Airtightness in new dwellings is about good design and construction – not just filling gaps!
Home energy use is responsible for 28 per cent of UK carbon dioxide emissions which contribute to climate change. By following the Energy Saving Trust’s best practice standards, new build and existing housing will be more energy efficient and will reduce these emissions, saving energy, money and the environment.
- air leakage
A building’s design and the quality of its construction will have a major effect on the amount of air leakage. Other factors include wind against the side of the dwelling and the buoyancy effect (warm air rises and creates a drawing effect, pulling air in through gaps in the ground floor and walls).
- improving air tightness
While the main focus of this guide is the achievement of an airtight dwelling, ventilation must not be forgotten. It is necessary for a comfortable and healthy environment as it removes or dilutes pollutants that accumulate in the building.
- understanding air tightness
Many of the paths shown were found in the dwellings investigated in the Energy Efficiency Partnership for Homes study, referred to earlier. Here, they are examined in more detail.
- solid ground floors
Junctions between the ground floor slab and the external walls are usually hidden from view by the skirting board. However, air can leak through the small gap under the skirting board, causing a cold draught across the floor.
- suspended floors
Suspended timber floors have a myriad of gaps around the boards, at the junctions with walls (internal and external) and around service pipes. Air can leak down through these gaps into the unheated floor void.
- windows and doors
It is common to find gaps between the window frame and the wall. They may also occur between the frame and the opening casements, lights and sashes. The window’s opening/closing mechanism can become loose with time, and may not close as tightly. If this is the case, a draughtstrip may not be sufficient to seal the widening gap.
- masonry walls
Masonry external walls, especially those built from lightweight concrete blocks, have varying degrees of porosity, allowing air through. Unfinished mortar joints also provide air leakage routes.
- service pipes
Gaps in the air barrier are common around water pipes, gas pipes, boiler flues and electric cables that pass through external walls (Figure 13), although they are generally hidden from view behind kitchen cupboards, bath panels, sink pedestals, toilet basins and vanity units.
- principles of air tightness
Airtightness is all about avoiding gaps – gaps in contractual arrangements and the design process as well as those between components and those left during installation. They are usually caused either by a lack of awareness of the importance of airtightness or by a lack of contractual responsibility for ensuring airtightness.
Consider the appointment of an independent airtightness adviser.
- construction practice
Building work must be sequenced so that each part of the air barrier is completed before following trades cover the work. The ‘out of sight – out of mind’ approach will not achieve airtightness: a subsequent pressure test will show up any inadequacies.
- inspection and supervision
All works will need to be inspected as construction proceeds. Once the building work is completed it may be difficult to examine the airtightness layer as it is usually covered by internal fixtures and finishes.
- inspection checks
Recommended inspection points and checks for ensuring air tightness within a house extension
If the air barrier is damaged, it must be repaired. The materials used must provide at least the same level of airtightness as the original construction. Gun-applied sealants are usually suitable for sealing around service penetrations. Larger openings through walls and floors can often be filled with mortar.
There are other issues to be considered in the case of flats.
- air pressure testing
Fan pressure testing allows the permeability, or airtightness performance, of the dwelling to be assessed. The technique is straightforward and can be carried out when the envelope is complete.
- air leakage audit
A draught can be difficult to locate. The occupant may feel cold air, but may not be able to trace exactly where it is coming from. Often the source of the problem (the air leakage path) is hidden by kitchen units, bath panels or other boarding and so it can be very difficult to pin point it without a thorough audit.
- post construction
Having constructed an airtight dwelling, it is important to make the owner aware of the fact. The owner needs to know the location of the air barrier in order not to compromise its effectiveness when carrying out future alterations.
- existing dwellings
To upgrade existing dwellings, the same constructional elements need to be considered. In some instances, the methods will be the same as in new buildings, for example gun-applied seals around service penetrations and the gaps around windows and doors.
Airtightness is an increasingly important part of a ‘whole house’ approach to building. Airtight buildings will be more comfortable for occupants and also be more energy efficient.