is! w
HIP
Reflective Insulation GORDON B. WILKES Massachusetts Institute of Technology, Cambridge, Mass.
A brief historical review of reflective insulation, including the theory, is presented. A summary of laboratory and service experience with reflective insulation covering a ten-year period is given, and the results show that this type of insulation is effective and permanent when properly installed under suitable conditions. The remarkable insulating value is shown for this type of insulation when in a horizontal position with heat flow downward. A single reflective sheet placed midway in the air space of a flat roof can give an insulation value equivalent to 3 inches of corkboard if the direction of heat flow is downward as in the summer. With heat flowing upward through the same roof, the insulating value may be reduced to about 1 inch of corkboard. The importance of heat capacity of insulation for intermittently heated or cooled chambers is mentioned. The heat capacity of aluminum foil, as installed, is about one fortieth that of an insulating material weighing 8 pounds per cubic foot.
shortly after he learned to make metal cooking vessels. The rate of heat loss from a copper kettle would be materially less than that from an earthenware vessel. For well over one hundred years, the physicist has been familiar with the advantages of light-weight bright-metal containers for calorimetric work on account of the relatively small heat loss and particularly their low heat capacity. He recognized that he could reduce heat losses with a n insulation such as cotton wool but that such insulation was not satisfactory for his purposes because of its greater heat capacity. I n the middle of the last century, Peclet (7) carried out experiments showing the excellent insulating value of multiple layers of tin-coated steel separated by definite air spaces. He even went so far as to find out the effect of varying the thickness of the air space between the reflecting surfaces. During the next seventy-five years we find occasional references to the insulating effect of metallic surfaces, but there was practically no commercial use of this type of insulation prior to 1925. At that time Schmidt and Dykerhoff fled patents in Germany for the use of reflective surface as insulation. They experimented with extremely thin aluminum foil, less than 0.0005 inch thick, which had not been available to previous investigators, and developed a n effective and inexpensive form of insulation. These two are undoubtedly responsible for a great advance in the a r t of heat insulation. Since the issue of these patents a number of other investigators have discovered important applications of the principles of reflective insulation, and during the past ten years millions of square feet have been applied in this country alone.
Theory An air space has long been recognized as somewhat of an insulator a t moderate temperatures. If this space is more than 0.75 inch across, such as would be found in a frame house wall, radiation is responsible for approximately two thirds of the total heat flow. If the above air space is faced with metallic surfaces that are very poor radiators and good reflectorsof heat radiation, this same air space will become roughly three times as effective as a n insulating medium because the radiation can be practically eliminated. The National Bureau of Standards (6) states that such an air space faced with ordinary paper is equivalent to 0.26 inch of good insulation while the same space faced on one side with clean alumiis equivalent to 0.75 inch of good insulation. e three means of heat transfer-radiation, convection, and conduction-follow three fundamentally different laws as follows:
ORE than ten years have passed since modern rewas first Weed commercially. is based Won Since its insulating different principles from those of the customary insulating materials, it has taken much longer to pass through the infant stage than would have been the case with a new insulation of and educathe usual type. Although much tion is desirable in connection with reflective insulation, sufficient time now seems to have so that we can draw reasonable conclusions as to its value based on laboratory tests and more than ten years of service. I n this paper a n attempt will be made to sketch a brief history of the development of this type of insulation, to give insulating values for various uses whenever possible, and to cite some instances where it has been in service over a period of years with the results obtained.
Historical
e,tr. = effective emissivity of the two surfaces =
The advantage of metallic over ngnmetallic surfaces in reducing heat losses was undoubtedly recognized by man 832
1
INDUSTRIAL A N D EYGlNEERlNG CHEMISTRY
JULY, 1939
A = area, sq. Et. T, = abs. temp. of wzrmer surfiice, ' F.(tt 460" F.) T, = abs. temp. ofcooler surface, F.(& 460" P.) D Staran constant = 17.2 X 10-*'B. t. u., hr.-', ft.?, F.-4 abs.
++
-
Convection: q.lmrt.tion = C A ( h - W'' qco.v..tiun = quantity of heat by Convection, E. t. u. per br. (1, - &) = difference in temp. between two surfaces, ' F. C = coefficientof convection Conduction:
KA(k - t 4 1 qtOnduc(io. = quantity of heat by conduction, B. t. u. per hr. K = coefficient of thermal conductivity qeonduation =
1 = thickness, in.
