The eco lodge’s design stage SAP calculations

We’ve found a local energy assessor to do our SAP (Standard Assessment Procedure) calculations. At the design stage of a dwelling you have to show that it will have a sufficiently low carbon footprint when it is being used (its energy running costs if you like). Sufficient that is to meet current building regulations. Every dwelling has to have both a design and final stage SAP (and after September 2010 the rules for the design stage get even stricter). Our assessor is John Harding (07810 388263) – Ian picked him because he showed a real interest in our unusual build and seemed keen to get involved in sustainable building (plus he’s local to us in Devon, living in South Molton, and says he can sharpen a mean scythe!) The SAP gives you a score on a scale where 100 would be complete carbon neutral (ie running the building has no net carbon footprint).But to get the score you have to assess a lot of component parts – and big parts like floors, walls and roof can themselves be complex. For example, between inside and outside of the building you have an insulated cavity (straw in our eco lodge) but in places structural components bridge that cavity – called ‘non-repeating thermal bridges’ – like the window sills and where the rafters join the tops of the walls. All these have to be detailed so you can calculate the thermal transmittance (or resistance) of the wall. To get the figures right you need to consider the materials used, their thickness (eg the stud work) and how much of the wall they represent. The windows also need to be considered. They will lower the SAP score because they transmit heat out. But they have their good side too – through solar gain. So their contribution to the score depends where they are on the building and what you use to glaze them. We have big South facing windows (they gain heat in the winter when the sun is low but absorb less in summer when the sun is higher in the sky) and also East facing openings (West facing windows can absorb too much heat during the summer, meaning you need to cool the building, without offering that same heat in the winter when the Sun is more southerly).

We’re pretty sure we’re going to put double glazing in the big south facing windows because they’ll give us more solar gain. Triple glazing is more about sound proofing, and although it does retain more heat it also keeps out warmth from solar gain in winter (but windows on the northern side, where the entrance door is, will be triple glazed). The design stage SAP should help us understand the consequences of that sort of choice.

And then again your SAP score also depends on how you are planning to run the building – what you’ll be using for heating and hot water etc. Interestingly, a less well insulated building that is efficiently heated and powered can have a similar SAP score to one that is well insulated but more poorly run.

Of course, we’re aiming for the highest (best) SAP we can get. John said that if we’d known exactly what we were going to build (and how) before even picking up a trowel we could also have applied for the higher levels of the BREEAM code for sustainable homes.

But our build is more ‘as you go’ than that, partly because we’re researching it as we go along – finding our local source of timber, and our straw bales from a neighbouring farmer ‘across the fence’. And BREEAM certification also comes with a hefty price tag attached.

So we reckon the best way to get the highest SAP score we can is to take advantage of the requirement for design stage calculations and then see where we can make improvements.