Hygrothermal assessment in cold roof buildings with reduced ventilation

Thermally inefficient buildings are prevalent throughout Ireland creating a barrier to ending fuel poverty and reducing CO2 emissions. The building sector in the EU typically accounts for up to 40% of total energy consumption, with 69% attributed to space heating. Dwellings that are poor at conserving energy can lose up to 30% of their heat through the roof.

Retrofitting homes with energy saving technology can provide significant benefits to the consumer, economy and the environment. Major progress has been made in upgrading the building energy rating (BER) of domestic dwellings over the last 15 years in low-risk, high-yield projects. Although the dormer bungalow has been identified as problematic for deep energy retrofit due to the building designs impact on ventilation requirements. It has been previously excluded for deep energy retrofit schemes such as the warmer home scheme.

Throughout Ireland dwellings with dormer attics built pre 2006 were subject to a varying degree of building design standards and/or poor building practices. Instances where the space between the rafters and the dormer rooms are restricted leads to lower levels of insulation being installed or comprised ventilation flow rates. In addition, the ventilation requirements are higher for a dormer attic due to the increased surface area where heat loss occurs in comparison to without a dormer conversion (i.e. insulated at ceiling level).The issues associated with dormer attics occur due to the restricted space between the roof rafters and plasterboard ceiling of the dormer room. Meeting the current building regulations requires a significant depth of insulation that is typically not available where the ventilation requirement must be also considered. This suggests that either reduced ventilation is accepted which could lead to interstitial condensation or that the insulation is not sufficient to achieve the minimum U value.

Finally, the effect on the building structure that the relationship between the relative humidity, indoor/outdoor temperature and insulation thickness, pre and post retro fit is poorly understood. This proposal aims to address these gaps in knowledge, provide a cost optimal model for thermal energy retrofitting of dormer attics that meet building regulation U values, and demonstrate the effect reduced ventilation has upon condensation in a cold roof attic under Irish climate and building archetype conditions through a hygrothermal analysis.


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