What ETTV actually measures, and why manual prep fails

What the number actually is

ETTV, or Envelope Thermal Transfer Value, is a single figure for how much heat passes through a building's external walls and windows over a year. It combines three routes: conduction through solid wall, conduction through glass, and solar radiation through glass. Each is weighted, summed, and measured against a ceiling set by the building code.

The formula has not changed in years. What changes constantly is the input: window sizes, shading depths, glass specification, orientation, and the wall build-up. Every one of those moves during design.

The calculation is not the hard part. Keeping it true to a design that is still moving is.

Why manual preparation fails

A manual study is a snapshot. Someone exports areas from the model, transfers them into a spreadsheet, applies coefficients by hand, and produces a number. That number is correct for the model as it stood that morning.

Where it comes apart

• Areas are re-measured by hand every time the facade changes
• Glass and shading specs live in a schedule the spreadsheet never reads
• One orientation gets checked properly, the others get assumed
• By the time it is submitted, the model has moved on again

None of this is carelessness. It is what happens when a check that should run continuously is done once, by hand, near a deadline.

What changes when it runs continuously

When the calculation reads the model directly, the number stops being a milestone and becomes a reading. You see the effect of a deeper reveal or a different glass the same afternoon you draw it, which is the only point at which the answer is still worth having.