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Considering Ventilation and Air Management in Basements as part of an Overall Waterproofing Strategy

Introduction

This paper will raise some of the issues associated with the implications and challenges associated with the control and management of atmospheric moisture in basements.

The utilisation of basements and underground structures as living and working spaces, stores, plant rooms and utility spaces is reliant on its ability to resist the ingress of water. It is the job of an architect working in partnership with a waterproofing design specialist to engineer and install solutions that provide levels of ground water resistance that are commensurate with the design brief and that meet the expectation of the client. Detailed guidance setting out the processes that underpin this design methodology and expected performance grades for waterproofing underground structures are set out in BS 8102: 2009 ‘Code of practice for protection of below ground structures against water from the ground’.

The table below is an extract from BS 8102: 2009. Though some degree of water ingress is acceptable in basements built to meet the requirements of a performance “Grade 1” building, both performance Grades 2 and 3 clearly state that “no water penetration is acceptable”. However in the description of performance level, ventilation and air management is acknowledged as being “necessary” and must therefore be a consideration for the design team. If air management is not given appropriate consideration there is a high risk that the underground space may not be fit to perform its intended use. Table 2: Grades of waterproofing protection

Special characteristics that make underground rooms susceptible to problems

created by atmospheric moisture

Basement spaces are by definition wholly or partially below ground. This means that the same opportunities to

promote air exchange through doors and windows to the outside can be restricted. In addition natural “incidental”

air exchange through discontinuities, cracks and other openings in the fabric of the building should be all but

eliminated if the waterproofing system has been designed and installed correctly.

This largely watertight and (to all intents and for the purpose of this document) airtight box sits in the ground. In

most situations in the UK the temperature of the ground below 500mm from the surface is very static and

predictable and does not fluctuate greatly between the summers and winter months. Generally the ground

temperature in the UK ranges from 8 to 12°C. The temperature of the surrounding earth will dictate the

temperature of the man-made structure buried within it.

Grade Example of use of structure Performance level

1 Car parking; plant rooms (excluding electrical equipment); workshops

Some seepage and damp areas tolerable, dependent on the intended use B)

Local drainage might be necessary to deal with seepage

2 Plant rooms and workshops requiring a drier environment (than Grade 1); storage areas

No water penetration acceptable Damp areas tolerable; ventilation might be required

3 Ventilated residential and commercial areas, including offices, restaurants etc.; leisure centres

No water penetration acceptable Ventilation, dehumidification or air conditioning necessary

appropriate to the intended use

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(Source: BS8102:2009)

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Considering Ventilation and Air Management in Basements as part of an Overall Waterproofing Strategy

The temperature of the primary earth retaining structure may be influenced by the internal air temperature but this influence will be negligible unless the walls or floor were insulated externally during construction. Even where earth retaining structures have been isolated from the ground by insulation, they are often very thick and so have considerable thermal mass. In some situations this can be so great as to negate some of the effects of the externally applied insulation unless the basement is heated constantly.

In a situation where the air temperature is 21°C and relative humidity (RH) is 60% then no condensation will occur on surfaces in equilibrium with air temperature. If however the same background RH is present but the air temperature drops to 12°C at its interface with an earth retaining wall, then a relative humidity of 100% will be achieved and condensation will occur.

In the summer it is not uncommon to see ambient air temperatures rise to say 24°C with humidity values around 65%. If the earth retaining wall remains at 15°C then the air at the interface of the wall would achieve 100% RH and liquid water will be liberated from the air as condensation.

This situation can make some basements more susceptible to the effects of high humidity and condensation during the warmer summer months than they are in the winter. That said, the example above illustrates that unless the humidity within the basement is controlled then mould growth and condensation are a distinct possibility.

It is worth remembering that water will be liberated from the air when the relative humidity reaches 100% but mould growth will occur on some substrates where humidity is held around or above 75% for prolonged periods of time.

Underlying Principles

In very simple terms, pressure is exerted on the air by water held within it. The greater the amount of water held

in a fixed volume of air the greater the vapour pressure that can be recorded. Vapour pressure is measured in

Kilopascal (KPa).

