938 Calculating Ufactors for Metal Building Walls

ASHRAE Standard 90.1 recommends The Johan-nesson-Vinberg Method7 for determining the U-factor for sheet metal construction, internally insulated with a metal structure bonded on one or both sides with a metal skin. This method is useful for the prediction of U-factors for metal girt/purlin constructions which can be defined by the geometry described in Figure 9.12.

The dimensions of the elements shown, as well as their conductivities are evaluated in a series of formulations, which are in turn combined in two parallel paths to determine an overall component resistance. The advantage of this method is that metal gauge as well as thermal break thickness and conductivity can be taken into account in the calculation. Caution is advised when using this method to evaluate components with metal-to-metal contact. The method assumes complete thermal contact between all components, which is not always the case in metal building construction. To account for this effect, values for contact resistance can be introduced in the calculation in place of or in addition to the thermal break layer.

Table 9.5 Wall insulation installed between 16 gauge "thermal" Z-girts.

Nominal

Table 9.5 Wall insulation installed between 16 gauge "thermal" Z-girts.

Nominal

Unbridged

Thickness

R-Value

R-Value

R-Value

R-Value

R-Value

R-Value

(Inches)

2' O.C.

4' O.C.

5' O.C.

6' O.C.

8' O.C.

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