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)


John Stumbles said...

Here's a Crazy Idea That Might Just Work.

Taking up carpets and stripping floorboards on suspended ground floors may look trendy but it's thermally terrible: there are usually significant gaps between the floorboards and there's a space underneath with outside air blowing through it (which is necessary to keep the under-floor void from getting damp and encouraging rot in timbers). But what if we were to seal up the air bricks and blow warm air in under the floor? Assuming the air being blown in was reasonably dry it would keep down humidity and prevent damp and rot under the floor and the warm air would percolate up through the gaps between the boards giving ersatz underfloor heating! When you've got warm feet you can be comfortable at lower air temperatures in the room so you should need less heat input overall. (Although you would lose some heat keeping the sub-floor void warm it would be less than heating the room alone with its bare floor, even without the icy draughts contributing to heat loss: a solid floor on earth has a U value of about 0.36 W/m^2/K (according to BS5449(1990)) compared to a bare suspended floor at about 0.61 (with one air brick) to 0.82 (>1 air bricks). (And it wouldn't be so much worse than your insulated floor's U of 0.25.)

If you could get enough warmth through to heat the room this way you could not only dispense with radiators, freeing up wall space, but you could use an air-to-air heat pump rather than a boiler to supply the warm air.

Justin Needham said...

@David. As you say it's about comfort and feel good as much as cost savings. The physical comfort is hard to communicate to people though. I have worked hard on remedial IWI for a 1970's conventional filled cavity house. The whole feel of the place changes remarkably. We are all willing to pay a lot for property, but many draw the line at insulation, looking only at the ££. Somehow this palpable difference needs to be conveyed to people. It's more than the money.
@John: perhaps not as crazy as it initially sounds for a quick-fix improvement at least. Sourcing dry air the biggest hurdle though!

Ray Alexander said...

Have you heard about underfloor heating? That's taking off here in the UK which is good.

In fact, when I wanted my underfloor heating I got some quotes - but opted with STL Heating as the price was good, plus they're local (North West UK).


Mark Thompson - Innovate UK said...

Just came across your blog and very interested to see your calcs and thinking on insulating the underside of suspended floors. I lived in a house with this floor structure many years ago and the thermal comfort was truly awful, aggravated by very poor airtightness round the edge of course as joists had settled/moved in a few places to make it even worse.

I viewed a suspended 60’s bungalow a couple of years ago when house hunting for my in-laws and there were fan heaters everywhere as a result!

A little while ago I came across this startup company http://www.q-bot.co/services.php which is doing very well in developing a small tethered robot to insulate underfloor structures with spray foam. DECC are running some trials which I believe are going extremely well.

I agree with you that the comfort driver carries great weight in such an investment decision.

I was also looking at U values of carpet underlay a little while ago, and there are some surprisingly high TOG value/Low U-Value underlays (TOG 3.4 – U value circa 3) out there. We have some of this down in our bedroom and it is surprising how thermal comfort feels really good in bare feet (albeit this over an internal upstairs floor void rather than ground floor)

Mark Thompson - Lead Technologist, Innovate UK

GCSE.com said...

David - any news on how well your underfloor insulation has performed through the Spring?

Henrietta Fuller said...

I have very bad circulation. My hands and feet are always much colder than the rest of my body and it's worse in the winter. I hate having to get up in the morning since my warm and toasty toes always become really cold when they touch the floor. I think underfloor insulation might just be the way to go. You're right about comfort. I would not mind paying just a bit more for warmer toes.

Henrietta Fuller @ Bri-Tech Heating and Cooling