Environmental Controls I/IG Lecture 9 Heat Flow in Opaque Materials Thermal Mass.

Environmental Controls I/IGEnvironmental Controls I/IGEnvironmental Controls I/IGEnvironmental Controls I/IG

Lecture 9Heat Flow in Opaque Materials

Thermal Mass

Conductive Heat FlowConductive Heat Flow

Conductive Heat Flow through opaque materials:

Q= U x A x ΔT

Q: heat flow (Btuh)U: transmission coefficient (Btu/h-ºF-ft2)A: area (ft2)ΔT: temperature difference (Ti-To)

Transmission CoefficientTransmission Coefficient

Transmission Coefficient (U):

U= 1/ΣR

U: transmission coefficient (Btu/h-ºF-ft2)ΣR: sum of resistance values (R-values) for layers of a construction assembly

Summing R-valuesSumming R-valuesSum of R-values (ΣR):

ΣR= 1/hO+R1+R2+R3+…+1/hI

hO,hI: film surface conductance coefficients

R1,R2,R3,…: Resistance values (R-values) for each layer of a construction

assembly

Air FilmsAir Films

Film surface conductance coefficient

Outdoor air film: R= 1/hO

Indoor air film: R=1/hI

Finding hFinding hOO and h and hII – – EmittanceEmittance

Emittance(ε): absorption of radiant heat

S: p.1570, T.E.3B

Finding hFinding hOO and h and hII

Film surface conductance coefficient (S: p. 158, T4.3)

Position of Surface

Direction of Heat FlowEmittance

Air Motion

S: p. 1570, T.E.3B

Finding hFinding hOO and h and hII – – EmittanceEmittance

Emittance(ε): absorption of radiant heat

Effective Emittance (εeff):

1/εeff=1/ε1+1/ε2-1

S: p.1570, T.E.3B

R-values for Enclosed Air R-values for Enclosed Air CavitiesCavities


S: p. 1571, T.E.1

Emittance

Position of Air Space

Air Space Width

Air Space Temperatu

reDirection

of Heat Flow

R-values For Solid MaterialsR-values For Solid Materials

Table 4.2 Thermal Properties of Typical Building and Insulating MaterialsDensity

ConductanceConductivity

Resistance

S: p. 1549, T. E.1

Conductivity and Conductivity and ConductanceConductance

Conductivity (k) heat flow through a material per unit thickness

Conductance (C): heat flow through a material of stated thickness

C=k/x

where x= unit thickness (in.)

Conductivity and Conductivity and ConductanceConductance

Conductivity vs. Conductance

1’

1’

1”

1ºF

x”

1ºF

Conductivity k=0.25 Btuh

Say x=4”

Conductance C=k/x=0.25/4”=0.0625 Btuh

S: p. 182, F.7.8

Example 1

Converting to ResistanceConverting to Resistance

Resistance (R): measure of resistance to the passage of heat (h-ft2-ºF/Btu)

R=1/C or R=x/k

Converting to ResistanceConverting to Resistance

Conductivity vs. Conductance

1’

1’

1”

1ºF

x”

1ºF

Conductivity k=0.25 BtuhResistance R=1/k=1/0.25= 4

Say x=4”

Conductance C=k/x=0.25/4”=0.0625 BtuhResistance R=x/k=4/0.25=16

Example 1 (cont.)

S: p. 182, F.7.8

Table 4.2 Thermal Properties of Typical Building and Insulating Materials

Thermal Properties TableThermal Properties Table

S: p. 1522-3, T.E.1

U-Value CalculationU-Value Calculation

Wall 1indoor air film½” gypsum board2”x4” nominal stud (pine) w/3.5” Ins.½” fiberboardwood shingles (16” long, 12” exposure)outdoor air filmSection View

At AtInsulation Frame

Component (RI) (RF) Ref.


indoor air film 0.68 0.68 T.E.3A

½” gypsum board 0.45 0.45 T.E.1

2x4 stud (3.5” pine) n.a. 4.35 T.E.1

3.5” Insulation 13.00 n.a. T.E.1

½” fiberboard 1.32 1.32 T.E.1

wood shingles 1.19 1.19 T.E.1

outdoor air film 0.17 0.17 T.E.3A

Totals ΣRI 16.81 ΣRF 8.16

UI 0.059 UF 0.123

Finding Indoor Air Film Coefficient Finding Indoor Air Film Coefficient –h–hII


S: p. 1570, T.E. 3A

Indoor air filmVertical surfaceHorizontal heat flowNon-reflective surface

hI=1.46R=0.68