Brochure thermal mass - The Concrete Initiative · 2017. 4. 13. · 6 THERMAL MASS - BEST PRACTICE GUIDELINES FOR OFFICES AND COMMERCIAL BUILDINGS The use of concrete to provide thermal

1

General guidelines

for using thermal mass

in concrete buildings

2

THERMAL MASS - BEST PRACTICE GUIDELINES FOR HOUSING

In warm climates, the thermal mass in concrete andmasonry helps provide a comfortable livingenvironment and reduce overheating problems,whilst in cooler climates it can be used to absorb solargains and reduce the need for heating energy.As the basic design requirements for both seasonsare not incompatible, housing in more temperateclimates can be designed to take advantage ofthermal mass on a year–round basis. Examples ofsuch regions include much of northern Europe andthe UK. For case studies and more information on thebenefits of thermal mass visit: www.cembureau.eu/default.asp?p=Case_studies01.asp

Cooling - During hot weather, the thermal mass inconcrete and masonry housing will soak up internalheat gains, helping stabilise the temperature andmaintain comfortable conditions. This is because the

high thermal capacity of the building fabric allows agreat deal of heat to be absorbed with only a smallincrease in the internal surface temperature of thewalls/floors. As a result, the surface temperature staysbelow that of the ambient air for much of the day,resulting in both radiant and convective cooling ofthe occupants. At night, the heat absorbed by thewalls/floors is removed by using the relatively coolnight air to ventilate the building, enabling the houseto repeat the cycle the following day. The risk ofoverheating can be further reduced by locating anoverhang above south facing windows to shadethem from the sun during the hottest part of the day.This is of course a well understood approach todesign in the warmer parts of Europe, but is alsobecoming increasing relevant in other regions wherethe impact of climate change is beginning to increasepeak summertime temperatures.

Illustration © The Concrete Centre

Heating - - - - - During the heating season thermal masscan be used to reduce fuel requirements, by allowingthe low angle winter sun to shine into the buildingthrough the south facing windows during thewarmest part of the day (shading overhangs are onlyeffective during the summer). The solar gains areabsorbed by thermal mass in the floors and walls,

and then slowly released at night as the temperaturedrops. This heating and cooling cycle is similar to thatused in the summer, the difference being that solargains are encouraged during the heating season asthis is useful heat, and windows are kept shut at nightto minimise heat loss.

3


DESIGN CHECKLIST FOR YEAR-ROUND USE OFTHERMAL MASS

Orientation - - - - - Dwellings should be orientatedtowards the south, or within about 30º of south, tomaximise solar gain during the heating season and tosimplify shading in the summer through the useoverhangs, balconies, brise soleils etc.

Windows – – – – – where heating is the mainconsideration, the basic requirement is for relativelylarge south facing windows and relatively small northfacing windows (over the course of a year, northfacing windows generally have a net energy loss).The area of south facing glazing will need to be sizedto take account of a range of factors including theinsulation performance of the glass, level of thermalmass, and general design requirements for thedwelling. Windows that are too large may be counterproductive as their heat loss on winter nights canoutweigh their ability to capture solar gains duringthe day. They may also lead to an increased risk ofoverheating during the summer. As a rough guide,windows should be at least 15% of a room’s floorarea to provide adequate daylight, and not morethan 40% of the façade area if excessive heat gain/loss is to be avoided1. Larger areas may be possiblewith triple-glazing or specially coated highperformance glass. Whilst large south facingwindows are preferable, worthwhile savings can stillbe achieved with a conventional area of glazing. Ifcooling performance is the main consideration, amore modest window area will reduce solar gains,

however they should still be of a sufficient size toallow adequate daylighting.

Room layout – – – – – For optimal passive heating, the mostfrequently used rooms should be on the south side ofthe dwelling so they enjoy the greatest benefit to behad from solar gain during the heating season.Bathrooms, utility rooms, hallways, stores etc. shouldbe located on the north side.

