HomeMy WebLinkAboutThermal Properties of Metal Stud Walls 1984| Alaska Energy Authority CON ; LIBRARY COPY
023
THERMAL PROPERTIES
of
METAL STUD WALLS
Egae ee) | | State of Alaska Department of Transportation and Public Facilities ' Report No. AK-RD-85-05
HHH ha il lll Hut FUTEVVUULO ER UAT THU Ea Ml
Rh
THERMAL PROPERTIES OF METAL STUD WALLS
by
John P. Zarling
W. Alan Braley
Institute of Water Resources/Engineering Experiment Station
University of Alaska
Fairbanks, Alaska 99701
and
James S. Strandberg
Scott V. Bell
J. S. Strandberg Consulting Engineers, Inc.
P. 0. Box 319
Fairbanks, Alaska 99707
July 1984
Prepared for:
STATE OF ALASKA
DEPARTMENT OF TRANSPORTATION AND PUBLIC FACILITIES
DIVISION OF PLANNING
RESEARCH SECTION
2301 Peger Road
Fairbanks, Alaska 99701-6394
The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein.
The contents do not necessarily reflect the official views or policies of
the Alaska Department of Transportation and Public Facilities. This report
does not constitute a standard, specification or regulation.
TABLE OF CONTENTS
DESCRIPTION seein ean ee see ee eo F abe eed dle woe bbe» CALIDPATTON Jools oe ee ee ee eth be tue b elds weber ebb
TESTING PROCEDURE
Test Walls..
Guarded
Test Wall Computations
Hot Box Testing Procedure............eeeeeee
ASHRAE Calculation Method.....
Finite Element Method..............cccccccccccccccccccecs
RESULTS... 2500.5
REFERENCES.......
APPENDIX.........
ASHRAE Wall R-Value Calculation.............c cece cece eee Project Photographs......... cece cece cece cccceccccccceeace
Figure
10
11
12
13
Typical test wall
LIST OF FIGURES
Sections: 6 inch metal stud panels...........
Sections: 2 x 6 inch wood stud panels.......
Exploded view of guarded hot box.............
Schematic of thermal energy transfer in
guarded hot box
ASHRAE "Zone Method". ........cecceeceeee cence
Finite Element Mesh.............. taete ress aes
Isothermal Plot:
Isothermal Plot:
Isothermal Plot:
Isothermal Plot:
Wood Stud Wall With Foam...
Wood Stud Wall Without Foam
Wall Panel 2 Under Construction...........06.
-ii-
10
22
23
27
28
29
30
36
37
Table
10
11
L
Test wall components
IST OF TABLES
Thermal properties of test wall components..........
Test result summary:
Test run #1.........
Test result summary:
Test run #2.........
Test result summary:
Test result summary:
Test result summary:
metal stud wall with foam
metal stud wall without foam..
wood stud wall without foam...
Summary of wall R-values using standard air film resistances...
-iii-
15
16
17
18
19
20 ’”
24
25
35
SUMMARY
The use of exterior-applied foam insulation boards to increase the
thermal resistance, or R-value, of walls is a common practice with both
metal and wood stud walls. This report presents the results of tests
conducted to compare the experimentally measured R-values of metal and
wood stud walls, with and without exterior-applied foam insulation
boards, using two calculation methods.
Both the Finite Element Method (FEM) and the American Society
Heating Refrigerating Air Conditioning Engineers (ASHRAE) method of
calculating the thermal resistance of metal and wood stud walls were
used to predict the insulating ability of four different walls. Four
wall panels were also tested in a guarded hot box to experimentally
measure the thermal resistance of the different wall configurations.
Based on the test results of the wall panels, the average R-values
of the walls are 22.2 hr-ft?-°F/Btu for a 6-inch metal stud wall with 2
inches of exterior foam insulation, 28.2 hr-ft2-°F/Btu for 2 x 6-inch
wood stud wall with 2 inches of exterior foam insulation, 12.8
hr-ft2°F/Btu for a metal stud wall without foam insulation, and 19.1
hr-ft?-°F/Btu for a wood stud wall without foam insulation.
Results of the ASHRAE calculation exceed the tested wall R-values
by 9% to 19%, and the FEM calculation results exceed the tested R-values
by 3% to 21%.
The ASHRAE and FEM R-value calculations agree within 0.6% to 6.6%.
INTRODUCTION
The use of metal studs in commercial and institutional building
wall construction is an accepted practice in Alaska. Metal stud walls
are preferred over wood stud walls because of their uniformity and
structural superiority. In interior Alaska, the installed costs of
medium height (8-foot), wood and metal stud walls are the same. Wood
stud walls are more cost effective for shorter walls and metal stud
walls are more cost effective for taller walls.
The high thermal conductivity of steel reduces the insulating
ability of a metal stud wall below that of a comparable wood stud wall.
-1-
To offset the high thermal conductivity of steel, rigid foam insulation
boards of polyisocyanurate, polystyrene, or polyurethane are often added
to the building exterior to provide a thermal break for reducing the
thermal energy loss through the studs.
The use of exterior-applied foam insulation board has recently
become a standard construction practice in the construction of wood stud
walls as well. Therefore, both wood and metal stud walls were tested
for this report. Both types of walls were tested with and without the
exterior foam insulation boards, and the results were compared with
calculated wall thermal resistances.
