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