Build tight ventilate right was a slogan first used in the latter years of the last century

What does it mean now

How tight is tight, in ACH/h at 50Pa is. It 10, 5, 2, less than 1 or some multiple of 0.1? What does tight mean?

Then what is right in terms on ventilation? Depressurising an air tight building with an extract fan? Opening windows, trickle vents, PIV, MVHR, using poor levels of air tightness?

Anyone like to put numbers on ACH and acceptable heat losses due to ventilation?

OK then tell me a number for tight in m3/m2h then

OK - I'd go for about 2m3/h/m2@50Pa for commercial and 1m3/h/m2@50pa for domestic

Both are achievable in practice without complex detailing etc - but are dependent on "type" of construction

Values above are about 1/10th of what's considered traditional however so it does need some attention from designers and builders - basically there needs to be a clear air tightness strategy from the beginning and someone to champion that on site

I wouldn't depressurise any building personally - if you want control (and that may include filtering the incoming air to achieve reasonable indoor AQ) then you want positive overpressure - the degree of which will and should vary IMO depending on what's to be achieved

For all buildings with mech ventI would put in G4/F7 filtration as a starting point - then enhance from there depending on location

That said, I like windows that open - people aren't stupid - they will use them but in shared spaces often need an indicator that the AQ is dropping and a bit of fresh is needed

I'm still not convinced by the need for MVHR in pretty airtight domestic properties - windows are perfectly capable of being used to maintain IAQ without presenting a significant energy burden

Regards

Barney

They would laugh at you in Canada!

I call 5 bad practice and 2 not quite good enough!

In Canada they only want low decimal points

Posted By: gyrogearGuess it just goes to show that we all live in different places on the Planet, with different climates and requirements

Tell that to PHPP - which adjusts physical requirements to account for climate - but airtightness not negotiable.

Who knows what real ventilation requirements are. I think there's widespread agreement that 0.4 AC/h is a bit high so I'm going to guess 0.2 AC/h. For a 240 m³ house that comes to a heat-loss coefficient of just under 20 W/K.

Heating degree days to 20 °C for RAF Benson in Oxfordshire is just over 3500 so, without heat recovery, heat loss would be 70 kWd (1680 kWh) per year or about 190 W or nearly a tenth of a 2000 W society person's use. Heating with mains gas, that'd be about £84/year.

Seems worth reducing to me.

Posted By: gravelldSimilarly, do you not think 'common or garden' dumb on demand extract fans should be left off but not sealed?

It doesn't really matter what I think; it's what ATTMA thinks that matters. But personally, yes I agree, any extract fan should have its duct closing mechanism tested as part of the test.

Posted By: tony: “Any view on air tightness number?”

What's difficult is knowing how the airtightness tests relate to infiltration in practice.

E.g., 50 Pa corresponds (via ½ρv² [¹]) to a wind speed of 8.77 m/s (19.6 mph; Beaufort force 5, fresh breeze [²]) which is probably a bit higher than average but not at all exceptional, particularly during the parts of the year when the heating matters. Of course, wind only blows from one direction but then it tends to produce a negative pressure on the other side of the house. But then many houses are sheltered from direct wind by trees, other buildings, etc.

Plus there's stack effect.

So it's hard to say but I think that if the results of the airtightness test are an order of magnitude more than the ventilation requirements then something's likely wrong. Subject to more information, my guess would be that PH 0.6 AC/h or similar (e.g., 1 m/h) seems a sensible target. Lower if you're somewhere windy.

What I worry about more is lack of windtightness on the outside of the insulation.

[¹] https://en.wikipedia.org/wiki/Stagnation_pressure
[¹] https://en.wikipedia.org/wiki/Beaufort_scale#Beaufort_Number_5

Posted By: Ed Davies50 Pa corresponds (via ½ρv²) to a wind speed of 8.77 m/s (19.6 mph; Beaufort force 5, fresh breeze)

and (correct me if wrong) about 10mm water. We can easily suck orange squash up a straw 150mm or even 300mm or more, so have at our instant disposal a test rig capable of 30x the 50Pa standard, for a small sample (give it a wipe first!)

Posted By: fostertom: “and (correct me if wrong) about 10mm water.”

Less than that, about 5 mm.

Water has a density of about 1000 kg/m³ so 1 m³ weighs 10'000 newtons (if we round gravity from 9.81 m/s² to 10). The pressure at depth h in millimetres is therefore 10'000 N/m² × h/1000 = 10·h Pa. So 50 Pa is just 5 mm of water.

http://www.kylesconverter.com/pressure/pascals-to-millimeters-of-water agrees, give or take the rounding.

But I agree with Tom's basic point that testing at 50 Pa is pretty weedy.

Posted By: Ed DaviesBut I agree with Tom's basic point that testing at 50 Pa is pretty weedy.

It's based on average wind pressures, I think. Some places with a more serious climate test at 75 Pa or even 100 Pa. In this country, testing at more than 100 Pa is deemed to possibly damage the building's fabric!

There's a good summary of airtightness from the passivhaus point of view, including testing and what they think should be sealed, at:
http://www.passivhaus.org.uk/filelibrary/Primers/Passivhaus-Airtightness-Guide.pdf

What makes us (or anyone) think that actual infiltration is linearly proportional to pressure? Is fluid flow thro an orifice linear to pressure?

FT - no it's not linear

At 50Pa infiltration for an average new build house may be say 5 m3/hour/m2env@50Pa which for my planned house (370m2 env) equals 513 litres/second. A house leaking that much air would be loosing around 600 Watts per degree C temp difference (adding the equivalent of 1.6 to the U values of the house). Obviously a new build house does not leak that much air.

I therefore propose that the test at 50Pa is too high to represent average infiltration for a typical house (but may be about right on the windiest days) - however testing at a lower pressure would mean that the test would only be valid if taken on a windless day - 50Pa is probably about right for testing. because of the non-linearality of infiltration it means that small holes and cracks are under represented and larger cracks and holes are over represented compared to average conditions.

Test value can be adjusted by dividing be a number (e.g. 20 for SAP) or using a formula for calculating heat loss though infiltration. The number 20 is questionable for it's simplicity but it seams about right - the real number would be different for each mix mix of small and large cracks for any given tested house.

error above - for sap divide by 20 for an average 4m/s (not 4kph)

air density changes at about 0.005kg/m3 per degree centigrade - which equals 0.05 Pa per m height per degree C

stack affect for a 5m height difference and 20 deg C temp difference would be 5Pa. Assume 2.5 sucking at the bottom and 2.5 blowing out the top balanced at mid height - strangely about 1/20th of the blower test.

In cold still weather stack affect would be significant.