SCOPE
The scope of this project was to:
1. test the insulating qualities of four full-scale test wall
panels in a controlled testing environment;
2. use the ASHRAE and finite element methods to calculate the
thermal resistances of test walls; and
3. compare the calculated thermal resistances of test walls to
their measured thermal resistances.
TEST EQUIPMENT
TEST WALLS
Four test wall panels were constructed to reflect present
construction methods in interior Alaska (Figure 1, 2, and 3). Figure 1
shows a two-view presentation of a typical test wall. Figures 2 and 3
illustrate horizontal sections through test wall panels 1, 2, 3 and 4
showing the components of each wall panel.
The test wall designs are based on the metal stud wall system with
exterior-applied foam insulation board used at the Pearl Creek
-2-
Elementary School constructed near Fairbanks during the winter of
1982-83. Construction of the test walls varies from the actual
construction at the school in two respects:
1. the use of nails in the school walls and screws in the test
walls; and
2. the use of a single layer of 3/8-inch AC plywood in the test
walls to simulate the 5/8-inch gypsum board used in the school
walls.
The second change was dictated by the design of the guarded hot
box. It required the interior side of the test wall to have a
2-inch-wide lip of 3/8-inch-thick structural material. Besides being
too thick, a gypsum board lip would have been destroyed while being
moved and handled. The difference in thermal resistance between the
plywood and gypsum board is minimal; the plywood is rated at 0.09
hr-#t2-°F/Btu less than the gypsum board, ASHRAE (1981).
Each panel is 71 inches high by 91 inches wide, and the panels vary
in thickness from 7 inches to 10.5 inches. Table 1 lists the four walls
and their components. Table 2 lists the four walls, their components
and the thermal conductivity of each component as used in the ASHRAE and
finite element methods of calculating the wall R-values.
THE GUARDED HOT BOX
The guarded hot box used for this test was constructed by the
University of Alaska-Fairbanks (UAF), and the Alaska Department of
Transportation and Public Facilities (DOTPF) Research Section (Figures 4
and 5). It was constructed according to ASTM (1980) guideline C-236-71.
The guarded hot box was used to measured the heat flow through the
test wall panels. It consists of three boxes (Figure 4).
1. The cold box held the test wall panel and simulated the outdoor air
temperature.
+ FoR ENLARGED WALL SECTIONS <26 FIGLUees 2ES
ARYAN LLL LL ELL PEEL ERS COL ENR]
Ce ——
2/e' Ac PLYWooD
GMIL POYETHYLENE
UAPoR BARRIER
Gx Ve" 20 GAUGE METAL Tub
G' FIBER CLASS
INSULATION
Yt! cex PLYKooD
t' STYROFOAM,
TYPE <M
V2! epx PLYKCOoD
16% FET PAPER
Be! T1-11
OUTSIPE
A, WALLS 1- DETAL Sib: ITH FOAY INSLILAT od
4'= Io!
a} Ac PLYNOOD INSIDE.
GMIL POLYETHYLENE VAPOR BARRIER
G1 Ve" 2 GAUGE METAL sTup
G' FiBERGLASS INSULATION
122" cok Pornicop
1S FELT PAPER
Be Ti-i1
OUTSIDE
@. WALL #S-VEAL SUS UTHOUT FOAM INELILATION
Ya\ to!
FiGuze 2 :SECIONS-G' METAL sip ANELS
eo
ae Ac PoAcop
GMIL POCTETHYLENE
VAPOR BABRiga.
2x @! Woop sup
Sa Fipezalace MSULATIOM
Ve! cpr ercop
2! STYROFOAM,
Tre <M :
V2! cox FYKCoD
153 Fecr Apes
3/]' T1-11
OUTS pe
A. WALL - HOOP STURS WITH FOAM IRSULATON
VA's o-f!
. 9/8" Ac PLYkleoPp IMSIDE |
GMIL ne pelea Ce =
312' tpepases
INSULATION
V2" Cox Forkleo
1S FELT Aree Se! T1-11 —_
OUTSIDE.
B. WALL 4 - CCD STUDS WITHOUT FoaM HeULATICN
4'= t'-o!
FGUze 2° SECTONS-244 |lcabp SUP FANELS
TABLE 1. Test wall components.
Wall
Wall
Wall
#1: metal studs with foam insulation
3/8 inch AC plywood
6 mil clear polyethylene vapor barrier 6 inch x 1-1/2 inch C-channel, webbed, 20 gauge galvanized steel studs 6 inch Johns-Manville unfaced fiberglass insulation with an installed density of 0.48 pounds per cubic foot 1/2 inch CDX plywood
2 inch Styrofoam Type SM extruded polystyrene smooth skin insulation with a density of 2.2 pounds per cubic foot 1/2 inch CDX plywood
15# felt paper, one layer, unmopped 5/8 inch T1-11 siding :
#2: wood studs with foam insulation
3/8 inch AC plywood
6 mil clear polyethylene vapor barrier 2 inch x 6 inch wood studs, common grade (1-1/2 inch x 5-1/2 inch actual size) 5-1/2 inch Johns-Manville unfaced fiberglass insulation with an installed density of 0.52 pounds per cubic foot 1/2 inch CDX plywood 2 inch Styrofoam Type SM extruded polystyrene smooth skin insulation with a density of 2.2 pounds per cubic foot 1/2 inch CDX plywood
15# felt paper, one layer, unmopped 5/8 inch Tl-11 siding .
