Building Regulations standards

The Building Regulations control the overall energy performance of the building, which takes into account its size, form and type of fuel. The Regulations do not stipulate the performance of the construction except to lay down minimum values of insulation of elements.

The thermal performance of an element of construction is expressed as its U-value, a measure of the amount of heat that will pass through one square metre of the construction with a temperature difference of 1°C between the inside and the outside of the building, W/m2K – the lower the better. Minimum acceptable values of W/mrK are: a. An area-weighted average of all elements of a particular type: Wall 0.35, Floor 0.25, Roof 0.25, Windows, Rooflights & Doors 2.2. This is roughly equivalent to 100mm of air-based insulation in the walls, and between 150 and 250mm in the roof. Windows need to be double-glazed with low-emissiv- ity coatings to achieve the required U-value.


Building Regulations standards


In addition, there is a lower standard to be applied to any particular element or part of an element with the aim of preventing condensation risks.

  1. Individual element values: Wall 0.70, Floor 0.70, Roof 0.35, Windows, Rooflights & Doors 3.3.

Thermal bridges

The Regulations also control the effect of thermal bridges which are non-insulating parts of the construction allowing heat through and thus reducing the overall performance. Repeating thermal

bridges can occur within an element of construction and common examples include wall ties in cavity wall construction, studs in timber construction and insulation between but not over ceiling joists in a loft. These effects can reduce the performance by 10% or as much as 50%. They can be overcome by using plastic wall ties, composite timber studs and ensuring that insulation is of a thickness that covers the ceiling joists. The effect of repeating thermal bridging is taken into account when calculating the U-value of an element of construction. Non-repeating thermal bridges can occur at the junction between elements. Common examples are around windows and where the intermediate floor or the roof meets the wall. These effects can also reduce the performance of the envelope by up to 50%. The effect of these nonrepeating thermal bridges is taken into account in the SAP by assuming a default figure based on common construction. You can improve on this by using accredited good practice details or by devising a particularly efficient form of construction which significantly reduces cold bridges and providing calculations which show the actual effect of the cold bridging.

Air leakage

The Building Regulations control the unwanted infiltration of air into buildings and homes. From April 2006 homes have to be air-tested. The building is pressurised by a large fan installed in the front-door opening and the amount of air leaking to the outside can be measured. The air permeability of the building will have to be less than 10 m5/hour/m2 of exposed external surface at an applied internal pressure of 50 Pascals. For small buildings, this measure is roughly similar to 10 air changes/hour (ac/hr).

This is not a tough standard. The average for all houses, including very draughty Victorian ones, is estimated to be around 14 ac/hr and for houses built since 1960 10 ac/hr. A modern house would normally achieve around 8. A reasonable standard

Above: Timber around windows and at the floor level can create thermal bridges which may significantly reduce the thermal performance of the construction.

Using a smoke generator, it is very instructive to test where the draughts come from in a building. Old windows (especially sliding sash windows) and doors are very draughty. Draught-stripping old windows and doors is probably the most cost-effective energy-saving measure for an old house. Modern joinery is generally reasonably well draught- stripped, but make sure that casements are rigid enough for the ironmongery to properly close the casements against the seals, particularly on double doors. Make sure too that the frames are sealed to the structure. Other common problems occur where the first floor meets the external walls, allowing draught into the floor void which comes out at skirtings and electrical sockets and switches. Loft hatches, timber suspended floors, service entries, service penetrations into the loft and around rooflights are also common sources of air leakage. Plasterboard dry lining creates a void in the walls which can carry draught throughout the building if the external envelope is leaky.

