Misc. Factors for High-performance Basements and Foundations

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Where basements or semi-basements are used for habitable accommodation, or cellars have been converted to living areas, similar standards of performance to above ground accommodation are expected, especially in relation to noise, thermal insulation, ventilation and condensation, daylighting, fire and means of escape and precautions against radon. Practical considerations often mean, though, that these ideals cannot be fully met.

Cellars in older properties are still frequently used for accommodating service entries and meters.

Characteristic details

Basic description of types of below ground constructions

Topics discussed are influenced to a considerable degree on the shape and size of the cellar or basement in relation to the building, to ground levels, and to means of access.

In the last case, whether there is independent access from the outside or whether access is provided only from within the building.

--- Services in the cellars of older properties are frequently maintained in hazardous and damaging conditions

--- Access to this basement is provided from street level independently of the dwelling above

--- No access from street level is provided to these basements from the fronts of the dwellings Five types of below-ground construction need to be considered:

  1. a building which is entirely below ground. It may or may not be stepped
  2. a full cellar or basement, which is completely located below ground level, to an above-ground building
  3. a partial cellar or basement which is completely located below ground level
  4. a semi-basement or cellar
  5. a stepped construction

A cellar or basement may extend under the whole area of a building or only part of a building. Occasionally a cellar or basement may extend beyond the floor plan of the building: E.g., where part of a cellar once formed a coal hole under an adjacent pavement. Semi-basements or cellars are similar to basements or cellars, but have at least one or more walls exposed as an external wall instead of acting as a retaining wall. Stepped constructions are buildings located on a sloping site and in which the lower level rooms are dug into the sloping ground to form a partial or full semi basement.

Main performance requirements and defects

Noise and other unwanted side effects

Where basement walls separate dwellings, they are required to resist the transmission of sound as provided by the Regulations. Care should be taken to detail the junctions of separating walls with other elements such as perimeter walls, floors and partitions.

--- Types of basements and cellars: Entirely below ground Full basement Partial basement Semi-basement Stepped construction

--- Criteria for calculating the perimeter-to-area ratio and depth (H) of a basement for establishing the U-value.

--- An underground house.

Thermal performance

The thermal performance of basement floors is also briefly mentioned in Floors and flooring.

As principles of modern building pointed out, construction laid directly in or on the ground does not lose heat in the same way as walls and roofs.

The ground acts to some extent as an accumulator of heat, and in fact most of the heat loss occurs within a short distance of the edges adjacent to external walls. Consequently the average heat flow is dependent on the size and shape of the construction.

The heat flow pattern in basements is rather complex and time dependent.

The U-values can be calculated using the steady-state component averaged over the basement, which provides an approximation of the heat losses which is adequate for most purposes.

The procedure is explained in Govt. Agency Information Paper. The deeper the floor level below the ground, the better the U-value.

Alternatively, U-values of uninsulated basement floors in terms of the perimeter-to-area (P/A) ratio and the basement depth. Linear interpolation is appropriate.

In theory, of course, a basement is well insulated, being in contact with the ground which constitutes an infinite thickness of insulation.

However, domestic basements are seldom entirely below ground and areas of the enclosing walls are often exposed to exterior conditions. In addition, a basement wall just below ground level will be exposed to temperature fluctuations similar to those for an exposed wall. For this reason, wall insulation should extend well below ground level.

While the ground around a basement has insulating properties, it also creates a structure with a high thermal capacity which will respond slowly to heating. To provide a more rapid response, it will be necessary to consider insulating the external walls and basement floor.

If internal or external insulation has already been added, it will be important to find out if the insulation is of a type which is suitable for use in potentially damp conditions; exterior insulation will need to be frost resistant.

In summer, basements tend to provide good thermal comfort, provided cross-ventilation is available and heat gains (e.g. from cooking) are controlled.

Inadequate insulation of basement walls in contact with the ground and above ground level is the most common problem. Water saturation of insulation on external walls below ground level can occur where water permeable insulation is used. Thermal bridging by other structural members such as beams and columns may occur.

The insulation value of solid construction may be reduced if the structure is saturated.

Ventilation and condensation Many domestic cellars are poorly ventilated. This is due to insufficient air vents having been provided at the time of construction, and those that have been provided are often blocked up. As a consequence, the air quality is likely to be poor which results in musty smells. Any increase in ventilation of the cellar using unheated air will have an impact on heating costs for the house.

