Green Building Forum - Build tight ventilate right

Build tight ventilate right

tony — Thu, 25 May 2017 11:49:45 +0100

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?

Build tight ventilate right

DarylP — Thu, 25 May 2017 12:17:45 +0100

I cannot comment on AC/h, as SAP in UK uses m3 / m2h (cubic meter of air per m2 envelope area x hours).
Air tightness testing is for uncontrolled ventilation - leaks.
Controlled ventilation is sealed during tests: trickle vents, int. extracts, etc etc
Losses due to ventilation can range from v small, to considerable, depending on the m3 of the dwelling, the numbers/types of vents.

Build tight ventilate right

tony — Thu, 25 May 2017 13:06:15 +0100

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

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djh — Thu, 25 May 2017 13:28:31 +0100

Posted By: DarylPControlled ventilation is sealed during tests: trickle vents, int. extracts, etc etc

Trickle vents should not be sealed - they should be closed but not sealed. Ventilation systems and extract fans should be sealed, but only the ducts themselves, not the connections to the walls or windows. As usual, this is if the job is being done properly, not according to English tradition.

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barney — Thu, 25 May 2017 13:57:27 +0100

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

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Sigaldry — Thu, 25 May 2017 14:07:46 +0100

I'd call tight anything below 5mÂ³/mÂ²/hr @ 50 Pa <- Good Practice.

Below 3mÂ³/mÂ²/hr @ 50 Pa is best practice (combined with a mechanical ventilation system (with a low fan power and heat recovery - essential below this level; recommended below AT=5).

See EST GPG 224 and EST GPG 268 for some good practice guidance on ventilation and low air tightness.

Attention to detail can get you below 1mÂ³/mÂ²/hr @ 50 Pa with a SIPs dwelling (and AT=2-3 is relatively easy it seems), Timber frame seems to be able to get below AT=3-4 reasonably with airtightness layers, attention to detailing/filling gaps; Most masonry dwellings seem to be able to get AT=5-7 easily enough with some care and attention and lower levels doable with a parge coat or plaster. All down to workmanship and thinking about it.

Build tight ventilate right

tony — Thu, 25 May 2017 16:08:41 +0100

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

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gyrogear — Thu, 25 May 2017 18:25:53 +0100

Guess it just goes to show that we all live in different places on the Planet, with different climates and requirements.

One should be comparing apples with apples, no ?

gg

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fostertom — Thu, 25 May 2017 19:49:09 +0100

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.

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gravelld — Thu, 25 May 2017 21:22:02 +0100

Posted By: djh
Trickle vents should not be sealed - they should be closed but not sealed. Ventilation systems and extract fans should be sealed, but only the ducts themselves, not the connections to the walls or windows.

Similarly, do you not think 'common or garden' dumb on demand extract fans should be left off but not sealed? They contribute to the leakiness of the house day-to-day do they not?

Or are they considered to be offering background ventilation when turned off? Seems a bit finger-in-the-air.

Mine were even worse chimneys than my downlighters, fortunately far fewer of them.

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Ed Davies — Thu, 25 May 2017 21:22:26 +0100

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 mÂ³ house that comes to a heat-loss coefficient of just under 20â€¯W/K.

Heating degree days to 20â€¯Â°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 2000â€¯W society person's use. Heating with mains gas, that'd be about Â£84/year.

Seems worth reducing to me.

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tony — Thu, 25 May 2017 22:01:42 +0100

Good numbers thanks

Any view on air tightness number?

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djh — Thu, 25 May 2017 22:15:42 +0100

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.

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fostertom — Fri, 26 May 2017 09:26:06 +0100

Posted By: djhany extract fan should have its duct closing mechanism tested

if any - traditionally these are just on open hole in the wall.

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Ed Davies — Fri, 26 May 2017 09:45:18 +0100

Posted By: tony: â€œAny view on air tightness number?â€

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

E.g., 50 Pa corresponds (via Â½ÏvÂ² [Â¹]) to a wind speed of 8.77 m/s (19.6 mph; Beaufort force 5, fresh breeze [Â²]) 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.

[Â¹] https://en.wikipedia.org/wiki/Stagnation_pressure
[Â¹] https://en.wikipedia.org/wiki/Beaufort_scale#Beaufort_Number_5

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Ed Davies — Fri, 26 May 2017 09:53:08 +0100

PS, my assumption is we're comparing against a house built properly to current building regulations and wondering what the next step should be. I think it's airtightness and MHRV rather than more insulation for a number of reasons:

1) It's probably the most cost-effective step.

