I'm already taking my ceiling closer to R5, and my walls R2.5. Getting double paned windows will bring those into line with the walls, but at significant cost - I'm yet to get actual quotes, but I've had back-of-laptop estimates of over $10k for my place. So with the ceiling and walls improved significantly, the value proposition of bumping a much smaller surface area up as well becomes much harder to justify.
So, I'm looking in a different direction: engineered windowblinds.