PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Ventilative cooling in noisy environments:
practical options for the UK
Nick CONLAN1; Jack HARVIE-CLARK2
1,2Apex Acoustics, UKABSTRACT
It is only very recently that a proper consideration of the internal acoustic environment when mitigating
overheating has started to be taken seriously in the UK. Previously this issue has not been addressed by thevarious regulatory systems, and developers have been reluctant to pay for what they have seen as anunnecessary consideration. The Association of Noise Consultants has produced the criteria and guidance inits draft Acoustics, Ventilation & Overheating: Residential Design Guide. At this crucial stage in theestablishment of new approaches for the construction industry, we are setting out “rules of thumb” withtypical options for ventilative cooling in varying external noise environments. We are developing solutions
in collaboration with mechanical engineers and product manufacturers that are suitable for UK building
typologies. Vents in the external facade are found to present the easiest adoption into current building
designs. As acoustic attenuation typically involves airflow resistance, mechanical fans are required toensure ventilation rates are achieved in noisier environments or where the apartment has single-sided
ventilation opportunities.
Keywords: Passive attenuated ventilative cooling, vents, acoustics ventilation overheating

1. INTRODUCTION
The need to mitigate overheating in modern dwellings can require ventilation rates much greaterthan the rates requires to simply provide reasonable air quality. The provision for purge ventilation,
usually opening windows, is typically relied upon for this purpose. However, high noise levels arefrequently cited as a reason that residents are reluctant to open windows to provide increased
ventilation, and they may suffer from over-heating as a result. The regulatory framework in the UKdoes not currently require noise and overheating requirements to be considered together and this has
led to the situation where the majority of planning application include noise assessments based on
closed windows and overheating assessments which assume that the win dows are open, as describedby Conlan et al (1).
This paper presents practical methods to provide ventilation rates that may be suitable to mitigate
over-heating using façade mounted ventilation systems and compares the performance with openwindows. Other methods such as good acoustic design for the building location and orientation, theuse of balconies or enhanced window configurations can also reduce the noise ingress to the building ;these are discussed by Conlan et al (2)(3).

2. AVO GUIDEThe Association of Noise Consultants has produced the draft Acoustics Ventilation andOverheating (AVO) Residential Design Guide (4). The AVO Guide recommends an approach toacoustic assessment that takes regard of the interdependence of provisions for ven tilation to mitigate
overheating with external noise ingress. It provides guidelines for indoor ambient noise levels during
the time periods when cooling may be required, and the proposed levels are higher than the generally
accepted annual-average noise levels on the basis that only occur part of the year. This is based on anassumption of “adaptive acoustic comfort” as described by Harvie-Clark et al (5) - where occupants
may exercise control over the internal environment by opening windows, for example.
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NC@apexacoustics.co.uk
JHC@apexacoustics.co.uk

3. VENTILATIVE COOLING
3.1 Definition
Ventilative cooling is a way to cool indoor spaces through the use of natural (passive) ventilation,mechanical ventilation strategies or a combination of both. Ventilative cooling uses outside air toremove heat from indoor spaces to mitigate overheating. Ventilative cooling can save cooling energy
and gives more flexibility and design options for buildings, enabling a broader range of design
solutions to fulfil building energy legislations (6). Ventilative cooling aims to cool a dwelling without
using coolants, which are typically referred to as air conditioning systems.
3.2 Natural ventilation options
In its simplest form, ventilative cooling would be the opening of a window to allow cooler external
air to reduce the internal temperature. For noisy environments an attenuated ventilator could be used toprovide a similar airflow as an open window. The efficiency of the natural ventilation can beinfluenced by design decisions such as the opening dimensions, the number of openings and theirposition in a building e.g. is it single sided ventilation, or can the dwelling be ventilated by openings
on more than one façade – cross flow ventilation. In addition to the design influence, the air flow at any
one time will be determined by variable uncontrolled factors which are the wind speed, wind direction
and temperature difference between outside and inside of the building.
3.3 Mechanical ventilation and hybrid options
A full mechanical ventilation system would consist of a fan unit within the dwelling which is ductedto the building facade to bring air into the dwelling with a separate duct for the exhaust air. For amechanical extract system the air usually enters the building via a façade mounted ventilator andwould be extracted mechanically by a fan. In the UK these systems would typically be referred to as
MVHR and MEV systems respectively.
The advantages of using a mechanically assisted system is that the airflow r ate can be controlled by
the fan and is not subject to external factors. Façade air inlets can be much smaller as the pressure
differences induced by the fan can be much greater than the natural pressure differences from ambientconditions. The disadvantages are that the system would use energy compared to a natural ventilation
system and that the system is likely to require greater maintenance during the lifetime of the building.