833
reflectivesurfacesandover0.75inchin thicknessisprimarilyby convection. Frequently the term "coefficient of thermal conductivity" is used to describe the insulating value of an air space (over 0.75 inch across) faced with a reflective surface, in which case theheat transferislargelybyconvection,someradiation, but no conduction in the ordinary meaning of the word. The fact that many types of reflective insulation will vary in value with position is also responsible for variation of test results. These troubles do not arise with the ordinary insulating materials. As we become more familiar with the properties of reflective insulation, many of these difficulties will be eliminated but at present there is certainly a considerable amount of confusion in regard to this new type of insulating material.
Plain Reflective Surfaces with Definite Air Spaces
A casual glance at the above laws will show the difficulty of One of the simplest ways of converting an ordinary air attempting to determine coeRicients that will combine any space into an effective insulation is to subdivide the space into two of the above methods of heat transfer except for very two or more spaces with one or more sheets of material with limited cases. Heat flow by conduction is proportional to the reflective surfaces. In general, the spaces should be at least temperature difference, inversely proportional to the thick0.5 inch across, and i t would be better to have them 0.75 inch ness, and independent of position. Radiation for small across. The following list will illustrate some of the more temperature differences is approximately proportional to the common materials and methods used to accomplish this temperature difference but is independent of thickness and result: position. Convection, on the other hand, is proportional to ALUMINUMFOIL(Alfol Insulation Company). In ~iiost the E/, power of the temperature difference, is independcut of cases this foil is rolled to a thickness of about 0.0005inch and is thickness in most cases where the thickness is more than 0.75 usually crumpled to some extent before application. It is inch, and varies greatly with position, direction of heat flow, attached to the side walls of the air space by means of cardheight, etc. It is common practice for many engineers to use board strips and a stapling machine. Whenever practicable, a conductance value in calculating the rate of heat transfer the original air space should be subdivided into one or more across an air space. The term "conductance" is commonly air spaces of approximately equal thickncss. This foil is defined as the quantity of heat flowing per imit time, per unit frail and should be applied by an experienced man. Once area, and per unit temperature difference. Consequently, if applied properly, however, i t has been demonstrated to he we use a conductance coefficient in order to determine the rate sufficiently strong for the required purpose. of heat flow across an air space, we are assuming that radiation ALUMINUM FOILox PAPER(Reynolds Metals Company). and convection obey the same laws and that they are both Verv t h i n aluminum foil i s proportional to the temperacemented to kraft paper, either ture difference. In the case on one side or both as desired where one of the surfaces is a reThis material may be attached flective surfacc, the heat transfer is largely by convection, to the side walls of an air space in much the =me manner as the and considerable error in the plain foil, but the material has calculation may be introduced, much more mechanical strength since the temperature difference and can be applied by any earto the 6/4 power increases much penter or skilled workman. more rapidly than the temperaLEAD-TIN A L L O Y COATED ture diffcrence. Furthermore, there are many published tables STEEL (American Flange and giving conductance values for Manufacturing Company). air spaces, but they do not state This material consists of sheet that these values vary tremcnsteel about 0.006 inch thick, prodously with position if reflective tected on both sides with a leadtin alloy. It is sold in sheets 24 surfaces form the two sides. X 32 inches in size, and these The available data in the sheets are crimped at 4inch inliterature on the insulating value tervals to produce additional of reflective type insulation stiffness. T h i s m a t e r i a l i s shows considerable variation. me ch a n i e a 11y much stronger This is not surprising because than any other commercial prodthe majority of the experimentuct used for this purpose. ers attempt to treat reflective ALUMINUM FOILON CoKnuinsulation in the same way as QATED CARDBOARD (Aluminum the ordinary insulating mateAircell Insulation Company and rials. The heat transfer through others). Corrugated paper ('/s ordinary insulating materials inch thick) is coated on both such as corkboard, fiberboard, APPARATUSUSED IN DETERMININO RATE OF sides with aluminum foil. The rock wool, etc., is primarily by FIEATTRANSFER ACROSSALUMINUM-FOl%-LlNED c a r d b o a r d adds considerable conduction, where= the heat AIR SPACESIN Vanions POSITIONS, HEAT rigidity to the foil and makes transfer across an air space with MEASUREMENTS LABORATORY, M. 1. T.
