How to ensure custom windows comply with high wind requirements?

[Geoff_G]

I found my way here via a google that brought me to this thread.

I am building a cottage in a high elevation area in the North Carolina mountains where the code requires that windows be certified to a performance grade of 50 (PG50). The bulk of our window package calls for standard/readily available windows that come with the PG50 certification, but the plans also call for six small (i.e. 17"w x 36"h) "push out" all-wood single casements and two similar french (double) casements. Window manufacturers can make these casements as a custom order and certify that they adhere to the county's requirements, but the cost is painfully prohibitive.

For a different project, my cabinet maker made several windows for me (pictured) that are similar to the casements I'd like to install in this cottage. Windows such as these, made by my cabinet maker, would be far more affordable (even with more substantial sashes and tempered glass) than the quotes I received for custom windows made by a manufacturer. Unfortunately, without any sort of certification, the inspector wouldn't be able to approve them.

The inspector has said that, provided I have a NC structural engineer "sign off on them", I can use whatever windows I want. Because the cost difference is so great between custom windows provided by a manufacturer and what I could make locally, it's worth it to me to explore this.

Any advice on how I might satisfy the county's wind load requirements while building my own windows locally? I suppose I'm looking for an engineer who could consult on a window design and certify that it will meet the county's requirements, but I don't even know if that's a thing that structural engineers can do! I appreciate any suggestions.

 

[r13]

I think you should look into the standard and procedures for PG50 certification, and see if there is anything you can do to meet the requirement.

 

[rowingengineer]

Lived in a high wind speed in Australia and yes we did this type of works as a structural engineer, giving the strength of the window for expected windloads was straight forward for small windows. It was the water/air penetration ratings that we couldn't provide input on and you either had to meet some deemed to comply requirements or give up.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Geoff_G]

Thanks! I just reached out to the organization that manages the certification (the FGIA

https://fgiaonline.org/

), so I am standing by for more info. However, from what I can tell from their website, the certification process involves physically testing a manufactured window model, rather than merely approving a design.

The code actually requires a Design Pressure rating of 50 ("DP50"), and not a "PG50" rating, tho PG rated windows meet or exceed DP ratings. Hopefully the FGIA can help me, but if anyone has any other ideas for me, I'm all ears.

 

[NorthCivil]

From what I understand on the topic, residential windows are meant to comply to NAFS.

NAFS is a standard created to create a uniform performance target for windows to comply to, to allow windows to be supplied from any winodw production factory to any housing application across north america. There will be structural, weatherproofing, and thermal performance criteria.

Specific design engineering is the alternative. structural engineering is one part of this, weatherproofing performance is the second part. thermal performance may or may not be the third part, depending on the requirements of your jurisdiction. I work in this business - it wont be worth our consulting fees to get involved to certify windows on a job this small (in fact, its never worth our time to get involved on a house). your engineering fees, if you do it right, will be more than the custom window costs. you may get away with lesser engineering, by hiring an engineer who will only certify for structural, and the building inspector accepting that alone, when they should request weathering proof of performance as well.

the benefit of NAFS is it allows a window maker in arkansas to supply windows to a homebuilder in oregon. as such, there is huge efficiencies in scale available. the down side is custom windows are effectively no longer compliant. you may have an easier time trying to source these particular window types from a different manufacturer. expand your search right across the country.

 

[jjl317]

The NFRC (National Fenestration Ratings Council) has an online database of window manufacturers and products that have already received certifications (

https://www.nfrc.org/certified-product-directory/).

The database isn't particularly straight forward, but might allow you to narrow down a search of who even makes windows that could meet the criteria.

But having also worked in the window industry, I agree with NorthCivil that I would expect it would be difficult to get a full custom window to meet current requirements.

 

[Geoff_G]

Thanks jjl317 and NorthCivil. NorthCivil, that is pretty much what I was expecting to hear, but was hoping to get a different answer. It does seem like the standard would (should?) allow for some alternate path, even if slightly burdensome, for someone to build their own stuff. Absent that, it forces me to buy from a distant manufacturer who operates at a large scale and prohibits me from self-performing the work or using a local small business. I appreciate y'all's feedback. Very helpful.