Tinkering with L1A 2013 compliance

Tinkering under the bonnet of SAP

Jon Ducker’s been getting under the bonnet of SAP to get a building to comply with the new Building Regulations for L1A 2013. Here he shares his experiences…

I’ve given up on the BRE cSAP calculator for modeling, as it takes too long to get at the results of minor changes to a calculation. Instead I’ve started using a SAP2012 development version of JPA Designer’s SAP calculation software (Build 21), which seems to be reasonable for the purpose for now.  Other software manufacturers have also got ‘beta’ versions of their software out too, but as far as I’m aware, none of it is BRE approved yet, so there’s a caveat attached to any results coming out of any of the software – any results should be taken with a pinch of salt.

The available calculators are generally missing some product data files (SAP appendix Q primarily), which complicates matters a little, but you can work around if familiar with SAP Appendix Q. They also cannot guarantee that all of the non fabric (TFEE/DFEE) information is fully up to date with SAP2012; but it’s still useful to try out the software, to gain an indication of what might be needed come April and to help pick out anything that needs to be worked on (providing feedback).

From initial modeling, the TFEE doesn’t seem to be too hard to achieve, provided the fabric specification is in the ballpark of the Notional dwelling SAP Appendix R values (including good thermal bridging details) and providing the remainder of the specification is also good.  Achieving better than the TER seems slightly trickier, but not overly so.

I’ve been looking at a variety of construction types, using ACD Psi values, Constructive details Psi values, Kingspan TEK Structurally Insulated Panels details  (and even a timber frame version with TRADA’s calculated details) and I have managed to get each to pass with a fabric led approach, picking up the DFEE requirement along the way in each case.

A key issue is knowing what air tightness is reasonable to target. The notional specification is based on a target of 5 m³/m²/hr @ 50 Pa.  Approved Document F however, recommends that mechanical ventilation be considered when designing dwellings that may achieve an air permeability of 5 m³/m³/hr @ 50 Pa or below, and BRE have previously suggested that any dwelling achieving an air tightness of 3 m³/m²/hr @ 50 Pa or below, must have adequate mechanical ventilation (or if achieving below that level, presumably the dwelling should be made more leaky).

If not air tightness testing every property, a developer needs to ensure that dwellings that aren’t pressure tested will still comply with an adjustment of +2.00 m³/m²/hr added to the average of tested results.

This means, that if targeting an air tightness of say 6 m³/m²/hr @ 50 Pa. for compliance, the tested properties would need to achieve  an average of below 4 m³/m²/hr @ 50 Pa, whilst also not achieving below 3 m³/m²/hr. This narrow band might prove tricky to achieve.

I’ve therefore  been targeting an air tightness of 6 m³/m²/hr @ 50 Pa.  for Timber frame or 7 m³/m²/hr @ 50 Pa for Masonry, as a reasonable design air permeability target when modelling houses with natural ventilation, to allow for more wriggle room for that issue.

The thermal bridging details seem to be more important at the levels now required and can make a considerable difference to compliance levels required.

As a case in point, for a typical small semi-detached masonry house (around 76 m² of total floor area over two floors), modelled with both Accredited Construction Details (ACD) and with Constructive Details (CD) the following specifications might be necessary to pass SAP 2012 / L1A 2013:

If following ACD’s (actual detail lengths and ACD Psi values):

  • Horizontal ceiling insulation to achieve 0.10 W/m²K.
  • External Walls to achieve 0.15 W/m²K (Brick/Block cavity wall with 150mm cavity with 100mm Kingspan Kooltherm K8 Cavity Board insulation and a 0.11W/mK internal block).
  • Floor 0.11 W/m²K (e.g. 150mm Kingspan Kooltherm K3 Floorboard on a beam and block floor).
  • Windows 1.20W/m²K (0.63 g-value).
  • Solid front door to achieve 1.00 W/m²K.
  • Half glazed door to achieve 1.20 W/m²K (0.63 g-value).
  • A Waste Water Heat Recovery system for main mixer shower.

If following partial fill Constructive Details (Psi values appropriate to the U-value ranges and actual detail lengths):

  • Horizontal ceiling insulation to achieve 0.11 W/m²K.
  • External Walls to achieve 0.18 W/m²K (CD) e.g. brick/block cavity wall with a 125mm cavity with 75mm Kingspan Kooltherm K8 Cavity Board and a 0.11 W/mK AAC Block.
  • Floor 0.13 W/m²K (e.g. 120mm Kingspan Kooltherm K3 Floorboard on a beam and block floor).
  • Windows 1.40W/m²K (0.63 g-value).
  • Solid front door to achieve 1.00 W/m²K.
  • Half glazed door to achieve 1.20 W/m²K (0.63 g-value).

Remainder of specification:

  • Party wall sealed and filled (assumed as 0.00 W/m²K).
  • A design air permeability of 7 m³/m²/hr @ 50 Pa.
  • Natural ventilation with 4 No fans.
  • A regular gas condensing boiler (a real one used for modeling 79.70% Summer /90.40% Winter).
  • A central heating circulation pump that is A rated (2013 or later) in heated space.
  • A weather compensator.
  • Time and temperature zone control; Boiler interlock.
  • Radiators.
  • Range Tribune 150l hot water cylinder (1.31 kWh/year declared loss factor).
  • Total water use designed to be 125 l/person/day.
  • Cylinder in heated space, thermostat and separate timer for domestic hot water.
  • All primary pipework insulated
  • No secondary heating.
  • 100% Low energy lighting
  • No renewables.
  • Assuming average overshading, 2No sides sheltered and East/West orientation.

The ACD version modelled achieved a TER of 19.36, DER of 19.26; TFEE of 50.40 and DFEE of 48.90.

The Constructive Detail version achieved a TER of 19.36, DER of 19.28; TFEE of 50.40 and DFEE of 47.60.

The new TFEE and DFEE metric ensures that dwellings will achieve a fair level of performance from the fabric; However it still seems reasonably easy to come up with a variety of routes to compliance, suggesting there’s still plenty of opportunity for tinkering.

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Jon Ducker is a qualified energy assessor working for Kingspan Insulation Ltd. He has an extensive knowledge of energy efficiency, renewable energy systems and sustainability in buildings with an expert knowledge of the relevant sections of buildings regulations and standards and their interactions with SAP. He provides authoritative advice regarding energy assessments for a wide range of public and private sector clients.

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