Designing Windows for the BC Energy Step Code
Whether it is a single family dwelling, townhouse development, mixed-use building or multi-storey apartment/condo building, collaborating with your most trusted trades early on in the process is crucial. At the design stage of your project, speaking with experts will allow you to better understand and appreciate how each component of your building envelope and mechanical systems are going to work together to achieve the overall desired building performance. Whether building to step 1 or step 5, there are major savings opportunities available.
Let’s take a closer look on how windows play a significant role in find those savings.
When choosing the right windows for your project, you need to know the following:
- Step code level target
- Desired u-value factor & solar heat gain coefficient
- Performance grade and related NAFS ratings required
To help understand the chart below, you will need to understand the following:
- Sliding windows vs. casement windows
- LoE coatings, otherwise known as low emissivity
- Double glazing vs. triple glazing
By referencing the ‘Window Design’ chart below, you will see what window products can be used and how manipulating the glass selection (‘IGU Glazing’) can improve performance:
Case Study #1
This chart shows how sliding windows can be improved from roughly U1.60 to U1.40 with high performance double glazing, but that’s where slider performance stops. Sliding windows cannot receive triple glazing so if you want sliding windows or doors on your project, it’s important to understand this limitation for your energy model. Whether for reduced cost or egress concerns, triple glazing is not possible for sliding windows. Do not model a U1.20 sliding window as this will only cause confusion at the tender stage.
Case Study #2
Starting at a slightly higher u-value of U1.45 a casement window is typically the window of choice for high performance homes. A casement window can range anywhere from U1.45 to U0.90, depending on LoE choices and whether you’re using double or triple glazing. Do not model a casement window at minimum building code U1.80 ratings. This product doesn’t exist and you are going to receive a much higher performing product, which needs to be entered into the model to identify those trade-off savings discussed above.
Case Study #3
When building to steps 3 and above, typically windows will fall between U1.40 and below. It is important to understand that when transitioning to triple glazing, significant design changes need to be considered to accommodate the extra glass weight.
As a result, operable vents and the overall window size needs to be smaller. A site distribution and installation plan needs to be reviewed as well. Do not specify triple glazing on its own. “Desired u-value factor & solar heat gain coefficient” ratings drive whether double or triple glazing is required, which is why it is listed above as the 2nd item you need to know at the design stage.
Be sure to utilize the last column to understand the budget effects upgrading or downgrading your windows will have. A common example would be to upgrade your sliding windows from a U1.60 to a U1.40. This will have a roughly 31% cost increase. To simplify the math, a $10,000 window package now becomes $13,000, an increase of $3,000. However once this upgrade is reflected in your energy model, this upgrade could eliminate the need for exterior insulation. At a cost of $15,000, that is a net savings of $12,000!
To wrap up, speak with the experts around you. Trust your trades and energy advisors. Look at your projects from a holistic perspective and you can find savings the next time you find yourself navigating the BC energy step code. Lastly, get a window geek in your corner!