Problems associated with Airtight buildings

Last post I looked at the importance of airtightness when it comes to low-carbon buildings. The lower the air infiltration, the greater the thermal comfort and less heating or cooling required to keep the indoor temperature comfortable.

The dangers of air-tight buildings are three-fold:

1. POOR AIR QUALITY

• reduced fresh air into the building results in high CO2 levels which is bad for health

2. MOULD GROWTH

• warm, moist air internally – as a result of breathing / laundry/ bathing etc – cools on external walls and windows and causes condensation and mould growth

Diagram showing moisture within walls

Common sight at well sealed window frames as moist warm air cools when coming into contact with cold surface of external openings

 External wall details showing typical problems due to moisture (A,B&C) and ideal solution (D)

3. OVERHEATING

• Highly insulated and air-tight buildings are susceptible to overheating in the summer. The Chartered Institute of Building Services Engineers / Arup Study defines WARM as 25C and HOT as 28C. Designers must consider this to allow for natural ventilation solutions that incorporate opening windows and roof vents to encourage cross ventilation and stack-effect.

Older buildings often make use of air-bricks high up on external walls so that warm, moist air can escape and allow fresh air into the building. But this totally conflicts with the intention of restriction indoor – outdoor air flow to preserve indoor temperature. Air-bricks are useful where the difference between indoor and outdoor temperature is not massive, for example climates where artificial heating or cooling is not generally used. Windows typically have trickle-vents incorporated into the frames to assist with the exhaust of moist air, but these also let unwanted cold air inside.

Victorian houses were actually designed and made to be quite leaky (rattling sash windows and open floor boards) to allow the smoke from coal fires to escape in an attempt to keep air healthy, however, thermal comfort was poor due to the draughty interiors and they were expensive to heat.

There is now a serious effort to retrofit these buildings to improve air-tightness and increase building envelope insulation.

Where mechanical heating and / or cooling is required, air-tightness is vital to energy efficiency and thermal comfort. The best way of ensuring good air quality (low CO2 levels) and good humidity levels (below 60%) is to make use of MVHR – mechanical ventilation and heat recovery system.  It is also worth noting that the MVHR can be switched OFF in the summer months to allow occupants to open windows and doors to naturally ventilate the building.

Schematic of MVHR installation

MVHR is only really necessary with air change rates of less than 4 per hour. However, I live in an apartment that has about 8 air changes per hour and while we have not issue with CO2 levels – typically 1000ppm, we have a big problem with humidity levels and condensation. In winter we use a de-humidifier permanently to control moisture levels. In summer we open the windows and the natural ventilation works just fine.

In summary, designers should design air-tight buildings but must consider efficient ventilation heat recovery systems which work to ensure healthy fresh air levels and extract moist air from kitchens and bathrooms to discourage mould growth. These should be commissioned properly before occupation and filter cleaned regularly. It is also imperative that the design considers good passive ventilation during summer months when the MVHR unit can be switched off.

INTERESTING LITERATURE ON THE SUBJECT:

• Zero Carbon Hub published a ‘Practical Guide to building Air-tight buildings’
• Useful article here… ‘Build tight, ventilate right’
• Mould growth and high humidity levels are a very common problem for buildings in cold climates and increasingly uncovered as an issue in modern renovated or new airtight buildings. This article…  describes a situation in America where an office building was renovated to improve air-tightness but it because a party ground for mould growth and resulted in the building being shut down and occupants relocated due to the hazardous air quality.

USEFUL PRODUCTS:

• Aereco, a french compancy that specialises in humidity sensitive ventilation systems, uses sensor technology to switch on an off depending on humidity levels. This works well to conserve energy but ensure good air quality too. 
• MVHR system from Green building store

Airtightness – is it all just a load of hot air??

No one likes a leaky building! You may not have thought about it before but I guarantee you have felt the discomfort of being in a draughty room. Much of sustainable building design relies of appropriate building materials and the orientation of windows and openings, these are the obvious aspects that are considered towards energy efficient buildings. However, if the building is not adequately airtight, any effort you might have made for insulation and orientation will be seriously compromised. Think about it – the idea is to keep heating or cooling INSIDE but if the nice warm/cool air is simply escaping out through poorly sealed building envelope, any insulation is rendered mostly useless. So to answer the question in the title – It is all about hot (or cool) air to achieve optimum thermal comfort at minimum cost.

Typical heat-loss pathways

The diagram above shows the common spots that contribute to poor airtightness…Junctions – roofs to walls, walls to floors; and openings – doors and windows and wall penetrations for services.

Regulations and guidelines vary on the level of airtightness required. From the best (Passivhaus) at 0.6 air changes per hour (ach) at 50Pa to British building regs at 10 air changes per hour.

Maximum air permeability	(m3/(h.m2) at 50Pa*
UK Building regulations – poorest acceptable standard	10
Building regulations indicative Part L 2010 target	7
Netherlands	6
Germany	1.8 – 3.8
Energy Saving Trust best practice	3
Super E® (Canada)	1.5
PassivHaus Standard	0.6
* Some values are actually air changes per hour @ 50Pa.

Buildings have to be constructed VERY well to achieve the passivhaus requirement of 0.6 and generally require the use of special membranes and taping to minimise air leakage. Airtightness tests are conducted during construction to check where levels are at and if more work needs to be done to achieve the desired levels (see image below)

Airtightness test carried out during construction

Where a building has less than 4 ach @ 50Pa, artificial ventilation will be required or the air quality will become unhealthy. (See previous blog post on this). Great for heat retention but bad for oxygen levels! The best systems in use are the Mechanical Ventilation Heat Recovery (MVHR) which uses heat from stale air being extracted, to warm incoming fresh air. You might be thinking, whats the point of making a building so airtight if you need energy to regulate air quality. But the energy used to run MVHR is minimal and costs about 50 pence per week and when you compare this to the reduced heating or cooling demand, there is no contest.

So, what kind of design decisions are required to achieve exemplar airtightness?

During my studies, SIGA did a workshop with us to educate us about their many products and how these should be used…

SIGA workshop at Oxford Brookes University

Membranes line out the entire inside of the building and these are fixed in place with tape and double sided sticky strips. Nails or staples should never be used as the piercings would compromise the integrity of the membrane.

All openings and penetrations are taped up:

SIGA product diagram showing where the products are used on a typical house envelope

There are typically two layers of BARRIER MEMBRANES in a good roof and wall design:

1. (RED LINE) the VAPOUR BARRIER (Airtightness layer) stops warm, moist internal air from getting into through the insulation and causing mould growth.
2. (BLUE LINE) the BREATHER  MEMBRANE (Weather-tight layer) allows moisture OUT but stops weather driven air and moisture INTO the building.

It is important to note that there is a difference between AIRTIGHTNESS and AIR INFILTRATION – the former refers to gaps in the building envelope which allow valuable warm or cool air to escape. Infiltration, however, is affected by wind loads / occupant behavior / natural ventilation strategies.

Energy Savings Trust has produced a document with some good case study information regarding this subject.

Find membranes and tapes at SIGA or INTELLO.

Next post I will look at the problems associated with Airtight buildings, namely poor air quality and mould and moisture issues, and how these can be avoided.