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Air tightness

‘Build tight – ventilate right’

Building Regulations

The subject of ventilation is fully incorporated into the Building Regulations and is likely to become more strictly controlled in the near future. Good ventilation goes hand in hand with air tightness.

The Approved Document for Part L1A of the regulations covers air tightness under Air permeability and pressure testing

England Wales England and Wales – Part L

Scotland Scotland – Domestic handbook

England Northern Ireland – Approved Document, Part F

There are two ways of measuring air tightness –

  • the number of air changes per hour experienced by the whole building (at a fixed test pressure – usually 50Pa). This is known as the n50 value and is the value used in Passivhaus calculations.
  • the number of air changes in m³/m² of floor area (at a fixed test pressure – usually 50Pa). This is known as the q50 value and is the value used in SAP calculations to many people’s consternation.

So the latter method assumes a ceiling height (of say 2.5m) which may actually vary and be incorrect in some areas of the building, particularly hallways and high rooms.

Draught and combustion air

Green Spec have a good diagram showing the main problem areas in a house where draughts occur. Draughts can have a tremendous effect on cooling houses in winter and hence on energy use and fuel bills. It is not only the net cooling but also the fact that large parts of rooms can become unusable when draughty. This becomes very noticeable when you are sitting still and trying to read or work and also for the elderly and disabled. Also, draughts can locally cool room surfaces so that condensation (and mould) forms from moist air. A really draughty old house in an exposed position might easily have 20 air changes per hour (a.c.p.h.). Compare this with the 0.6 a.c.p.h. @ 50Pa. (maximum) recommendation for what is actually needed in a Passivhaus in order to reduce heat loss.

On the other hand, it is possible to over-seal a room or house, especially if there are any heating appliances which need an air supply coming in through the room (rather than room sealed appliances). If they don’t get enough air they can start to produce carbon monoxide into the room where they are and this is poisonous.

Ventilation for heating appliances

The Building Regulations have a good deal to say about permanent ventilation required by heating appliances such as open fires and wood burning stoves. These appliances require a certain amount of air to keep them burning properly and to avoid carbon monoxide (which is poisonous) escaping into the house. The requirements are laid out in the Approved Documents, part J, particularly

Section 1: Provisions which apply generally to combustion installations,

Section 2: Additional provisions for appliances burning solid fuel (including solid biofuel) with a rated output up to 50kW

Appendix F: Assessing air permeability of older dwellings in relation to permanent ventilation requirements

Passivhaus ventilation

The rate of ventilation necessary for good healthy air quality as applied in the PassivHaus standard is 1m³/(hr x m²). With Passivhaus this is supplied by MVHR. In this way a healthy environment is maintained while at the same time loosing less than 10% of the heat from the waste air.  See the bre PassivHaus Primer.

The Building Regulations on fan testing can be seen at Approved Document L1A – Conservation of fuel and power in new dwellings

Interestingly there is a peculiar British contradiction about “fresh air” and some people get quite jumpy at even the thought of draft excluders……see more on Why the aversion?

When faced with the idea of controlled ventilation a considerable amount of people go into a slight panic mode and start saying things like “I hate stuffy rooms”, “I need to be able to fling all the windows open” or “You’re not going to coop me up”. Occasionally they complain that ventilation never works properly.

At the same time people complain about draughts, fuel bills and damp patches behind the wardrobe.

Given that draughts, high fuel bills and the damp patches are largely to do with draughty houses (along with poor insulation) the question arises why people get so edgy about controlled ventilation.

There are several possible overlapping explanations –

There is a historic fear of TB. Since near the beginning of the industrial revolution until less than a century ago there were enormous areas of working class housing where people lived at very high densities. There was massive overcrowding and TB was endemic. TB is a respiratory disease which spreads well when people are in close proximity and there is a lack of fresh air. Ventilation was a massive issue, viz. the way sanatoriums tended to put patients out in the open for long periods of the day to get fresh air. These kind of fears go deep into a national consciousness.

Maybe the UK climate is partly to blame. It is so changeable that it is never worth preparing too thoroughly for anything. ‘Don’t waste money insulating or draught proofing – we may have a mild winter’. ‘Not worth building a swimming pool – it may be a chilly summer’. Not the sort of thinking that goes on in north continental Europe.

Maybe it is the nation’s historic attitude towards sex and morality. Digging down into the 1968 edition of the RIBA Metric Handbook there is a list of recommended temperature for rooms in houses. It happens to give the equivalent German standards.

It is noticeable how the standard for Bedrooms (sleeping only – No sex please we’re British) puts the recommended (by whom?) temperature at 13-16ºC compared with the German one at 20. Maybe it was just so draughty you had to do it under the covers or in the living room. Notice also that the woman in the kitchen had to keep working to keep warm. And as for the bathroom! 1962 it was but attitudes persist.

You could always rely on cheap coal (-gas – oil – but seemingly not nuclear) to provide a bit of heat when you needed it. There’s plenty of it down there. Well that one is coming home to roost.

Maybe the experience which many people have had with noisy fans and badly designed ventilation ductwork has left a bad taste in the mouth and nose.

There is the possibility of lingering historic sexism to do with the quality of housing. Men designed and built all the houses. It was never in a man’s interest to make them warm and cosy and cheap to heat. He could go off down to the pub after work and leave his wife to figure out how to stay warm and dry the nappies. Bit sissy to do any of that draught proofing stuff.

Fan testing

The latest set of building regulations stipulate air permeability of no more than 10 m³/h/m² @50 Pa. in other words ten times leakier than the PassivHaus standard. However these standards will be drastically tightened if the goal of zero carbon new housing in the EU is implemented in 2021.

