Air Tightness
see also Ventilation because this is linked to the level of air tightness achieved.
and see also Vapour barriers
‘Build tight - ventilate right’
The subject of ventilation is now incorporated into the Building Regulations.
England and Wales - Part L1A and Part L1B
Scotland - Domestic handbook section 6
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)
- the number of air changes per hour experienced above 1m² of floor area (at a fixed test pressure - usually 50Pa)
So the latter method makes an assumption about the ceiling height (of say 2.5m) which may be incorrect in some areas of the building.
Green Spec have a good diagram showing the main problem areas in a house where drafts occur. Drafts 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 drafty. This becomes very noticeable when you are sitting still and trying to read or work and also for the elderly and disabled. Also, drafts can locally cool room surfaces so that condensation (and mould) forms from moist air. A really drafty 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 house, especially if there are any heating appliances which need an air supply. If they don’t get enough air they can start to produce carbon monoxide into the room where they are and this is poisonous. Hence the maxim - ‘Seal tight, ventilate right’. The rate of ventilation necessary for good healthy air quality (assumed in the PassiHaus standard) is 1m³/(hr x m²) and this is supplied by MVHR (mechanical ventilation with heat recovery). See the bre PassivHaus Primer.
Interestingly there is a peculiar British contradiction about “fresh air”. more +/-»
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 UK is to be achieved by 2016.
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. see also Ventilation
fan test being carried out
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%
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 drafts, fuel bills and damp patches behind the wardrobe.
Given that drafts, high fuel bills and damp patches behind the wardrobe are largely to do with drafty 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. (Like the way that houses in London have extended party walls above the roof because of the deep fear that the great fire of London engendered. Or the way that rats got a bad press after the outbreaks of the plague.)
- There is the possibility of 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 draft proofing stuff.
- 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 draft 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 line - 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 drafty 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.
