humidity. A different signal conditioningcircuit is tested in conjunction with the sensor (Figure 10). The sensor is an element in an oscillator circuit. Changes in the resistance of the sensor change the output frequency.Frequency responds approximately linearly to changes in relative humidity, and frequencyinformation is easily transmitted to the data collection point without loss of accuracy.Figure 11 shows some typical early test results that look promising. If individuallycalibrated, the sensor's error may be limited to 3% relative humidity over a range of 15% to 100% relative humidity. However, recent repeated tests over longer time periods have indic­ated that the sensor is not very stable and that there is interference between adjacent sensors at low humidities.

OTHER METHODS

Physical phenomena other than electric resistance or capacitance can be employed to measure MC or humidity in buildings or building components. Some of these are currently available or in development. A short description of two with potential for in-place measurement follows.

Dimensional Change Sensor

Hedlin and Nicholson (1980) describe a humidity sensor that is a cellulose crystallitestrip, which expands or shrinks with the rise and fall of relative humidity. This bends a stainless steel strip, and this deformation is measured by a pair of piezo resistance strain gages. Hedlin and Nicholson found a significant change in calibration over time and showed that maximum hysteresis varied between 2% and 7% relative humidity, depending on tempera­ture. Response time was approximately three minutes to reach 63% of total change.

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Nuclear Magnetic Resonance

The phenomenon of nuclear magnetic resonance (NMR), although somewhat exotic in comparison to typical diagnostic practices in the building sciences, nevertheless has potential as a means of identifying and quantifying water in building elements. An NMR device is normallythought of as a laboratory instrument. A test sample is placed inside a radio frequency(rf) coil and the unit is mounted between the pole pieces of a large electromagnet as shown in Figure 12.

Other geometries, however, are possible. A permanent magnet as small as a 3-in cube with a 1-in diameter cylindrical hole in the center for an rf coil and a sample has been used for soil moisture tests. N M R sensors of this type have been implanted under highwaysin Arizona and Pennsylvania to monitor moisture variations in highway subgrade soil over extended periods of time (Matzkanin 1975). Recent advances in magnetic materials suggestthat these units can be made as small as 1-in cubes.

When considering other geometries, it is well to remember that basically NMR onlyrequires that there be two uniform magnetic fields perpendicular to each other imposed on the test sample, one constant, H, and the other, H 1’varying over time (Abragam 1961).

A (Of

course, appropriate field strength and electronic characteristics must be achieved.)variant of the previous case has only the rf coil that produces H1 embedded in the samplewhile the constant field is achieved by properly shaping the pole pieces of an external magnet.

This configuration has the advantage of removing the high-mass H -field supply from the test region. With only a lightweight air core coil embedded, there is1much more likelihood of the test region being in thermal and hygric equilibrium with its surroundings. The logical extension of this idea is both field sources outside the sample region. AnNMR sensor configuration based on this concept has been successfully developed and used to measure the MC in concrete bridge decks at depths down to 3-1/2 in (Matzkanin 1981). A similar arrangement attached to a tractor has been used to continuously measure the water content in agricultural fields (Matzkanin 1983b).

The feasibility of NMR for moisture measurement in building systems is currently beingaddressed at the Southwest Research Institute (Matzkanin). Preliminary laboratory NMR measurements on moistened fiberglass, fiberboard, and expanded polystyrene samples (approxi­mately 16 cm3) have provided strong signals that are easily measured. Three significantquestions are being addressed: (1) is the NMR signal simply related to MC, (2) can one-sided measurements be carried out, and (3) can the measuring volume be defined well enough to make the technique useful for local moisture measurements?

DISCUSSION AND RECOMMENDATIONS

This paper is by no means complete in its discussion of available instrumentation, mostlybecause published information on moisture-measurement techniques is limited. Much of the available data is from the 1960s or early 1970s and widely scattered throughout the litera­ture. While many researchers have undoubtedly gained valuable experience with instrumenta­tion since then, very little has found its way into the literature. A survey of researchers and their experiences would therefore be of great value. A common approach to evaluatingexistent and future instruments would also be very helpful.