View of the sky – – – – – To maximise solar gain during theheating season, the southern façade should have arelatively clear view of the sky to allow solar radiationfrom the low angle winter sun to pass directly into thebuilding i.e. underneath any shading overhangs. Tomaximise the view of the sky, adjacent buildings andstructures creating an obstruction angle greater thanaround 30º above the horizon should be avoided:Every percentage point increase in obstruction over30º results in roughly the same percentage pointincrease in energy use2. New housing developmentsprovide the best opportunity for optimisation as asympathetic layout can often be adopted.

Indirect solar gains - - - - - Where obstructions limit theamount of direct solar radiation to be captured,some heat is still obtained from diffuse radiation, andalso reflected radiation from the ground (especiallyfrom light coloured paving), as well as from adjacentbuildings. It is a common misconception that passivesolar design only seeks to maximise the amount ofdirect sunshine entering a building.

4

Illustration © CIMbéton

Shading – An overhang of around 0.5 to 1.5m(depending on window height) will block the highangle sun during the hottest part of the summer.During the heating season, overhangs will notobstruct the low angle sun, which is able to shinedirectly into the dwelling. This very simple form ofshading requires no user control, but does not offersome of the additional benefits of other shadingsystems such as glare control and the addedinsulation provided by shutters on cold nights.Overhangs may therefore be best used incombination with other forms of shading to optimiseyear-round performance.

Thermal mass and insulation – In most climates,both thermal mass and insulation are importantfactors in optimising the thermal performance ofbuildings. The positioning of thermal mass in relationto the insulation will result in distinctively differentresponses, and as far as practicable, the internalsurfaces of heavyweight walls floors and ceilingsshould be left thermally exposed to aid heatabsorption. Internal finishes such as plasterboard andcarpet will to some extent act as a barrier to heat flowby acting as an insulating layer. The insulation inexternal walls should be located behind the concrete/masonry inner leaf. Some types of concrete wallconstruction may use interior insulation inconjunction with a thermal break; however asignificant level of thermal mass can still be achievedwithin such a building if concrete floors are used. Thesimple rule for maximising thermal mass is that, as faras practicable, concrete floors and walls should be left

thermally exposed inside the building e.g. use finishessuch as paint, tiles, or plaster.

In warmer climates where cooling is the mainconsideration, thermal mass performs anotherfunction: In addition to absorbing heat gains throughwindows and from internal sources (as previouslydescribed), it will also slow and reduce theconduction of heat gains through external walls androofs. Heavyweight construction is particularly goodat doing this. If the gains are delayed sufficiently, theywill not be felt until the evening/night-time, when therisk of overheating will have moderated and thecooler night air should be sufficient to offset theslightly warmer internal surfaces. To take fulladvantage of this effect, the objective is to design foran appropriate time lag.

For east facing walls a very short or very long delay isgenerally best, however the latter requires a very thickwall which may not be practicable. For south facingwalls a lag of around 10 -12 hours will delay themidday heat until late evening/night. The same delayor slightly less (8 hours) is also suitable for west facingwalls since there are only a few hours to sunset.North facing walls have little need for a time lag asthe solar gains are small. For roofs exposed to solargains all day, the time lag needs to be very long todelay the penetration of heat until the evening.However, this is often impracticable, as it requires veryheavyweight construction, so the use of additionalinsulation is typically a better option3.

5

Occupant control - The basic control strategy on hotdays is for windows to remain closed to preventwarm air entering the dwelling, and shading to beused to limit direct solar gain. Cooling is provided bythe thermal mass. In the evening when the ambienttemperature drops below the internal temperature,windows are opened to provide night-time ventilationand cooling of the building. During the heatingseason, windows are kept shut, with ventilationprovided by trickle vents or some other form ofcontrolled background ventilation. Where applicable,shutters, blinds, curtains etc. can be used to reduceheat loss from windows at night.

Architectural appearance - A commonmisconception about passive solar design and theuse of thermal mass generally is that it results, ofnecessity, in dwellings of unconventional appearancewhich do not fit comfortably into the townscape.However, this is not the case, and passive solarfeatures can be introduced into existing ranges ofvolume house design without major changes inappearance, cost or marketability4.

