Detailed Construction Drawings
The following guidance may be considered good practice for delivering insulation continuity and airtightness in construction. The guidance considers projects at three stages – Design, Construction and Testing.
Design Stage
The complexity of the modern building envelope requires that consideration is given to achieving insulation continuity and done at both strategic and detail level.
Consideration at the strategic level involves the primary construction and insulation method (masonry cavity insulation, insulated timber frame, etc.) and selecting the primary air barrier elements (plaster finishes, sheathing boards, etc.). The choices made dictate the philosophy for the remainder of the design and construction process.
At the detail level it is important that the design builds upon the above strategy showing the builder how to maintain insulation continuity and airtightness. Achieving continuity in practice requires that the designer:
Identifies the components which form the insulation layer and air barrier in each part of the construction
Develops details that achieve continuity of the insulation and air barrier between each part of the construction and the next
Communicates the intentions clearly to the builder
The air barrier line – The air barrier is a layer within the building envelope which will adequately restrict the passage of air between the internal and external environments. The barrier should closely follow the line of the inside face of the insulation in the exposed elements of the fabric of the building.
Consideration should be given at an early stage as to which layer of each exposed element of the fabric will form the primary air barrier, and to the junctions between them. The details in Section 2 assume the air barrier will be formed largely by internal plaster or plasterboard finishes.
Pen-on-section drawings – It is good practice to mark up the air barrier line on the architectural main section drawings as a bold distinguishable line. If the air barrier is continuous, it should be possible to trace around the whole section without lifting the pen. If you have to lift the pen, you have discontinuity and a potential air leak. The details in Section 2 show the air barrier as a bold dark blue line.
Larger scale drawings - It is good practice to prepare large scale drawings of sensitive points in the design. These drawings should clearly identify the insulation and the air barrier. The drawings should be issued to all relevant parties identifying how the integrity of the insulation layer and air barrier is maintained at particularly complex interfaces.
The following will help achieve insulation continuity and airtightness:
Keep it simple! Simple designs are more likely to get built right.
Decide which layer of the construction provides the air barrier. Stick with this. Use the pen-on-section test to check continuity and to identify key details.
Minimise the number of different types of construction within the thermal envelope – wherever one form of construction meets another, problems are likely to occur.
Pay careful attention to the design of junctions between elements to ensure continuity of the air barrier. Think the construction sequence of each detail through, to ensure that it can be built. Change details if it becomes apparent they do not work, or if site staff identify better ways of doing them.
Favour simplicity of form – complex forms increase the number of junctions within the thermal envelope, each of which increases the likelihood of discontinuities.
Minimise penetrations of the thermal envelope, whether by services or structure or construction. A services space inside the air barrier line can help reduce service penetrations.
Where penetrations are unavoidable (soil stacks, ventilation exhausts and intakes, water supply, electricity and gas supplies), develop appropriate details for their proper execution, for making good damage to insulation, and for re-sealing pipes and ducts to the surrounding air barrier.
Construction Stage
Three basic principles should be addressed during construction to ensure insulation continuity and effective air barriers: Management, Communication, and Quality Control.
Management – On-going review of the design is required. The project management should ensure that details of all design changes involving elements of the external envelope are distributed throughout the design, procurement and construction teams.
It is important that the project programme reflects the required sequence for effective formation of the air barrier and insulation installation e.g. eaves insulation. All trades must be permitted access to form not only the part of the insulation layer or air barrier for which they are responsible, but also to ensure that continuity is achieved between their works and that of other contractors.
It may be prudent when compiling the programme to include an “Air Tight” milestone. Knowledge of this date may permit management to schedule thorough envelope pre-test inspections and test dates in advance of the end of the project. Testing during the construction stage along with good quality control procedures allows problems to be identified and corrected early in the construction process prior to final testing.
Communication and Education – Personnel involved in procurement and constructing the building fabric should understand the need for insulation continuity and airtightness. The more aware people are of the issues, the less likely essential components will be engineered out of the design for cost savings, and the more receptive site staff will be to requests for a higher standard of workmanship.
Awareness may be raised at key stages by briefing procurement offices and site tool-box talks. The detailed pen-on-section drawings may be issued to all parties clearly identifying where and how insulation continuity and the air barrier will be maintained.
Operatives directly involved in constructing the insulation and air barrier should be encouraged to draw attention to difficulties and request direction rather than to muddle through.
Operatives not directly involved in the building fabric should also be made aware of the importance of insulation continuity and the air barrier and of flagging up any breaches through these “lines of defence”. They should also be required to remedy potential thermal bridges or air leakage routes brought about by their own activities, or to seek help from other trades, depending on the nature of the breach.
