For the past few weeks, I've been doing loads of research about roofing. Our main goal for the cottage is to have something with substantial r-value. Living in a trailer for two winters has made the importance of insulation abundantly clear. Our secondary goal is being able to see our beautiful timber frame rafters from the inside—we worked so hard on them!
Given my criteria and limited knowledge about roof building, it would seem we have two good options. One, we could build a box on top of the rafters to fill with rigid foam insulation. This is what Rick, our maple sugaring and retired construction friend suggested. Or two, we could purchase structurally insulated panels (a.k.a SIPs). SIPs are essentially an insulation oreo—foam sandwiched between two layers of wood.
The SIP idea came from our other construction buddy, Rick #2. He wrapped his entire reclaimed timber frame barn home with them after finding a great deal on "seconds." As a home energy efficiency auditor, he is positively obsessed with creating a "sealed envelope" and then controlling the flow of fresh air using an air-to-air exchanger.
I'm not particularly persuaded by this argument (our house is going to be "breathable", sheathed with strawbales and lime plaster), but I was persuaded when he pointed out that building a box on top of our roof and filling it with foam would essentially be like creating our own SIP panels, except with a lot more effort and most likely less-effective (in terms of r-value) results.
Tyler and I waffled between these two options quite a bit, our opinions vacillating wildly when we heard the price of the panels, and then again when we calculated the cost of the lumber and foam it would take to build our own, and again when Rick found us half-price "seconds", and again when the company selling those seconds couldn't actually sell them until the company they were being bought by finished buying them.
In the end, mostly out of exasperation and a desire to get the roof taken care of quickly, we opted for SIP panels. Our deposit is in, the engineers are working on them, and now we need to get the roof ready for the installation.