What should air leakage be measured against?

[Ed Davies]

In another thread: @JSHarris wrote:

 

Quote

Why we have to relate airtightness to floor area is beyond me.

 

On reflection I'm beginning to think relating airtightness to floor area would make sense, which is probably why nobody does it. Ideally, it'd be done per person (m³/h/peep, or better l/s/peep) but you can't really determine the number of people who will occupy the house. Passivhaus does some other measures using finished-floor area as a proxy for the number of people, e.g., heat load, total heating and primary energy use. Apart from this tending to encourage larger houses with wasted space, I don't think it'd be completely mad to treat air leakage in the same way.

 

Using air permeability (m³/m²·h, as per building regs in the UK) fails to penalise awkward shaped houses (e.g., long and thin) which leak more air for the same permeability and internal area just because they have a larger envelope area.

 

Using AC/h fails to penalise unnecessarily large houses.

 

Say you have a 10 m x 10 m bungalow with a room height of 2.4 metres, then the envelope area is 296 m². If the leakage is 1 m³/m²·h then that's 296 m³/h and the air change rate is 296/240  = 1.233 AC/h. If you increase the ceilings to 3 metres the envelope increases to 320 m² and so the air change rate becomes 320/300 = 1.0667 AC/h. I.e., better even though the house is actually leaking more air.

 

I doubt this makes a huge difference in practice (other than it being very difficult to build tiny houses to Passivhaus standards) and there isn't really a single right answer I do think it's worth being aware of the tradeoffs between the different methods of measurement.

[ToughButterCup]

2 hours ago, Ed Davies said:

[...]

I do think it's worth being aware of the tradeoffs between the different methods of measurement.

 

Are we allowed to choose between methods? 

[A_L]

40 minutes ago, recoveringacademic said:

Are we allowed to choose between methods? 

 

No, Bregs are m³/m².hr and Passsivhaus ACH. Both at 50Pa over/under pressure. At low levels the numbers can be arithmetically similar, e.g. 0.6ACH corresponds to 0.48m³/m².hr  for a simple bungalow of 117m² I have details for.

Also the actual infiltration rate of the unpressurised house is about 1/20th the test number.

[Ed Davies]

50 minutes ago, recoveringacademic said:

Are we allowed to choose between methods?

 

Obviously not from the point of view of complying with a particular set of rules for some reason (wanting a gold star or not to be subject to the threat of state-sponsored violence or whatever). But it's worth at least thinking about if there's ever to be anything better than Passivhaus, for example.

 

6 minutes ago, A_L said:

Also the actual infiltration rate of the unpressurised house is about 1/20th the test number.

 

I've seen that said a number of times but I'm sceptical. 50 pascals corresponds to the extra stagnation pressure from a wind of 8.84 m/s (19.77 mph). If that wind is blowing on to the upwind side of the house the downwind side will be at roughly -50 Pa so, very crudely, there'll be in an inflow on the upwind side equal to test conditions and an outflow on the downwind side of the same. Combine that with the ”Bernoulli suction” of the roof (acting like a wing with lower pressure over the top) and the practical infiltration rate is likely to be a lot more than 1/20th of the test number.

 

For comparison, the wind so far this month at Wick airport (which I use as an proxy for the wind on my site) has averaged at 6.92 m/s (15.5 mph). Bearing in mind that the average of the squares is larger than the square of the averages it won't be far short of test conditions.

 

Maybe that 1/20th is a reasonable average for the whole year but that's not very interesting; the average for the heating season would be more relevant.

[D Walter]

Hi, picking up on this old thread, is there a simple way to convert the airtightness results from a SAP test into the Passive House equivalent or will I need to do a volume calculation for the house first?  Our SAP test has just been done and the I was told that the result is 1.71 which I assume is  m3/(m2*h).

[SteamyTea]

This is a similar to the problem of using heating and cooling degree days, if the base temperature is not set the same i.e. 15.5°C, then individual results become meaningless.

This is the problem we had over at the other place when we started to collect usage and house size data.  Too many people thought they would make their houses look better by changing the base temperature.

[ADLIan]

Dividing the SAP air infiltration rate (at 50 Pa) by 20 gives an approximation for ach in normal conditions. Method above, accounting for volume and exposed area, gives ach still at 50 Pa.

[D Walter]

Apologies, but I do not have a technical grounding and don't understand the response.  Are you able to clarify, thanks.

[Ed Davies]

1 hour ago, D Walter said:

Hi, picking up on this old thread, is there a simple way to convert the airtightness results from a SAP test into the Passive House equivalent or will I need to do a volume calculation for the house first? 

 

No, unfortunately, there isn't a simple conversion. It's not just the volume that matters but also the surface area; the conversion factor will be different for a nice cube-shaped house (low area to volume ratio) or a long thin house (high area to volume ratio) or one with an uneven outline (dormers or the like).

[SteamyTea]

1 hour ago, Ed Davies said:

the conversion factor will be different for a nice cube-shaped house (low area to volume ratio) or a long thin house (high area to volume ratio) or one with an uneven outline (dormers or the like).

I know that is the case, but it would be fun to reduce each house to a basic cube and seeing how much difference it actually makes.

Would need a reasonable estimate of how much energy each house has used (gas/oil/wood/electricity) and detached, semi, end terrace, mid terrace.  Then there is the really odd ones that may be triangular or third buried.

But as a start, just a cube would be a good starting point.

Or maybe a sphere.