#3: metal studs without foam insulation
Wall
3/8 inch AC plywood
6 mil clear polyethylene vapor barrier 6 inch x 1-1/2 inch C-channel, webbed, 20 gauge galvanized steel studs 6 inch Johns-Manville unfaced fiberglass insulation with an installed density of 0.48 pounds per cubic foot 1/2 inch CDX plywood 15# felt paper, one layer, unmopped 5/8 inch T1-11 siding
#4: wood studs without foam insulation
3/8 inch AC plywood
6 mil clear polyethylene vapor barrier
2 inch x 6 inch wood studs, common grade (1-1/2 inch x 5-1/2 inch actual size) 5-1/2 inch Johns-Manville unfaced fiberglass insulation with an installed density of 0.52 pounds per cubic foot 1/2 inch CDX plywood
15# felt paper, one layer, unmopped 5/8 inch T1-11 siding —
TABLE 2. Thermal conductivity of test wall components.
2
K-values (Btu-in/hr-ft*-°F)
-60°F Test +20°F Test FEM ASHRAE FEM ASHRAE
Wall #1: metal studs with foam
3/8 inch AC plywood 0.80 0.80 0.80 0.80 6 mil vapor barrier ---- ---- ---- ---- 6 inch x 1-1/2 inch steel studs 314.00 314.00 314.00 314.00 6 inch fiberglass insulation 0.29 0.26 0.31 0.30 1/2 inch CDX plywood 0.80 0.80 0.80 0.80 2 inch Styrofoam Type SM 0.17 0.17 0.19 0.19 1/2 inch CDX plywood 0.80 0.80 0.80 0.80 15# felt paper 1.04 1.04 1.04 1.04 5/8 inch T1-11 siding 0.80 0.80 0.80 0.80
Wall #2: wood studs with foam
3/8 inch AC plywood 0.80 0.80 0.80 0.80 6 mil vapor barrier ---- ---- ---- ---- 2 inch x 6 inch wood studs 0.80 0.80 0.80 0.80 5-1/2 inch fiberglass insulation 0.29 0.26 0.31 0.30 1/2 inch CDX plywood 0.80 0.80 0.80 0.80 2 inch Styrofoam Type SM 0.16 0.17 0.18 0.19 1/2 inch CDX plywood 0.80 0.80 0.80 0.80 15# felt paper 1.04 1.04 1.04 1.04 5/8 inch T1-11 siding 0.80 0.80 0.80 0.80
Wall #3: metal studs without foam
3/8 inch AC plywood 0.80 0.80 0.80 0.80 6 mil vapor barrier ---- ---- ---- ---- 6 inch x 1-1/2 inch steel studs 314.00 314.00 314.00 314.00 6 inch fiberglass insulation 0.27 0.26 0.30 0.30 1/2 inch CDX plywood 0.80 0.80 0.80 0.80 15# felt paper 1.04 1.04 1.04 1.04 5/8 inch T1-11 siding 0.80 0.80 0.80 0.80
Wall #4: wood studs without foam
3/8 inch AC plywood 0.80 0.80 0.80 0.80 6 mil vapor barrier ---- ---- ---- ---- 2 inch x 6 inch wood studs 0.80 0.80 0.80 0.80 5-1/2 inch fiberglass insulation 0.27 0.26 0.30 0.30 1/2 inch CDX plywood 0.80 0.80 0.80 0.80 15# felt paper 1.04 1.04 1.04 1.04 5/8 inch T1-11 siding 0.80 0.80 0.80 0.80
FIGURE +: GUARDED HoT Box - EXPLOpED Ole
HO €cALE
Poop:
PWALe :
3 LISS ITS SF
r " U See Ze SSZ SSS SIS ZS S2IZSZ IZ IV XN XY XS Zs HEAT REMOVED ROM colli Box Jo SIMULATE OUTSIDE Al@ TEMPERATURE (To/ad.
MEAT AppED To METER 0x TO MANTAIN 15° SIMULATED INSIFPE AR TEMPERATE (Tim.
HEAT ADDED To GUARD Box TO MAINTAIN GUARD Box WITHIN 1°9F OF METER Bok TEMPERATURE.
HEAT FLOWING THROUGH TEST WALL. OuALL = Pverse - Pula
HEAT FLOWING FROM METER fou To GlaARD Box.
2. The guard box enclosed the warm side of the test wall and it's
temperature was set to match the meter box temperature.
3. The meter box was nested inside the guard box and held tight
against the warm side of the test panel. It's temperature
simulated the indoor air temperature.
The guard and meter boxes are maintained as near as possible at the
same temperature to eliminate thermal energy transfer between them.
When there is no temperature difference between the guard and meter
boxes, then all thermal energy leaving the meter box leaves through the
test wall panel, and the thermal energy supplied to the meter box equals
the thermal energy transferred through the test wall panel.
The meter box is heated by lights mounted in the box and controlled
by a thermostat and an on-off controller. Fans mounted in the meter box
reduce air stratification and provide some thermal energy input. The
guard box also contains fans and lights for air circulation and heating.
The cold box is cooled by an externally located, two-stage
compressor refrigeration unit and a fan-coil unit mounted in the box.
The refrigerant, ethane, is cooled in an external condenser, expanded,
and then circulated through the evaporator coil in the cold box.
