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Is draught-proofing for airtightness a good thing?

Up to half of all heat loss in buildings can be due to air leakage, uncontrolled ventilation and wind, despite the fact that insulation levels have increased substantially over recent years. This means that, along with insulation, draught-proofing for greater airtightness is vital.

Airtightness = extreme draught-proofing

Thermographic images are very good at picking up where cold comes in and heat leaves. This picture shows the draught under a door before (left) and after (right) draught-proofing. Thermal imaging cameras can be hired.

Thermal imaging cameras are very good at picking up where cold comes in and heat leaves. They can be hired. Here you see the draught under a door before (left) and after (right) draught-proofing.

Draught-proofing is a step on the way to airtightness. Put another way, airtightness is extreme draught-proofing.

It’s always a no-brainer to draught-proof premises. The quickest and cheapest wins for retaining heat are achieved by reducing unwanted airflow in and out.

A check should be made of each room, and the exterior (where safe) for the following unwanted openings, which must be sealed:

  • around the doors and windows
  • where services enter or leave the building
  • gaps in flooring
  • letter-box (fit seals)
  • A draught-proofing solution for letter-boxes that maintains airtightness. The front of the letter-box is on the left. At the back is a draughtsealed, insulated box with a door. The box is within the wall.

    A letterbox that maintains airtightness. Behind the front of the letterbox is a draughtsealed, insulated box with a door. The box is within the wall.

    around the edges of walls at the floor and ceiling level

  • air bricks
  • trickle vents
  • construction joints between materials
  • at the eaves

Dampers should be fitted to any chimney flues, and intermittently-running extract fans should have well fitting, self-closing covers which securely close when not in use.

Unused chimneys should be blocked off. Used chimneys should be fitted with chimney balloons in the seasons when they are not being used for fires.

Draught-proofing windows and doors

A variety of common draught-proofing seals: brush seal, wiper seal, compression seal, service seal.

A variety of common draught seals: brush seal, wiper seal, compression seal, service seal. Source: Energy Saving Trust (first three); Chris Twinn.

Special attention should be paid to fittings on doors and windows. Badly fitting doors and windows are a major source of draughts. Small gaps around the frames can be filled with gun-applied sealants and fillers.

Draught-stripping around the openings themselves is inexpensive, simple to install and greatly improves comfort as well as reducing fuel bills. Many different types of seals are available, including compression seals, low fiction or wiper seals and for larger gaps around frames.

The loft hatch should be secured with a catch. It should be draught-stripped and have 30cm of overlapping insulation glued to the top-side.

Cat flaps should be sealed as well as possible, and preferably on entrances with a porch with a cat-flap on each door.

New sealed windows or secondary glazing?

Left: Three levels of seal and a top-performing handle/catch system guarantee no draughts on this Passivhaus-certified door. Right: insulation within the frame and between the panes of a triple glazed window or similar door removes thermal bridging and contributes to the Passivhaus level of performance.

3 levels of window seal guarantee no draughts. Insulation within the frame and between the panes removes thermal bridging.

Some doors or windows may need to be replaced. Existing single pane windows, and windows with metal frames, should definitely be replaced, If this is not possible, then secondary glazing may be fitted.

Secondary glazing fits inside the reveal, and may be removed for cleaning purposes. The edges should be compression sealed.

Replacement doors and windows should have insulated cores, i.e. with insulation between the two outer surfaces to prevent thermal bridging, and preferably be constructed from timber for environmental reasons. These measures typically pay for their cost in the value of the energy saved in three to four years. Sealed windows of Passivhaus standard have three levels of seal which guarantee no draughts.

Air quality and health in a relatively airtight home

Even with all these measures, this level of airtightness is not very airtight and open or gas fires are not a problem in terms of air quality.

Extra ventilation will not be needed, provided internal doors are not tightly draught-proofed and there are trickle vents on windows in these rooms, in accordance with Building Regulations rules on the number of air changes per hour required in order to have these appliances.

Very airtight homes do not use open fires or gas fires. They do not need them. Therefore there is no issue with air quality.

Air tightness is regulated under Part F of the Building Regulations. It specifies a minimum of 0.3 litres per second per square metre of floor space (see table 5.1b).

Air leakage standards

The above are the quick wins. To go further requires more major interventions.

Air leakage is measured as the rate of leakage per m2 of external envelope per hour at an artificial pressure differential through the envelope of 50 Pa. i.e. x m3/hr/m2@50Pa.

  • The Building Regulations 2006 standard is 10 m3/hr/m2
  • The Energy Saving Trust’s ‘Good Practice’ rate is 5m3/h/m2
  • ‘Best practice’ is 3m3/h/m2 (based on CIBSE Tm23 2000)
  • The AECB Gold Standard is 0.75m3/h/m2@50Pa
  • Passivhaus Standard is 0.6 air changes per hour @ 50 Pa

Installing wall insulation cuts cold draughts

Internal glasswool wall insulation between studs.