834
1NDUSTRIAL AND ENGINEERING CHEMlSTHY
i t easier to handle, but adds relatively little to the insulating value of the subdivided air space. The air spaces faced with foil a.re responsible for the bulk of the insulating value of this material.
VOL. 31, NO. 7
self. This method will not give quite as good insulation per inch of thickness as plain foil definitely spaced, but the ease of application will frequently offset the additional cost of foil for equally well insulated jobs. Crumpled foil has been used to a considerable extent by the United States Navy (several million square feet in the past few years), merchant vessels, refrigerator ears and trucks, ovens, the ill-fated Hindenburg, etc. Crumpled foil has also been constructed in panel form, particularly for use in the insulation of oil tanks. The new 2Winch telescope dome under construction in California is being insulated wit,h crumplod aluminum foil in panel form. Recently, crumpled aluminum foil has been made in blanket form for use bctween the studs in frame-house walls. Thus, two or inore layers of foil may be installed with one operation.
Calcutation of Insulating Value It is impossible to give a definite coeficient that %-illproperly describe the insulating value of reflective insulation under all conditions of audication. A Considerableamount of exuerimental and pradeal data has been coileked during the past ten years, however, and it is now possible to ALUMINUX FOIL AND PAPER, ACcoeDIoN TYPE(The Rubermake rather accurate predictions of the insulating value of reoid Company). Two layers of aluminum foil are definitely flective insulation when applied in various ways and under spaced about I inch apart by paper that runs diagonally from i u ~ ~ different conditions. The accompanying charts and tables one side to the other and forms triangles with a l ~ ~ m i ifoil shouhl make the calculation of the insulating value of reflective bases. The foil sheets are creased in such a way that the combination may be folded into long strips about 0.25 inch insulation fairly simple for most installations. The data are taken primarily from Mason (d), GrifEths and Davis (S), thick, 1.5 inches wide, and any length desired-say, 200 feet. These strips may be cut to the length desired in order to fit the U'ilkes and Peterson (f4),and a large numbcr of experiments height of the stud space and then opened to the desired width conducted in the Heat Measurements Laboratory of the Massachusetts Institute of Technology under t,he supervision and attached to the sides of the studs. In this way two layers of spaced foil may be applied in one operation. of the author. The latter tests were sponsored by the Alfol ALUMINUM Fait ON PLASTER BOARD 8. GypSUm company and others). Aluminum foil is attached to one side of plasterboard, and this in turn is applied over the studs of a frame wall thus coniwbing an air space with radiating surfaces to one with a highly reflective surface. PAPERS CoATEn WITII REFLECTIVE SURFACES.Paper may be coated with a semireflective nonmetallic surface or with an aluminum paint. The emissivity of these coated papers will vary between 0.30 a.nd 0.50, as compared to aluminum foil at 0.05 and ordinary paper at 0.90. ALUMINUM FOIL ON COllllUGATED ASRESTO~ PAPER. This material is essentially the same as foil on corrugated paper, but it can withstand higher temperatures and the corrugated asbestos sheet usually has greater thickness. ALUMINUM FOILAND SPACER S T R I P PANELS.Panels of aluminum foil have heen'constrncted with fiberhoard or other suitable spacer strips. The foil layers are usually spaced about 0.5 inch apart and built up to a thickness that will give satisfactory insulation for the particular job. This particular method has been used primarily in refrigeration work. cOur1eay. Ganmol Elcctrzc ComponU CRUMPLED ALUMINUM FOILis used primarily in the indusELECTRICALLY HEATED OVEN BEINGINSULATED WITH CRUMtrial field. The aluminum foil is crumpled by hand a s it PLED A L a f m u h c FOIL comes from the rolls on the job and then straightened out roughly. One crumpled sheet is laid on another until the Insulation Company, Inc., Reynolds Metals Company, and desired thickness is attained. Experience has shown that others. The work of other investigators, including Rowley approximately two and a half layers per inch of thickness give (S), Queer (8), Scliad (Iff), Gregg (8), Van Dusen (is), the most economical insulation. In the case of plain foil nd,h Babbitt (fA),Nichols (6), and Schmidt (Zf), lias been utilized definite air spaces, the thickness of the foil has little to do with to a more limited extent. These experimenters usually did the insulation value, but with crumpled foil it is important to not attempt to separate the three methods of heat transfer use extremely thin foil, 0.0003 inch thick, so that therewill not and generally used relatively small air spaces of one square be any appreciable conduction of heat through the metal i t foot or less. CRUMPLED ALUMlNUM F O I L PANELS FOR INSULATION OF STORAGE TANKS Similar panels will be wod on the new %inch telescope dome.