In occupied buildings, including basements, the direction of “vapour drive” is typically from within the building to

the outside. This is because the vapour pressure is almost always higher within the occupied rooms than it is

outside it. Increased water vapour in the air is the result of occupation or activities that happen within the

enclosed space (breathing, use of water for cooking and cleaning, bathing and recreation, as well and other forms

of water that evaporates from the buildings fabric and joins the natural water held in the contained air).

In all but extremely exceptional circumstances, any water that enters the building through the structure will be in

liquid form and is the result of defects in the waterproofing. If this water reaches the surface of the walls or floors

this may evaporate into the air and will in effect increase the vapour load of the air. However, a more likely

consequence of the ingress of water is a cooling the surface of the damp wall and may negate any insulation.

This slightly cooler surface can cause RH to be elevated very locally and condensation is therefore more likely to

occur.

In unoccupied basement spaces such as car parks and storage areas the levels of moisture vapour are likely to be

influenced by the ambient air entering through any gaps or openings in the structure. In these circumstances it

is more likely that the relatively high air moisture loadings that occur during the summer months will result in the

deposition of free water through condensation or elevated relative humidity at the interface with the cold earth

retaining structures.

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Considering Ventilation and Air Management in Basements as part of an Overall Waterproofing Strategy

Where unoccupied basements are heated or where plant or equipment generates heat summer condensation is less likely. However the same heat source that reduces summer problems can have the effect of allowing the air to take on higher moisture loadings during the winter that result in condensation or high humidity at the interface of the cooler, earth retaining structures.

With all the above observations set out it is always worth remembering that the surface of the earth retaining wall will be influenced by the internal temperature as well as the ground conditions. This influence will be greater where the affects of insulation within the structure are seen.

It is essential to understand that all the parameters that result in basement condensation are variable and may change relatively quickly in response to the effects of heating, ventilation and occupation. In many situations where problems with condensation are suspected “snap shot” inspections may not give conclusive results so longer term monitoring may be required in order to understand the processes at play.

Considerations

It can be difficult to control the surface temperatures of the primary earth retaining structures, and air

temperatures and moisture production will fluctuate dependant on the style and characteristics of occupation,

use and the time of year. The only remedies to combat these potential problems in normal occupied basement

rooms is air management through ventilation or insulating internal finishes from the underlying earth retaining

structure. In well-designed basements both techniques are often adopted.

Condensation between construction layers

Where humidity is high and walls are relatively cold, it is not uncommon to find water beading on or running off

the “dry” side of waterproofing systems. This can also appear as wet patches on the floor where water has run

down the waterproof membrane or surface applied waterproofing, sometimes taking advantage of the gap that is

created between the wall finishes and the earth retaining structural elements. When this occurs the wetting is

often assumed to be the result of failed waterproofing.

Interstitial Condensation

Interstitial condensation is the term used to describe condensation that occurs within the fabric of construction materials. It may be possible for this to occur within basements however as the earth retaining structures are usually constructed of high density materials that have little temperature variation throughout their thickness. The phenomenon is rarely seen. Where interstitial condensation is suspected, a process of diagnostic investigation using temperature and humidity profiling through the depth of the construction product may be necessary. As this would involve a destructive testing process the risk of causing damage to the primary waterproofing systems must be considered before any such diagnostic tests are undertaken.

Penetrations, cold bridges and insulation

It is likely that waterproofing systems applied to both new and existing structures will have to be designed to accommodate service penetrations and structural features that connect to the outside environment. As the

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Considering Ventilation and Air Management in Basements as part of an Overall Waterproofing Strategy

temperature of these elements may be affected by the atmospheric conditions outside the building or the

materials flowing through them there is a risk that these will be colder than the surrounding ground at times during

the winter. As a result they can result in surfaces that are at or below dew point. These elements can be

overlooked when considering the need for insulation and by definition potentially create a “cold bridge” to the

outside.