As well as considering ventilation of the cellar it may be worth considering ventilation in the rest of the house. Open flue heating appliances in the cellar or basement must be adequately ventilated.

Dampness and mould growth are often indications of condensation, but can also be a result of penetration of moisture from the ground. The diagnosis of the cause of dampness in basements tends to be complex; dampness in unoccupied buildings is rarely due to condensation.

However, condensation is common in occupied basements because ventilation and heating are usually poor.

Basements tend to be difficult to ventilate properly and may often require special measures to avoid condensation. Bathrooms and other utility rooms, which require only limited natural lighting, are often located in basements but can generate high moisture vapor levels. This vapor will need to be extracted or diluted by ventilation. There can be risks of surface and interstitial condensation with balconies and access ways above basements; these problems are analogous to those sometimes encountered with flat roofs. See also ‘Roofs and roofing’.

Basements which have been used for fuel storage are likely to be contaminated by hygroscopic salts.

Previous use of lightweight plasters in a relatively damp environment can accentuate dampness and condensation problems.

The type of construction and wall (and possibly floor) structure will need to be identified to determine the extent of thermal bridging.

Thermographic techniques, while expensive, may be valuable in identifying the exact location of thermal bridging. Consideration should also be given to the adequacy of cross-ventilation in hot summer conditions.

Federal Code refers to condensation risk relevant to basements. The audiovisual package Remedies for condensation and mould in traditional housing offers simple, practical advice.

Federal Code gives data for determining the insulation needed to achieve U-values of 0.45 W/m^2 K for basements, as currently required by the Elemental Method of satisfying the Building Regulations. However, new U-values are proposed for new housing, which will affect the above calculations. Federal Code also gives information on basement U-values which can be used either for the Target U-value Method in the Regulations, or for calculating heat losses in more general terms. The paper supported the 2000 edition of the Building Regulations.

Precautions against radon

The various options for reducing the levels of radon entry into cellars and basements are discussed in the feature panel.

Poor ventilation practice can increase radon entry. A combination of well sealed or rarely opened windows downstairs, and poorly sealed or regularly opened windows upstairs should be avoided, as this can increase the stack (or chimney) effect within the house which results in radon being drawn into the house from the ground. Similarly, unused chimneys or unsealed loft hatches can increase the stack effect. Vents should not be cut through timber floors to provide ventilation to combustion appliances such as open fires as these can become major radon entry routes. Existing vents should be sealed and alternative sources of ventilation, such as through-the-wall vents, provided.

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EXAMPLE: Dampness and condensation in a converted basement used for storing household goods

This basement was in a Coded listed building, the main part of which was built with a further extension. The house was in very poor condition. It had been converted to provide five flats for elderly people together with a storage room for each flat in the cellar. The structure was of brick with a slated roof; the chimneys had been removed above roof level. The cellar external walls were earth retaining with the outside ground level approximately 300 mm below the new ceiling level. Ventilation had been provided by means of air bricks inserted into the outside walls above ground level and below ceiling level. No dampproofing had been provided and the walls were painted with emulsion paint.

Dampness and fungal growth on stored items had been a continuing problem. An extract fan had been installed which operated intermittently, switched by a humidistat. Occupants complained about the noise produced by the fan when operating. The Feds were called in at this stage.

Measurements were taken of internal conditions using chart recorders. Values of temperature at 15-17 °C and relative humidity of around 89% were recorded for a long period of time. With the extract fan in operation, the humidity reduced to 65-70%. Rusting of metal components and mould growth were inevitable with these high levels of humidity. Discoloration of the emulsion paint and some visible dampness highlighted the inadequacies of these conditions.

The Feds report proposed that air louvers were inserted in the fire doors for each store room. These had to be of a type that would self seal in the case of a fire. It was also noted that external air ventilation was being impeded by growth of vegetation and a regular maintenance regime was recommended.

It was considered that the main extract fan should be retained, but fitted with a time clock to prevent operation during the night hours. It would have been ideal to have ventilated these store rooms by passive stacks which could have been routed through the old chimneys. The removal of these stacks had removed this option.