4. VENTILATION OPENING TERMINOLOGY
4.1 Single openings
There is much confusion regarding the terminology used to describe ventilation openings; theproposed definition used here are taken from Jones et al (7). A sharp-edged orifice is a circular opening
with unsmoothed edges and a length that is significantly shorter than its diameter. T he airflow rate, Q(m3 /s), through and sharp edged opening is proportional to its cross sectional area, often referred to asa free area, A f (m2 ). It is also a function of the pressure drop across the opening ΔP (Pa), the density of
the air U (kg/m3 ) and geometry of the opening such that:(1)ൌǤǤሺʹ'ȀUሻ
Where C d is the discharge co-efficient used to account for the restriction to the airflow for different
opening types, and for a circular sharp-edged orifice Cd o = 0.61. In practice the use of free areas lacks
definition and can lead to different estimates. In Approved Document F it is described as the geometricopen area of a ventilator The discharge coefficient Cd of a window can be estimated using aspreadsheet, based on measured data, and with variables for the windows dimensions and degree of
opening, as described by Daniels et al (8).
4.2 Equivalent and Effective areas
The equivalent area, A eq , is a measure of the aerodynamic performance of the ventilator and isdefined as the area of a sharp-edged orifice which air would pass through at the same flow rate for thesame pressure difference as the opening under consideration. The air flowing through the orifice is still
restricted by the discharge coefficient of the orifice. It can be represented by the equation:ൌǤȀ

(2)

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The effective area, Aeff, (or net area) is also a measure of the aerodynamic performance and isrepresented by the equation:ൌǤ

(3)

The effective area is considered the most useful parameter for defining the aerodynamic
performance of a ventilation opening.

5. ACOUSTIC PERFORMANCE OF VENTS
5.1 Individual Vents
The acoustic performance of vents are given as the element normalized level difference, D n,e , and isexpressed in decibels (dB).The performance is measured in a laboratory following the methodology ofBS EN ISO 10140 (9) and calculated using equation (4).

ǡൌͳȂʹȂͳͲȀͲ

(4)

For unsilenced air inlets, such as a simple opening or weather louvre in the façade, the value can becalculated using the following equation as described in BS EN 12354-3 (10):ǡൌǦͳͲሺȀͳͲሻ(5)

Where S open is the area of the opening in m 2 .
As the performance is based on a single vent, if more than one vent is used in a façade then theinternal noise will increase as additional sound energy will enter the building. For more than one ventwith the same acoustic performance, D n,e , the performance of N number vents is:ǤሺሻൌǡȂͳͲ

(6)

6. PASSIVE VENTILATION OPTIONS
6.1 Comparison of open windows and attenuated vents
To compare the acoustic performance of an attenuated vent with an open window it is necessary todetermine the equivalent ventilation performance of each. For this comparison we compare a square
façade opening with the physical open area of 1 m2 . Assuming a coefficient of discharge, Cd , for the
opening of 0.62, the opening has an effective area, A eff, of 0.62 m2 . For comparing acoustic vents andopen windows on this basis the effective area of the element (vent or open window) should also be 0.62
m2 . An open window which has the same effective area would typically be 1.5 m wide, 1.15 m height
and opened to 25 degrees, giving a 0.5 m throat opening.
We have compared test data for commercially available vents, which have both acoustic andventilation performance data. The tested vents have a face area of 1.2 m x 1.2 m and consist of an
external weather louvre, an attenuator and an internal damper (tested separately for the acoustic tests).
6.2
Acoustic ventilator performance
For each tested acoustic ventilator, the effective area of the unit and therefore the total area ofventilator required to achieve the same effective area as the 1 m2 opening, has been calculated. The
acoustic performance of the ventilator is adjusted to account for the area of the vent u sing equation (6).
The resulting acoustic performance for each of the tested acoustic vents is shown Table 2.

