I have approx 310kg of air in my flat. Heat capacity of air is 1.05 KJ/kgK. So that's 1.05*310=~330 KJ/K. To take open my windows and take the air away I'm going from ~20C to ~5C, so 15C diff. So 15*330KJ ~=5MJ of energy binned. Seeing as 1J = 1w.s, that's take a 3KW electric heater half an hour to replace if it was 100% efficient.

out of interest, how much does it cost to build 'a house'

nothing fancy or outstanding, just a detatched, 3-4 bedroom, bricks and mortar house, cavity wall insulation, normal central heating and radiators, double garrage. Just an average suburban house, built to a reasnoble standard (so not david willson [st]homes[/st]sheds etc)

try western log group

they're near afan so a good excuse for a ride if ever i've heard.

we're thinking of one too.

The standard insurance rebuild cost for 3-4 bedroom house is ca £90,000 (depending upon various rating factors such as location, materials etc….)

tinas – meant to be around the £100 per square foot region IIRC.

Standard off the cuff response is £1,000.00 per square metre.

Though I've read about people managing it for ~50 and as high as 250 /sqft

Just always wondered how much more people on grand designs were paying compared to if they'd just settled for a more normal house on the same plot.

There were plots for sale on the outskirts of Reading for £17,000, no planning permission, but in a place you could reasnobly expect to get it in 5-10 years time (technicaly greenbelt, but over the road form a new housing estate).

Clearly letting all the air out of your house is not ideal, I'd have to be an idiot to think that and as well all know, I'm not 😉 I was just pulling you up on a technical point 🙂 And the more practical point that even if you left all the windows open for half an hour your house wouldn't be as cold as it woudl be when you say came back from holiday having left the heating off.

What about calculating the rate of heat loss through the required ventilation compared to the rate of heat loss through walls/windows/roof etc? Would be interesting.

tinas – stay well clear of those unless your are v. well informed. most land bank resellers are scamming the ignorant.

as to construction cost, Build it and self build magazines cover it regularly.

What about calculating the rate of heat loss through the required ventilation compared to the rate of heat loss through walls/windows/roof etc? Would be interesting.

I've no idea what the required ventilation rates are at the moment TBH, but ventilation losses will always be in addition to loss through walls, so while it may be a small percentage, it's always there and always something to minimise. The heat exchanger systems may well be good for this.

If letting air out of the building isn't an issue why are we designing air tight buildings… Enhanced accredited details etc ? Mevhr system that mean you don't have to open a window for ventilation.

Coffeeking- with reference to your post at the top… That's enough!

If letting air out of the building isn't an issue

I never said it wasn't an issue.

Interesting how people don't read things properly, isn't it? 🙂

I'm nearing the end of a self build. I looked into SIPS and beco wallforms but eventually went with tried and tested timber frame and went over spec on insulation.
Easily the best VFM way to build a house imo.
I would still love to do a beco house one day but I found it hard to get a builder interested in it. They are very resistant to change generally I've found.
Most will take your plans and think, 'Hmm, never used this stuff before, best think of a healthy price then double it.'
I also looked at various 'green' things for heat power etc but they all seem to be eye wateringly expensive and just before they have paid for themselves, will implode and need replaced. Only one I'd consider would be solar hot water but I dont have a suitable south roof area due to dormers.
Throwing a few extra thousand pounds worth of insulation at it is easy and effective long term.

I'm 'experimenting' with photovoltaics at the moment, a lot better when it comes to awkward roofs. You thought about geothermal or did you not have the room?

My Dad works for a company that manufactures insulation products that are used in 'kit' houses, timber kit ones are a bit of a muchness though the best prefab ones are reckoned to be German or Scandinavian.

I think greenwoood make a small version of their MVHR, or you could try vent axia.

as for MVHR causing draughts – cant see it unless the vents/ducts were incorrctly placed. the fans are Dc powered, continuously running type and the actual flow rate is way less than that in one of the light/fan combo's you get in most bathrooms. 80% heat recovery is possble so it a nobrainer in new build IMHO.