One of the store rooms was in excellent condition due to there being a small freezer. Just enough heat was emitted by the freezer to raise the ambient conditions to allow storage of books and other papers without deterioration.

--- Fungal growth on items stored in a basement

--- Items stored in a basement adjacent to that illustrated. Sufficient heat was emitted from the small freezer to keep the moisture levels below dewpoint

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Options to reduce radon levels

Sealing walls and floors

Where a house with a cellar or basement has low-to-moderate radon levels (up to say 400-500 Bq/m^3), it may be sufficient to carry out simple sealing of the walls and floors - all cracks and gaps found in the walls and floors will need to be sealed if this is to be effective. Cracks and gaps are likely to be reasonably easy to locate within the basement or cellar, as there are unlikely to be any floor or wall coverings to hide them from view. However, owing to poor construction and less demanding standards of finishing there are likely to be many cracks and gaps needing sealing. Consequently effective sealing is likely to prove difficult and time consuming.

In most cases the floor above the basement or cellar will be of timber construction and major gaps and cracks, particularly large openings around services, will need to be sealed. Less common are vaulted cellars, with concrete floors above. With these there will probably be fewer opportunities for sealing. If the cellar is rarely used, or perhaps never used, one possible option might be to seal it off from the rest of the building while still retaining good ventilation.

Sealing to ground floors In cases where the basement or cellar is only located under part of the building, sealing of adjacent solid ground floors can be considered.

Sealing floors to the occupied part of a dwelling may be more complicated owing to the need to lift carpets, move furniture and possibly strip skirting boards. This solution is only likely to prove successful with low-to-medium radon levels (up to 400-500 Bq/m^3), and where it’s possible to seal major gaps and cracks. Large gaps or holes in suspended timber floors can, and probably should, be sealed.

However, completely covering a timber floor with an impervious material such as polyethylene sheet is not recommended as this could lead to timber rot.

Replacement floor

If the cellar floor is in a very poor condition - perhaps being just compacted earth or cut directly from the bedrock - laying a new floor may be an option. The new floor should have a radon-proof membrane included to help to prevent radon passing through the floor slab. This will also double up as protection from damp penetration. It’s doubtful whether replacing a single floor will provide a significant reduction in radon levels, and installing a sump within the soil or fill below the new floor may be a useful precaution. The sump can then be activated if radon levels remain high after the new floor has been laid. Whole-house positive pressurization Positive pressurization involves blowing air from the loft space down into the house to reduce the pressure difference slightly between the dwelling and the underlying soil, and to increase ventilation and therefore dilution of radon within the dwelling. The system works best in dwellings that are reasonably airtight. It’s usually only recommended for use with indoor radon levels up to about 750 Bq/m^3, although if the house is very airtight it might be effective with higher radon levels.

These measures may prove effective in reducing radon levels in the occupied part of the house, but the radon level might then remain high in the cellar. If this is the case, separate provision for the cellar may be needed, or the cellar sealed from the house.

An important advantage of using positive pressurization as a solution is that it’s simple to install.

Increased underfloor ventilation

Suspended timber floors at ground floor level are often poorly ventilated. Increasing underfloor ventilation can help to lower radon levels and will help to reduce the risk of timber rot. For low to moderate radon levels (up to say 400-500 Bq/m^3 ), it’s worth considering increased natural underfloor ventilation. This can usually be achieved relatively easily by upgrading existing airbricks or by providing additional ones. In some cases the underfloor void is open to the cellar.

If it is, additional underfloor ventilation may also help to ventilate the cellar space. For higher radon levels (above 400-500 Bq/m^3 ), or where it’s difficult to increase the natural underfloor ventilation, mechanical underfloor ventilation may have to be considered. See Floors and flooring.

Increased natural ventilation to a cellar

For low-to-moderate radon levels increased natural ventilation of the cellar may be worth considering. This works by diluting radon within the cellar, introducing natural ventilation through airbricks, wall vents, disused coal holes and, in semi-cellars, vents in windows. If the cellar is completely below ground, air will need to be ducted into it. As the cellar is not used as living accommodation, the localized draughts and reduction in temperature within it caused by increased ventilation are probably acceptable. However, increased ventilation is likely to result in a need for increased heating in rooms above. To minimize problems, and to prevent draughts entering the living accommodation above, any large gaps in the floor between the cellar and the rest of the house should be sealed.