(u.
INDUSTRIAL AND ENGINEERING CHEMISTRY
JULY, 1939
835
I
320 280
240
200
k+
y a 120
BO 20
40
60
BO
TEMPERATURE
FIGURE1.
u
T4
us.
v,
100
1%
40
TEMPERATCRE 0 L
FIGURE 4. CONDUCTAKCE us. POSITIOK 2
40
l&F€RATURE
FIGURE 2.
6 80 100 D k E R E N C E (AT1 %
ATV4 VS. AT
3 D
4' 4'
I"
A
5 0 6 8
2' 2'
I
4
.30 0
10
20
30
40
50 AT
FIGURE3.
COEFFICIENTOF CONVECTION us. POSITION
The transfer of heat by radiation and convection across relatively large air spaces may be readily calculated from the data in Figures 1, 2, and 3. These data should be used with caution if the dimensions of the air space are outside of the limits of those used as experimental evidence. I n Figure 2 , the coefficient of convection in the vertical position applies for temperature differences from 5" to 100" F., but in the other positions the experiments covered only a temperature difference between 5" and 30" F. The length and breadth of the air spaces varied from 2 to 8 feet in either direction, and the thickness varied from 1 to 4 inches. Radiation is independent of the position and thickness of the air space as well as the direction of heat flow. It depends only upon the emissivity and the temperature of the two surfaces. The net radiation for black-body conditions of the two surfaces may be found by subtracting the values found on Figure 1 for the corresponding temperatures of the two surfaces. If this net value is multiplied by the effective emissivity of the two surfaces, we obtain the quantity of heat per square foot per hour transferred across the air space by radiation.
'i
'Po
GRlFFlTHS GRlFFlTHS GRlFFlTHS GRlFFlTHS
70
h DAVIS L DAVE
h DAVIS h DAVIS
80
90
100
IIC
FIGURE 5 . CONDUCTANCE OF VERTICAL AIR SPACEus. A T
The amount of convection can be calculated from Figures 2 and 3. If we multiply the coefficient of convection (Figure 3) by At5/4 (Figure 2)) we have the quantity of heat per square foot per hour that is transferred by convection. The sum of the radiation and convection will give the total heat flow across the air space under the specified conditions. The above method of calculating the heat transfer by evaluating the radiation and convection separately is recommended whenever the conditions make it possible and where accuracy is desired. Frequently, however, conductance values are much easier to use and will give sufficient accuracy for the work involved. Conductance is a combination of radiation and convection or/and conduction. Figure 4 indicates the variation of the conductance of an air space (effective emissivity = 0.05) in various positions. The values were determined with a n air space 30 inches wide, 8 feet high, and 3 inches thick. The temperature of the cooler surface was 40" F. and of the warmer surface, 70" F. The figure shows clearly that the rate of heat flow in a horizontal position is more than three times greater when the heat flow is upward than when it is downward. Figure 5 gives the variation of conductance with ternpera-
VOL. 31, NO. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
836
ture difference for vertical air spaces (effective emissivity = 0.05) with heights between 2 and 8 feet. One curve covers air space thickness of 1 to 4 inches; while the second curve is for 0.5-inch thickness. These curves practically coincide a t the higher temperature differences, which indicates equal convection coefficients; but a t the smaller temperature differences the heat transfer is greater with the 0.5-inch space, owing to the retarding of convection and in'crease of conduction through the air. The loss by radiation on Figure 5 is based on the cooler surface temperature being 40" F. Since radiation is such a small part of the total heat transfer, the conductance
One curve represents a 10" and the other a 40" F. temperature difference between surfaces. The values decrease rather abruptly with increase of height up to about 3 feet, b u t a further increase in height makes rather small changes in the conductance value. Many experimental determinations have been made on air spaces in the neighborhood of 1 foot in height. Such values would be about 15 per cent too great if 1.8 I
f'T0 4"THICKNESS $TO B'HIGH
I
'40.:
3 ,4 5 .6 AIR SPACE WlDTk IN INCHES
FIGURE6. CONDUCTANCE us. WIDTH
.2
0
value is approximately correct for any cooler surface temperature within, say, 30" or 40" F. If the cooler surface temperature were actually 80" instead of 40" F., the error by using Figure 5 would be onIy about 3 per cent, assuming that all other conditions were the same. Figure 6 is based on the experimental work of Mason; it shows how conductance varies with thickness of a n air space faced both sides with aluminum foil (effective emissivity = 0.05). These values were determined on a n air space approximately 7.75 inches square, but since there was relatively
\
A WILKES h PETERSON 0 GRIFFITHS h DAVIS t STEENKAMP (M.1 T.1
\
VERTICAL AIR SPACE eEFF;=o.05 TI= ROOM TEMP 4" WIDE SPACES
\
\
'\.
