42" line of sight

Hello Rebecca
The credit addresses two aspects of fenestration design, the window height between 30” and 90” and the access to views at seated height, assumed to be 42" for desk work. The intent is to provide a straight line of view from seated position. The project can try submitting the credit showing the unobstructed line of sight going up to the window at 54". Some projects have come across that situation and got the credit approved with a narrative.

I think you've misinterpreted the 42" line of sight. 42" is the assumed center line of a persons eye when seated. The sills must be no higher than 30" AFF and the head no lower than 90" AFF for the window to count. 54" sills will disqualify the window openings from meeting the 30" maximum height.

Could someone confirm or clarify Rick's comment above that a window sill must be no higher than 30" and the head no lower than 90" for the window to qualify. Doesn't the window just have to be between these two heights to qualify, with no particular dimensions required. I have seen smaller windows that do not run from 30" up to 90" be used in this credit and not questioned by GBCI as long as the 42" line of sight and 90% of square footage with views is met.

Eric, it's my understanding that you are correct.

Eric and Rick. Combine what you are both saying and it is right. The 42" is the base (baseline) for the line of sight and NOT the absolute line of site. For example. If the viewer is seated (42" eye height), the window sill at 54" and the header at 90", assuming no other obstructions, there IS a unobstructed view to the outdoors. There are far too many examples of vague language like this in the standard which cause confusion. The graphic of the section is a bit more clear, showing that the clear line of sight is NOT defined by the 42" eye height but instead defines a baseline. Furthermore, this 42" is not the low end of acceptable line of site. The 30" is actually the low end and if the 42" eye height has a view to the 30" sill, you are still compliant.

Hi Ari,I don't think that's quite right. The credit language isn't clear on this, but the Reference is guide is very clear that the view has to be horizontal at 42" (for a seated adult). I've never understood how the 30" and 90" measurements apply, but I'm pretty sure that if you don't have vision glass at that 42" level (or whatever height your occupants eyes are mostly to be at), you don't qualify.

Nadav, please reread the reference guide. This is where people get confused and make huge leaps. "draw a line at 42 inches (typical seated eye height) across the section to establish eye height". What most people do is confuse that and say they are establishing line of sight. That is not supported by the language. What confuses people even more is the next bullet point which references horizontal views from a seated position. The 30-90" vertical measure IS the field that the writers have determined as the lower and upper bounds of a horizontal view.

The reason your line gets drawn all the way across is to aid in demonstrating the lack of visual obstructions and NOT to define some line of sight that would be both unrealistic and ultimately overly restrictive.

I agree with Ari on this, the credit language clearly states vision glazing will be between 30 and 90 and inches, which is the upper and lower limits of a typical persons field of view. The 42" is used as an example to demonstrate credit compliance because most interior activities are done while seated. It is what everyone thinks has to be used because it is the one in the Reference Guide. But, project teams that have spaces other than classrooms or offices could demonstrate the occupants are provided a view between 30 and 90 at what has been determined a typical eye height.

A manufacturing facility where the workers stand or sit on stools could earn the credit by showing that the workers typical line of site is, lets say 54 or 68 inches. As long as that line drawn at that height is not blocked and falls between the 30 and 90 inches on the vision glazing, then it is acceptable.

A window whose sill height is 54 inches would not work for an office where people sit all day. But a lab where people sit on stools working at counters already at 36 inches, 54 could work.

There are so many scenarios out there that the Reference Guide does not cover and the language provided tries to be written in a way that one could use to meet all those circumstances, it doesn;t always work that way as we know it. That is why there are erratas, That is also why project teams need to clearly demonstrate that they earn the credit and explain what is going on with the space and reviewers also need to realize that not every project is the same and there is always special circumstances.

Thanks, Todd. I agree with you as well. I was using the 42" height as an example for that setting, not as a universal value. What I'm still trying to confirm is whether the line from whatever eye-height you use has to be horizontal or not. In Ari's first comment, he says: "If the viewer is seated (42" eye height), the window sill at 54" and the header at 90", assuming no other obstructions, there IS a unobstructed view to the outdoors." That would be true only if the line of sight does NOT have to be horizontal. What's your take on that?

Technically there is nothing stating that it has to be horizontal. So, you could argue that an occupant has been provided views to the exterior that falls between 30 and 90 inches. If you read Heschong and Mahones study on Windows and Offices, page 47, there are 5 types of views. This report is the basis of exemplary performance. Type 1 does provide the occupants a view that falls what appears to be below or at 90 inches. It is not much, but you can see the sky. Now think how you you calculate the square footage that is compliant for views, its based off of a plan view., not a sectional view of the window wall.

However, Figure 4 shows the direct line of site as a horizontal line and one of the calculations requirements, and it states, is using the direct line of site.

So, based on the information provided throughout the credit, the direct line of site, whether it be 42" sitting or 54" on a stool, etc., must be horizontal.

Todd,

The Figure 4 you're referring to has a caption change (Addenda 5/9/11): "Unobstructed view over low partition". I read this as GBCI getting rid of the 'direct line of sight' language for this figure.

I'm trying to get to the bottom of this 42" issue and direct line of sight. Has anyone had experience where they have been awarded this credit with higher than 42" partitions (46"), with a direct line of sight beginning at 42" that ISN'T horizontal?

Thanks!