The benefits of improved construction detailing

The energy consumed by dwellings accounts for a large proportion of Ireland's total energy consumption, and of the carbon dioxide emissions, which contribute to climate change.

Much of this energy is accounted for in the space heating. Insulation standards for roofs, walls, windows and floors in the Building Regulations have increased over the years to improve efficiency by reducing heat loss. These standards relate to the average performance of specific elements.

As insulation standards improve, the significance of local areas of reduced insulation (thermal bridging) e.g. at joints and around the edges of window openings, and gaps in the building envelope leading to air leakage, becomes increasingly important in terms of contribution to overall heating and ventilation losses from the dwelling. Reducing heat loss due to easily avoidable air leakage can significantly improve the energy performance.

Effects of thermal bridging and air leakage include-

• Surface condensation, damaging decorations and enabling mould growth

• Deterioration of the building fabric caused by interstitial condensation

• Occupant discomfort caused by draughts and cold rooms

To reduce the impact of these and to address the potential health risks they may pose to building occupants, insulation continuity and airtightness need to be thoroughly considered at all stages of design and construction.

Insulation Continuity

The thermal performance of a plane building element (within a particular construction) is described by its U-value (W/m2K). This is a measure of the heat transmission through the element per degree of temperature difference (degrees Celsius denoted as degrees Kelvin to signal temperature difference) between the internal and external environments. Thermal bridging typically occurs at the junctions between planar building elements, e.g. at wall/roof and wall/floor junctions, and around openings, e.g. at window jambs, where the continuity of the insulation is interrupted. Thermal bridging increases the heat loss and the risk of condensation due to the lower localised internal surface temperatures.

BRE IP 1/06 describes a method of quantifying this extra heat loss at a thermal bridging by way of its linear thermal transmittance, or Psi (Ψ) value, in units of (W/mK).

Building Regulations 2021 TGD L Dwellings indicates that heat loss calculations should include the effects of thermal bridges when calculating the Maximum Permitted Energy Performance Coefficient (MPEPC) and Maximum Permitted Energy Performance Coefficient (MPEPC).

BRE Information Paper IP 1/06 also describes a method of assessing the effects of the low internal surface temperatures (that result from the construction) by way of the temperature factor f, or fRsi. Depending on the intended building function, the temperature factor (f min) of the detail must be no less than the critical temperature factor, f c, given in the paper. Use of the details in Part 2 meet with the guidance in Par 1.3.3.2 and Appendix D of TGD L 2021 Dwellings, for rooms within a typical dwelling, and make reasonable provision with regard to limitation of thermal bridging.

Airtightness

The airtightness of a dwelling, or its air permeability, is expressed in terms of air leakage in cubic meters per hour per square metre of the dwelling envelope area when the building is subjected to a differential pressure of 50 Pascals (m3/(h.m2) @50Pa).

The dwelling envelope area is defined in this context as the total area of all floors, walls and ceilings bordering the dwelling, including elements adjoining other heated or unheated spaces.

Air leakage is defined as the flow of air through gaps and cracks in the building fabric. Uncontrolled air leakage increases the amount of heat loss as warm air is displaced through the envelope by colder air from outside (infiltration). Air movement of warm damp air through the building structure to outside (exfiltration) can also lead to interstitial condensation within the fabric, which reduces insulation performance and can cause fabric deterioration.

The air permeability of a building can be determined by means of a pressure test. The procedure for testing is specified in I.S. EN ISO 9972:2015 “Thermal performance of buildings: determination of air permeability of buildings: fan pressurization method”. Additional guidance on testing procedure is given in the ATTMA publication “Measuring air permeability of Building Envelopes”.

Building Regulations 2021 TGD L Dwellings indicates that reasonable provision for airtightness is to achieve a pressure test result of no worse than 5m3/(h.m2)@50Pa. Current typical values achieved in NZEB dwellings are less than 3m3/(h.m2)@50Pa.

The airtightness appropriate for a particular dwelling design will depend upon the Building Energy Rating the builder is aiming to achieve. Care should be taken to ensure compliance with the ventilation requirements and permanent air supply of Part F and of Part J of the Building Regulations respectively. For further information, see TGD F and TGD J.

Adopting the details in this publication should help to achieve airtightness of 5m3/(h.m2)@50Pa or better.

All materials used for airtightness should meet the guidance in Technical Guidance Document D “Materials and Workmanship”, be fit for the use for which they are intended and for the conditions in which they are to be used.

In assessing the fitness for use and conditions of use of a material or a product, consideration should be given to durability, safety, local climatic conditions (e.g. wind driven rain, humidity etc.) and other such issues.