1
2
3 HEIGHT
4 5 IN FEET
6
7
FIGURE 7. CONDUCTANCE us. HEIGHT
little convection with thicknesses less than 0.5 inch, the values probably apply to larger spaces with the exception of the values corresponding to an air space thickness of 0.7 inch. The results were all determined with the space in a vertical position. Figure 7 indicates the variation of conductance with height of a vertical air apace faced on one side with aluminum foil.
20
40 AT
60
80
lo(
OF.
FIGURE8. CONDUCTANCE us. AT EFFECTIVE EMISSIVITY
AND
applied to a n 8-foot air space. The curves are all based on a thickness of 4 inches but probably could be used without much error on thicknesses from 1inch upward. Figure 8 indicates how the conductance of air spaces varies with effective emissivity and temperature difference. The radiation calculation is based upon the cooler surface being a t 40 " F. The curves apply only to vertical air spaces that are from 2 to 8 feet high and thicker than 1 inch. Figure 9 is based on Mason's work with approximately 7/16-inch thick air spaces faced both sides with aluminum foil. The values are expressed as coefficients of thermal conductivity since there was little convection in these spaces. The points on the curve above a mean temperature of 70 " F. were determined in a vertical guarded plate tester; the points below 70" F. were found by the box method when using only the four vertical walls and the top. The rather erratic placing of the points in the low range is probably due to the method, as pointed out by Mason. These tests represent rather a wide range of temperature since the temperature on one side of his specimens was always near room temperature but the other side varied from -56" to f290" F. Figure 10 shows the variation of the coefficient of thermal conductivity of crumpled foil with mean temperature based upon Mason's work on the guarded plate tester and upon pipe tests by the author a t relatively high mean temperatures. The resistance to heat flow through crumpled aluminum foil may be expressed as a coefficient of thermal conductivity because it varies directly with the thickness and is roughly independent of position. In other words, the calculation of the rate of heat transfer through crumpled foil may be made in exactly the same manner as would be used with the common type of insulating material. The difference between two and three layers to the inch is shown to be very small as would be expected, since it has been found that about two and a half layers per inch is probably the most effective spacing of thin crumpled foil.
JULY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
a37
The values in Table I are based on actual tests by the guarded box method with a temperature difference of approximately 70" F. between the inside and outside air. If the aluminum foil in these tests were replaced with aluminum-foil coated paper, thin ('/*-inch) corrugated cardboard covered with foil, or the tin-lead alloy coated steel, the values would be essentially the same. OF HEATTRANSMITTANCE FOR TYPICAL TABLE I. COEFFICIENTS FRAME AND BRICK VENEER WALLS U ,B. t . u /Hr./Sq. F t . / O F.
Uninsulated One side of stud space faced with A1 foil One foil layer, making two air spaces one sheet waterproof paper between plaster and foil Two foil layers, making three air spaces one sheet waterproof paper between plaster and foil Three foil layers, making four air spaces one sheet waterproof paper between plaster and foil
+ +
+
CRUMPLED FOIL 0 2 LAYERS PER I N C H . 0 0 0 3 5 FOIL WILKES PIPE TEST 3 LAYERS PER INCH , 0 0 0 3 5 FOIL MASON, PLATE TEST
0.26 0.20
,
+ 0.13 0.09 0.08
'50
100
200 MEAN T
300
400
50C
OF.
FIGURE10. THERMAL CONDUCTIVITY us. MEAN TEMPERATURE
~~
Pitched Roofs I n general, very little attention has been given to the fact that the rate of heat transfer across a n air space with a t least one reflective surface is dependent upon position and direction of heat flow (Figure 4). The data in Table I1 are calculated from values given by the American Society of Heating and Ventilating Engineers (1) and the conductance of air spaces in various positions.