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Table 2 – Acoustic performance of attenuated vents with an effective area of 0.62 m2
Effective

Vent Description

Area m2

No. vents required
to achieve
0.62 m2

Dn,e,w + Ctr
one vent

Aeff

Dn,e,w + Ctr
for vents withAeff = 0.62 m2

1 m2 opening

0.62 m2

1 m2

1010600 mm long 40% OA

0.17

3.692115600 mm long 25% OA

0.12

5.202518300 mm long 40% OA

0.21

2.91

1712300 mm long 25% OA

0.12

5.052114150 mm long 40% OA

0.22

2.88

1510100 mm long 25% OA

0.17

3.74159A summary of the dimensions and acoustic performance of a simple opening, open window andattenuated vents, which provide the same ventilation performance, is shown in Table 3
Table 3 – Comparison of window and attenuated vents required to provide the same effective area, 0.62 m2

1 m2 opening
Dn,e,w + Ctr 10 dB

1.5 m x 1.15 m window

2 m x 1.44 m x 0.15 m

2 m x 2.6 m x 0.6 m

open at 25q

attenuator

attenuator[varies]Dn,e,w + Ctr 10 dB

Dn,e,w + Ctr 18 dB

The values of acoustic performance against the size of the attenuators has been plotted to establish
the relationship between the volume of the attenuator and the required acoustic performance . An
estimate of the required volume for an attenuator, V, for a given acoustic performance and effective
area can be made using:ǡ͵ൌሾሺሺǡǡ൅ሻȂͳͲǤͶͷሻȀʹǤͻͳሿǤሾȀͲǤ͸ʹ](7)The estimated volume doesn’t include the space required for louvres, filters or dampers and is justfor the attenuation element.

7. COMPARISON OF OPEN WINDOWS
7.1 Napier University Study
A study of the sound insulation through ventilated domestic windows (11) gives the acoustic
performance of different window types opened to provide different free areas. As overheating control
generally requires larger window openings compared to ventilation rates for air quality, only the3537

largest tested free area of 0.2 m2 has been assessed.
The effective area for each window type has been calculated using Daniels (8) and the measured
acoustic performance has been adjusted based on the area required to provide an effective area of 0.62m2 . The predicted acoustic performance for each of the window types is shown in Table 3.
Table 3 – Comparison of window acoustic performance to provide the same effective opening
Dnew + Ctr

Glazed Area

Effective area

Dnew + Ctr

m2

m2dB

A1 – Side hung

0.45

0.124

1710

A2 – Top hung

0.15

0.133

158

C – Tilt and Turn

0.71

0.120

1710

D – Sliding Sash

0.45

0.106

157

E – Top hung

0.21

0.081

189

F – Top hung

0.28

0.093

1810

G – Side hung

0.38

0.112

1710Window reference

for windows withAeff = 0.62m2

The predicted performances of the different window types are very similar, with the sash window
being the poorest performing and potentially the small hinged windows performing slight ly worse thanthe large windows. None of the windows performed better than the theoretical performance of a simple
opening with an effective area of 0.62 m2 , which would be a 1 m2 opening and a predicted D new + Ctr of
10 dB.

8. PREDICTED REQUIREMENTS FOR APARTMENTS
8.1 Overheating study for typical apartment design
Hilson Moran (12) have undertaken a detailed study of parameters which can influence thepotential for overheating for a typical single bedroom apartment. They have compared the effects ofglazing area, vent areas, single or dual aspect, orientation, internal and external shading and thermalmass. A typical arrangement of the rooms is shown in Figure 1 which shows both the single and dual
aspect arrangements for the living room and bedrooms.

