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Airtightness and Ventilation

Airtightness and Ventilation

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Reducing heating demand in our homes is one of the most important steps to a zero carbon future. It’s then far easier to install enough renewable energy to meet our remaining needs. Upgrading insulation makes the biggest impact, but air infiltration also causes significant heat loss. Careful attention to airtightness and ventilation is key to keeping heating demand low.

In CAT’s ‘Zero Carbon Britain: Rising to the Climate Emergency’ report we recommend building new homes to the ‘Passivhaus’ standard or similar. That means stringent airtightness and mechanical ventilation with heat recovery (MVHR).

We also recommend a mass refurbishment of existing buildings. If you undertake a whole house retrofit, this may also involve using MVHR. If you’re not able to renovate to the highest level then passive (non-powered) ventilation, or a mixture, might be appropriate.

Unintentional vs intentional ventilation

‘Build tight, ventilate right’ is a long-standing energy efficiency mantra. The aim is to stop unintentional ventilation and instead provide intentional ventilation. Airtightness applies to the entire building fabric: gaps and cracks at the corners and edges where materials join, holes for pipes and cables, where joists penetrate a wall, and so on.

The benefits go beyond energy savings. The right approach to airtightness actually makes a house feel both fresher and more comfortable. People tend to crank up their thermostat in a draughty house to overcome the cooling effect of incoming air flows. In contrast, with low air movement a home remains comfortable at a lower temperature.

With good airtightness, effective ventilation replaces stale air – including carbon dioxide, cooking smells, water vapour, dust, off-gassing (for example from new appliances), and so on. If a building isn’t airtight enough, planned airflows for ventilation won’t work as intended.

In addition to draughts, ‘thermal bypass’ can cause further heat loss. This is when outside air gets past the insulation layer and into spaces within the building fabric. The ‘dot-and-dab’ technique for quickly fitting plasterboard creates voids prone to thermal bypass. It also happens when spaces in a roof or intermediate floor aren’t properly sealed when being insulated. As well as heat loss, these cold patches are at risk from mould growth.

Measuring airtightness

The first step is to know your starting point by having an airtightness test. Then you can set a target for improvement and a strategy to achieve it.

Airtightness in older buildings varies a lot, and you can’t tell just by looking. Using thermal imaging during a test helps to identify heat loss from air leakage, by comparing cold spots visible during the test to those when unpressurised. A local energy agency or community organisation may be able to provide an initial survey.

A blower door test shouldn’t be very disruptive or expensive. To carry it out you seal existing ventilation points like extractor fans, trickle vents, and flues. Then a powerful fan extracts air to create a pressure difference that amplifies air infiltration, allowing you to measure non-intentional ventilation. You may want the test to give two measures:

  • Air permeability relates to the surface area of a building. It’s used in UK building regulations. The units are cubic metres of air per hour per square metre of surface area: m3/h.m2@50Pa (tested at 50 Pascals pressure).
  • Air changes per hour (ach) relates to the volume of a building. It’s used in high standards such as Passivhaus (1ach@50Pa for retrofit) and AECB retrofit (2ach@50Pa).

These could be very different, especially for a less compact layout with a high surface area. When speaking to contractors make sure you’re both talking about the same measure.

Blower door test
A blower door test can be used to measure non-intentional ventilation.

Aim and strategy

Once you have a baseline, you can decide on an airtightness target that is practical for your project, considering also the type of ventilation you’ll then need.

A typical modern house probably has an air permeability of about 5m3/h.m2@50Pa. Some older homes are similar, but many are much leakier. Recommendations vary, but for mechanical ventilation to be appropriate you’ll probably want less than half that, and to make heat recovery worthwhile you should target 1.5m3/h.m2@50Pa or less.

To deliver a more exacting target you need an airtightness strategy. This specifies where the airtight layer will be, with details for all the junctions, gaps and holes. When going for a tested standard, set your target to give a bit of margin for error.

Thermal imaging
Thermal imaging can be used to find cold areas where air is coming in – as shown by the dark patches around the cables (the light areas are hot pipes).

Hitting the target

Make sure that the target and strategy are clear to your architect and builders, because effective ventilation depends on it. Attention to detail is vital and therfore you need a good relationship with contractors and they must be fully engaged with the process. As stringent airtightness is new to many in the UK, having a ‘no blame’ culture allows you to get mistakes fixed before it’s too late.

