Monopoly Roof Assembly with Nailbase Panels – GreenBuildingAdvisor

Energy-Smart Details

Monopoly framing is a great way to improve airtightness and get the insulation where it works best, on the outside

The monopoly roof (aka the chainsaw retrofit for remodels) is a wall-to-roof assembly that eliminates rafter tails in order to wrap a water-resistant barrier (WRB) and air barrier continuously from the wall sheathing onto the roof sheathing. This is one of a few good ways to achieve an airtight assembly where a wall and roof meet. While there are already some nice examples of this method here on GBA, the particular details shared here explore how it can be done using nailbase panels.

What are nailbase panels?

Nailbase panels are prefabricated roof (or wall) panels that adhere OSB sheathing to polyisocyanurate, extruded polystyrene (XPS), expanded polystyrene (EPS), or graphite polystyrene (GPS) foam board. These panels are available in various thicknesses and R-values and are used where continuous insulation over the structural sheathing is desired.

When adding foam insulation to the exterior of a roof, you need another layer of sheathing on top of it to fasten the roofing. In a typical built-up system, you would lay the foam sheets first and then take a second trip around the roof to lay the OSB. But with a nailbase panel, you are completing both steps at once, which reduces labor. It’s still good practice to use at least two layers of insulation (for example, a base layer of foam plus the nailbase panel) so seams can be staggered. This helps maintain continuity if gaps develop due to shrinkage.

An assembly using a nailbase can be either vented or unvented. I’m showing a vented panel in these details because I like to encourage drying wherever possible. I typically use vapor-open WRB/air barriers to promote drying into the vented cavity. Plus, it’s good practice for a vaulted ceiling to have a vented roof. But honestly, the venting is a belt-and-suspenders approach that helps…