Typical options for walls


Ventilation – For optimal summertime performancerooms should be designed to enable crossventilation, which is particularly effective for nightcooling. This is achieved by locating windows onadjacent sides of a room to maximise air flow. Single-sided ventilation, where the air enters and leaves viaone or more windows located in the same wall, isless effective but still adequate for smaller rooms,especially if the window(s) provide a relatively largefree area when opened. The optimum ventilation ratefor night cooling depends on the specificcharacteristics of the dwelling, but for maximumeffect there should be up to ten air changes per hour.Higher air change rates may improve the coolingrate, but only to a limited extent. This is because thecooling rate is also affected by the length of time theair is in contact with internal surfaces; high air changerates result in less contact time.

6

THERMAL MASS - BEST PRACTICE GUIDELINES FOR OFFICESAND COMMERCIAL BUILDINGS

The use of concrete to provide thermal mass in officesand commercial buildings is a well establishedapproach to passive cooling, which can alsoenhance the buildings’ structural and visual qualities.The mass is typically provided by the floor slab whichwill have an exposed soffit and/or an underfloorventilation system. The slab provides a large heat sinkto counter the relatively high internal heat gains fromequipment and lighting etc. As with other highthermal mass buildings, the internal environmentresponds slowly to changes in ambient temperature,helping stabilise conditions during warm weather.This is assisted by the relatively low radianttemperature of the exposed concrete, which helpsmaintain a comfortable working environment,allowing higher air temperatures to be tolerated thanwould otherwise be possible.

Cooling - In addition to reducing peak temperaturesby absorbing internal heat gains, thermal mass alsodelays its onset by up to six hours. In an officeenvironment this is particularly beneficial as the peak

temperature will typically occur in the late afternoon,or the evening after the occupants have left. At thispoint the warming cycle is reversed, with solar gainsgreatly diminished and little heat generated byoccupants, equipment and lighting. As the eveningprogresses the drop in external air temperaturemakes night ventilation an effective means ofremoving the accumulated heat, so the cooling cyclecan continue the following day.

Heating - The ratio of cooling to heating tends to behigh in offices and commercial buildings as a result ofthe significant internal loads from lighting, equipmentand people. This makes the summertimeperformance of thermal mass the main considerationin this type of environment. The effectiveness ofpassive solar design for heating may also be limitedby the occupancy pattern of office environments,which is typically limited to the daytime. However, thebasic design principle can still be used to maximisedaylighting without unduly increasing the risk ofoverheating from solar gains.

Typical options for floors


7

DESIGN CHECKLIST FOR THERMAL MASS INOFFICES AND COMMERCIAL BUILDINGS

Passive and active systems - For well shaded, lowoccupancy buildings, the combination of thermalmass and natural ventilation from windows can besufficient to provide comfortable internal conditionsand avoid overheating problems. More demandingenvironments may require the addition of mechanicalventilation to increase the cooling capacity andimprove year-round control. This will often take theform of a ‘mixed-mode’ system which optimises theuse of passive and mechanical ventilation throughoutthe year. Another option is water-cooled floor slabs,also know as thermo-active concrete, which providesa hybrid approach that maximises coolingperformance and can take advantage of naturalwater sources. For environments where conventionalair-conditioning cannot be entirely avoided, thermalmass can still provide a means of significantlyreducing energy consumption, and can shift the loadto the night-time when it is generally cheaper to runthe plant.

Optimal slab thickness - Floor slabs will typicallyprovide most of the thermal mass and, to someextent their thickness will determine the coolingperformance. Whist floor spans and loading are whatlargely determine thickness, the following pointsshould also be noted:

• It is generally accepted that heat will penetrate up to100mm into concrete during a simple 24 hour heatingand cooling cycle. However, for longer cycles i.e. thatexperienced during an extended period of hot weather,greater depths can be advantageous as the increasedheat capacity delays or avoids the concrete becomingsaturated with heat.