Quality Control – Many contractors now have systems in place for monitoring the quality of their processes and products. Experience shows that the Quality Assurance (QA) should be extended to check for insulation continuity and airtightness. The ACD sheets can be used for this.
An essential QA control is that insulation continuity and airtightness are considered during all design changes and material substitutions affecting the external envelope. An ill-formed design change may jeopardise the final performance of the building envelope.
The QA process should ideally involve inspection of finished works especially the building envelope. This will enable management to check that all works are properly constructed prior to being covered over.
Testing Stage
Insulation Continuity – Inspection of the insulation will largely be a qualitative assessment during construction. This should be a series of inspections as recommended above. These inspections might be recorded as a series of brief reports supplemented by photographs as well as the completed ACD checklists.
Airtightness – The air permeability test is usually undertaken as the building nears completion. The external envelope must be practicably complete with all windows, doors and service penetrations installed and air sealed. The test is a quantitative assessment which culminates in either a pass or a fail result against a design value.
It can be useful to perform air tightness checks on sections of the building during construction to identify areas of leakage prior to completion of finishes.
Where the building fails to meet the required airtightness standard, inspections might be undertaken utilising tracer smoke to identify areas of excessive air leakage. Remedial works must then be undertaken to improve the airtightness performance of the fabric. Depending on the design and the formation of the air barrier, this might be difficult and time consuming, ultimately delaying completion.
Implementing Acceptable Construction Details Into Design
To make best use of the ACDs, the following will be helpful:
Detail drawings (sections and plans) identifying the line of the air barrier
List of Acceptable Construction Details incorporated into the design
List of the builder’s own details incorporated into the design
Specification of the air barrier materials / elements
Details of air barrier junctions and interfaces including means of sealing service penetrations
Evidence of Site Quality Control during construction (photos, check sheets, etc.)
Use of Acceptable Construction Details
Section 2 of this publication provides a series of Acceptable Construction Details showing typical junction interfaces for various construction types.
Table G1 - Types of Construction
As well as the above, a small number of details are applicable across several construction types and are referred to as Type G, General details.
At the start of each section there is an index outlining the details described in that section. The specific psi values associated with the details are also provided at the start of each section.
To assist the user, all junction types for a particular construction are collated into one .pdf file, downloadable from the Department of the Environment Community and Local Government website – http://www.environ.ie.
Acceptable Construction Detail Sheets
The ACD sheets comprise a checklist together with an indicative illustration and general annotation. The purpose of the illustration (used in conjunction with the checklists and general notes) is to provide generic guidance on the key features that must be incorporated into the actual designs. They do not provide a complete solution to airtightness or insulation continuity on any particular project.
The general notes outline important issues regarding performance of each junction in terms of insulation continuity and airtightness, and raise potential conflicts such as ensuring adequate ventilation to roof voids.
Whilst the notes and the comments are not exhaustive users must satisfy themselves as to the fitness of their own designs for the intended purpose.
The checklists enable the builder to make reasonable provision for ensuring insulation continuity and airtightness. The ACD checklists may be used for this purpose for each type of dwelling. It is recommended that the design and construction be reviewed at key stages during the process by using the checklist.
Substitution of Contractors Own Designs and Proprietary Designs
If use of details other than Acceptable Construction Details is proposed for use in construction they should meet the alternative requirements given in Paragraph 1.3.3.2 in Building Regulations 2011 TGD L –(Dwellings).
DEAP Calculations
Where all ACDs are adopted for all key junction details for a dwelling type and are installed as per the ACD checklists, the dwelling fabric design as a whole will meet the guidance provided in Par 1.3.3.2 in Building Regulations 2011 TGD-L (Dwellings) and qualify for the value of y =0.08 in DEAP calculations. Alternatively the transmission heat loss coefficient (H~TB~) can be calculated for each of the key junctions for the specific dwelling using the psi values given in Tables D1 to D6 in Appendix D of TGD L 2011.
For use in DEAP it should be possible to identify the dwellings in which ACDs have been used through a unique identifier e.g. address with house no. or MPRN.
If not all the details within a dwelling are completed to the ACD checklists then the appropriate ψ value for the acceptable construction details used may still be claimed from Tables D1 to D6 to calculate a Y value for the dwelling. ψ values for details which are not Acceptable Construction Details should be calculated and certified in accordance with Building Regulations 2011 TGD-L (Dwellings) Par 1.3.3.2. Certified details may be provided by a third party certification body such as Agrement or equivalent or certified by a member of an approved thermal modellers scheme or equivalent.