The guarded hot box is instrumented for both temperature control
and data acquisition. On-off controllers regulate the thermal energy
input to the meter and guard boxes. A Fluke 2240C datalogger measures
and converts the thermocouple voltages to Fahrenheit temperature,
measures the energy supplied to the meter box, prints the data on paper,
and concurrently records the data on a Textronix 4293 Magnetic tape
recorder. After recording, the test data were transferred to a
microcomputer for reduction and organization.
The guarded hot box was not calibrated before being used in this
test. However, the output of each thermocouples was compared against an
ice bath and all agreed within +0.5°F.
-11-
TESTING PROCEDURE
TEST WALLS
Guarded Hot Box Testing Procedure
Each wall was tested in the guarded hot box at -60°F, -40°F, -20°F,
O°F, and +20°F simulated outdoor air temperatures. The interior (warm
side) air temperature was maintained at +75°F for all tests. Wall #1
was tested twice to investigate any hysteresis effects -- once in a
sequence of progressively cooler test temperatures (+20°F to -60°F) and
once in a sequence of progressively warmer temperatures (-60°F to
+20°F). Each test lasted eight hours and data were collected every 15
minutes, resulting in a total of 33 values for each variable. The data
were reviewed to assure steady-state conditions existed during the
duration of the test, and average values were used in the calculations.
Test Wall Computations
The basic formula for conductive thermal energy transfer is
Q= A(T,-To)/R
where
Q = rate of thermal energy transfer through a conductive element
(Btu/hr),
surface (frontal) area of the conductive element (Ft),
1° temperature on the one side of the conductive element (°F),
2 temperature on the other side of the conductive element (°F),
thermal resistance of the conductive element (hr-ft2-°F/Btu). " PAA > Solving the equation for R yields
R= A(T,-T)/Q
-12-
where R is the thermal resistance of the conductive element, which for
our application is the test wall.
The overall R-value of each test wall was computed using the air
temperatures on either side of the test wall panel, the energy supplied
to the meter box during the test and the elapsed time of the test.
Figure 5 shows that the thermal energy supplied to the meter box
(Qmeter) equals the thermal energy lost or gained between the meter box
and the cold box (Qwall) plus the thermal energy lost from the meter box
to the guard box (Qm/g). To calculate the energy transfer across the
test wall, the energy transfer between the meter and guard boxes was
first calculated using
Qm/g = (TD1)(ETIME)(73.71 ft?) (0.2931)/5.31
where
Qm/g = heat transfer between the meter box and the guard box
(watt-hours) ,
TD1 = temperature difference between the meter and guard boxes
(°F),
ETIME = elapsed time test (hours),
73.71 = meter box/guard box interface area (Ft?),
0.2931 = watts/(Btu/hr) conversion factor,
5.31 = combined R-value of the 1 inch of extruded polystyrene
foam and 1/4 inch plywood which comprise the meter box
wall.
Once the thermal energy transfer between the meter and guard boxes was
known, the overall R-value of the test wall was determined using
R = (MA1-CA1) (0.2931) (30.75 ft?) (ETIME)/(Qmeter-Qn/g)
where
R = overall R-value of the wall (hr-ft?-°F/Btu),
MA1 = average meter box air temperature (°F),
CA1 = average cold box air temperature (°F),
0.2931 = watt-hour/Btu conversion factor,
=13-
30.75 = meter box/test wall interface area (#t2),
ETIME = elapsed test time (hours),
Qmeter = thermal energy supplied to the meter box during the test
(watt-hours),
Qm/g = thermal energy transferred between meter box and guard
box
during the test (watt-hours).
The calculations were repeated for each test wall at each temperature,
and the R-values for each test wall were averaged (see Tables 5-9).
Comparing the tested wall R-values with calculated wall R-values
required that all air film thermal resistances be equal. The velocities
of the air circulating in the meter box and cold box were unknown, so
the test air film R-values had to be determined from the measured
temperature differences (surface temperature minus ambient air
temperature) and from the heat flux across the test wall. The measured
air film R-values were then subtracted from the calculated wall R-values
and replaced with standard air film thermal resistances of 0.17
hr-ft2-°F/Btu and 0.68 hr-#t2-°F/Btu outside and inside respectively.
Table 10 compares the resulting wall R-values with standard air film
resistances with wall R-values calculated using the ASHRAE and finite
element (FEM) methods.
ASHRAE CALCULATION METHOD
ASHRAE (1977) recommends the thermal resistance of a wall sandwich
containing a metal stud extending either wholly or partially through the
insulation should be determined by the guarded or calibrated hot box
method. If testing is not possible, ASHRAE recommends a "Zone Method"
as a means of approximating the wall R-value. The Zone Method involves
breaking the wall into two zones. Zone A contains the steel stud and
Zone B contains the remaining portion of the insulated wall section.
The thermal transmittance of each zone is calculated, and the results
are combined to arrive at the overall average thermal resistance for the
wall. The method is based on calculating the equivalent resistance of
the wall.
-14-
TABLE 3. Metal stud wall with foam (#1) January 9 to January 13, 1984.
