The following quotes from relevant standards and guidance set out the context under which architects and design certifiers should commission thermal bridge and hygrothermal risk assessment in many projects.
Many of the issues relate to retrofit of traditional or historic buildings (whether for use as dwellings or not) however many do not: modern green roof construction in Ireland, for instance, can be a higher risk measure. Of course many constructions such as new build cavity wall construction do not require formal hygrothermal risk evaluation (instead the designer should refer to guidance in Appendix B of TGD L(2011) and BR 262), but the designer or certifier should be very aware of how and when where risk grows and takes appropriate steps.
A list of forms of construction that are higher risk can be found at: .
BS 5250:2011 Code of practice for control of condensation in buildings
Foreword
The occurrence of condensation is governed by complex interrelationships between heat, moisture, air movement, building layout and the physical properties of building materials. The designer’s choice of plan form, envelope materials, construction details and systems of heating and ventilating can ensure the risk of damaging condensation is minimized under reasonable conditions of use. The builder’s understanding of the design intent, together with good workmanship under proper supervision, can then result in a healthy environment within a durable building envelope. Bearing in mind that occupants often fail to use buildings in the manner intended, be it by choice, lack of understanding or force of circumstance, designers are advised to err on the side of caution and adopt robust fail-safe solutions.
When it is proposed to re-furbish a building or make changes to its use, the risk of condensation has to be re-assessed in the light of the new usage.
D.3 Interstitial condensation
D.3.1 Principles
BS EN ISO 13788 describes a method (known as the “Glaser method”) for predicting the risk of interstitial condensation occurring under specified environmental conditions. The method indicates how much condensate will be deposited, how much will evaporate and any balance which might accumulate year on year. The method uses monthly mean external conditions to calculate the amount of condensate deposited and/or evaporated in each of twelve months, following the first predicted occurrence of condensation.
The method is a useful assessment tool, suitable for comparing different constructions and assessing the effects of design changes. It assumes one-dimensional, steady state conditions and does not consider air movements within or through the construction and makes no allowance for the moisture in the materials or rainwater absorbed during construction. Consequently, while it is useful for comparing the performance of different structures, it does not provide an accurate prediction of moisture conditions within the structure under service conditions. More advanced methods, which are standardized in BS EN 15026, are available and are described more fully in [5].
TGD L, Conservation of Fuel and Energy - Dwellings
0.2.1 General
0.2.1.1 The incorporation of additional thickness of thermal insulation and other energy conservation measures can result in changes in traditional construction practice. Appendix B contains general guidance on the construction and installation for common forms of construction for roofs, walls and floors.
0.2.1.2 Care should be taken in design and construction to ensure that changes do not increase the risk of certain types of problems, such as rain penetration and condensation. Some guidance on avoiding such increased risk is given in Appendix B of this document. General guidance on avoiding risks that may arise is also contained in the publication “Thermal insulation: avoiding risks; Building Research Establishment (Ref BR 262)”.
(Blog Note #1: where the guidance in Appendix B is not relevant, or sufficiently relevant, the onus on the designer to ensure what they are proposing is satisfactory is higher. In absense of direct guidance supplementary support should be sought)
0.6 Application to buildings of architectural or historical interest
0.6.4 In general, the type of works described above should be carefully assessed for their material and visual impact on the structure.
0.6.5 Historical windows and doors should be repaired rather than replaced, and internal insulation and damp-proofing should not disrupt or damage historic plasterwork or flagstones and should not introduce further moisture into the structure.
1.3.3 Thermal bridging
1.3.3.1 To avoid excessive heat losses and local condensation problems, provision should be made to limit local thermal bridging, e.g. around windows, doors and other wall openings, at junctions between elements and at other locations. Any thermal bridge should not pose a risk of surface or interstitial condensation and any excessive increase in heat loss associated with the thermal bridge should be taken account of in the calculation of average U-value.
Appendix B.3 Condensation in buildings occurs whenever warm moist air meets surfaces that are at or below the dew point of that air.There are two main types, surface condensation and interstitial condensation. Surface condensation occurs on the surface of walls, windows, ceilings and floors and may result in mould and mildew. Interstitial condensation occurs within the construction of the building and can damage structural materials or make insulating materials less effective.
Full checks should be performed on the likelihood of surface and interstitial condensation of a construction detail in accordance with I.S. EN ISO 13788. This standard contains recommended procedure for the assessment of the risk of: -
- surface condensation and mould growth;
- interstitial condensation.
I.S. EN 15026 can also be used to assess the risk of surface condensation and mould growth. The transient models covered in this standard take account of heat and moisture storage, latent heat effects, and liquid and convective transport under realistic boundary and initial conditions.
(Blog note #2: BS 5250:2011 which was issued after TGD L:2011 was completed is highly critical of I.S. EN ISO 13788. See D.3.1 above)
(Blog note #3: Not all Appendix B details are good. For instance the use of internal insulation of a hollow block wall, without first sealing the room face of the hollow block wall with a good scratch coat, is foolish considering how permeable the blocks themselves are to air, and the potentional impact on heat loss if even a small artea of the 45mm wide mortar joints was missing).
Appendix D.2 Mould growth and surface condensation
For dwellings, the value of fRsi should be greater than or equal to 0.75, so as to avoid the risk of mould growth and surface condensation. For three-dimensional corners of ground floors this value may be reduced to 0.70, for all points within 10 mm of the point of lowest fRsi.
TGD L, Conservation of Fuel and Energy - Buildings other than Dwellings
Introduction
Existing Buildings
In the case of material alterations or change of use of existing buildings, the adoption without modification of the guidance in this document may not, in all circumstances, be appropriate. In particular, the adherence to guidance, including codes, standards or technical specifications intended for application to new work may be unduly restrictive or impracticable. Buildings of architectural or historical interest are especially likely to give rise to such circumstances. In these situations, alternative approaches based on the principles contained in the document may be more relevant and should be considered.
0.2 TECHNICAL RISKS AND PRECAUTIONS
General
0.2.1 The incorporation of additional thickness of thermal insulation and other energy conservation measures can result in changes in traditional construction practice. Care should be taken in design and construction to ensure that these changes do not increase the risk of certain types of problems, such as rain penetration and condensation.
1.2.4 THERMAL BRIDGING
1.2.4.1 (see quote for 1.3.3.1 above) The additional heat loss associated with thermal bridges should be limited to less than 16% of the total calculated heat loss through the plane building elements.
(Blog Note #4: Unlike with TGD L-2011: domestic where 0.15 or 15% is allowed as a default value for how much thermal bridging occurs (when expressed as a percentage of the heat loss over the whole envelope area) regardless of how much actually occurs, the 16|% mentioned here IS a requirement: that is to say 17% could be judged non-compliant)