With the underfloor insulation finished we turned our attention to the ceiling. To get a reasonable amount of R-value AND a vent space into the 2×6 rafter bays, we are taking a layered approach of polyiso foam and rockwool. This roof detail devised by Derin Williams of Shelterwise, starts with 2 inch R max and backs it up with 3 inch rockwool on the bottom giving an overall R value of 28, which is actually decent for such a thin roof. To preserve the half-inch vent space on top of the foam we glued little plywood spacers on. This way tomorrow morning when we install it we don’t have to worry if we are pushing it up too hard. The minimalist in me is vaguely annoyed by the hybrid insulation approach in this tiny house. We have an XPS thermal break under the floor then rockwool on top, then rockwool overhead, then polyiso foam. It just seems inelegant, but every part of the system really is doing a specific job that it’s well-suited for. Personally I don’t love foam, Rockwool just feels more natural, but interestingly poly iso-foam actually has a slightly lower lifecycle carbon footprint. XPS on the other hand is through the roof. Of course this is just measuring carbon not all the other toxins that are left behind when the foam degrades. Rockwool is basically inert. One could argue, however, that a better insulated (polyiso foam) building is going to consume less fossil fuel inputs in the form of heating, none of which are without their own toxic footprint. It’s a complex subject. We educate ourselves the best we can and try to make good choices.