Wet plaster on a masonry wall, or cellulose fibre insulation fully filling the cavities in a timber-frame wall, are better than dry lining or mineral wool insulation respectively; even so, they are not sufficient. You have to be positive about sealing the building at the design stage and follow through by ensuring high-quality construction on site. Sealing can be achieved either on the inside or the outside. The advantage of sealing on the outside is that you seal over the critical floor-to-wall junction. The disadvantage is that, if you have a loft, it is difficult to seal the ceiling to the outside of the wall at the eaves. This can be avoided if you design out a cold ventilated loft by having the top-floor ceiling following the underside of the sloping roof. With timber-frame construction, sealing the inside can be achieved either by bedding the plasterboard lining on mastic or by incorporating an air/vapour barrier. Sealing the outside can be carried out by a ‘housewrap’ of moisture- permeable building paper with taped joints. The choice of air-flow retarder is also linked to your strategy for dealing with moisture movement and countering the risk of condensation.

The standard of construction is a critically important aspect of achieving an airtight building; contractors have to understand the need for high-quality construction and pass this on to their subcontractors, particularly the electrician and plumber, who are both used to banging holes in the structure to let pipes and wires into the building without paying much attention to sealing them up afterwards. Service penetrations often account for half the air leakage. Close supervision is required on site.

Methods which have proved effective in timber- frame construction include providing a service void for pipes and wires on the inside of the external walls so that the air-flow retarding membrane does not need to be punctured by services and requiring the timber-frame subcontractor to hand over a weathertight and TESTED airtight structure. The services are then installed and the airtightness is tested again before the plasterboard finishes are fixed.


When warm moist air meets a cold surface condensation will take place, as on a cold window surface in winter. Condensation repeatedly running off the glass may cause the window-frame to rot. Ventilation will avoid condensation and is essential in a loft and below a suspended timber floor. If a house is well insulated and fitted with double- glazed windows, the inside surfaces will be relatively warm and there will be no surface condensation. However, moisture vapour will be driven through the construction by a higher vapour pressure inside the building towards the cold outside layers of the construction where it may condense and reduce the effectiveness of the insulation and cause decay and mould growth. This process is happening repeatedly in a masonry wall, but the materials used do not generally decay.

However, the same is not true of a timber-frame construction.

Condensation can be prevented by:

  • Providing adequate ventilation to prevent the build-up of excessive moisture, providing adequate heating to make sure that surfaces are warm enough inside to prevent condensation taking place on the surface (it may still take place within the construction when it is termed interstitial condensation) and providing adequate insulation, again to raise internal surface temperatures. Minimum standards of ventilation and insulation are laid down by the Building Regulations.
  • Ensuring that the construction is airtight. Much more moisture is carried into the construction through air than through vapour diffusion.
  • Ensuring that the inside is resistant to vapour passing into the construction. Materials such as metal and glass are impervious to moisture; others such as a polythene sheet are very resistant. Most building materials, on the other hand, are relatively permeable to moisture vapour. Vapour resistance is usually achieved by a vapour-check layer, commonly a polythene sheet or plasterboard with a thin plastic film on the back. It is important that this layer does not have gaps which allow warm moist air into the construction. Electrical switches and sockets are a common source of discontinuities. This can be avoided by installing them in a service void inside the vapour control layer.
  • Ensuring that moisture can always pass easily to the outside. In this way you make sure that moisture cannot build up within the construction. The rule of thumb is that the outside should be at least five times more permeable to vapour than the inside. In a timber-frame construction, the outside of the construction may be a fibreboard or building paper which has a ventilated cavity behind a rainscreen cladding of tiles on a roof or render, masonry or timber on a wall. You can use relatively vapour-permeable materials and a hygroscopic insulation material, that is one that can absorb a certain amount of moisture (cellulose fibre for example, rather than fibreglass which cannot) to form a ‘vapour-balanced’ or ‘breathing’ construction. This is similar to traditional building construction (soft bricks, lime plaster and lime mortar) in contrast to most modern construction which uses hard and nonabsorbent products such as glass, metal, hard bricks, Portland cement mortar and gypsum plaster. A ‘breathing’ construction is a risk-free solution, and some claim creates more even levels of humidity inside the building.

What is vital is that the construction must be airtight, and with high vapour resistance on the inside and low vapour resistance on the outside for condensation to be avoided.

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