Hi,
I had a query whether ASHRAE 90.1 mentions what value has to
be taken for wall and roof absorptance as it mentions for U value . As
absorptance value has great impact on heat gains and further on energy
consumption.
Can we have a better wall absorptance in proposed case from base case
so to improve energy performance
Any response will be highly appreciable.
Thanks
Sambhav Tiwari.
Table G3.1 5c (for Proposed) and 5d (for Baseline) mentions how to model the reflectance (1-absorptance) of both the baseline and proposed roof. The baseline must be modeled with a reflectivity of 0.3. The proposed must be either 0.3, or 0.45 if the proposed roof has a reflectance of 0.7 or greater.
I have not seen anything in 90.1 about modeling the reflectance of walls.
Frank
Hello Sambhav,
In regard to exterior surface solar reflectance, ASHRAE 90.1 focuses only on
roofs and not walls. In the 90.1-2007 version, Table 5.5.3.1 has U-Factor
requirements for high albedo roofs (formerly referred to as "cool roofs".)
This single table is a replacement for the roof U-Factor requirements found
in Tables 5.5-1 through 5.5-3 (i.e., climate zones 1 through 3.) The
definition of "high albedo" is found earlier in Section 5.5.3.1.1 as roofs
with a minimum Solar Reflective Index (SRI) of 0.82. Typically, this is a
roof of high emissivity as well as high reflectance. The 90.1 User's Manual
shows values that achieve an SRI of 0.82 in Table 5-D, ranging from an
(e=0.75, refl=0.7) to (e=0.1, refl=0.81) combinations.
Regards,
Larry O. Degelman, P.E.