Living room

Bedroom

Living room

Bedroom

Dual aspect

Single aspect

Single aspect

Dual aspect

Figure 1: Arrangement of typical rooms modelled for overheating3538

For this assessment we are considering a design which doesn ’t incorporate measures to control
overheating and has a glazed area equal to 40% of the façade area. This area has been used as thelowest modelled area which would be likely to achieve the desirable daylighting requirements for theapartments.
The analysis predicted the effective area required to achieve ‘thermal comfort’ – compliance withCIBSE TM 59 (13) - and for some orientations it isn’t possible to achieve this even with fully open
windows. Dual and single aspect rooms were considered; dual aspect living rooms required larger
open areas as the increased (solar gain) heat loads were not offset by the improved ventilation
effectiveness. No living rooms achieved the thermal comfort criteria using open windows, apart fromsingle aspect north facing rooms. For the bedrooms, thermal comfort criteria could be achieved with
open windows; the effective areas required for different room orientations, single and dual aspects isshown in Table 4.
Table 4 – Effective area of façade opening to achieve thermal comfort in bedrooms of a notional London
apartment with no overheating mitigation included
Effective area ofAspectOrientation

ventilation openings
to achieve thermal

Effective area ofAspectOrientation

comfort m2

ventilation openings
to achieve thermal
comfort m2

Single

N

1.84

Dual

E/N

1.39

Single

E

2.14

Dual

N/E

1.39

Single

S

2.45

Dual

E/S

2.09

Single

W

2.45

Dual

N/W

2.09

The lowest calculated effective area required is more than twice area of the vents and options
shown in Table 2. Using equation (7), and for a D new of 15 dB, which could be suitable for external
levels up to 52 dB LAeq,8hr at night, the volume of the smaller ventilator for a dual aspect arrangement
would be 3.5 m3, or 10 % of the room volume.
Given that the living rooms cannot be controlled with natural ventilation and the large size ofattenuation required for the bedrooms, natural ventilation appears to be impractical for controlling
overheating in London apartments unless they incorporate overheating mitigation measures and / orimprovements are made to the ventilation efficiency of the attenuated vents.

9. HYBRID OPTIONS
As a natural system seems to be impractical for controlling overheating in London apartments,
where noise levels exceed much above 50 dB LAeq,8hr at night, a hybrid system could be consideredwhich incorporates a façade mounted vent with an integrated fan to provide air flow.
9.1 Case study
The refurbishment of a 26 story building, next to the A12 road in London, required a hybrid
solution which had 0.12 m 2 free area vent for natural ventilation and an integrated fan which could
provide the ventilation rates required to purge ventilate the rooms. The system provides 37 dB
reduction from external to internal noise levels during operation, and is fully controllable fromswitches within the bedroom. A sketch of the internal layout of the building is shown in Figure 2.

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Figure 2: Interior view of attenuated hybrid vent (image courtesy of TEK Ltd)

10. CONCLUSIONSThe size of an attenuated ventilator which can provide the same ventilation performance as an open
window can be estimated for a given sound reduction requirement using the equation (7). The
overheating assessment of a typical London apartment with no particular overheating mitigation
indicates that attenuated vents would need to be very large to have any significant control of noise
ingress and may be impractical. Hybrid or mechanical systems for ventilative cooling are therefore
likely to be more appropriate, unless the building includes appropriate overheating mitigation, or the
aerodynamic performance of the vents can be significantly improved. Hybrid systems can be used toprovide ventilative cooling; these systems can provide natural whole house ventilation, with fansproviding increased ventilation rates for mitigating overheating.

ACKNOWLEDGEMENTS
The authors acknowledge the technical support for the overheating study from Hilson Moran anddetails of the acoustic and aerodynamic performance of the attenuated vents from TEK Ltd.

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