Jan 09 Jan 10 Jan 11 Jan 12 Jan 13 Test temperature -60°F -40°F -20°F O°F +20°F AVERAGES
Elapsed Time 8.00 7.98 8.02 8.03 8.00 8.01 (hours )
Supplied Power 461 388 314 244 189 (watt-hour)
MAl: meter air (°F) 74.08 74.15 74.19 74.08 73.97 74.09
CAl: cold air (°F) -59.07 -40.07 -20.40 0.53 20.56
GAl: guard air (°F) 74.14 74.48 74.28 74.47 73.94 74.26
TD1: mtr-grd (°F) 0.3094 0.0691 0.1564 0.0655 0.2867
MA1-CA1 (°F) 133.15 114.22 94.59 73.55 53.41
Mtr/guard area (#t2) 73.71 73.71 73.71 73.71 73.71
Watts/Btu conv. 0.2931 0.2931 0.2931 0.2931 0.2931 factor
Meter box R-value 5.31 5.31 5.31 5.31 5.31 (hr-ft°-°F/Btu)
Wall/mtr. area (Ft?) 30.75 30.75 30.75 30.75 30.75
Qm/g based on TD1 1.28 2.24 5.10 2.14 9.33 (watt-hours )
Overall R-value 20.88 21.30 22.13 22.01 21.43 21.55 (hr-ft"-°F/Btu)
-15-
TABLE 4, Metal stud wal] with foam (#1) January 16 to January 20, 1984.
Jan 20 Jan 19 Jan 18 Jan 17 Jan 16 Test temperature -60°F -40°F -20°F 0°F +20°F AVERAGES
Elapsed Time 8.08 8.05 8.02 8.00 8.02 8.03 (hours )
Supplied Power 456 401 341 271 191 (watt-hour)
MAl: meter air (°F) 74.10 74.06 73.99 74.08 73.85 74.02
CAl: cold air (°F) -59.30 -39.14 -20.31 -1.29 20.96
GAl: guard air (°F) 73.52 73.64 73.07 73.57 73.76 73.51
TD1: mtr-grd (°F) No Value 0.4506 0.7918 0.5636 0.3542
MA1-CA1 (°F) 133.40 113.20 94.30 75~37 52.89
Mtr/guard area (#t2) 73.71 73.71 73.71 73.71 73.71
Watts/Btu conv. 0.2931 0.2931 0.2931 0.2931 0.2931 factor
Meter box R-value 5.31 5.31 5.31 5.31 5.31 (hr-ft°-°F/Btu)
Wall/mtr. area (#t2) 30.75 30.75 30.75 30.75 | 30.75
Qm/g based on TD1 0.00 14.76 25.84 18.34 11.56 (watt-hours )
Overall B-value 21.30 21.26 21.63 21.51 21.31 21.40 (hr-ft°-°F/Btu)
-16-
TABLE 5. Wood stud wall with foam (#2) January 30 to February 3, 1984.
Jan 30 Jan 31 Feb 01 Feb 02 Feb 03 Test temperature -60°F -40°F -20°F O°F +20°F AVERAGES
Elapsed Time 8.03 8.02 8.05 8.00 7.98 8.02 (hours )
Supplied Power 327 281 227+ 204 139 (watt-hour)
MA1: meter air (°F) 73.98 73.89 73.88 73.74 74,47 73.99
CAl: cold air (°F) -61.54 -42.28 -19.66 1.13 19.26
GAl: guard air (°F) 74.00 73.88 74.22 73.66 75.07 74.17
TD1: mtr-grd (°F) 0.1212 0.1509 -.0445 0.2800 -.1510
MA1-CAl (°F) 138;52 116.17 93.54 72.61 55.21
Mtr/guard area (#t2) 73.71 73.71 73.71 73.71 73.71
Watts/Btu conv. 0.2931 0.2931 0.2931 0.2931 0.2931 factor
Meter box R-value 5.31 5.31 5.31 5.31 5.31 (hr-ft°-°F/Btu)
Wall/mtr. area (#t2) 30.75 30.75 30.75 30.75 30.75
Qm/g based on TD1 3.96 4.92 -1.46 9.11 -4.91 (watt-hours)
Overall B-value 30.36 30.42 29.71 26.86 27.59 28.99 (hr-ft°-°F/Btu)
-17-
TABLE 6. Metal stud wall with no foam (#3) February 6 to February 10, 1984.
Feb 06 Feb 07 Feb 08 Feb 09 Feb 10 Test temperature -60°F -40°F -20°F O°F +20°F AVERAGES
Elapsed Time 8.10 8.00 7.98 8.08 7.88 8.01 (hours )
Supplied Power 771 656 540 441 326 (watt-hour)
MAl: meter air (°F) 73.73 73.88 74.13 74.17 74.16 74.01
CAl: cold air (°F) -62.00 -41.43 -21.75 -1.64 19.05
GAl: guard air (°F) 72.79 73.26 73.74 73.78 73.77 73.47
TD1: mtr-grd (°F) 0.1361 0.3173 0.2018 0.2573 0.3861
MA1-CA1 (°F) 135.73 115.31 95.88 75.81 55.11
Mtr/guard area (Ft?) 73.71 73.71 73.71 73.71 73.71
Watts/Btu conv. 0.2931 0.2931 0.2931 0.2931 0.2931 factor
Meter box R-value 5.31 5.31 5.31 5.31 5.31 (hr-ft°-°F/Btu)
Wall/mtr. area (#t2) 30.75 30.75 30.75 30.75 30.75
Qm/g based on TD1 4.49 10.33 6.55 8.46 12.38
(watt-hours )
Overall R-value 12.93 12.88 12.93 12.76 12.48 12.79 (hr-ft°-°F/Btu)
-18-
TABLE 7. Wood stud wall with no foam (#4) February 13 to February 17, 1984.
