Which "m2" do you count, in cost per square meter (sqm)?

[puntloos]

When you hear, say, "1000/sqm", which sqm are counted? Purely the actual livable rooms? What about toilets, halls, garage? Or simply the raw footprint of the house because 'cheap' rooms counter the 'expensive' ones and it all averages out in the end?

 

https://abi.bcis.co.uk/measuring_your_house/measuring.aspx seems to be saying 'raw footprint' - is that correct?

 

My current house design gross external size is 240sqm (not counting the loft).. but for example we have a double-height hallway in the current design and a 6x3 garage.. etc

 

 

 

Edited November 21, 2019 by puntloos

[Jeremy Harris]

The normal metric is the net internal floor area, so the area of all habitable floors inside the building shell.

[SteamyTea]

Be fun to calculate the energy usage for the sum of all built areas.

 

[puntloos]

Just now, Sensus said:

 

It depends what you're counting it for.

 

But more often than not for the purposes we're usually interested in, its the gross internal area (GIA) that's the 'normal' metric.

 

See RICS code of practice here. Note in particular pages 7, 11 and 15, which tell you what form of measurement is 'standard' for what purposes.

 

 

 

Sigh. So I guess it's 'up to the builder' to decide how to count?

 

Still, as rules of thumb go, perhaps just stick with gross. 

[puntloos]

Just now, Sensus said:

 

No, absolutely not.

 

Read again what I wrote, then read pages 7, 11 and 15 of the document I linked.

 

The RICs offer a very clear, well-defined standard, and tell you for what purposes you should be using each form of measurement.

 

Sorry you're right. I assumed that builders could just 'pick the own metric' - but still if you assume they're saying GIA then it can only be better

[joth]

59 minutes ago, SteamyTea said:

Be fun to calculate the energy usage for the sum of all built areas.

 

 

 

The very definition of passive house is <= 15kWh/m2 per year heating load. 

PHPP uses the concept of annual energy use per "total treated floor area" pretty much equivalent to useful floor area. 

I always think of our house as 160m2 but it's actually 151 when I subtract the area used by internal walls, first floor void (above stairs and hallway) etc. which slightly penalises the efficiency

[SteamyTea]

9 minutes ago, joth said:

The very definition of passive house is <= 15kWh/m2 per year heating load. 

And this penalises small houses that use less energy overall.

[nod]

I just realised while my build was 284m2 coming in at 830m2 

in with our costing are two double skinned double garages We must have spent at least 25 k on these 

Done better that we thought 

perhaps I should re calc ?

[Jeremy Harris]

10 minutes ago, joth said:

 

 

The very definition of passive house is <= 15kWh/m2 per year heating load. 

PHPP uses the concept of annual energy use per "total treated floor area" pretty much equivalent to useful floor area. 

I always think of our house as 160m2 but it's actually 151 when I subtract the area used by internal walls, first floor void (above stairs and hallway) etc. which slightly penalises the efficiency

 

 

Indeed it does.  The difference for us between the two is about 22m², which makes a significant difference in PHPP.

[joth]

9 minutes ago, SteamyTea said:

And this penalises small houses that use less energy overall.

It certainly does. Also smaller will have less embodied carbon and generally use less resources / have lower environmental impact. While not perfect, it does exactly do the thing you said would be fun to do (unless I misunderstood?)

[SteamyTea]

If we reduced our houses to cubes, for calculation purposes, then divided the annual energy usage by that area, and then, divided by hours in year, we would get a number.

The lower the number, the better.

Mine is 3.5 [W.m^-2.year^-1]

[Ed Davies]

@SteamyTea, I think you want to have another look at your units there. Perhaps W·m⁻² or something.

 

But W/peep would be my preference.

[joth]

1 hour ago, Ed Davies said:

@SteamyTea, I think you want to have another look at your units there. Perhaps W·m⁻² or something.

 

+1

2 hours ago, SteamyTea said:

Mine is 3.5 [W.m^-2.year^-1]

I'm 2.9 Wm⁻²

 

For unit-whacking I do like to use the Google onebox calculator:

(25 (kwh / m2)) / (1 year) =

2.85198882 watts / m2

 

1 hour ago, Ed Davies said:

But W/peep would be my preference.

