Construction/Insulation Strategy for New Build - Roof

matrat

Hi all,

I will be setting out in building a new house in the not too distant future and having gone through the process before i have a much better understanding of the importance of insulation, heating, ventilation, glazing, construction and how these all need to work together for a full solution. My current house is warm and comfortable and cheap to run (for me, not everyone) and i want to at least keep to that standard. I believe however, with a little planning and some better understanding of some things, i can only improve on what i currently have.

I wont write up an essay on where i am at with each element of the construction here in one thread as it would annoy me to proof read it back let alone for you folks to read it so i will start with the roof. Current house has sprayed insulation from wall plate to ridge with no vent card and sprayed onto breathable felt, and not counter battened under slates. No vapor barrier behind plasterboard. As much as i like the current setup, i have my doubts about its overall longevity in terms of the roof timbers etc. One of the great things about it however is that i didnt have to worry about an airtight layer between the attic and the rooms below. For light fittings, attic door, mhrv vents, wiring, plumbing etc, this was an added plus. It has also left us with a usable attic that isnt cold and draughty. I have been spending time reviewing the possible
best options, excluding my current, for roof construction insulation and would love to get peoples opinions on what that is. The idea of a cold ventilated attic space seems a waste to me so i have looked primarily at warm options(osb on rafters covered in a vapor tight layer, insulation, membrane, battens, slates), which i think allows for a completely sealed roof space. Has anyone any experience with a method like this? I assume you carry the vapor membrane down over eaves and continue it to you meet the external leaf of house? Is this right? Or even a practical solution? Cant see how the cost would be huge as i would just be moving the position of materials.
The second solution that keeps coming up is SIP roof panels. Again, anyone got any experience with them? Are they very costly? Assuming that you can just carry cavity insulation up to meet the underside of them, seems like detailing them would be easier? Has anyone any experience with using SIP roof panels on top of a cavity wall construction? Say if it is a pumped cavity, how is this detailed where the panel meets the wall plate and external leaf? Cant find any details showing how this is done on google. Is a traditional wall plate removed and instead, some kind of cavity closer at an angle that the SIP panel is nailed through and into?

I understand fully that it is impossible to say what will be cheapest or exactly how someone decides it would be done, all i am looking for is general advice and experienced opinions. For example, during the last build, i let myself get talked out of using geo thermal heating at the very outset because of people saying it would be too expensive to install etc etc. By pure chance i was doing some research on oil boiler prices in a local heat merchants and the manager was super helpful and through the course of the pricing, asked what type of boiler it was replacing. When i said it was for a new build, he took great interest in wanting to tell me all about their air to water heat pumps and about how people get the prices so wrong, and he opened the door to actually going out and getting some prices. So if someone says that a SIP roof costs about double of a standard roof with insulation etc factored in, then i will prob spend more time researching other options.

Thanks in advance for any help, opinions etc.