2) It's, in some ways [Â¹], easier to check that it's been done properly.

So I'm interpreting the question as what level of airtightness is needed to make sensible use of MHRV (i.e, to recover a significant proportion of the ventilation heat loss). There's the separate issue of not over ventilating on windy days which applies if you're aiming for natural ventilation only but can be ignored if you're aiming for MHRV levels.

[Â¹] Easy to check at completion, harder to know how it'll last.

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fostertom — Fri, 26 May 2017 11:04:52 +0100

Posted By: Ed Davies50 Pa corresponds (via Â½ÏvÂ²) 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!)

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gravelld — Fri, 26 May 2017 11:29:51 +0100

Posted By: fostertom
Posted By: djhany extract fan should have its duct closing mechanism tested
if any - traditionally these are just on open hole in the wall.

I think Dave, from the point of view of measurement, was rightly identifying that the duct can be closed anywhere from 0 to 100% (ignoring the surely immaterial impact of the fan blades), and you're right, a vast, vast majority are at 0%.

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Ed Davies — Fri, 26 May 2017 12:02:59 +0100

Posted By: fostertom: â€œand (correct me if wrong) about 10mm water.â€

Less than that, about 5â€¯mm.

Water has a density of about 1000â€¯kg/mÂ³ so 1â€¯mÂ³ weighs 10'000 newtons (if we round gravity from 9.81 m/sÂ² to 10). The pressure at depth h in millimetres is therefore 10'000 N/mÂ² Ã— h/1000 = 10Â·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.

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fostertom — Fri, 26 May 2017 12:36:52 +0100

Yeah you're right - I miscalculated 'two tenths of a inch' from an American source (probably NASA!)

I've yet to try sucking on a piece of OSB - could be a good check at the builders merchant, as air permeability is said to vary from batch to batch.

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djh — Fri, 26 May 2017 12:41:25 +0100

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

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goodevans — Fri, 26 May 2017 12:58:00 +0100

Largely it doesn't matter what the pressure value is so long as it's in the ballpark for windy days - when you look at SAP average infiltration is assumed at 1/20th of the 50Pa m3 value (formular 18) assuming an average 4kmh wind location and no shelter (linearly increasing/decreasing depending on location shelter factors). If you test at different values just take a different fraction. Testing at 50Pa seems about right to me.

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fostertom — Fri, 26 May 2017 13:34:18 +0100

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

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Ed Davies — Fri, 26 May 2017 13:36:19 +0100

Posted By: goodevanswhen you look at SAP average infiltration is assumed at 1/20th of the 50Pa m3 value

But is that assumption reasonable given that 50â€¯Pa seems not to be huge compared with plausible air pressures on a house (even though those pressures wouldn't uniform in the way that a pressure test is)?

E.g., 4â€¯km/h might be average over the whole year in the south of England but seems very low as the average for the worst parts of the heating season when it matters most.

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goodevans — Fri, 26 May 2017 16:09:38 +0100

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.

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fostertom — Fri, 26 May 2017 16:15:59 +0100

And stack effect - which keeps going, either up (mostly) or down (sometimes in summer) 24/7 even when no wind, and Fraunhofer consider it by far dominates wind effects to the extent that WUFI's (horribly rule-of-thumb) methodology for infiltration ignores wind weather data, instead looks at the height of the stack i.e. how many storeys. Maybe PHPP's 50Pa isn't about wind at all, but stack effect.

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goodevans — Fri, 26 May 2017 16:31:00 +0100

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

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djh — Fri, 26 May 2017 16:41:00 +0100

goodevans already explained that the test pressure isn't especially connected to anything at all. In fact the results stated at a test pressure of 50 Pa are actually calculated by regression from results measured at a range of pressures. It's just a standard value for tests.

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goodevans — Fri, 26 May 2017 16:55:30 +0100

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.

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fostertom — Fri, 26 May 2017 17:20:13 +0100

Interesting -

Posted By: goodevans5m height difference and 20 deg C temp difference would be 5Pa

is a bungalow with walk-height loft, or a v skimpy flat roof 2 storey. 2 storey with loft more like 7m, so 14C - not just deep winter. A 2-storey maisonette above shop say with GrdFl entrance would be 9m, 11C.