I know where you're coming from, but I don't like it so much as it is very dynamic number based on usage and  seems even easier to "game". (A developer can "design" a house for say 6 people, knowing full well the owner will only put 3-4 ppl in it, or whatever, whereas self-builder would feel more obligation to use the number they know they're actually going to put in the house. PHPP is bad enough for this already with e.g. "personal electronics" being a weird heuristic. I originally tried to model all the computer, network, CCTV, etc gear I might choose to install but then realized I was needlessly penalising myself vs a developer making an empty house to sell on, so just stripped it back to use the built in heuristic basics. OTOH their circulation pump electricity consumption heuristics are just plain broken, so I took the choice to override that with more sane numbers. Not perfect, but it's what I've got.)

 

[Ed Davies]

1 hour ago, joth said:

I know where you're coming from, but I don't like it so much as it is very dynamic number based on usage and  seems even easier to "game".

 

Indeed, almost impossible to define sensibly. But it's worth remembering that m² is really just a proxy for people kept comfy. One or two people living in an enormous house is likely wasteful even if it's a creditably low W/m².

 

On the other hand, the W bit doesn't make much sense either. For example, sunlight coming in through a window is counted as free energy but sunlight falling on a PV panel, converted to electricity used to heat water (for space heating or DHW) does count which seems totally arbitrary to me. I get that sunlight in July shouldn't offset electricity in January but sunlight in March which is used instantly shouldn't be included in you W/m²

 

All reasons why getting too precious about which m² count is not overly productive.

[SteamyTea]

4 hours ago, Ed Davies said:

Perhaps W·m⁻² or something

Yes, not sure why I put the year in, probably still had 8760 in my head.

W.m^-2 is the mean power the house is emitting, regardless of anything else.

[joth]

9 hours ago, Ed Davies said:

sunlight coming in through a window is counted as free energy but sunlight falling on a PV panel, converted to electricity used to heat water (for space heating or DHW) does count which seems totally arbitrary to me

 

I can rationalize this a bit. The primary reason for windows is light and aesthetic pleasure, and they're reasonably low embodied carbon, so any heating from them really is "free" as you'd be getting the cost of them anyway.

PV panels on the other hand are more damaging to manufacture (citation needed) and have no other purpose so need to justify themselves purely on the useful energy generated. Installing them and then use their full capacity on simultaneous  heating and cooling due to poor design elsewhere, justified because the energy is "free", would kind of defeat the goal (Jevons paradox)

 

 

Edited November 22, 2019 by joth

[ToughButterCup]

17 hours ago, SteamyTea said:

And this penalises small houses that use less energy overall.

Interesting, how does a standard that should be universal penalise small houses? 

You going to show me some hard sums?

[Ed Davies]

3 hours ago, joth said:

The primary reason for windows is light and aesthetic pleasure, and they're reasonably low embodied carbon, so any heating from them really is "free" as you'd be getting the cost of them anyway.

 

Wookey, on another forum, was mostly joking when he suggested a house with no windows, just cameras and flat screens, but what you say is only true up to a point. There are lots of reasons for windows: light, outside awareness/aesthetic pleasure, means of escape… but relatively small windows will cover those requirements and it's arguable from an energy point of view that beyond that size PV or solar thermal panels are a better net investment considering the extra heat loss through windows, etc. AIUI, PHPP used as intended doesn't allow you to explore that trade-off.

 

3 hours ago, joth said:

PV panels on the other hand are more damaging to manufacture (citation needed)

 

Indeed, a citation is needed. If cost in £ is considered as reflecting cost to the environment then per m² PV panels are cheaper than windows. For net energy harvested, I'm not so sure as that'd be very site and user-behaviour [¹] dependent.

 

[¹] E.g., whether or not insulated blinds are used behind the windows when there'd otherwise be a large heat loss.

[SteamyTea]

1 hour ago, AnonymousBosch said:

You going to show me some hard sums

No, but Wikipedia will.

https://en.wikipedia.org/wiki/Surface-area-to-volume_ratio