Feb 13 Feb 14 Feb 15 Feb 16 Feb 17 Test temperature -60°F -40°F -20°F O°F +20°F AVERAGES
Elapsed Time 8.10 8.05 7.88 7.93 7.92 7.98 (hours )
Supplied Power 495 428 340 282 216 (watt-hour)
MAl: meter air (°F) 74.23 74.17 74.18 74.08 73.98 74.13
CAl: cold air (°F) -61.53 -41.58 -20.29 -0.70 17.80
GAl: guard air (°F) 74.17 74.07 74.53 74.22 74.39 74.28
TD1: mtr-grd (°F) -.0776 0.0106 -.1790 0.0145 -.0215
MA1-CA1 (°F) 135.76 115.75 94.47 74.78 56.18
Mtr/guard area (#t?) 73.71 73.71 73.71 73.71 73.71
Watts/Btu conv. 0.2931 0.2931 0.2931 0.2931 0.2931
factor
Meter box R-value 5.31 5.31 5.31 5.31 5.31 (hr-ft°-°F/Btu)
Wall/mtr. area (#t2) 30.75 30.75 30.75 30.75 30.75
Qm/g based on TD1 -2.57 0.35 -5.84 0.48 -.70 (watt-hours )
Overall B-value 19.92 19.64 19.41 18.98 18.51 19.29 (hr-ft°-°F/Btu)
-19-
TABLE 8. Summary of wall R-values using standard air film resistances*.
2
Test Wall R-values (hr-ft"-°F)/Btu Temp. ; Wall Number (°F) ASHRAE FEM TESTED
1: Metal stud with -60 28.32 28.91 22.05 foam +20 24.95 24.51 22.25
AVERAGE 26.34 26.71 22.15
2: Wood stud with -60 34.69 33.41 29.89 foam +20 30.66 27.90 26.55
AVERAGE 32.68 30.66 28.22
3: Metal stud without -60 15.20 15.31 13.10 foam +20 13.85 | 13.20 12.45
AVERAGE 14.34 14.26 12.78
4: Wood stud without -60 21.89 21.26 19.97 foam +20 19.64 17.92 18.18
AVERAGE 20.77 19.59 19.08
* Standard inside air film thermal resistance = 0.68 hr-Ft?5°F/Btu and standard outside air film thermal resistance = 0.17 hr-ft‘-°F/Btu. The tested R-values have been corrected for standard indoor and outdoor air films.
-20-
The ASHRAE (1981) "Zone Method" has been applied to the two steel-
stud wall panels tested. The width, W, of Zone A (Figure 6) is defined
as
W m+ 2d
where 3 " width of metal path terminal (m = 1.5 inches for steel stud),
d = distance from wall cold surface to metal surface but not less
than 0.5 inches (d = 3.625 inches for Wall #1 and d = 1.125
inches for Wall #3).
The above values of m and d yield a Zone A width, W, of 8.75 inches for
Wall #1 and 3.75 inches for Wall #3. The basic zone area for both walls
is 2 feet (studs 24 inches o.c. by 12 inches). Zone A and Zone B areas are 0.729 ft® an 1.271 ft® for Wall #1 and 0.313 ft2 and 1.687 ft2
Wall #3. Calculation details for the average thermal resistance are
given in Tables 9 and 10 for the two metal stud wall sandwiches tested
in this study.
The building material thermal conductivities used for the ASHRAE
and finite element calculations are listed in Table 2.
for
FINITE ELEMENT METHOD (FEM)
The finite element technique was used to determine the steady-state
heat transfer rate and temperature profiles at the stud and insulation
for each wall sandwich tested in this study. The finite element
computer program used was the "Dow Model" written by F.S. Wang (1979)
which has been implemented on the University of Alaska's School of
Engineering VAX computer. This model allows the use of both triangular
and quadrangular elements and was originally written for performing
transient analyses. However, by setting the capacitance of all the
elements to zero, the model solves the steady-state heat transfer
problem. A typical mesh for a steel-stud wall sandwich is shown in
Figure 7. Numerical values of the thermal conductivity of the
-21-
=2.6 SNE me Se pees A, = ZON
} = -—-44—-—— -22-
D, N VIN IN UP PAV AY TAY i > V RV RV RYAN Aa oe ae oe Padded Pend otloehe de sdabsebe de cpeat on
IN TN TT A TT
Pad Se a eee
“ — a A
A <!} Bs LN
Raa ‘i De ihr et WO ATRE WOE INCE CREE ICL TRE CCR AEE WL NCCE NIE tS ger WO en ea et oer BAN A NIC INI INIA i \ \ | ] at SA EY AN
PH) (3 Hea YY 1 a PAIN \\ ALS Lowe 5) == nm Ws AP I PASSE SBS SS SS ESS SS SES ESSE -23- i —~ MENT MESH UceD IN ANACTSIS CF IAL Ficuze 7: SINE ELE!
