BUILDING PERFORMANCE INSULATION
is nothing to stop bulk air movement through the insulations itself – removing heat as it goes. There is a very simple solution to this problem: avoid ventilated cavities next to insulation. There are two methods of achieving this in the cavity wall example: either use a wind-proof membrane between void and insulation; or fully fill the cavity and get rid of the ventilation. The latter is much easier to build well, but does rely on a quality render to prevent moisture ingress (see Figure 4).
The major underlying cause of the ‘reality gap’ is a lack of appreciation of the less obvious heat loss mechanisms
Air tightness Air tightness has come to the fore since 2006 with the publication of the Part L requirements. Good air tightness will improve occupant comfort and fabric durability, as well as energy consumption. However, in order to achieve the best results, it must always be coupled with a well-designed ventilation system; relying on a leaky building for ventilation is not a good idea for a low energy design. Air and wind tightness are different. Air tightness means absolutely preventing air from penetrating through the shell, while wind tightness merely stops air movement through the insulation. Both are needed on a good construction. The easiest way to achieve a high standard of air tightness is to simplify the air-tight layer as much as possible – junctions will always be difficult, so
minimising these is the first step. An air-tightness strategy should then
be discussed (preferably before planning) and fully developed as the construction is chosen. There are a number of air- tightness consultants and manufacturers that can provide advice, but a good, basic understanding of these principles is needed within the design team.
Thermal bridges Even though an understanding of what constitutes a thermal bridge is commonplace, a clear strategy to reduce its impact is not. The most significant thermal bridges
occur where there is a conflict between structural and thermal elements around complex junctions. The most successful way of reducing thermal bridging is to challenge the architect to make the thermal envelope as simple as possible. A construction method is needed that separates the structural elements (which generally have high conductivity) from the insulation as much as possible, preferably with the structure inside the thermal envelope. This strategy ensures that the thermal
bridges are kept to a minimum. Where they do occur, thermal analysis software can be used to quantify their effect. However, the calculation should be undertaken with caution, as the result is primarily a small difference between two large numbers, and so is easy to get wrong.
Factfile Basic rules of good insulation
These basic rules below will help to deliver a better performing building fabric. A typical corner incorporating these elements is shown in the diagram. 1. Keep a separate insulation zone, free from thermal bridges or cavities. Externally, this is covered in a windproof layer to stop blowthrough. Internally, an airtightness layer stops air movement through the fabric, and reduces moisture penetration; note that the airtightness membrane can also be located on the inside of the structural zone if it makes the construction simpler, but must be on the outside of the services zone. 2. A separate structural zone, which might be in-filled with insulation to improve the performance, but this is not the primary role.
38 Insulation bounded by:
Continuous airtight layer internally Continuous windproof layer externally
Conclusion By bringing an understanding of building physics to the table, building services engineers can undoubtedly change the way buildings are designed, creating thermal envelopes that are simpler, easier to build and better performing. The wider field of building physics
1. Insulation zone 2. Structural zone 3. Services zone
4. Weatherproof protection
3. All services should be kept clear of the insulation and airtightness layers – a services zone is the easiest way to achieve this. 4. Weatherproof cladding
CIBSE Journal November 2011
to provide protection from water ingress and aesthetic, in some cases (such as fully filled brick cavity) the weatherproof protection and windproof layer may be the same thing.
covers this topic in much more detail to include areas such as comfort and moisture transfer. The Passivhaus Designer course, run by CarbonLite, allows participants to fast-track their understanding of this and other aspects of low energy design highlighted within this article. Visit
www.carbonlite.org.uk for more information.
l Sally Godber is a chartered engineer and partner at WARM: Low Energy Building Practice.
www.peterwarm.co.uk CIBSE Guide A: Environmental Design can be obtained at
www.cibse.org/bookshop
www.cibsejournal.com
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