Structural progress

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.

Comments

[Russell Baillie]

Hi,
One of the wall options that I evaluated that looked good was standard 90 x 45 framing with vertical 45 x 45 strapping on the outside of the standard framing. That allowed 2 layers of standard insulation: continuous vertical rolls of 50mm insulation on the outside between the 45 x 45 timber and regular insulation thickness between the standard framing. Ply sheet or weatherboard cladding can fix straight to the 45 x 45 as long as the 45 x 45 is soundly fixed to the 90 x 45 studs.
Another thing to think about is the framing companies are tooled up for everything to be 90 x 45 and may need to reconfigure their work benches to run a house lot of frames through at 140 x 40. Also 3 nails a join instead of 2, etc, etc. It would pay to talk to some of them. Maybe they can handle it and you can run 140 x 40 studs with 90 x 40 nogs with the insulation as described above.
The 45 x 45 couldn't go on the inside as the gib lining wouldn't have the same support against kids running into walls.
At the end of the day though, when I ran Alf on the various options, for the Auckland climate the 6" walls didn't make that much difference over the 4" walls and I decided that it would be more effective to put that money into better glazing where Alf did show a greater improvement.

The roof trusses & ceiling insulation:
again i preferred a standardised truss manufacture and 2 layers of insulation. one between the truss bottom plate and the other layer of insulation at right angles running over the tops of the bottom truss plate, again minimising cold bridging.