Natural Ventilation Compliance Calculation

We've seen cases where the LEED reviewers interpreted this situation differently from local code officials, who were also following ASHRAE 62.1 language to determine the ventilation provided by operable windows and the distance of compliant spaces.

On a residential project pursuing LEED NC 2.2, we had a similar condition to yours where a back room without windows had a large opening to a front room, and the front room had operable windows for ventilation. Most of the back room was beyond the 25' distance from the front room windows. For local code compliance the back room was considered adequately ventilated, but the LEED reviewers indicated any area beyond the 25' would not be considered adequately ventilated. We were able to comply with LEED requirements by providing a small transfer fan between the front and back rooms that would mix the air of the front room with the back room.

To your question, since this was an NC project and under 2.2, it may not predict exactly how your situation would be reviewed, but may give you an idea how it has been interpreted in the past.

I think your scenario could comply, since you are essentially mechanically ventilating the one interior space. In our situation the "transfer fan" was just to mix the air between the inner and outer rooms, not to supply dedicated outside air. The window openings of the outer room were greater than 4% of the combined total area of both rooms, since they were providing the outside air for both. Hope that helps.

I have a slightly different variation that I'm trying to resolve. One large room (4,000 sf metal shop) is naturally ventilated by two large garage doors (totaling 336 sf). Per your transfer fan example, can I get around the 25' requirement with ceiling fans, which are moving the fresh air around?

Jared, are those doors actually open whenever the metal shop is in use? In all weather?

Not ALL the time, but occupancy often correlates to activity going in and out of the doors. It's admittedly a low-tech solution, but one that seems to work for their situation. It seems plenty ventilated overall, but the back corner in winter is clearly the worst-case scenario. Any work-arounds you can think of?

Jared, I'm not sure if this helps, but I don't see how you can get around showing that it complies with ASHRAE Standard 62.1-2007, paragraph 5.1 (with errata but without addenda), per the credit requirements.Unless you can apply the 10% exemption here, but the space may be too large.

Jared,
You'll probably want to get some engineering input that takes into account local climate and code; as an existing space I'm assuming it may not even meet the current code requirements for ventilation, especally given the use. There is an exception in section 5.1 where a natural ventilation system approved by the local code officials can trump the prescriptive requirements, but that route may not apply here to your existing building that was designed to meet an older version of the code.

Alternatively, it might be worth seeing if an exhaust-only system could work.

In other situations I've seen LEED reviewers rule that entry doors would not qualify as operable windows, so given the importance of this prerequisite you may need a CIR and/or mechanical engineering input to resolve the issue with greater certainty. Hope that helps.

Thanks, Tristan and David ... are there any considerations for occupancy and activity? Occupancy is very low and not regular in this space, so it seems like overkill to over-engineer it. Is this where a CIR could possibly help?

David,

I'm curious about your comment about entry doors and natural ventilation. We had an engineering firm do an IEQp1 assessment for us, and I believe they are using the free opening of the door (42 square feet) to demonstrate natural ventilation in the lobby and kitchen in a residence hall. Any idea whether doors normally count or if only windows can be used to demonstrate compliance? Thanks.