TABLE 9. Wall 3 Zone A Calculation
R= 9659 “15-2 “BE
Section Area_x Conductance CxA R/A
Air (outside, 15 MPH) -313 x 6.0 1.88 ~53 5/8 in plywood -313 x 1.29 -40 2.48 15# felt paper -313 x 16.70 5.23 .19 1/2 in plywood +313 x 1.60 -50 2.00 Steel-stud (20 GA) -125 x 7930 991. Fiberglass -188 x .0433 1.42 0.0 Steel-stud (20 GA) -0033 x 52.3 -17 Fiberglass -310 x .0433 -0734 5.45 Steel-stud (20GA) -125 x 7930 991. Fiberglass (20GA) -188 x .0433 1.43 0.0 3/8 in plywood -313 x 2.13 .67 1.50 Air (inside, still) -313 x 1.46 -46 2.19
Total £R/A = 14.34
Wall 3 Zone B Calculation
Section Area_x Conductance CxA R/A
Air (outside, 15 MPH) 1.687 x 6.0 10.12 .10 5/8 in plywood 1.687 x 1.29 2.18 -46 15# felt paper 1.687 x 16.70 28.17 .04 1/2 in plywood 1.687 x 1.60 2.70 .37 Fiberglass 1.687 x .0433 .073 13.69 3/8 in plywood 1.687 x 2.13 3.59 28 Air (inside, still) 1.687 x 1.46 2.46 41
Total =R/A = 15.35
= (.0697)(.313) + (.0651) (1.687) _ Btu Usve ; -0659 Tiel
hr-ft"-F° 1 hr-ft?-F°
-24-
TABLE 10. Wall 1 Zone A Calculation
Section Area_x Conductance CxA R/A
Air (outside, 15 MPH) -729 x 6.0 4.37 -23 5/8 in plywood -729 x 1.29 94 1.06 15# felt paper -729 x 16.70 12.17 08 1/2 in plywood -729 x 1.60 1317 86 2 in polystyrene foam ---extruded -729 x .08 062 16.14 1/2 in plywood -729 x 1.60 1.17 86 Steel-stud (20 GA) -125 x 7930 991 Fiberglass -604 x 7.83 4.73 .00 Steel-stud (20GA) -0032 x 52.3 .173 Fiberglass -726 x .0433 -032 4.89 Steel-stud (20GA) -125 x 7930 991 Fiberglass -604 x 7.83 4.73 00 3/8 in plywood -729 x 2.13 1.55 -64 Air (inside, still) -729 x 1.46 1.06 94
Total =R/A = 25.70
Wall 1 Zone B Calculation
Section Area x Conductance CxA R/A
Air (outside, 15 MPH) 1.271 x 6.0 7.63 13 5/8 in plywood 1.271 x 1.29 1.64 -61 15# felt paper 1.271 x 16.70 21.23 .05 1/2 in plywood 1.271 x 1.60 2.03 .49 2 in polystyrene foam
--- extruded 1.271 x .085 . 108 9.26 1/2 in plywood 1.271 x 1.60 2.03 -49 Fiberglass 1.271 x .0433 -055 18.16 3/8 in plywood 1.271 x 2.13 2.71 .37 Air (inside, still) 1.271 x 1.46 1.86 -54
Total =R/A = 30.09
-038 -729) + (. 1.271 Usve (.0389)(.729) + (.0332) (1.271) = 9353 Btu
2 hr-ft?-F°
1 1 2 R = >—— = meee = 28.3 «hr-ft°-F°/Btu Usve -0353
-25-
components making up the wall sections tested and analyzed are listed in
Table 2.
The warm side air temperature in all computer runs was set at
+75°F, and the cold side air temperature was set at either -60°F or
+20°F. Warm side air film thermal resistance was assumed to be 0.68
hr-ft?°F/Btu, and the cold side air film resistance was assumed to be
0:17 hr-ft2~°F/Btu. Isothermal contours plotted for each wall sandwich
are shown in Figures 8 to 1l. Dividing the finite element calculated
total heat transfer rate through the wall by the warm side to cold side
air temperature difference gives the average R-values. ASHRAE and FEM
calculated R-values are given in in Table 8.
RESULTS
WALL PANEL TEST RESULTS
The test results listed in Tables 3-7 are the wall R-values with
the measured air film resistances. Table 8 lists the R-values with the
standard air films for the -60°F and +20°F tests, and these are the
“results used for comparison in this report.
1. The average tested wall R-values range from 12.8 hr-#t2-°F/Btu for
a metal stud wall without foam insulation to 28.2 hr-ft?-°F/Btu for
a wood stud wall with foam insulation (see Table 8).
2. The thermal resistance of the tested metal stud wall without foam
insulation is 58% of a metal stud wall with foam insulation.
3. The thermal resistance of the tested wood stud wall without foam
insulation is 68% of a wood stud wall with foam insulation.
4. The thermal resistance of a metal stud wall with foam insulation is
22% less than that of a wood stud wall with foam insulation.
-26-
-L@-
-8z-
-12 -40 8 -6 -4 -2 0 2 4 6 8 10 12
FIGURE 4: Finn ELEMENT ISOTHERMAL METAL STUB WALL iditHouty FOAM
-62-
= =
a DU oo
euUrP= INTE ——o I= AN ig = Je i eo™ PLOT:
-0€- SS —— RW OTN ( Omar.) M/S JV INIA IAL WZ Nie ase \f \/\ ER SW pA TAS gto) TTP
FIeURe |1: FIP BLEMENMT IScoTHERAMAL FLOT: Kloop arub WALL WITHOUT FOAM
5. The thermal resistance of the metal stud wall without foam
insulation: is 33% less than that of a wood stud wall without foam
insulation.