There are special tapes and grommets for sealing junctions and problem areas, such as where electricians and plumbers run pipes and cables out of the building or into an uninsulated loft. Other weak points include recessed lights and the loft hatch. For some older houses secondary glazing might be a less costly option than replacement glazing for getting the required airtightness at window frames.

Ventilation types

With the best airtightness you’ll need mechanical ventilation with heat recovery (MVHR). This extracts warm stale or moist air, generally from the bathroom and kitchen. A heat exchanger transfers the heat from this to the incoming fresh air for other rooms. MVHR should have very low running costs, saving much more energy than it consumes. However, as a decent Passivhaus-certified system will cost a few thousand pounds, the retrofit must be to a suitable standard to make it worthwhile.

At lower levels of airtightness, simpler mechanical extract ventilation (MEV) may be enough. Instead of heat recovery, fresh air is drawn in through trickle vents or similar. This can use humidistats to regulate air flow. It may be possible to get the same effect without power, through a ‘passive stack’ system. You can supplement a mostly unpowered approach with a small heat recovery fan for the bathroom (perhaps also the kitchen).


Passive or mechanical ventilation can fail to deliver if there are mistakes in the design or installation or if it’s not maintained – resulting in a stuffy, humid and unhealthy house. A successful approach must work under normal living conditions, for example don’t rely on leaving bedroom doors open.

Poor design or installation of a mechanical system could make it noisy. Large diameter ducts and a low air resistance heat exchanger keep the noise and energy use of fans very low. Rigid ducting is preferable to flexible, which may be easier to install but tends to not perform well. In general, flexible ducts lead to more noise, are harder to clean, and don’t last as long. For heat recovery to work efficiently, the MVHR unit and ducting need to be within the insulation layer, not in an uninsulated loft where they’ll lose heat.

Check the maintenance required to keep a ventilation system efficient and healthy, and that there’s access for this. For example, with an MVHR system you’ll need to check the filters every few months and clean or replace as necessary. Replacement paper filters might cost £10 to £15. You may need a more thorough servicing and cleaning of the heat exchanger and fans every few years.

Contacts and further information

CAT offers short courses including the long-running eco-refurbishment course, which will include more about airtightness and ventilation options. See also this blog post about eco-refurbishments by our course tutor, Nick Parsons.

To find someone to oversee a retrofit project, see the AECB member listings or the Passivhaus Trust (with EnerPHit for retrofits). The AECB and Passivhaus Trust list professionals trained to high standards for both new build and renovation. Another listing site for finding eco-builders is the Green Register. Or you could try the listings of the Government’s TrustMark scheme, which now covers professionals that carry out energy efficiency work including retrofits.

Getting good advice upfront should give big savings later. Local community-based organisations are ideal as trusted intermediaries – bringing together householders and local tradespeople, and supporting both. At the moment, there are organisations like this dotted around the UK, with more gradually springing up. If you contact me I may be able to give you some local contacts.

This page was written by CAT’s Information Officer Joel Rawson. You can contact me with further questions (choose ‘free information service’ on the form).

Related Questions

What is breathability?

This can be a confusing term for two reasons. Firstly because it refers to moisture not air and secondly because the exact physical processes which go on in a building are complicated (and still being researched). However the metaphor of breathing captures the important idea of continual movement. Moisture in itself might not matter, as long as it doesn’t take up permanent residence in your walls.

Broadly, breathability is the ability to buffer moisture – to soak it up and then release it – thus avoiding a situation where water vapour is retained by the fabric of a building. The technical term for this property is hygroscopic. Natural materials tend to be more hygroscopic than synthetic ones. The term “vapour permeable” also gets used in this context, referring to materials that allow water vapour to pass through at a certain rate.

A breathable construction will be made up of a gradation of vapour permeable materials, with the materials towards the inside being less vapour permeable than the materials towards the outside. A rule of thumb is that the material on the inside (warm side) should have five times the vapour resistance of the material on the outside (cold side). Without using a very high resistance material such as polyethylene or foil-backed board, this can be achieved with a suitable vapour-check layer on the warm side of the insulation. This layer slows down the passage of moisture to a safe level.

The construction also needs to be airtight on the inside to ensure the vapour-check layer works as intended. So it’s important to not put gaps in the vapour check layer for services (wires, etc), and instead have a thin space for these services between the vapour check and the internal finish (e.g. plasterboard). The final layer of external cladding might be waterproof (such as tiles) but there must be a ventilated gap between this and the sheathing board that actually completes the breathable construction.

