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Are Prescriptive Specs Obsolete?

Builders Developers

Recently on LinkedIn’s #TwoCentsTuesday, we asked whether new code requirements have made prescriptive specs obsolete. 91% of you said they were. This Tuesday, in a related post, we asked you to spot the contradiction in this spec: 

All windows to have LoE2 on surface #2, with argon gas and warm edge spacer achieving a U1.40 metric performance level.   

If you said this spec includes both a prescriptive and performance method, then you know your stuff. In this particular scenario, the spec calls for a LoE2 coating on glass surface #2 but also requires a 1.40 U value. The problem is, this U value cannot be achieved with a LoE2 coating. Therefore, these specs will not make sense to a manufacturer

Prescriptive Vs. Performance Methods 

When it comes to designing a home, one of the first considerations should be whether to take the prescriptive or the performance path to achieve code compliance. The prescriptive method specifies how the building must be constructed and what components are desired for products, like asking for windows to have a certain coating on the glass. The performance method, on the other hand, specifies what standard these components and the building itself should be able to achieve

A problem arises when both are specified, as manufacturers often encounter contradictions between the two as demonstrated by the example above. This can be a major headache and is a sure-fire way to slow down your project. 

Choosing the Right Path: Why Prescriptive is Obsolete 

So how do you know which method is best? At Centra, we work to the BC Building Code and regularly work to the BC Energy Step Code as well. Prescriptive requirements can be good for setting minimum product expectations. By using the term minimum expectations with a known performance outcome, the manufacturer can better navigate your requirements. For example, specifying a warm edge spacer as a minimum criteria will eliminate the use of an aluminum spacer as a path to meeting a performance spec.  

The problem with the prescriptive method is that it allows for no flexibility when it comes to trade-offs in energy performance. Unless you’re only specifying minimum requirements, it forces you to meet pre-specified standards that may not be best for your building. They often result in higher costs, and a home that doesn’t perform as well compared to a home that was built under the performance path.  

Dropping the rigidity of the prescriptive approach and opting for the performance method not only helps make your building code compliant, it also does so much more efficiently and often at a lower cost. It tends to result in a more integrated approach to home design which leads to more successful projects overall. This approach incorporates changes in building design, layout, framing techniques, system selection, and materials. We often refer to this as ‘the house as a system’. 

For example:  

“An energy advisor has indicated that if we use a U1.25 window in a step 3 building in most cases you can eliminate the need for exterior insulation.  The window to wall area needs to be below 25%.  Consider this in which windows you select.  You will pay a bit more for windows, however you will save a considerable amount on the project in insulation.” 

Trust the experts 

At Centra, we’re the window experts and we know what components are needed to make an energy-efficient home. You can trust that by choosing the performance route, we will select the right combination of products to ensure the building meets the required standards in the most cost-effective way possible. To be candid, we want your project to succeed because your success is our success. It leads to a fruitful working relationship for both parties going forward, so your priorities are very much our own priorities.

We Are Your Certified Window Geeks 

We love to talk windows! The demand for glass is at its all-time highest, but we are working with our customers to ensure you get the products you need. Contact us as early in the process as possible so we can help secure your order and keep your project on schedule. We will guide you early on so that your supply requirements are met. 

For more information on this topic (and many more!) contact our local Employee Owners at 1-888-534-3333 or drop us an email at Join the club and become a Window Geek yourself by signing up for our biweekly newsletter, which provides news, advice, and tips about the window industry and upcoming events, and test your window knowledge by following us on LinkedIn. 

Written by: Les Ferris , January 30, 2023
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