Please help me understand air tightness rating

[Tony L]

I know the PH airtightness target (ie max allowed) is 0.6 ACH @ 50Pa.

 

My understanding of this is: if one of the external doors is opened then sealed up with an airtight membrane with a fan in it that sucks air out of the house to create a pressure difference of 50 Pascals between the inside of the house & the air pressure outside, & we keep the fan going & the pressure difference stable for an hour, if the house is going to score 0.6 this means that, in crude terms, 60% of the air inside the house at the start of the hour will have been sucked out of the house by the and of an hour & replaced with air from outside that will have found its way in through gaps and air-permeable surfaces.

 

I also understand, that even a score at twice the value of the PH minimum, ie 1.2 ACH is still good, & if I can get a score this low, MVHR (or similar) will be required.  If somebody wanted to rely on trickle vents & other holes in the house, rather than MVHR to provide ventilation, then they would have to ensure their house scored 3.0 ACH or greater, otherwise they’d be falling foul of building reg.s – in England – I suspect there are different rules for other areas.

 

So, if I haven’t already gone wrong, my question is: what does this sentence following mean & how does this relate to the 0.6/1.2/3.0 ACH figures from my examples above?  I can't make sense of it.  Obviously, "that" is supposed to be "than", & presumably "p.a." is another way of writing "Pa".  What is "h."?  Hours?  Perhaps this is a different way of measuring the leakage/permeability of the building in terms of the length of the building's perimeter.   The sentence is from an example Building Construction Notes document that I’ve received from an arch tec, I’m considering using to complete my house design & provide drawings:

 

[IanR]

While Passivhaus targets are quoted in Air Changes per Hour @ 50Pa, as is common on the continent, UK Building Regs uses the units m³/m².h @ 50Pa, where cubic meters or air loss, per hour, are measured as a ratio of the square meters of surface area of the "treated envelope". ie the internal wall, floor and ceiling/roof area of the airtight envelope.

 

There is no general correlation between ACH@50Pa and m³/m².h @ 50Pa, each house is different, and needs to be calculated.

 

The Air infiltration test doesn't actually run for an hour, and it averages two tests that pressurise and depressurise the house at 50Pa and -50Pa, in the manner you describe. From memory, the tests run for about 10 mins each.

 

If you have an air infiltration rate of less that 3m³/m².h@50Pa then English Building Control require you to have whole house mechanical ventilation. If you are above that figure, with trickle vents closed, then whole house mechanical ventilation is not required, but you will require extractors in wet rooms.

The 10m³/m².h@50Pa figure you have shown is an old figure. Currently new builds are required to be better than 8m³/m².h@50Pa, although that was due to change in April this year to 5m³/m².h@50Pa, but I don't believe the update to Building Regs has been introduced yet (although I may have missed it).

Edited Thursday at 23:06 by IanR

[JohnMo]

As @IanR says is pretty much what Scottish rules say also, for any Scottish viewers.

 

Next steps are dMEV or MEV, 3m³/m² or worse continuous extract. Almost silent fans run at a low rate, with trickle vents. But you can add a condition based system. So trickle vents that modulate automatically in response to humidity levels. And fans that do the same.

 

Then there is intermittent fan and trickle vents. Generally for 5m³/m² or worse from memory.

 

Trickle vents can be through wall or window frame, can be fixed or automaticy.

 

You also passive stack, and PIV.

 

 

[Nickfromwales]

IIRC, the test is 10 positive + 10 negative, and the score is the average of the 20 tests.

 

Things change, standards move, but the basics remain the same; the devil is in the details and airtightness will always be my #1 recommendation for a comfortable, efficient home, then insulation, then fabric and roof choices for acoustics and decrement delay.

 

Fabric choices can either help or hinder with airtightness, but understanding what you’re doing before you do (or attempt to do) it is paramount, so ask a lot of questions and prepare well.

[Tony L]

That's an excellent explanation.  Thank you, @IanR.

 

12 hours ago, IanR said:

The Air infiltration test doesn't actually run for an hour

Yes, I didn't think it did - the way I explained my understanding of how the test works could have been better.

[Tony L]

4 hours ago, JohnMo said:

But you can add a condition based system. So trickle vents that modulate automatically in response to humidity levels. And fans that do the same.

I've just put one of these fans in my rental property.

 

There's a PIV in there too.  V good.  It was part of my strategy to deal with a black mould problem.

[Nick Laslett]

@Tony L, I feel your pain. I resisted MVHR for the longest time, but finally rolled up my sleeves and did the reading. The earlier you can get your head around this the better it will be for your build. MVHR ducting is more of a challenge the later it is added to a design. 
 

Just play with the ACH value on Jeremy’s spreadsheet to see the effect on the heating requirements. 
 

There is a very good MVHR design spreadsheet here on BuildHub too, but this is a little more daunting than the heat loss spreadsheet. 
 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=23283

[Tony L]

21 minutes ago, Nick Laslett said:

@Tony L, I feel your pain. I resisted MVHR for the longest time, but finally rolled up my sleeves and did the reading. The earlier you can get your head around this the better it will be for your build. MVHR ducting is more of a challenge the later it is added to a design. 
 

Just play with the ACH value on Jeremy’s spreadsheet to see the effect on the heating requirements. 
 

There is a very good MVHR design spreadsheet here on BuildHub too, but this is a little more daunting than the heat loss spreadsheet. 
 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=23283 21.17 kB · 75 downloads

 

Thanks, Nick.  I've been completely into the airtight + MVHR way of thinking since I first read about it here 18 months or so ago.  I've just been improving my knowledge over the past few days to help keep my partner on board with these ideas after she was fed some mis-information by somebody who we were considering using to deal with our building reg.s & construction drawings.  I'll have a look at that link when I get a chance, just to improve my understanding.  I will likely be paying an expert from Wales to deal with my MVHR when the time comes.  My design will leave spaces in all the right places.