ASHRAE-90.1-2022- - Page 275

Extracted Content

Step 4: Calculate the R-values inside the girt and adjacent to the web. The R-values inside the girt are the air space (RAUP) added in series with the insulation (R2UP); their combined value is then added in parallel to RPUP. Depending on the thickness of the air space, it can be modeled as conduction as shown in Equation A9.4-31:

H 3 RAUP = ------ ka

(A9.4-31)

where H 3 = thickness of the air space, ft ka = thermal conductivity of air, Btu·ft/h·ft [2] ·°F When appropriate, the air space can be modeled as convection, which is a constant R-0.92 h·ft [2] ·°F/Btu for walls .

The R-value for R2UP is expressed in Equation A9.4-32. The insulation thickness is also not limited by the girt height and can extend beyond it.

H 4 R2UP =

= ----------------------------------------------

(A9.4-32)

• B --------- - + C --------- -

H 4  o  o

A + B

 o  o

--------- o - o + C --------- [H] [4] H 4  

where H 4 is the thickness of the mineral fiber at x = 0 in feet.

The R-value of the web (RPUP) is calculated using 26.2 h·ft [2] ·°F/Btu as the thermal conductivity of the girt in Equation A9.4-33:

Web Height RPUP = --------------------------- -

kp

(A9.4-33)

where kp = thermal conductivity of the girt, Btu·ft/h·ft [2] ·°F Web Height = height of the girt, ft The addition of the air space and insulation in series are combined to be in parallel with the girt, which is expressed as Equation A9.4-34:

----------- Lf = ----------------------------------- Lf - tp - + ------------- tp - RUP RAUP + R2UP RPUP

Equation A9.4-34 can be rearranged and solved for RUP as presented in Equation A9.4-35:

 RAUP + R2UP  RPUP RUP = ---------------------------------------------------------------------------------------  Lf - tp  RPUP + tp  RAUP + R2UP  Lf

(A9.4-34)

(A9.4-35)

Because the thickness of the girt is significantly less than the flange width ( Lf ), Equation A9.4-35 can be simplified as Equation A9.4-36. However it is important to note that RUP will be close to 2 or lower (depending on how the air is modeled) because of the significant effects of the steel girt:

 RAUP + R2UP  RPUP RUP = ------------------------------------------------------------------------ Lf RPUP + tp  RAUP + R2UP  Lf

(A9.4-36)

Step 5: Calculate the R-value outside the girt. Typical construction above the girt consists of a thermal spacer block and compressed mineral fiber insulation. These two insulations are in series, and the total R-value ( ROPI ) is expressed as Equation A9.4-37. If there is thermal break tape present it is included as the third insulation in this series.

(A9.4-37)

ROPI

= ------- + ------ kf kI

H 1

H 1 H 2

------- + ------kf kI

where H 1 = thickness of thermal spacer block, ft H 2 = thickness of compressed mineral fiber insulation, ft kf = thermal conductivity of the thermal spacer block, Btu·ft/h·ft [2] ·°F kI = thermal conductivity of the compressed mineral fiber insulation, Btu·ft/h·ft [2] ·°F

ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 273