6. The thermal resistance of all test walls is better at the colder
test temperature. This is as expected due to the higher thermal
resistances of fiberglass and polystyrene at colder temperatures.
However, test Wall #1 showed only a slightly lower resistance at
the colder temperature.
ASHRAE AND FINITE ELEMENT METHOD CALCULATIONS
The results of the two calculation methods correlate more closely
for metal stud walls than for wood stud walls. The ASHRAE results vary
from 0.1% to 5.0% from the FEM results for metal stud walls, and from
3.3% to 8.8% from the FEM results for wood stud walls. The wood stud
walls do exhibit two-dimensional heat flow in the vicinity of the studs
which is accounted for in the FEM analysis, but not in the
one-dimensional ASHRAE analysis.
Although thermal conductivity values used in each set of
calculations varied slightly (see Table 2), the level of correlation is
high.
TESTED VERSUS CALCULATED WALL R-VALUES
The average calculated wall R-values exceeded the average tested
wall R-values by 3% to 21%. The FEM results generally were closer to
the tested results than the ASHRAE results, exceeding them by 3% (Wall
#4) to 21% (Wall #1). The ASHRAE results ranged from 9% to 19% higher
than the tested results. Walls #1 and #2 had a greater difference
between measured and calculated R-values than Walls #3 and #4.
The lower tested wall thermal resistances may be partially
accounted for by the screws used to attach the foam insulation boards to
the exterior of the test walls. The screws pierce the insulation,
creating areas of high thermal conductance and lowering the walls'
R-values. Nominal values for thermal conductivity were used for the FEM
-31-
and ASHRAE calculation procedures. Because the tabulated values are
averages, a plus or minus 10% error in R-values is not unexpected.
EXPERIMENTAL ERROR
In all the Wall #1 tests, and for Wall #4 at -20°F, O°F, and +20°F,
the test wall's cold surface temperature was cooler than the cold box
air temperature. This colder surface temperature is most likely
explained by radiant cooling caused by the coil "seeing" the test wall.
CONCLUSIONS AND RECOMMENDATIONS
1. The difference between measured and calculated R-values was larger
in Walls #1 and #2 (the walls with exterior applied, foam
insulation boards). While the insulating value of a wall is
traditionally derated to account for the lower thermal resistance
of the wall studs, foam insulting boards are considered continuous
and are not derated for fasteners or gaps between the boards. This
could explain the need to compensate more for nails and screws that
pierce exterior-applied foam board insulation than for those which
only pierce the exterior plywood sheathing on the walls without
foam insulation.
The test results indicate that when designing walls to meet a
specific R-value, the thermal resistance of the walls should be
increased over the initially calculated value by 10% to 20%,
depending on the wall configuration and the calculation method
employed.
2. Because metal stud walls lose more heat than wood stud walls, their
use should be limited to those cases where they are economically
justified. If used, additional insulation should be installed to
increase their thermal resistance.
3. The effects of metal fasteners on the thermal performance of walls
with exterior-applied foam board insulation should be investigated.
-32-
The use of plastic fasteners or other methods that do not degrade
the thermal insulating qualities of walls may be a cost-effective
construction practice.
IMPLEMENTATION
The findings of this report should be considered by the design
community and design managers when selecting the structural elements and
insulation system of the thermal envelope of a new state facility.
Selection or development, by the designers, of a low conductivity
fastener to secure exterior insulation board and siding is also
encouraged.
The results of this report will be incorporated into the State of
Alaska Design Standards Manual for buildings being developed by the
Department of Transportation and Public Facilities Statewide Standards
and Technical Services.
REFERENCES
1. 1981 ASHRAE Handbook of Fundamentals. American Society of Heating,
Refrigerating and Air Conditioning Engineers, Atlanta, GA.
2. 1980 Annual Book of ASTM Standards, Pt. 18 Thermal Insulation;
Building Seals and Sealants; American Society for Testing
Materials, Philadelphia, PA.
3. Kreith, F., and W.Z. Black, Basic Heat Transfer. Harper and Row,
New York, NY. 1980.
-33-
APPENDIX
This appendix contains a standard ASHRAE methodology calculation
for the U-value of the wood stud wall with exterior foam insulation and
Photographs of the DOTPF-UAF guarded hot box and a test wall under
construction.
-34-
TABLE 11. ASHRAE R-Value Calculation
Wall 3 Wood-Stud Wall R-Value
Thermal R (with Section Resistance insulation) R_ (stud)
Air (outside, 15 MPH) si? .17 .17 5/8 in plywood .77 .77 .77 15# felt paper .06 06 -06 1/2 in plywood .62 .62 -62 2 in rigid foam
(extruded polystyrene) 11.76 11.76 11.76 1/2 in plywood -62 -62 -62 5-1/2 in fiberglass 21.15 21.15 --- 2 x 6 wood stud 6.88 --- 6.88 3/8 in plywood -47 -47 47 Air (inside, still) -68 .68 .68
Total =R/A = 36.31 20.80
Ay 1, Re 1 29375 (gp ray) 4-0625 (oy) 2 U = "I + °S = . — = .0288 Btu/hr-ft"-F° ave Ry Re 1
Ar
- 1 on 2 ° R= v = 34.69 hr-ft°-F°/Btu
ave
Where
#0205 Al sr (1) = .9375
as = 48(1) = .0625
-35-
-36-
-37-