• A slab thermally exposed on the upper and lowersurface (e.g. exposed soffit and underfloor ventilation),can utilise a slab thickness much greater than 100mmas the surface area for heat transfer is effectivelydoubled.

• In addition to allowing heat flow to and from the top ofthe floor slab, underfloor ventilation can also beconfigured to create turbulent air in the floor void,which enhances the cooling rate and allows heat topenetrate further into the top surface.

• Profiled soffits (e.g. coffered, troughed, wave form, etc),provide an increased surface area which improvesconvective heat transfer, increasing the overall coolingperformance.

Taking account of the points above, buildings withexposed soffits and underfloor ventilation can oftentake advantage of the thermal mass available inconcrete floor slabs of 250 mm or more.

Control - Night cooling should take maximumadvantage of ambient conditions whilst avoidingovercooling, which will cause discomfort at the startof the day, and may result in the subsequent need toreheat the space. Mixed-mode systems should defaultto natural ventilation whenever possible so theenergy consumed by running fans is minimised. Toachieve these objectives a number of different controlstrategies, which vary in their approach can be used.However, Research by BSRIA5 in the UK has shownthat a complex control strategy is often unnecessary.The careful selection of the control set-point to initiatenight cooling was, however, identified as being ofgreat importance. As a result of the monitoring, andfurther research using computer simulations, BSRIArecommend the following night cooling strategy forthe UK (set points may need changing for otherclimates):

1. Select one, or a combination of the following criteria,to initiate night cooling:

• Peak zone temperature (any zone) >23ºC• Average daytime zone temperature (any zone)

>22ºC• Average afternoon outside air temperature >20ºC• Slab temperature >23ºC

2. Night cooling should continue providing the followingconditions are satisfied:

• Zone temperature (any zone) > outside airtemperature (plus an allowance for fan pick-upif mechanical ventilation is used)

• Zone temperature (any zone) > heating set point• Minimum outside air temperature > 12ºC

3. Night cooling should be enabled (potentially available):• Days: seven days per week• Time: entire non-occupied period• Lag: if night cooling is operated for five nights

or more, it should be continued for a furthertwo nights after the external air temperaturefalls below the control set-point

8

Daylighting - Exposed concrete soffits can help toprovide good daylight penetration when designed inunison with the façade. The objective is to maximisethe daylight within the space without causingexcessive glare and solar gains. A high window headallows light to be reflected off the soffit and travelwell beyond the perimeter zone. The use of profiledsoffits (e.g. coffered) running parallel to the path ofdaylight can enhance daylight penetration. Slabs canalso be angled slightly upwards towards atria orwindows to improve performance. In addition toaiding daylighting, profiled slabs can provide apositive visual aspect to the lighting design bycreating areas of contrast which help to define roomgeometries. Ideally, a high surface reflectance of atleast 70–80% should be achieved, and a gloss factorof no more than 10% to prevent lamps frombecoming visible. A simple painted finish using whiteemulsion is a particularly effective way to achieve this,and provides a cost-effective solution that has beenwidely used. Another option is to use white cementin the mix to provide a light surface finish that islargely maintenance free. The use of an unpaintedsoffit made with white cement requires a highstandard of casting to achieve a consistent, fair-facedfinish.

Shading - Internal blinds intercept and absorb solarradiation after it has entered the building and thenreradiate a significant proportion of this into theroom. Consequently, when used as the only meansof shading, they generally provide insufficientattenuation of solar gain in passively cooledbuildings. Ideally the main shading should beexternal to reduce this problem. Recent advances inglass technology have provided coatings that candistinguish between longer-wavelength solar heatand shorter wavelength visible light. This can bebeneficial; however, given the large overlap inwavelength between the two, there is a limit to howfar this technology can be used to limit heat gains.Horizontal overhangs and projections on south-facing facades work well in mid-summer, but unlessthey are practically deep, will be less effective in thespring and autumn when the sun is lower in the sky.To counter this and provide some glare control, acombination of fixed external shading incombination with some form of adjustable blind canprovide a good overall solution.

