We engaged a structural engineer this week. This is pretty exciting as it means we are moving into the next stage of the design, "Detailing". A full set of plans for Council consent will be produced in this stage.
Presently we are looking closely at the best way to build the walls and roof to minimise thermal bridging without significantly increasing cost. There are a number of important factors to weigh up through this process. We want to minimise the additional material that we use and the additional labour that is needed to build the frames.
From the cost assessments we have done so far it looks like we can build a wall that uses no more timber, with no more labour (possibly less) than a standard 90x45 framed wall. The wall will be 50% thicker, meaning we can fit 50% more insulation. The framing configuration significantly reduces thermal bridging, in combination with the higher insulation levels the R value for the wall is more than 50% higher than standard. There are 2 downsides; 1) The wall will be thicker slightly increasing the footprint of the building, 2) The wall will be "non standard", requiring our structural engineer to sign it off rather than the simpler route of compliance to NZS 3604:1999.
We will be adding some more information on the types of wall construction that we are considering under the Building Envelope section.
We are also looking closely at the roof construction to achieve a high R value. We are finding that the insulation thickness can be the determiner of the size of rafters and purlins rather than the structural requirements. Higher performing insulation can be used which achieves the same R value while being thinner, this is however more expensive. We are carefully weighing up the cost of larger timber vs the cost of higher spec insulation.