ESB, Ervia and Airtricity are the three main electricity and gas suppliers in Ireland. There are 19.82 billion cubic metres of proven reserves of gas. Natural gas extraction previously occurred at the Kinsale Head until its exhaustion. The Corrib gas field was due to come on stream in 2013/14. In 2012, the Barryroe field was confirmed to have up to 1.6 billion barrels of oil in reserve, with between 160 and 600 million recoverable. That could provide for Ireland's entire energy needs for up to 13 years, when it is developed in 2015/16. There have been significant efforts to increase the use of renewable and sustainable forms of energy in Ireland, particularly in wind power, with 3,000 MegaWatts of wind farms being constructed, some for the purpose of export. The Sustainable Energy Authority of Ireland  has estimated that 6.5% of Ireland's 2011 energy requirements were produced by renewable sources. The SEAI has also reported an increase in energy efficiency in Ireland with a 28% reduction in carbon emissions per house from 2005 to 2013.
Question: How many years did it take to reduce carbon emissions per house by 28%?
Answer: 8