This paper does not cover all instrumentation needs. Reliable and instantaneous measurement of surface moisture, for instance, would be useful. Some infrared absorptiontechniques have been used in industry (Whiting 1984), but to our knowledge no portableinstrument is available.

Convective moisture transport is important but difficult to measure. Quantification of the often minute and oscillating airflows is the problem here, which we considered outside the scope of this paper.

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SUMMARY

Measurement is required to provide accurate practical information on moisture in buildings.Although numerous laboratory methods for moisture measurement in materials are available,little instrumentation exists for in situ measurements of the building envelope. Most often measurements have been made of electrical resistance. This method has the advantage of being relatively cheap, but many interfering factors limit the accuracy obtainable and the method is inherently intrusive. Capacitance measurements are nonintrusive, provided that the area of interest is close to the surface. They are potentially less sensitive to interference from some extraneous factors but difficult to use quantitatively at present.These electrical methods face major problems, both because they assume a homogeneous dis­tribution of moisture and because conducting material layers can greatly interfere with measurement. Yet several recent developments in electric measuring techniques may yield some improvements. A promising alternative to electrical techniques is nuclear magnetic resonance, but it does not yet yield reliable quantitative results.

The optimal technique for measurement depends on the degree of accuracy needed, the measurement location, response time requirements, and cost restrictions. Response time is often not a critical factor because of the moisture storage capacity of many buildingmaterials.

Relatively little is known about actual performance of instrumentation in buildings or building components. A survey of building researchers and others who could document their experience with various moisture-measurement techniques would be very valuable. Developmentof a common evaluation procedure would also be helpful.

REFERENCES

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Figure 1. Five-daysequence of humidity and temperature conditions measured across the insulation of a wall panel facing south (Duff 1971)

Figure 2. Relation between electric Figure 3. Relation between dc and ac resistance and moisture content (1000 Hz) conductivity of a for several materials (Pande twisted pair of cotton threads 1975) at different relative humidities

(Murphy 1929)

Figure 4. Plan of the wood electric Figure 5. Signal conditioning circuit for

resistance sensor the wood electric resistance sensor

Figure 6. A simplified capacitor circuit Figure 7. Schematic electric field distribution for capacitor electrodes in the same plane

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Figure 8. Five equally spaced electrodes. Circuits are connected between 1 and 2, between 1 and 3, Figure 9. Electric resistance sensor between 1 and 4, and between consisting of filter paper 1 and 5. The penetration depths flued on printed electric are respectively 2D, 4D, 6D, circuit board and 8D

Figure 10. Oscillator circuit used in conjunction with the filter paper humidity sensor

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Figure 11. Preliminary results of response tests of a filter paper humidity sensor

Figure 12. The basic nuclear magnetic resonance (NMR) measurement configuration

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Discussion

C. HEDLIN, National Research Council of Canada, Saskatoon: Often it is sufficient simply to determine whether or not moisture is present in roofs. The roof is likely to have large amounts of moisture or to be dry. Moisture measuring devices measure moisture, not roof leaks. The latter may exist without affecting moisture resistant insulation; the water mayleak into the building without being detectable by moisture-measuring devices.

TENWOLDE: The authors agree with this comment.

M. MECKLER, The Meckler Group, CA: What can be done to deal with homogeneous versus nonhomogeneous mixtures?

TENWOLDE: This paper discusses several devices with the potential to detect non-uniform moisture distributions. In principle, any device that is able to measure at different locations and depth and that has a sufficiently small measurement region can detect moisture gradients. The most common example is the electric resistance moisture meter with insulated electrodes. Other potential devices are NMR devices and the multielectrode capacitance device in development at Oak Ridge National Laboratory.

TenWolde, A.; Courville, G. E. Instrunentation for measuring moisture in building envelopes. In: ASHRAE Transactions 1985, v. 91, part 2: Proceedings of ASHRAE annual meeting; [1985]; Honolulu. Atlanta, GA: American Society of Heating, Refrigerating and Air-conditioning

Engineers, Inc.; 1985: 1101-1115.

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