MEAN
T
OF.
FIGURE9. COEFFICIENT OF THERMAL CONDCCTIVITY us. MEANTEMPERATURE
The enormous insulating value of a n air space with one reflecting surface in a horizontal position with the heat flow downward has been little appreciated by the engineer or the public. I n this case, both radiation and convection have been reduced to a minimum. A single sheet of aluminum foil placed midway in a horizontal air space with the heat flowing downward gives the equivalent of approximately 3 inches of corkboard in insulation value. In addition to the building roofs and ceilings mentioned in Table 11, the same principles apply to many industrial situations such as the top of cold storage rooms, the bottoms of ovens, furnaces, etc.
TABLE 11. COEFFICIENTS FOR HEATTRANSMITTANCE FOR ROOFS AND CEILINGS Pitched Roofs and Ceilings
U,B. t. u./Hr./Sq. Ft./O F. Shingles, sheathing, and rafters (uninsulated) (1) Same with one layer A1 foil under rafters: Heat flow upward (winter) Heat flow downward (summer) Shingles, sheathing, rafters and plaster (uninsulated) (1) Same with one layer of A1 foil midway in rafter space: Heat flow upward (winter) Heat flow downward (summer) Same with two layers of AI foil in rafter space: Heat flow upward (winter) Heat flow downward (summer) Same with three layers of A1 foil in rafter space : Heat flow upward (winter) Heat flow downward (summer) Flat Roofs and Ceilings Flooring or roofing, joists and plaster (uninsulated) (1) Same with one layer of Al foil midway in air space: Heat flow upward (winter) Heat flow downward (summer) Same with two layers of A1 foil in air space: Heat flow upward (winter) Heat flow downward (summer) Same with three layers of A1 foil in air space: Heat flow upward (winter) Heat flow downward (summer)
0.56 0.24 0.19 0.32
0.16 0.12
0.12 0.09
0.10 0.07
0.30
0.155 0.074 0.122 0.052
0.100
0.040
Permanence of Aluminum Foil Insulation After i t has been determined that reflective type insulation can be made effective in retarding the flow of heat, the question arises as to its permanence. Its value as an insulator is due largely to the low emissivity of the surface, and we are all familiar with the change of the nature of the surface of many materials with time. Edwards (1.2)states: "The eye may be a good judge in appraising the reflectivity of a surface for
visible light but cannot evaluate the infrared radiation characteristics. A piece of aluminum may have high reflectivity for visible light and low reflectivity for infrared radiation, or it may have only fair reflectivity for light and be an excellent reflector of infrared radiation, depending on the presence or absence of surface films." The author exposed aluminum foil
838
INDUSTRIAL AND ENGINEERING CHEMISTRY
for 30 days in a furnace a t a temperature of 1050” F., and to the eye the surface changed to a dull gray; but the emissivity a t room temperature increased only from 0.05 to 0.075 and it still would have made a n effective insulator. The conductance value for a vertical air space faced with this gray surface would be only about 5 per cent greater than with new aluminum foil. Aluminum foil has now been in use for a sufficient number of years to establish the permanence of the surface when properly installed under suitable conditions. The following examples are taken from sources that are believed to be authentic : 1. Aluminum-foil-insulated house in Skovshoved, Denmark. This house was built and insulated in 1927. Samples of the foil were removed in October, 1935, by engineers from the Teknolgisk Institute, Copenhagen. The official report states: “We can testify that the brightness of the aluminum foil was unchanged in comparison to new foil of the same kind.” 2. Aluminum-foil-insulated provision chambers of the motorship Leverkusen, fitted out in May, 1928. In January 1934, samples of this foil were removed by representatives of Lloyd’s Register of Ship ing, Hamburg; they reported: “The general impression gainejas a result of the examination made is that the insulation examined by us is in exactly the same condition as it was five years ago when built into the vessel.” 3. Aluminum-foil-covered cardboard taken from foil-insulated dry-ice cabinet. It was exposed to the weather on a beach at Coney Island from September, 1934, to April, 1935. The emissivity of the foil removed from this cabinet was 0.04 ( l a ) . 4. Aluminum foil suspended vertically in laboratory for three years and measured with accumulated dust and fume. The emissivity was 0.05 (12).
