Tuesday, January 20, 2015

Underfloor insulation - thinking about the business case

Having had a go at the walls, the windows, the roof, and air-tightness, I've decided to get the last physical improvement done to our house - the floor.

(image kindly supplied by devanainsulation.co.uk)

The area of wooden suspended floor is about 35 square metres. According to page 290 of my book, the U-value of the floor is 0.7 W/m2/K, so the floor has a leakiness of about 25 W/K, which assuming an average temperature difference of 6 degrees, implies a conductive heat loss of 147 W (3.5 kWh/day). (The floor may also contribute some of the ventilation-leakiness of the house, but I'll neglect that.) The floor doesn't just lose heat, it also feels cold, and thus (a) affects quality of life, and (b) perhaps causes us to turn up the thermostat sometimes to improve our feeling of warmth. If we manage to keep the whole house (leakiness 240 W/K) on average 1 degree cooler thanks to improved floor insulation, then that would be an extra 240 W of saving (6 kWh/day).
I didn't do an economic calculation before deciding to get the underfloor insulation. It just feels like the right thing to do, and Retrovive was recommended by someone from Max Fordham, whose judgement I respect highly. Anyway, let's work out a pay-back time. The anticipated new U-value is about 0.25 W/m2/K. It looks like the work will cost about £2900 including VAT (including insulating central heating pipes that run under the floor) (for comparison we are perfectly happy to put down new carpets over a smaller area for a cost of £1150). If the insulation eliminates two thirds of the floor's conductive heat loss (i.e., about 2.2 kWh/day) and delivers say one quarter of the notional 6 kWh/day saving guessed above (if we managed to turn the thermostat down a bit), the total saving might be 3.7 kWh/day. With gas costing 5.2p per kWh, that's 20p per day, or £70 per year. So the payback time might be about 41 years.
If I went to the high end of all my estimates, I might imagine a saving of 9.5 kWh/day on average, in which case the payback would be about £180 per year and the payback time would be 16 years.
I am expecting that the main value of this work will be the improved feeling of comfort. People are happy to pay £36,000 per year to rent a family home [That's what we paid to rent a flat in London, at least]. If the home's main room feels really cosy, how much extra would we be willing to pay? I could imagine 5% or 10%. On those grounds, the comfort of cosiness is worth £1800 to £3600 per year. So the payback time, taking into account this benefit, is just one or two years.
I will post again when the work is done! (April 2015)