In a UK-type climate the interior of a house usually has a higher relative humidity (RH) than the outside because it contains people exhaling, cooking, and washing. It’s a very general rule that, left to themselves, things will always migrate from an area of higher concentration to one of lower concentration, so there will be movement of water vapour towards the outside (or, to be more technical, the interior has a higher vapour pressure).

At times of high interior RH, water vapour will enter the walls, lowering the RH (both high and low RH cause problems and the ideal range is 40-60%). At times of lower interior RH, almost all of the water vapour will migrate back into the interior to be taken away by air currents. Any water vapour that might have managed to penetrate the wall deeply will be drawn out towards the exterior because of the lower permeability of the outer layers, so no harm done. Any water coming in from the outside will be unable to get very far as it is pushing through layers of decreasing permeability. The breathability of materials interacts with both temperature (which affects RH) and ventilation (one of whose functions is to carry away water vapour).

How can I reduce condensation in my house?

Condensation is due to excessive moisture, cold conditions, cold surfaces or inadequate ventilation. It can cause mould, heat loss and building damage. To address these issues, the room should be properly insulated and adequately heated (to keep the surfaces warm).

So do take all feasible insulation and draught-proofing options, and look into improving single glazing with either replacement glazing or with secondary glazing (a cheaper option).

Condensation may still occur on replacement windows, as they’ll still be the coolest surface. New windows will be more airtight than old ones, so warm moist air will be no longer be escaping through cracks in the frame and around the seals. This means that existing damp issues may become more pronounced. Many windows will include trickle vents in the frame, to allow a small amount of ventilation, but to keep the house warm and dry you may need to take a few other measures to avoid producing lots of moist air.

Drying clothes indoors can easily cause problems of damp and condensation, leading perhaps to mould, etc. So if you need to dry indoors, it should be in a room that can be shut off and ventilated (perhaps with heat recovery, as mentioned below).

The bathroom and kitchen in particular should be able to be ventilated in a controllable way, to stop moist air circulating into the rest of the house. For example, after a shower or bath, leave the bathroom door shut and the window open for a while until moist air & condensation on the window/mirror has cleared. Do the same when cooking if you can; if your home has an open plan layout at least stop the moist air circulation where possible (e.g. to the upstairs rooms).

To avoid heat loss from a room like a kitchen or bathroom, where lots of moist air is regularly produced, you could consider a heat recovery extractor fan. It may be worth getting a slightly bigger heat-recovery fan unit than you need, as they can be a bit noisy if they are operating on full power. This may be fine if are just switching it on for a short while to clear the bathroom, but it could be obtrusive in the kitchen. If a fan unit is not supplied with more solid covers over the plastic slatting, it could be worth fitting something if you live in a house that is a bit exposed to the wind (as they could let in draughts when not in use). An openable wooden casing could be fitted quite easily.

If drying clothes indoors is not an issue, and you’re already careful about venting away moisture from bathroom & kitchen, then excessive condensation may be from some other cause, such as a water leak somewhere (e.g. from a pipe under the floor or in the loft), or water penetrating the structure from outside (such as rainwater coming in cracks in masonry, or if gutters are broken). If problems persist, it would be worth investigating these issues, as over time they’ll cause damage to the building.

What is a U-value, and what is thermal conductivity?

The U-value is a measure of how many watts (representing the rate of flow of energy) pass through one square metre (m²) of a construction detail (such as a wall) for every degree difference in temperature between the inside and the outside. Temperature is measured in kelvin, and 1K = 1°C (degree centigrade).

As an example, a U-value of 6.0W/m²K for a single glazed window means that six watts will be escaping through each square metre of glass when the temperature difference is one degree. If it is 20°C in the house and 0°C outside, then the heat loss is 20 x 6 = 120 watts per square metre.

U-values are generally used to describe the thermal performance (heat loss) for a section of construction that involves several materials – such as a wall made up of timber, insulation, board & render.

Thermal conductivity

For individual materials, such as a type of insulation, you’ll come across the term ‘thermal conductivity’, also known as a k-value or λ-value (lambda). This is the rate at which heat flows through a particular material, and good insulation will have a low thermal conductivity. It is measured in watts (heat flow) per metre (depth of material) per degree difference (inside to outside), so the unit is W/mK.

Most natural insulation materials (hemp, wool, recycled paper or textile) will have a thermal conductivity of about 0.035 to 0.040 W/mK, which is similar to the performance of conventional mineral wool insulation. Foil-backed plastic foam insulation boards are slightly better, with thermal conductivity about 0.023 W/mK. So about 100mm of the plastic foam board will give equivalent insulation value to about 150mm of the various woolly types.


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