Green Building Forum - What air flow rate counts as noticeable / slightly draughty? Fri, 17 Apr 2026 03:57:19 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=306936#Comment_306936 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=306936#Comment_306936 Mon, 07 Oct 2024 07:41:19 +0100 stonecold
The situation is that the ground floor has no corridor and one huge lounge that you walk through to reach the other downstairs rooms - so all the air going into the 3 bedrooms upstairs is going to need to go through the lounge in order to be extracted via the kitchen and downstairs loo extract valves. Is the guidance somewhere suggesting a particular limit for flow rate?

Design is getting finalised soon but the lounge has 44m3/h coming through a single door undercut from upstairs, plus 34m3/h from the supply valve (meaning 78m3/h leaving the lounge via a single door). That sounds like a lot to me. The lounge is about 20sqm with fairly low ceilings. I'm super sensitive to air movement /draughts even if they are warm air, so is this flow rate too high? We could reduce flow through the lounge by adding an extract valve in the entrance hall/stairs area, before it reaches the lounge. Or we could increase the flow to the bathroom extract.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=306939#Comment_306939 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=306939#Comment_306939 Mon, 07 Oct 2024 09:46:58 +0100 owlman Like I said I don't have MVHR.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=306940#Comment_306940 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=306940#Comment_306940 Mon, 07 Oct 2024 10:02:51 +0100 Doubting_Thomas - ceiling heights. You say these are low. What are we talking 2.3m, or even lower?
- occupancy. How many people are likely to be in the living space?
- flow rate of supply extract terminals involved. I got a bit confused where the air was going to and from in your description above.

The passivhaus guidance is to aim for 0.3-0.4 air changes per hour, which I think would be a bit lower than the flow rate you describe above, but is also guidance for the whole building rather than a single room.

Most of the industry guidance is based around relative humidity and how 'dry' the air feels rather than physical draughts. In my experience, at Passivhaus flow rates you only really feel the air flow if you are holding a hand close to the terminal. There is something called the Coanda effect that means (I think) that air supplied at high level near the ceiling will stay buoyant due to static pressure in the room and be thrown across the room rather than falling immediately and causing draughts.

There's more on this here if you want to get into the deeper physics & maths:

https://www.cibsejournal.com/cpd/modules/2022-12-air/]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307808#Comment_307808 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307808#Comment_307808 Sat, 22 Feb 2025 10:18:44 +0000 minisaurus What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307811#Comment_307811 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307811#Comment_307811 Sun, 23 Feb 2025 22:46:24 +0000 Mike1
Other possibilities include inserting a transfer grille in the wall above the door, or using a transfer duct spanning between two extra ceiling vents, one each side of the door, with a loop of pipe to connect them.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307815#Comment_307815 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307815#Comment_307815 Mon, 24 Feb 2025 12:33:30 +0000 sgt_woulds What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307822#Comment_307822 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307822#Comment_307822 Mon, 24 Feb 2025 17:36:36 +0000 djh Posted By: sgt_wouldsAnd intumescent closure devices to prevent the spread of flame between roomsSorry, but under what circumstances are they necessary (or recommended)? Many people, including us, leave internal doors open or ajar.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307829#Comment_307829 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307829#Comment_307829 Tue, 25 Feb 2025 07:53:33 +0000 revor Posted By: djhSorry, but under what circumstances are they necessary (or recommended)? Many people, including us, leave internal doors open or ajar.

My take would be where building regulations would require them. HMO's, public buildings, Care homes etc. Normal dwellings most unlikely but could depend on the size of rooms I think. I had a conversation with my BCO when I was running ventilation ducting from one ceiling void to another as to whether the ducting needed boxing in where it went from one room to another and he said rooms were too small to come under the regulation but I do not know what that size is. Think it is a sq M value.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307831#Comment_307831 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307831#Comment_307831 Tue, 25 Feb 2025 09:49:09 +0000 WillInAberdeen
There's some stuff in AD B about the protected stairway should have completely separated ventilation system and ducts from the other rooms shouldn't pass through the stairway except with fire or smoke dampers. IDK how often this is complied with.

Also there's fire separation between house and an attached garage, so eg you couldn't put the mhrv unit in the garage and duct it into the house.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307832#Comment_307832 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307832#Comment_307832 Tue, 25 Feb 2025 09:59:52 +0000 WillInAberdeen What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307836#Comment_307836 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307836#Comment_307836 Tue, 25 Feb 2025 11:15:10 +0000 Mike1 Posted By: WillInAberdeenPrevious versions of AD B had self closing doors with intumescent seals (my houses from the 90s had them, all propped open) but I can't immediately see that in current AD B.Fire door sets are still required in E&W but, except for doors to an integral garage, the requirement for closers was dropped for domesic homes (around 2010?) after a report found that many people disabled the closers, and among those who didn't there were over 30,000 finger-trap injuries each year.

Posted By: sgt_wouldsAnd intumescent closure devices to prevent the spread of flame between rooms
I've not seen any requirement for that in a domestic setting, any more than there is a requirement for them on conduits or waste pipes that pass between rooms or floors. Except, presumably, on ducts through a compartment wall between a house and garage, when the MVHR unit is within the latter.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307909#Comment_307909 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307909#Comment_307909 Sat, 08 Mar 2025 17:40:36 +0000 tony
Generally ventilation rates suggested are way too high. Flow rates through ducts and terminals can cause noise problems.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307911#Comment_307911 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=307911#Comment_307911 Sat, 08 Mar 2025 20:27:45 +0000 djh Posted By: tonyVentilation rates guidance is a mess! Occupancy is barely considered, l/s by number of bedrooms , floor area, then air changes per hourI agree that the ventilation system has to be able to deal with the maximum number of residents, but that seems like common sense?

Generally ventilation rates suggested are way too high.
There's nothing that requires you to operate your ventilation at the design rate. I frequently have mine at the 'off' setting of 50 m³/hr since there's normally just two of us at home.

Posted By: tonyFlow rates through ducts and terminals can cause noise problems.
That's a design issue with the ducts, not a problem with the flow rates. PH design requires the speed through the ducts to be calculated and kept low; i.e. the ducts need to be big enough to supply the required flow rate quietly. Anybody can follow the same methodology.]]> What air flow rate counts as noticeable / slightly draughty? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=308147#Comment_308147 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18209&Focus=308147#Comment_308147 Mon, 07 Apr 2025 12:55:14 +0100 stonecold
I did find the air flow valves from the Passivhaus Trust Good Practice Guide MVHR for Single Dwellings https://www.passivhaustrust.org.uk/UserFiles/File/Technical%20Papers/2018%20MVHR%20Good%20Practice%20Guide%20rev%201.1.pdf

(2018 version)
Air flow rate per room

Each space should have mechanical ventilation; either supply, transfer or extract, room.
Supply —consideration should be given to the planned/likely use of spaces.
A good starting point for bedrooms is ~20m³/h for single rooms and ~30m³/hr for double rooms.

Extract air flow rates are sized based on moisture generation rates and thus are based on periods of moisture generation, i.e. air flow rates required for peak ventilation (fan speed 3) typical figures used for Passivhaus design are below, note that certification does not require compliance with these:

Kitchen 60 m³/h
Bathroom, utility room 40 m³/h
WC 20 m³/h

:angry::shocked:]]>