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 airtightness early in design. This two stage process should be 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, membranes, etc.). The choices made at the strategic level may 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 follow the line of the inside face of the insulation in the thermal envelope.
Consideration should be given at an early stage as to which layer of each element of the thermal envelope will form the primary air barrier, and to the junctions between them. The details in Part 2 assume the air barrier will be formed largely by internal wet plaster, airtightness membranes 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 the pen has to be lifted, there may be discontinuity and a potential air leak. The details in Part 2 show the air barrier as a blue line. It is important that the designer is clear on the location and materials used to create the single air barrier layer in order to be able to communicate this effectively to the construction team.
There are four options indicated in the ACDs:
• Masonry inner leaf with wet-finish plaster, or
• Masonry inner leaf with scratch coat, and finished with plasterboard, or
• Insulated plasterboard system sealed to achieve appropriate airtightness, bedded on dabs and mechanically fixed, with continuous ribbon of adhesive around all openings, along top and bottom of wall and at internal and external corners, or
• Airtightness membrane and tapes
Generally, an airtightness finish of wet plaster on masonry or airtightness membranes and tapes will achieve the most effective airtightness layer and is likely to achieve the optimum airtightness performance. These systems also facilitate early airtightness testing before finishes or services are complete, avoiding any potential disruptive remediation works.
The use of plasterboard-on-dab finishes to achieve airtightness is mainly reliant on the skill and care of the installer to achieve the required airtightness performance. Any subsequent disruption of the boards also has the potential to compromise performance.
Where the airtightness performance only becomes evident close to final completion, remediation of the air barrier at that point in the construction process may involve significant disruption and delay completion.
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 general approach to the design will help achieve insulation continuity and airtightness:
• Keep it simple! Simple designs are more likely to be constructed correctly.
• Decide which layer(s) of the construction provide the air barrier. Stick with this. Use the pen-on-section test to check continuity and to identify key details.
• There is only one effective air barrier. There is no benefit to be had from multiple air barrier layers; the barrier performance is always defined by its weakest link.
• Minimise the number of different types of construction that are used to form 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. Where details are
changed during the course of construction the changes should be approved by the designer, or competent person.
• 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 building services, structural elements or construction components. In general, a services cavity provided on the warm side of the air barrier will help to ensure that service penetrations are avoided and to simplify the installation of pipes and cables.
• Where penetrations are unavoidable (soil stacks, ventilation exhausts and intakes, water supply, electricity and gas supplies), appropriate details should be developed taking account of the sequence of construction.
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 team should ensure that details of all design changes involving elements of the external envelope are notified to the design, procurement and construction teams.
It is important that the project programme reflects the required construction sequence for effective formation and testing of the air barrier and for the installation of insulation.
It may be prudent when compiling the project programme to include airtightness milestones. Knowledge of these dates may permit management to schedule envelope component inspections and airtightness tests in advance of the completion 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
All personnel involved in design, procurement and construction of 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 in pursuit of cost savings, and the more likely it is that site staff will be able to provide the higher standard of workmanship involved.
Awareness may be raised at key stages by briefing the construction management team and through the use of on-site tool-box talks. Detailed drawings should be issued to all parties clearly identifying where and how insulation continuity and the air barrier continuity will be maintained.
Operatives directly involved in constructing the insulation and air barrier should be encouraged to draw attention to difficulties and request direction.
Operatives not directly involved in the building fabric should also be made aware of the importance of insulation continuity and air barrier integrity and report any breaches to the supervisor responsible.
QUALITY CONTROL
All contractors now have systems in place for monitoring the quality of their processes and products. Quality Assurance (QA) should be included 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 thermal envelope.
The QA process should 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.
Particular care is required in construction types which rely on a vapour control layer and sufficient time should be allocated to permit its detailed inspection prior to covering up.
Testing Stage
A series of tests are required as the building nears completion including ventilation validation and airtightness testing. Other elements will be subject to ongoing inspection for conformity with specification and compliance with building regulations.
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 should be recorded and it is recommended that geo-located digital photographs are included with the completed ACD checklists.
AIRTIGHTNESS
The final air permeability test is usually undertaken close to 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 and should meet the provisions of section 1.5.4 of TGD L. Mechanical ventilation should be provided where airtightness is less than 3m3 (h*m2), see TGD F, section 1.2.1.1 (a) and (b).
It can be useful to perform airtightness checks on the dwelling during construction to identify areas of leakage prior to the completion of finishes.
Where the dwelling fails to meet the required airtightness standard, inspections can be undertaken utilising tracer smoke and/or thermal imaging 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, and may ultimately delay completion.
