Credit for Natural Ventilation?

You've hit one of my pet peeves - "natural ventilation" means different things to different people. To building codes, it means the windows can open. To green builders, it often means passive conditioning systems. We shouldn't confuse these two very different approaches, but we do.

Let's tackle the ASHRAE question first.

As I said, "natural ventilation" comes in two flavors:
1) operable windows, which don't necessarily provide adequate ventilation when the windows are closed.
2) passive ventilation (and conditioning) schemes, which can provide ventilation, and in many cases, temperature control.

As I understand the proposed changes to ASHRAE 62, the committee is considering requiring mechanical ventilation in case 1 above. They hope to do away with the typical mid/high-rise residential scheme that has living spaces rely on operable windows, while corridors are pressurized in the hope that fresh air leaks into the living spaces when the windows are closed. This kind of scheme, while very common, can lead to problems with IAQ, and can contribute to building envelope failures.

On to your second question - How does LEED/ASHRAE 90.1 handle natural ventilation?

If your ventilation is as per flavor 1 above (operable windows>4% of floor area), then you are correct - base case and design case gets the same system. If you leave out the cooling system, for example, you have to model a minimally code compliant cooling system in your design case. ASHRAE does this because, without an engineered passive cooling system, the space will be hot, and the occupants will likely install window units, etc.

For passive conditioning systems ASHRAE 90.1 -2007 is still fairly strict, but LEED is a little more lenient. Engineered passive conditioning systems have successfully been modeled using the Exceptional Calculation Measure pathway. The Aldo Leopold Legacy Center (LEED Platinum) in Wisconsin is a prime example of this.

There are also a few other tricks you might try. For example, for a project with no cooling system, a recent CIR (4/19/09) suggests:

" Although a cooling system must be modeled in both the Baseline and Design case, there are no requirements for Temperature Setpoint. Therefore, both cases may have the Cooling Temperature Setpoint elevated such that both systems do not ever run and thus does not consume any energy."

Determining the SC2 value for an irregularly shaped shading device:

We have office building that is aiming for NC 2009 Gold. The building is located at Taipei and has a irregularly shaped external shading device and is a little complicated to find the SC2 value directly. During these situations, what we do in Singapore is that we run a insolation simulation in ecotect analysis with and without shading device for each facade. From the percentage of shading(x%), we find out the SC2 value: SC2=(1-x%). From this, we find Total SC=SC1(of glass) x SC2. Kindly advice if this methodology is accepted by USGBC. we use IES for energy modelling. In IES, we have modelled without these shading device(as designing of those would make the model very heavy) and planning to assign the SC value to the external facade glazing. Is this a accepted methodology?

Many Thanks.

If anybody is aware of the above issue, please help us get clarified. Thanks.