Internal glasswool wall insulation between studs.

A total house renovation project is required. You would need to adhere at a minimum to Part L1B requirements on airtightness.

Many old houses, whether cavity or solid wall, have gaps in the masonry, or air bricks, which allow unwanted  draughts.

The solution is to install external or internal wall insulation, and lap the damp proof membrane (DPM) beneath the ground floor up the wall to meet the insulation, or the plaster/plasterboard, or follow the other instructions in our article on floor insulation.

Achieve a similar effect with the loft/roof, securing with tape and caulking the membrane to the exterior walls.

This will go much of the way towards airtightness control and elimination of cold draughts.

Detailing for key building element intersections is available for free to explain how to treat the airtightness barrier at these tricky places, from the Energy Saving Trust’s Enhanced Construction Details (ECDs).

But experience shows that even when following these, it is very hard to achieve airtightness and its ‘cousin’, wind-tightness, which gives protection from wind-leakage.

Wind-tightness

Wind-leakage occurs when strong winds tug air through insulation from the inside.

The most common example is when wind enters a loft space beneath the eaves. It is sometimes sufficiently strong to pull air through even several inches of insulation, from the warm space beneath, rendering the insulation ineffective. It can also happen with walls facing prevailing winds.

These challenges are dealt with by installing the right membranes or barriers to complete the totally unbroken airtightness and wind-tightness layers that must surround the building.

Vapour permeable membranes

Taping the airtightness membrane up against the wall and around service entries to prevent draughts entering through a suspended ground floor.

Taping the airtightness membrane up against the wall and around service entries to prevent draughts entering through a suspended ground floor.

In a loft or roof, we want excessive moisture to leave the building to prevent damp and rot setting in. Choosing the correct vapour permeablility of membranes and tile underlays is key to achieving this balance:

  • High vapour permeable underlay membranes are watertight but permit water vapour diffusivity to protect the structure from seepage and water condensing under roofing due to wind-driven precipitation. Using them means that there is no need for any ventilation gaps in the roof or wall structures.
  • Low permeability underlay membranes are used as an initial roofing layer beneath tiles in buildings with habitable as well as non-habitable loft spaces, when buildings are constructured from timber or metal frames, and where there is little risk of condensation.
  • Glueing the skirting board to the airtightness membrane.

    Glueing the skirting board to the airtightness membrane. Source: Chris Twinn

    Non-permeable underlay membranes have an aluminium layer on the lower side to reflect heat back into the building, and would be used if mechanical ventilation with heat recovery is employed in the building.

An airtightness membrane, which is also vapour permeable, should be beneath the insulation.

Vapour permeable membranes are available with reflective layers as well.

There are several manufacturers of these membranes and the associated tapes: Proclima, Protect, Isover and SIGA. They all offer a full range of products to meet all requirements.

The value of airtightness tape

Poor taping can easily come undone!

Poor taping can easily come undone!

In attempting to achieve airtightness, it’s the joins that need the attention.

Double-sided tape is used for airtight and moisture-proof sealing of overlapping membranes to metal sections, such as grommets for pipework and other service entry points that pierce the building envelope.

Tapes need to be very carefully applied to any membranes or insulation batts to completely eliminate gaps. And these seals must last for 20 years!

Party wall insulation

Party walls can also be a significant source of heat loss, where there is a significant heat difference on either side, and when the party wall contains a cavity.

The solution for the cavity wall is to take insulation across the top of the cavity in the loft, especially taking care to block the top of the cavity. Beneath this layer an air barrier membrane is installed, and on top of the insulation, a wind barrier membrane is installed.

Airtight recessed lights

The fashion for recessed lights in ceilings means that airtightness can be compromised. Warm air can leak out through the fittings.

The ideal solution is not to have recessed lights. If you are set on having them, then avoid halogen lamps, use LEDs, and take the following measures:

  • install so-called “airtight” versions (although they are not fully airtight)
  • if the chosen brand can accommodate trim with weatherstipping, install it
  • to deal with the risk of their escaping through the gaps with the electrical cable penetrations at the top of the unit, install a box from the attic side, sealing its seams well (tape, caulk, silicone, or spray foam) and superinsulating around it.

MVRH?

Conquering draughts in an older home requires considerable effort, but you’ll feel the difference. If aim to achieve near perfect airtightness, you may then be in the market for mechanical ventilation and heat recovery (MVHR), especially if the house suffers with damp. Read the separate FAQs on MVHR to decide if this would be the right choice for your house.

© David Thorpe Jan 2013. David is Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency

Also see:
Draught-proofing at free green open house events in September
Do I need MVHR?
Insulating a solid wall
Insulation vapour barrier
Interstitial condensation

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