The following examples give some of the author’s personal experience over a period of ten years with regard to the permanence of aluminum foil insulation: 1. Aluminum-foil-insulated residence, Wellesley Hills, Mass.,, built and insulated in 1933. The author personally removed samples of this insulation from the underside of the roof in June, 1938. The samples ap eared in perfect condition, and the average emissivity of the Pour samples removed was 0.054 as compared with 0.045 for new foil. 2. Aluminum foil after two-year exposure to salt spray and moisture on underside of roof of log boat house in Newington, N. H. The foil was spotted with salt that had been left on the foil by evaporation of the salt spray. The emissivity was found to be 0.10. 3. Aluminum foil exposed in a vertical position since 1929 to the dust and fumes in the Heat Measurements Laboratory, M. I. T. Samples of this foil have been removed from time to time and the emissivity has been determined. Over a period of ten years no appreciable change in emissivity has been found. 4. Aluminum foil insulation placed over ceiling in attic of residence in Newton Centre, Mass. After three years no appreciable change could be noted. 5 . Aluminum foil insulation in ceiling of a cellar with no covering of any kind. After three-year exposure this foil appeared in perfect condition except over the laundry tubs where
VOL. 31, NO. 7
soap had apparently come in contact with the foil and attacked the surface in spots. This cellar was very damp every summer. 6 . Hundreds of samples of aluminum foil have been stored in the laboratory for various periods of time up to ten years with no visible signs of deterioration of the surface. Alkalies attack aluminum readily, and foil should always be protected from direct contact with wet plaster. Aluminum foil may be coated with a thin transparent lacquer where prolonged exposure to moisture is expected. If this coating is properly applied, the emissivity will not be changed sufficiently to affect the insulating value more than a few per cent. Thick coatings of clear lacquer will increase the emissivity to a very high value, thus destroying the insulating value, but the coated aluminum will still appear bright to the eye ( l a ) . An ordinary glass mirror, silvered on the back, is a n excellent example of a good reflector of light but a very poor reflector of infrared radiation. An interesting point in connection with aluminum foil insulation is its extremely light weight. Because of this fact the heat capacity is very low, a n important factor in connection with intermittently heated ovens, furnaces, refrigerator cars, dry-ice containers, etc. The heating or cooling time is greatly reduced if the walls have low heat capacity with consequent saving in fuel and labor. Crumpled aluminum foil insulation, as usually installed, weighs approximately 3 ounces per cubic foot; most other insulating materials weigh between 3 and 10 pounds per cubic foot. A man can carry in his arms sufficient aluminum foil to insulate a n ordinary residence. This extreme lightness in weight is also of great importance on board ship, railroad cars, trucks, and aeroplanes.
Literature Cited
1
(1) Am. Soo. Heating and Ventilating Engrs., Guide, Chap. V, 1939. (1A) Babbitt, J. D., Heating, P i p i n g , Air Conditioning, 9,577 (1937). (2) Gregg, J. L., Refrig. Eng., 23, 279 (1932). (3) Griffiths, E., and Davis, A. H., Food Investigation Board, Special Rept. 9 (1922). (4) Mason, R. B., IND.ENG.CHEM.,25, 245 (1933). (5) . , Natl. Bur. Standards. A l u m i n u m Foil Insulation, Letter Circ. 535 (1938). (6) Nichols, J. T., Mech. Eng., 57,621 (1935). (7) Peolet, J. C. E., “Trait6 de la chaleur”, 4th ed., Vol. 1, Paris, Libraire de Victor Masson, 1878. (8) Queer, E. R., Trans. Am. SOC.Heating Ventilating Engrs., 38, 77 (1932). (9) Rowley, F. B., and Algren, A. B., Ibid., 35, 165 (1929). (IO) Schad, L. W., Ibid., 37,285 (1931). (11) Schmidt, E., Gesundh.-Ing., 50, 121 (1927). (12) Taylor, C. S., and Edwards, J. D., Heating, Piping, A i r Conditioning, 11, 59 (1939). (13) Van Dusen, M. S., Trans. Am. SOC.Heating Ventilating Engrs., 26, 385 (1920). (14) Wilkes, G. B., and Peterson, C . M. F., Heating, Piping, A i r Conditioning, 9, 505 (1937).
