Older existing building - no VLT available

Two ways to go about this; 1.If the windows can be opened, then use a light meter with the window open to get your base, close it and take a reading on the inside of the glass. Should get a rough idea of the VLT. Taking a reading inside and then walking outside will be less effective since the sky illuminance changes frequently. That is if its a long walk to the outside. We have tested this with a known VLT piece of glass and a good meter and have been within 5% of the actual VLT. 2.Use table A8.2 in 90.1 as a default value for your VLTs. Its a valid methodology.

This is pretty common actually. You could try putting the engineer on the spot and asking them what they would do if they had to replace a broken window. Depends on your relationship with them. Of course, this might not work because they might have a different answer if they have a broken piece of window to work with. Todd lists some good methods for estimating. Todd, do you think that ASHRAE 90.1 is a good source for default VLT in this case? Are you suggesting using the current version of 90.1 or the version for when the building was built? I believe the first version didn't come out until 1975.

Jill, I personally would not use the ASHRAE values, but would attempt the measurement option; especially with older buildings and especially if I'm doing daylight analysis. But, using the ASHRAE method is acceptable in EAp2, so there would not be any question in regards to using these values for the daylighting calculations. If you are just documenting the credit and not trying to change the daylighting performance of the space, then this might be an easy option. The more recent version of ASHRAE is probably more reflectant of values of the glass we currently run across in windows from the past decade or two. Not sure whether there is a table like that in the older ASHRAE versions. But, for LEED purposes, i can;t see an argument to not using the most recent version.