Four Times Better

Tired of living in a cold house and spurred on by the prospect of' ‘Peak Oil' and the likelihood of considerably higher energy bills in future years, I decided to take a look at how my house performs thermally. It is a typical two bedroom brick built end of terrace Victorian house dating from 1893, with single glazing and solid 9" brick walls.

Having already upgraded the loft insulation and with no cavity to even consider filling, it might be thought that double glazing was the obvious answer. Indeed my calculations showed that double glazing the 15m² of windows in the house (reducing the U-value from 5.6 to 2.8W/°C /m²) would reduce the heat loss of that element of the building from 84W/ºC to 42W/°C, a reduction of42W/°C.

U-value is a measure of how much heat energy a particular form of construction will transmit, and in the metric system it is expressed in W/°C /m². It thus measures how much energy a square metre of the construction will lose when subjected to a degree difference in temperature across it. Multiplying up by both the actual area concerned and the expected temperature difference tells you how much energy that part of the construction is likely to lose. Grossing up the areas and adding together the different elements of my house, the overall heat loss is about 400W/°C, this figure representing the amount of power in Watts needed to maintain the temperature 1°C higher than the temperature outside. Thus on a cold winter's day at -2° outside, if I want to maintain 18° inside, I will need 8,000 Watts (8kW) burning!

The possible saving of 42W/°C with double glazing thus represents 10% of my total energy costs, to be weighed up against a conservative estimate of around £4,000 to double glaze my eight windows to achieve that. Lf my energy costs were £1,000 per annum, it would take me 40 years to achieve payback!

Unconvinced of the economic case, I decided to take a closer look at the brickwork. Because it is an end terrace house, there is a total of some 95m² of 9" brickwork with U-value of 2.45W/°C /m² actively losing heat to the environment. An upgrade using external insulation could improve this U-value to 0.34W/°C /m². One area where this could easily be applied is the 40m² of blank gable end along the side of the house, where there are no windows, pipes or vents in the way. Just upgrading this section of the brickwork would reduce the heat loss on this element of the building from 98W/°C to 14W/°C, a reduction of 84W/°C. This represents a 20% possible saving in total energy costs, twice that of the double glazing option. Lf this can be achieved for less than £2,000 then this option of a 'gable warmer' is four times more cost effective than double glazing and will have a realistic 10 year payback period.

This all seemed good on paper, so I decided to proceed with a pilot project to construct such a gable warmer, which I will present in the next issue.

Patrick Taylor, Conservation Architect