If a house is made airtight, ventilation becomes critical to maintain air quality. To achieve good ventilation conditions, stale, humid, contaminated, oxygen depleted air needs to be removed from where it is generated and fresh air needs to be supplied to all habitable rooms. This is difficult to achieve and to control ventilation by simply opening windows or by depending on small permanent vents. Wind conditions vary, where some exterior surfaces will be exposed to face positive wind, blowing against the house.  Simultaneously, other lea-ward surfaces are in negative pressure causing suction effects. This can occur where chimneys downdraught and create conditions of smoke fumes. Rooms fitted with open fires or boilers or heating appliances that are not sealed against the indoor air, when ignited combust oxygen and can deplete the room of oxygen or worse, create a build up of carbon monoxide to dangerous levels. Radioactive radon gas levels can also be augmented in indoor environments, by open chimney- stacks, drawing warm air up and out. This can happen in circumstances where there’s a lack of ventilation vents, the interior spaces can create a negative air pressure with the ground, thereby vacuuming radon up from below the ground and trapping it the interior environment of the house.  Odours from toilets and grease and fumes from cooking all create health problems. However, the most common health damage caused by poor ventilation is a build up of humidity and condensation. High relative humidity conditions are now quite common in Irish homes, especially since the fitting of draught sealed window replacements and closing of permanent vents. If you think of the amount of vapour we create in a home from washing, cooking, breathing, perspiring etc, that can be sealed within our homes during the long heating season, often seven months.  This needs to be ventilated, but we don’t want the discomfort and heat loss from uncontrolled cold draughts.  Asthma has increased four fold in Ireland over the last twenty years and much of this increase is triggered from high humidity, generating condensation and moulds, and favouring conditions for dust-mites to multiply. An effective solution to ventilation for airtight homes is fitting a Heat Recovery Ventilation System. I will deal with this in a later blog. This is to prevent warm interior air that’s heated during the heating season, from driving through the exterior building elements, to balance with the outside ambient colder atmosphere. The warmer interior air typically suspends more water vapour then the outside colder air, causing a differential vapour pressure. This pressure drives the warm air to the outside, to balance the vapour pressure with the outside environment. It is therefore critical to prevent this vapour-laden air from driving through permeable materials like plasterboard and timber that are fitted on the inside of insulation. This is achieved by laying a vapour resisting, airtight membrane, ‘smart-ply’ OSB boards or by applying coats of wet plaster rendering. Its also important to eliminate all small gaps at joints and junctions that leak moist, warm, interior air to the colder ambient air outside, which contains much less water vapour. This is achieved by hermetically sealing all joints and junctions with a proprietary tape.

In my next blog I will be discussing Practical measures of upgrading in the home.

-Duncan Stewart