SAP Calculations + Rainwater tank

SAP Calculations
Are calculations to measure the energy rating of residential buildings and prove that minimum criteria are achieved.  Calculations of typical annual energy costs for space and water heating, lighting and CO2 are included.

Calculated Rating for 70 Upper Perry Hill

For low energy buildings SAP calcs. are not very reliable in predicting actual performance accurately and the way certain building features are rewarded may not make complete sense, but these are hoops that we must jump through!

The result for no. 70 are OK, see right, but no. 68 (terraced house) is some considerable way below an acceptable figure.

The reason for this seems to be that because the party walls either side of the house are fixed, the scope for increasing the level of insulation to floors, walls and roof is limited.

The two major components that influence heat loss from a building are insulation and airtightness, so the following options have been put forward to improve the SAP value:

• Reduce the number of air changes per hour from 8 to 4 (Maximum Building Regulation requirement is currently 10 and the 'energy efficient' 70UPH is 3)  So this would be a significant change to the specification that the contractor has priced, but may be achievable by introducing sealing tapes and membranes to 'seal' the external envelope, at additional cost.
• Increase the insulation particularly in the flooring and roof and try to relax the air change rate.
• Reduce the size of the large folding sliding doors to the garden.  This will only have a small effect, but since they face NW they do not benefit from thermal gain and the cost saving in the doors can be used to fund increased insulation.
• Install an air source heat pump in place of a gas boiler.  This scores bonus points in the SAP calculation because although greater capital costs the saving in energy use over time helps to recoup this.

So introducing an air source heat pump may alone solve the problem, but alternatives to this will need to be costed as an 'extra' by the contractor to establish which option is most efficient.

Rainwater Tank
A while back I visited Ecobuild and had a chat with staff at the Rainwater Harvesting stand who confirmed that the minimum cover depth is 700mm for tanks. 
I have this past week got round to checking this against the current drawings (in a break from packing) and realised that at this depth the proposed tank is getting too close to the 'zones of influence' of foundations. My previous assumptions were based on the fact that we have a concrete slab over the top anyway so the depth could be reduced.  Alternatives are to forget rainwater harvesting, or possibly install a pair of linked 1500 litre tanks to achieve the desired 3000 litre capacity.  This is all getting a bit complicated and more expensive!

I emailed the tank manufacturer in Germany (by-passing the UK supplier) who thankfully confirm that shallow depths are possible if the slab is designed to impose no load on the tank and bears on a 1 metre zone around the tank.

Quick phone call to the Engineer suggests this is feasible if the slab over the tank is increased from 150 to 200mm and has additional mesh in the lower part to provide structural capability.  I need to draw this up in section to check that it all works.
There will be increased costs in additional concrete and mesh, but in theory the excavations will not be so deep and cart-away costs and fill will be reduced and hopefully it will fit!