A first attempt at an OpenBEM 3 assessment

28 Mar 2015

This evening I've spent a surprisingly enjoyable time inputting all my house measurements, U values etc into OpenBEM v3. OpenBEM is an open source building energy model based on SAP 2012. It is implemented and hosted over at emoncms.org.

A bit of background: this is the house I (and my family) live in, a 1950s masonry cavity walled property, detached, about 285m². Oil fired heating, not on the gas grid.

I'm just inputting stuff for space heating, this being my first major concern. I also added solar PV, because I happened to have the numbers for that.

Here's our initial score:

House graphic, assessment overview

The result is:

  • SAP Rating: 68.5
  • Annual Energy cost: £1589
  • Floor 85.3 W/K
  • Roof 27.9 W/K
  • Walls 99.8 W/K
  • Windows 168.5 W/K
  • Ventilation 178.5 W/K
  • Annual heat demand: 33316 kWh
  • Therefore 116.77kWh/(m².a)

Some thoughts...

The cost is actually not far off, given their slightly out of date oil price. We pay less because we've paid less for oil plus we don't heat the whole house all the time, being a user of Heat Genius.

Ventilated losses are the most significant. The first thing to say here is that the ventilation losses are assumed given a count of known vents, chimneys, extractor fans, other holes in the wall, and the wall and floor construction. A air tightness test would give me a more accurate figure, but I want to leave that until all of the holes I know of are dealt with (definitely the subject of another blog post... or ten).

Despite the fact that it gives an assumed figure, ventilation being the largest factor doesn't surprise me. I don't know, but I'd also guess there are a number of factors "averaged" that aren't asked about in the model. Things like the use of plasterboard (I felt a draught through a plug socket in a plasterboard wall a few days ago!), sealing around window/door frames, joist ends causing gaps into the cavity and thus over ventilating the ground/first floor void, and more.

So addressing uncontrolled ventilation will continue to be my main priority in my quest for energy efficiency.

The windows, though, really surprised me. They are the second largest source of heat loss, but I keep reading in most cases they don't make the difference people expect. I know they are of a good size (not particularly large) and most of them are old fashioned 6mm 2G air fill - I inputted U=3 for these. But I always thought windows were unlikely to payback in any way in a short time. In this case if we replaced with U=0.8 (ambitious maybe) we'd save 23% of energy per year.

The floors were also a more significant loss than I expected. Our floors are all solid, but mostly uninsulated (assuming the minimum building regs require was applied at build time). I used the following formula for the U values:

0.05 + (1.65 * (perimeter/area)) - (0.6 * ((perimeter/area)^2))

It's useful to perform this exercise, because it helps with "what if" scenarios. Given this model if I change all windows to U=0.8 (say) "mega windows" that lowers energy use by 23%. I could do similar exercises for adding EWI (which would also help with air tightness) or ripping up the floor and going through the considerable effort of retrofitting floor insulation.

Of course, there are a number of items which will make this model inaccurate (although, as I said, the financial cost at least appears roughly accurate) as they aren't considered. Examples would be:

  • Unfilled external cavity walls being brought into the thermal envelope following extension, acting as 'chimneys' into the loft (we have this... another blog post)
  • External walls bridging the thermal envelope into ground floor extensions (and this)
  • Other thermal bridges like lintels (we probably have this too)

It's good to get some numbers down, even if they might be a little off, because it helps with the next stages of planning.

An interesting figure to consider is the delivered heat demand per m². At 116.77kWh/(m².a) that's almost three times higher than AECB Silver level (a potential target to aim for) and over four and a half times higher than EnerPHit (probably out of reach given time, expertise and money constraints).

Thanks to owlpacino, for the image above.
blog comments powered by Disqus