I have heard that in the UK/England the Building Regs state there can be "no more than 10 air changes per hour". Is this actually true? Or is in just a standard/advisory figure?
I can't see anything documenting this. I am guessing there is no legal requirement buy it would be very useful for me to get some documentation that confirms this as part of my research.

thanks

Jason,

You might be confusing two measurements?

Part L Building Regs, AD L1A England/Wales limits air permeability to 10m3/m2h tested at 50 Pascals.
ACH is a PH standard (0.6 or less).

The two are connected, but unless you are building to PH-certification, the m3/m2h is the metric to use.
Cheers....

thanks DarlyP

yes I am confusing them a little bit - I watched this very interesting documentary on airtightness and it seems to bring those terms together:

https://www.youtube.com/watch?v=W250wFGiCtM

The chap doing the air testing states that new build can be up to 10 ACH an hour to be compliant......I guess he is technically wrong....but is he broadly correct?

thanks alot

It perhaps depends on your definition of "broadly"

a house sitting on a 1 square metre foundation and 1 metre high has a cubic volume of 1m3. The walls have a surface area of 6 square metres (6 x 1 square metre). at 10 cubic metres per hour per m2 surface area, this box is allowed to lose 60 cubic metres of air per hour. As it is only 1 cubic metre in size it can change its air 60 times per hour

Do the maths again for a 10m x 10m x 10m box: volume = 1000 m3, total surface area of walls/floor/ceiling = 600 m2, loss allowed = 6000 m3; it's 1000m3 in size and can lose 6000m3/hour so it's 6 ACH

Let's do a terraced house, 5x10x(eaves 5/ridge 8). Surface area = 288 m2, Volume = 325 m3, Loss = 2880 m3/h, 8.8 ACH
6x6 bungalow with hipped roof 2.5 -> 5m: 139 m2, 113 m3, Loss = 1390 m3/h, 12.3 ACH


You might say that "on average the regs as applied to an average house might average out at about 10 ACH" but when you're dealing with compliance, and it's this easy to work out, why guess/approximate/average and potentially fail?

yes woefully lame and the real situation is even worse! As far as I know you are still allowed to fail and many new homes are never tested.

Posted By: Jason MatthewsThose calculations are somewhat over my head and the ACH seems to make more straight forward sense to me!

It's easy enough when you come to appreciate that:

ACH is "the volume of air the house may lose per hour, expressed as a percentage of the volume of air the house holds"
m3/m2/h is "the volume of air the house may lose per hour, expressed as a multiple of the total surface areas of the walls/floors/roofs"


To work out the surface area of the walls, we add them up. To work out the volume is more involved; mostly, houses are made up of cuboids and prisms, but for these examples I 'cheated' and drew a model house in google sketchup, a 3d modelling program, then asked it the volume of that i'd drawn


The maths is simple, once you have the volume and the surface areas:

In a passivhaus, take the volume of the house (in whatever units, let's use m3) and multiply it by 0.6. That's the most volume of air (in whatever units, let's use m3) the house may lose in an hour before it fails

In a regs house, take the surface area total and multiply it by 10. That's the most volume of air (in m3) the house may lose in an hour before it fails

If a house is 300 m2 and 300 m3, then the regs will permit it to lose 3000 m3 of air an hour (3 million litres). Passivhaus permits it to lose 180m3 of air an hour (180,000 litres)

So from what you have said, if I have this right, is that a housem (using your last example) could be compliant but in in fact end up being 12.3 ACH?

Yes. Remember, the tiny box I mentioned (imagine living in one of those white cube shaped plastic water tanks - they hold 1000 lites == theyre 1 cubic metre). According to regs, that cube may change its air 60 times per hour and still be OK!

... Yes, if the SAP calcs listed at ATT target at 10m3/m2h, and BC were happy to leave it at that.

However, if the DS SAP calcs indicate a specific ATT value, then they they will have to be tested, or have a 'penalty' of +2m3/m2h added on to the original dwelling's ATT test result.
And that penalty may well preclude the dwellings from meeting Part L Building Regs.
So increasingly, all dwellings will be ATT'd, and no bad thing, if it drives up site/construction stds!

Posted By: ringithought that would be good enough without first plastering the inside of block work

No - even if that's effective as air barrier (is plasterboard surface (not its joints) non-air permeable?), if the principal air barrier (against principal air pressure differences like buoyancy) is inboard you still need an outboard 'wind barrier' (e.g. joint-sealed breather felt) to prevent wind buffetting from driving air currents right through the fabric to the inside barrier. Best anyway to put the principal air barrier fairly well outboard.