Starting January 1st, 2023, all low-slope Re-roofs, and roof overlays will require the roof system to have at least an R10 of continuous insulation above the roof deck. While this may not sound like much, this will add a lot of complication and cost to your roofing project.
The goal for California is to continue to improve the energy efficiency of existing buildings. Continuous insulation not only acts as a secondary insulation layer to the "lid" of the building, but also, due to its "continuous" properties it combats thermal bridging. Thermal bridging occurs most commonly when you only have "batt" insulation on the underside of the roof deck. When "batt" insulation is your only source of insulation, thermal bridging occurs through heat or cold transfer through your framing members.
Essentially, thermal bridging is hot or cold transfer through your steel or wood framing not covered by insulation. Thermal bridging can account for a considerable amount thermal inefficiency, 10-15% of hot or cold thermal transfer that will negatively affect the buildings energy performance.
Now that all projects in this category require continuous insulation, in most cases your best option to achieve the new requirements is to apply Spray Polyurethane Foam as your roofing layer and insulation to your existing roof system. Adding a SPF system over your existing roof will be your most cost-effective option and can be warranted up to 20 years.
With any other roof system option, you will need to either first tear off the existing roof layer in order to install your continuous insulation or, if possible, secure your "board" insulation over your existing roof layer and apply your new roof substrate over the top. In either of those two scenarios, you will be spending a considerable amount more than if you were to choose an SPF roof system.
Not only will you spend more if you install "board" insulation as your continuous insulation layer, but "board" insulation is usually mechanically fastened with many metal plates and screws. As you can imagine, these metal plates and screws will conduct thermal transfer and allow for approximately 50% more thermal bridging compared to an SPF system with no metal fasteners. On top of thermal bridging through fasteners, "board" products also have seams that negatively affect the system's overall performance.
SPF roof systems are also indefinitely renewable when properly maintained for a total lifecycle cost that is much less than anything else available.
Please reach out if you have any questions regarding the subject, we are happy to help.