Project planning - Traditional responsibilities andboundaries within the design team may bechallenged in projects featuring a high thermal masssolution as the floor slabs shift from being a purelystructural element to something that has implicationsfor a range of design issues including aesthetics,lighting, acoustics and thermal performance.

An early appointment of the team is important if theoutcome of a high thermal mass project is to besuccessful, and the agreed terms should ensure that:

• Additional duties relating to the passive cooling strategyare defined and recognised. For example, with highthermal mass projects there is more likely to be arequirement for a design assessment usingComputational Fluid Dynamics (CFD), and for post-handover monitoring to enable fine-tuning of thecompleted building.

• There is a clear demarcation between the responsibilitiesof the architect relative to the building-services engineer;in high thermal mass building there is far more overlapbetween the roles of these parties and, hence, potentialfor confusion of responsibilities.

• The structural and building-services engineers’ views aregiven equal weighting alongside the architect’s onmatters that will affect the eventual performance of thecooling solution. Opportunities for increasing thecooling output from the thermal mass may be lost whereaesthetic considerations alone are given precedence.

It is essential where an option to use the thermal massfor cooling is being contemplated, that this forms anintegral part of the brief, and key decisions regardingthis certainly need to be taken before any significantarchitectural design work on the building isundertaken.

9

Natural lighting

Illustrations on this page:Lycée du Pic-Saint-Loup(School of Pic-Saint-Loup)Architect: Pierre TourreProject Manager: Serge SanchisPhotos: © Abbadie

10

Slab options: Exposed slab with naturalventilation - Flat concrete slabs are quick and easyto construct and economical for spans up to about9m (13m with post tensioning). They are also thesimplest way of providing a high degree of thermalmass. When used in conjunction withnatural ventilation from openablewindows, the slab can provide around15-20W/m2 of cooling.

The increased surface area of profiled/coffered slabs improves thermalperformance. While this has little effecton radiant heat transfer, the increase insurface area improves convective heattransfer, which can be doubled in someinstances6. The cooling capacity of aprofiled slab is typically in the order of20-25W/m2. In addition to theiraesthetic qualities, profiled slabs assist inmaximising daylight penetration andprovide improved acoustic control.

Formwork costs are generally higher, but pre-castingis an option, which brings with it the potential forsavings in site time and the quality benefits that amore controlled casting environment can offer.

Slab options: Exposed slab with mechanicalunderfloor ventilation - Raised floors are generallyconsidered essential for routing small power andcommunications in office buildings,and can also provide a useful means toprovide ventilation, via floor outlets. Thishas the benefit of reducing ductworkand allowing outlets to be easilyrelocated to meet organisationalchanges. A further benefit of thistechnique is the direct contact betweenthe air and the top of the slab, whichallows utilisation of the thermal mass inthe upper portion of the slab (see‘Optimal slab thickness’). If used incombination with a profiled soffit,which is not uncommon, the overall

cooling capacity from the floor slabs will be in orderof 25-35W/m2.



11

Slab Options: Exposed hollowcoreslab with mechanical ventilation - - - - -Precast, hollowcore slabs withmechanical ventilation supplied via thecores, provide excellent heat transferbetween the air and concrete, enablinga cooling capacity of up to40W/m2. Performance during theheating season is also excellent, makingthe use of hollow core slabs for heatingand cooling an attractiveyear-round design option.The technique was originally developedin Sweden and is marketed under thebrand name «Termodeck», and hasbeen used in many hundreds oflow energy buildings.

Slab Options: Exposed slab with water cooling/heating - - - - - The use of water rather than air to coolfloor slabs enables higher cooling capacities to beachieved by significantly increasing the rate of heattransfer. As the uncertainties and limited duration ofnight-time ventilation do not apply, water cooledslabs provide a more predicable outputthat can, if necessary, be maintained 24hours a day. The system typicallycomprises of polybutylene pipeembedded in the slab about 50mmbelow the surface, through which wateris circulated at approximately 14-20ºCduring the summer and 25-40ºC duringthe heating season. The technology isequally applicable to cast-in-situ andprecast slabs, and can provide a coolingcapacity of around 60-80W/m2.

