ASHRAE-90.1-2022- - Page 272

Extracted Content

b =

c =

---------------------------------------------------- - - 1   o 21 -  o 22  Yc  12 

----------------------------------------------- -   o 21 -  o 22  Yc  12

 o 2 W 2  o 2 -  o 2 1 2

 o 1 W 1 +  o 2 W 2

2 -  o 2

1 2

 o 2 1

2 -  o 2

1 2

Select the smaller value of Y 2, a and Y 2, b as Y 2. Y 1 shall be calculated as the difference between Yc and Y 2. Next, the R-values for the two compressed layers of insulation shall be calculated and converted to a U-factor . This process shall be repeated along the entire profile and the results numerically integrated using maximum 0.04167 ft increments. It is important to note that Equation A9.4-14 does not apply when the two layers of insulation are the same material. In this case, each compressed layer has the same thickness, which simplifies the U-factor calculations. The numerical integration still needs to be completed to determine the Uco .

Step 3: Determine the U-factor over the structural framing member. The variable Yo represents the thickness of the thermal spacer block and the thickness of the compressed insulation. The density of the compressed insulation is determined by Equation A9.4-16:

 c

=  --------- oto tc

where

 c = density of the compressed insulation over the framing member, lb/ft [3]

tc = thickness of the compressed insulation over the framing member, ft The thermal resistance of the compressed insulation is determined by Equation A9.4-17:

(A9.4-16)

(A9.4-17)

Rc

= ------------------------------------- tc - A + B  c + C   c

Determine the overall framing U-factor ( Ufo ) at the structural framing member, including the air film resistances, using Equation A9.4-18:

Ufo

1 = ------------------------------------------- - RTB + Rc + Ri + Re

(A9.4-18)

where Ufo = U-factor over the structural framing member, Btu/h·ft [2] ·°F RTB = R-value of the thermal spacer block, h·ft [2] ·°F/Btu Rc = R-value of the compressed insulation, h·ft [2] ·°F/Btu Step 4: Determine the overall area-weighted U-factor for the entire system using Equation A9.4-19:

Ues

= L --------------------------------------------------------  Uco +  wf  2   Ufo L +  wf  2 

where Ues = area-weighted U-factor for the entire system, Btu/h·ft [2] ·°F

Step 5: Calculate the adjusted overall U-factor ( Uadj ) using Equation A9.4-20:

(A9.4-19)

(A9.4-20)

Uadj

1 = --------------------------------------------------0.8676  Ues + 1.1423

where Uadj = adjusted overall U-factor represented by correlation with the finite element modeling, Btu/h·ft [2] ·°F Step 6: If there is any continuous insulation present, calculate the overall U-factor using Equation A9.4-21:

Uo

1 = ---------------------- - 1 -------- Uadj - + Rci

(A9.4-21)

A9.4.6.3 Single-Layer in Cavity and Double-Layer Walls. The U-factor of metal building walls that are insulated with a single-layer in cavity or multiple layers of mineral fiber insulation (see Figure A9.4.6.3) shall be calculated using the procedure outlined in this section. For double-layer walls, the procedure assumes that the outer layer of insulation is compressed between the wall panel and girt. There may also be

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