Air Leakage AKA Air tightness

When people ask me what is the first thing I should look at when trying to reduce energy consumption in a house I always reply draught exclusion. I have visited properties many times on the pretence of giving advice on how to reduce heat loss and turn around to see a 20mm gap under the front door. “Get a sausage dog”. Using an old fashioned draught excluder will help keep the heat in the building, not very high tech but probably the most important item to consider.

For example what is the point in installing high performance windows and leaving a gap between the frame and the wall – expanding foam is not airtight!

Air leakage is a description of the airtighness of a building but it does not give an accurate description of the problem. Un-designed holes with the fabric of a building can allow cold air to enter the house or warm air to escape; a two way street so to speak.

Attempting to quantify the cost is difficult but research suggests that leakage of air from a building can be responsible for up to 40% of heat loss (Webb – Barton 2002, BRE report BR448 Airtightness in commercial and public buildings).

In years past we had buildings which had numerous un-designed holes in the fabric of the building allowing air leakage such as chimneys, suspended timber floors with gaps in the floorboards, one reason why carpets became so popular – to stop the draughts through the floorboards.

As the house building industry strives towards lower carbon emissions un-designed holes within the fabric of the building need to be reduced by improved design and improved labour. For this reason attention to airtightness must be paid at design stage and the early phase of construction. The air tightness is checked at a fairly late stage in the construction process and if this highlights a problem then rectifying the problem can be destructive and expensive.

The airtightness test is measured in M³ of air lost from a dwelling per hour divided by the internal floor area when pressurised at a pressure of 50 pascals. All very technical but to relate this to air speed the Lawrence Berkeley Institute has made comparisons between 50Pa air pressure and air speed and have concluded that taken as an average this relates to a wind speed of 4.4 MPH.

The UK building Regulations give a maximum air leakage of 10m³/(h.m²) @ 50Pa, Holland has a maximum allowed value of 6, Germany around 3.6. The Passive House Standard of 0.6ACH @ 50 Pa. is a different metric and is not a direct comparison to m³/(h.m²) @ 50Pa, however, the AECB gives guidance that the Passive House standard relates to between 0.75 and 0.6m³/(h.m²) @ 50Pa.

There are numerous opportunities for air to permeate a property which we will attempt to consolidate as follows:

1-2           Suspended floors, gaps between floorboards and around the perimeter of the floor Junction between walls and floors, gaps left around services that penetrate floors.

3               Windows and doors, poor quality window and door seals allow air leakage.

4               Built in joists, gaps left around joists allowing air leakage into the cavity, gaps around air membranes not sealed properly

5               Window and door junctions to reveals, air leakage between poorly sealed aindow and door frames, poor sealing around spacers for example. Expanding foam is not air tight. Gaps at threshold, gaps between UPVC cills and reveal, hollow window cills breaching air barrier.

6               Gaps between walls and ceiling, poor sealing.

7               Gaps at the junction between internal partitions and ceilings, gaps at electrical socket and switch positions. Dry lining generally covers poor workmanship of the fabric of the building.

8               Poor seals in loft hatches and poorly fitted loft hatches.

9               Gaps behind electrical ceiling roses and downlights

10            Gaps around services such as SVP

11            Gaps around extraction fans and cooker extraction hoods.

12            Gaps around soil pipes through air tightness layer, gaps around services where they penetrate air tightness layer – commonly hidden behind kitchen or bathroom units.

13            Gaps in mortar joints allowing cold air to permeate the cavity.

14            Gaps between timber soleplate and ground floor slab – due to uneven slab or warped timber.

To reduce the air leakage in a building the air barrier should be defined, the air tightness

barrier can be a number of different materials, for example tapes and seals, membranes, parging coat or wet plaster. Communicate to all site staff the importance of the air tight layer and what exactly is providing the air barrier at all locations throughout the building and to inform the supervising officer of any breaches.

There are a number of practical things which can be done to reduce the air leakage from a building as follows:

• If there is a suspended floor ensure there is a membrane on top of the floor joists but below the floorboards, use a bitumen tape on top of the floor joists and screw through this tape when fixing the floorboards to the joists.

• Seal around the perimeter of the wall to floor junction with an approved sealing material such as Blowerproof which will stay flexible for the life of the building.

• Use approved sealing collars such around any penetrations through the air tight membrane use Gerband 586 Uni Tape to form a seal.

• Check quality of window and door seals to ensure an airtight seal.

• Fill gaps around joists with an approved sealant (Blowerproof) or if using an internal membrane use Gerband 586 Uni Tape to form a seal.

• Use air tight tapes between window and door frames and either the air membrane or the plastered reveal. For Passive Houses use airtight tapes (Illbruck or Iso Chemie) around the frames.

• Do not use hollow UPVC window cills, use either solid cills or even better face fixed aluminium cills (see RBB aluminium cills).

• Do not rely on plasterboard for the airtight layer, even if it is sealed, the homeowner will more than likely break the layer by putting a hole in for pictures or adding sockets or switches. Ensure the air tight layer is behind the plasterboard by using a membrane, parge coat or wet plaster.

• Use proprietary air tight loft hatches and ensure correct installation.

• Use proprietary pipe collars for sealing around SVP, always maintain the air tight layer. This may mean sealing to a membrane or the parge / plaster coat.