The speed of response using watercooling is relatively fast, taking around 30minutes for a change in the flowtemperature to have a discernible effectat the surface of the slab. This give a level of controlnot easily achieved in the other systems. In practicalterms, this ensures a steady slab temperature can bemaintained, preventing it from rising to a point whereoverheating might otherwise be experienced duringpeak conditions.

A number of options can be used to supply chilledwater, including mechanical chilling, natural watersources, or a combination of the two. The relativelyhigh chilled water temperature (required to avoidcondensation forming) allows use of water fromsources such as rivers, lakes and boreholes.

Mechanical chilling can also be used where naturalsources are not an option. Capital savings arepossible with the chiller plant which can be relativelysmall as the thermal mass will reduce the peakcooling load (which is used to size the chiller).



12

Slab Options: Exposed slab with chilled beams - - - - -In recent years, the combination of chilled beams andexposed concrete soffits has become an increasinglypopular solution in both new and retrofit projects. Inparticular, multi-service chilled beams have foundfavour with many architects and clients. This can belargely attributed to the simplification of ceilinglocated services by using what isessentially a packaged system that can,if required, completely avoid the needfor a suspended ceiling. Another keyfeature of chilled beams is their ability towork with the fabric of the building bysupplementing the passive coolingfrom thermal mass. The maximumcooling output from chilled beams is inthe order of 100-160W/m2, withadditional cooling provided by theslab’s thermal mass and potentially fromthe ventilation system as well if the air isconditioned. Ventilation is essentially aseparate provision, generally via eithernatural ventilation or a mixed-modeunderfloor system. The beams typicallyoperate with chilled or cooled waterbetween 14ºC and 18ºC, offering the potential toutilise water from sources such as lakes andboreholes. Passive cooling from the thermal mass isprovided in the usual way, with the chilled beams

operating during the daytime to boost the overallcooling capacity. In some installations, especiallythose using natural water sources, it may beadvantageous to also operate the beams at nightduring hot weather to supplement night coolingfrom ventilation.

1 Thermal mass for Housing, The Concrete Centre, 20082 Edwards B. Rough Guide to Sustainability, 2nd Edition, RIBA Enterprises, 2005.3 Balaras C. Passive Cooling of Buildings, 1997.4 Planning for Passive Solar design, Energy Efficiency Best Practice Programme, department of Trade and Industry, produced by BRECSU/BRE.5 Building Services and Research association, Bracknell, England.6 Barnard N, Concannon P, Jaunzens D, Modelling the Performance of Thermal Mass, Information Paper IP6/01, BRE, 2001.

Footnotes

Copyright:European Concrete Platform ASBL,

April 2009

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means,electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the European Concrete Platform ASBL.

Published by the European Concrete Platform ASBLEditor: Jean-Pierre Jacobs

1050 Brussels, BelgiumLayout & graphic design: .....

Printing by the European Concrete Platform ASBL

All information in this document is deemed to be accurate by the European Concrete Platform ASBL at the time of going into press. It is givenin good faith. Information on European Concrete Platform documents does not create any liability for its Members. While the goal is to keepthis information timely and accurate, the European Concrete Platform ASBL cannot guarantee either. If errors are brought to its attention, theywill be corrected. The opinions reflected in this document are those of the authors and the European Concrete Platform ASBL cannot be heldliable for any view expressed therein. All advice or information from the European Concrete Platform ASBL is intended for those who willevaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including for negligence)for any loss resulting from such advice or information is accepted. Readers should note that all European Concrete Platform publications aresubject to revision from time to time and therefore ensure that they are in possession of the latest version.


Documents

Brochure thermal mass - The Concrete Initiative · 2017. 4. 13. · 6 THERMAL MASS - BEST PRACTICE GUIDELINES FOR OFFICES AND COMMERCIAL BUILDINGS The use of concrete to provide thermal