Any gaps in the insulation will also result in localised fluctuations in internal surface temperatures. These are also

frequently referred to as “cold bridges.”

In order to prevent condensation occurring on these “cold bridges” it is important that locations and implications

of any potential “cold bridges” are understood at an early stage. It may be prudent to design waterproofing sys-

tems in a way that reduces or eliminates the “cold bridges” and penetrations to an absolute minimum. This will

not only reduce the likelihood of waterproofing defects but will also eliminate opportunities for condensation

problems. Where “cold bridges” are unavoidable special care must be taken to insulate or protect these features

with appropriate insulation or vapour control measures.

Special care to prevent cold bridging should be taken around ventilation ducts, cold water feed pipes, pipe conduits

and structural steelwork.

Insulation in basements, as in all applications, must be continuous, and unbroken as far as possible. In addition it is

important to ensure that the performance of the insulation is approximately the same throughout the scheme.

It would be counterproductive to insulate walls and floors to a high standard then neglect the insulation

performance of a buried roof. Any element that is colder than another will result in localised fluctuation of relative

humidity at the interface of the surface and the air.

In all situations where insulation is installed inside the envelope of a building, vapour control systems should be

used on the warm side of the system. In the same way as the insulation must be continuous and unbroken so must

the vapour control layers. Failure to maintain continuity will allow vapour movement and cold bridging that can

result in localised condensation.

Similarly insulation applied beneath floor slabs, above buried roofs or behind earth retaining walls must also be

continuous and offer similar performance characteristics. Discontinuity will result in cold bridging and variable

performance that can lead to differences in finished wall temperatures with colder surfaces becoming susceptible

to mould growth and condensation.

Ventilation and air management

Ventilation systems in basements are often critical in the control of both heat and moisture. It is crucial that air

management systems are designed and commissioned by specialist practitioners who can ensure that systems

work and perform as intended.

The removal of moisture laden air must be balanced with the buildings ability to accept air from outside. Special

care should be taken in areas where ground gas contamination or radon is a possibility. The use of ground gas

membranes, specialised air management and extraction systems may be required to negate the possibility of

creating dangerous or harmful subterranean spaces where radon, and other harmful ground gas is present.

Conduits, ductwork and trunking drawing air in from outside can be susceptible to condensation on their outside

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Considering Ventilation and Air Management in Basements as part of an Overall Waterproofing Strategy

surfaces as they enter the heated building. Conversely air extraction systems can be prone to condensation within

the ductwork or trunking as the air from the occupied space is cooled as it leaves the building. The build-up of

water within a ventilation system can lead to failures and water damage. If water is allowed to sit for long periods

within an enclosed ductwork then Legionella or other dangerous viral, microbial or biological organisms could also

contaminate the system.

Both situations can be negated to some degree by good design of the ventilation systems and the facilitating

ductwork but consideration must also be given to insulating duct runs, the use of measures to heat air before it

enters the building and measures to reduce the volumes of water created within the occupied space.

It is very important to ensure ventilation systems that utilise fans, ducts and trunking are properly designed,

installed and commissioned in order to ensure effective performance, and safety. It is also imperative that

ventilation systems are cleaned and serviced regularly to ensure safety and reliability.

Conclusion

Condensation and the effects of atmospheric moisture can be highly significant in underground structures. The

effects of water condensing on otherwise waterproof construction elements can be as significant to the occupant

as water ingress through the waterproofing system.

It is imperative therefore to ensure that the risks associated with atmospheric moisture in basements are

understood by all people involved in the design and delivery of successful underground waterproofing projects.

Although condensation may not be of primary concern to the waterproofing specialist when delivering a

waterproof structure some knowledge of the subject could prevent costly or damaging problems.

Stephen Hodgson

Chief Executive, PCA

Property Care Association

11 Ramsay Court

Hinchingbrooke Business Park

Huntingdon,

PE29 6FY

Tel: 0844 375 4301

E-mail: pca@property-care.org

www.property-care.org

SW-DP-VAMBasement-0217-v1

© Property Care Association, 2017. All rights reserved.

Discussion Paper