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Part L and U Values

Part L and U Values

 

In October 2010, Approved Document L of the Building Regulations (England & Wales) was introduced. There were a number of important changes to the 2006 version all of which aim to improve CO₂ emissions by 25% over the 2006 regulations. The key changes include:

• Improved limiting u-values for building elements
• Party Wall u-values (aka Party Wall Bypass)
• Improved Airtightness Requirements
• Linear Thermal Bridging

These measures are part of the ‘fabric first’ approach which basically attempts to get the building as thermally efficient as possible before adding on the renewables and other energy-saving techniques. This is the most sensible approach and it is anticipated that the next revision of Part L in 2013 will continue this concept.

Limiting U-Values

The table opposite shows the maximum permitted u-values for walls and floors in Part L1A (New build dwellings) and Part L1B (Existing dwellings).
In practice, lower u-values are aimed for to help the required % reduction in CO₂ from the SAP calculation be achieved easily. Currently for new build dwellings, walls tend to be 0.28 and below, floors 0.22 and below.
On the right-hand side of this page are tables of u-value solutions for meeting these targets using our range of concrete blocks.

Party Wall Bypass

A new concept introduced into Part L 2010 is the Party Wall bypass. The theory is that heat can be lost from the dwelling up through the cavity party wall and that therefore this heat loss must be accounted for in SAP.

Robust Detail Party Walls as approved for use with Thomas Armstrong’s range of blocks have been updated to include fully-filled options and therefore allow a party wall u-value of 0.00 W/m²K to be applied.

Furthermore, solid wall options (traditional blocks laid flat or solid Airtec blocks) allow a u-value of 0.00 W/m²K to be applied.

Air Permeability

The limiting value for air permeability has been set at 10m³/hr.m² tested at 50 pascals, and the requirements for on-site air permeability testing has been increased over previous regulations. For non-tested properties, the regulations say that an additional 2 m³/hr.m² must be added to the figure achieved for tested figure for a same house type. Therefore in practice designers are aiming for a maximum of 8 m³/hr.m² to ensure compliance with Part L.

Masonry block walls using any type of Thomas Armstrong concrete block have been shown to easily achieve these figures provided that good workmanship is employed. Figures of around 5 m³/hr.m² are commonplace and are desirable.

A parge coat on the internal wall faces can contribute to exceptionally low air permeability figures of 2 or less which help to achieve Passivhaus and Code Level 4 requirements. However, the designer has to be aware that air quality might suffer and mechanical ventilation systems may have to be installed to avoid condensation and related health issues for the residents.

Linear Thermal Bridging

All of the above measures result in dwellings with higher thermal insulation and greater airtightness. This has increased the significance of heat loss at junctions and must be accounted for in SAP calculation.

Simply put, the detail of each building element junction in the property (e.g. wall/window, wall/floor junctions etc) must be assessed for heat loss to give a ψ (psi) value for that junction. The values are then added together to give an overall linear thermal bridging figure for the property, its ‘y-value’. There are 3 routes to determining the overall y-value of a building:

Option 1
Do nothing, use a y-value of 0.15
If the designer does not account for each thermal bridge, then a default y-value of 0.15 must be applied in SAP 2009 which will be significantly disadvantageous in achieving desired DER and % reduction in CO₂ requirements. The only way to gain a pass in SAP would be to use PV or renewables at great expense.

Current Status: For aggregate and concrete blocks, use table K1 values (option 2). For Airtec blocks, Option 3 may be used. As part of the Aerated Products Association there are over 30 modelled junctions available which are being continually added to. Please contact us for details.

Option 2
Use Accredited Details from table K1 in the guidance to SAP 2009
In the absence of any approved government scheme, it is permitted to use the accredited values in table K1 found in the guidance for SAP 2009 for the purposes of SAP calculations. An overall y-value of around 0.08 would be expected. Once an ACD scheme is up and running, on-site inspections of the as-built vs design will need to be carried out.

Option 3
Use specific modelled junctions
ψ values for specific junctions, using specific building materials can be calculated by trained, approved persons. These can lead to y-values of 0.04 or lower which can greatly reduce the need for PV etc in order to achieve SAP target DER figures.
No site inspection is required but a 25%, or 0.02 confidence factor (whichever is the greater) must be added to each ψ value. Nevertheless using modelled junctions result in better values than any ACD scheme and can result in significant cost savings for the builder.

At the time of writing there is no government approved ACD scheme in place and the full Airtecψ values can be used without the 25% penalty.