Air tightness can be tested using a specially designed fan which is fixed in an open front or back door. All the other openings such as windows, flues, cat flaps etc are fixed shut and the fan is turned on. A meter indicates how easy it is for the fan to suck air out of the building and this then gives a value for air tightness. For a one-off test the cost might be around £300. It is not uncommon to have a two stage fan test with the first stage being when the building envelope is complete. In this way any problems can be remedied before floors and internal walls etc. make it difficult to get at crucial junctions.

Achieving good air tightness can be a considerable challenge. Materials which you might have thought would be air tight such as concrete blockwork or OSB sheets may well be quite leaky. Even the odd screw or nail hole in a ceiling or floor will show up when pressure testing is carried out.

Do you actually need to do a fan test?

To quote the BSRIA web site:

For one or two [housing] units, airtightness testing is not mandatory, but an ‘assumed’ air leakage rate above the industry norm must be made. Often, it is simpler and more cost-effective for builders to test to prove a lower air leakage rate than to find more complex ways to demonstrate energy saving.

This is because the SAP calculation has a fallback situation if the airtightness is not known but it does assume a rather low value. This would mean you had to improve other energy saving aspects of the house such as insulation. Probably better to seal the house well and go for the fan test. A test might cost between £250 – £400 and bear in mind that if you fail first time you will need a retest which will probably cost the same again.

The problem with joints

Nearly all forms of construction (with the exceptions of in situ concrete, hemp/lime construction and rammed earth) contain lots of small joints. These may each be very narrow but can add up to a large total over the whole area of the wall, floor, roof etc. For instance, brickwork, which you might consider to be air tight is often anything but due to the perpends being imperfectly sealed (either because the bricklayer didn’t do a very good job or because of subsequent thermal movement causing hairline cracks). Fairface brickwork (unplastered) tends to leak like a sieve. One of the main functions of render and plaster is to seal these many tiny cracks. While this works well (especially for plaster) on the visible parts of internal walls, there is a risk of air gaps around floor joist ends within the thickness of the floor (and ceiling etc.). see Timber floors.

Timber frame

The same goes for timber frame houses such as the Segal method or kit houses which rely on hundreds of individual joints between timber frame members and the panels between them. (SIPs is a notable exception because the panels are very large and stable and the joints between them can be designed to be airtight). With timber frame there are several aspects to the problem:

  • There are so many joints. When you actually do the maths and add up the running length of joints between all the posts, beams, joists, sole plates, head plates, panels, windows, doors, meter boxes, cat flaps etc. it can come to hundreds of metres so the work to seal them all properly is enormous, many would say it was impossible in practice.
  • The use of an air tight membrane to solve the problem has its own challenges.
    • The joints between different parts of the membrane require sealing and this is often only really possible by double lapping and then physically trapping. This is very labour intensive and tends to be difficult at corners or where three planes meet.double lap joint
    • It is particularly acute where the membrane needs to be inserted at the point where structural members cross.
      air tight membrane

      Normally the membrane would go on after the beam had been fixed to the post. However its position (shown by the orange line) would then be impossible, so it becomes necessary to place a strip of membrane (shown red) between the post and the beam prior to fixing them together. The main membranes can later be taped to this strip. Argh!

    • Taped joints may work well on a membrane/membrane joint but can be difficult on a membrane/timber joint (especially if the timber is rough sawn, dusty or damp). Anyway most of the adhesives used for sealing membranes may not last the decades (or centuries) required. Membranes can easily be damaged by later trades such as the fitting of electrical or plumbing outlets.
    • If a membrane is badly sealed or gets damaged then it is extremely difficult to locate the problem at a later stage such as fan testing because the air leak may have traveled around within the frame cavities in the wall before it shows up. The only way then is to rip off big areas of internal lining to find the damage.
  • Timber frames may actually move quite a lot as the timber dries out and this can lead to cracks opening up between joints. This is a particularly serious problem if the timber is used wet such as with green oak construction

The most reliable way of dealing with this type of construction (provided the framework is not used wet) is to firmly fix wood panels such as OSB directly onto the timber frame using a timber batten. This way a seal between the panel and the frame can be inspected visually during construction. Accidental damage to the panel is much more obvious than damage to a membrane.

post panel joint

The diagram only shows the panel/frame connection, not insulation, vapour barrier, surface finishes etc.

The fixing between the panel, the batten and the timber member must be firm enough that movement cannot occur if the panel attempts to move relative to the frame (panels can tend to buckle with changing moisture content). Fixings would need to be at 100mm or 150mm centres depending on the thickness of OSB used and on whether the panels are also being used for stiffness . This would imply the use of a nail gun.

Another method is the use of silicone gaskets to seal OSB to the frames. This was successfully used on the Stirley Farm Enerphit project.

The BRE do a Passivhaus Primer on air tightness

Air movement within insulation (thermal bypass)

A somewhat different aspect of air tightness concerns the cooling effect of cold outside air which blows into insulation and through it (particularly fibre insulation) and then back out somewhere else without actually entering the house.

This can happen when there is a cross wind or when there is a strong thermal stack effect and it carries warm air out of the insulation. It is sometimes known as thermal bypass and it can be prevented by having an airtight vapour permeable membrane or boarding on the outside of the insulation behind the main cladding. It is also required on any other external surfaces which may be subject to air infiltration such as suspended floors and roofs.

How serious the heat losses can be in some instances is covered in an article by Mark Siddall, particularly pages 2 and 3. In some cases ventilated roofs were shown to be degraded by nearly 40%

Fabric First by the Energy Savings Trust is one of the most useful studies on energy saving for new homes (though currently being updated)

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