-- Options to reduce radon levels (cont)

Increased natural ventilation to the rest of the house While improvements to the way in which a house is ventilated can help to reduce indoor radon levels, increased ventilation can affect indoor comfort so this may not be the best solution. Nevertheless, for low levels of radon , it may be worth considering. Any changes to ventilation must be permanent; simply changing window opening patterns is unlikely to be sustainable in the long term.

Mechanical ventilation of a cellar

Where the cellar is large, or the radon level is moderate to high (above 400-500 Bq/m^3), natural ventilation may not be sufficient to lower the radon level adequately. In these cases a mechanical system may be more appropriate.

A fan can be fitted to blow fresh air into the cellar. This will have two effects:

  • dilution of radon in the cellar
  • slight increase in air pressure within the cellar so as to counter the natural flow of radon from the ground into the cellar

If the cellar is not well sealed from the dwelling above, the increased airflow may cause cold drafts within the home. Therefore it’s best to seal cellar doors and any larger gaps in the floor above the cellar.

The increased air movement in the cellar will cool the air, so this is generally not an acceptable solution if the cellar is to be used for long periods as, say, a workshop or games room. Care must also be taken that any water pipes are lagged and that goods such as wines stored in the cellar are kept away from the direct fan draught. Increased air movement, however, can help to dry out dampness within the cellar, and may help to avoid fungal growth and timber rot. This solution may actually improve the general condition of the cellar, making it a more usable space.

Mechanical ventilation can also be fitted into the cellar to extract radon laden air. This can be achieved by:

  • fitting a fan to extract air from the cellar, in which case the cellar then acts as a large radon sump under the house
  • providing of additional fresh air inlets on the opposite side of the house to the fan to provide cross-ventilation and dilution of radon levels

Extracting radon laden air can increase the flow of radon into the cellar through the cracks and gaps in the walls and floor, so radon levels in the cellar will equalize with levels in the ground beneath and may be higher than before. Therefore, in the absence of precise measurements, extract ventilation should only be considered when the cellar is used for storage and rarely accessed, or when it’s sealed from the dwelling above.

With both supply and extract ventilation systems there is a risk of some fan noise. Fans should not be located under noise-sensitive areas. If noise proves to be a problem a silencer may need to be fitted.

The increased air movement in a partial cellar may also benefit floor voids in any adjacent suspended timber floors. This can be assisted by providing ventilation holes from the cellar. If the radon level is very high or the size of dwelling is such that ventilation to the cellar is insufficient to influence the radon level in the adjacent floor void, or both of these, a second supply fan to ventilate the floor void might be needed.

Changes to the ventilation of a cellar can influence airflow through the earth under adjacent solid ground floors. E.g., mechanical extract ventilation from a cellar under one part of the house could result in radon being drawn from the soil beneath the rest of the house, so that the cellar acts as a large sump, reducing the need for action elsewhere.

Sump system to a cellar

If the radon level is moderate-to-high (above 400-500 B q/m^3), and the floor of the cellar is of reasonable quality concrete or is a good condition stone flag floor, a sump system is likely to be the most suitable option.

It’s doubtful that a sump system would be appropriate if the floor comprises severely cracked concrete, is made up of poorly jointed flagstones, has been cut directly from the rock, or comprises exposed earth. For the sump system to work effectively there will need to be some permeable soil or fill beneath the floor slab. Where the water table is above or at the same level as the floor of the cellar, radon is likely to enter through the walls rather than the floor, so installing a sump would prove ineffective as it would fill with water and prevent the flow of air through the soil.

If the sump is a viable option it can be located anywhere within the cellar that is convenient. The benefit of this system is that sumps have been shown to be the most effective method of reducing moderate-to-high levels of radon to below the action level. The sump system should not alter any of the indoor conditions within the cellar or dwelling above, although some form of isolation may be needed if a boiler or open flued combustion appliance is operated within the cellar.

The exhaust pipework and the fan unit to the sump can be located internally or externally, depending on preference. Internal fans may need to be insulated to reduce noise, and the routing of pipework be boxed in or carefully routed if taken through the dwelling above. External fans can be boxed in to improve aesthetics, or can be hidden from view with plants.

If the area of ground floor adjacent to the cellar is large, or the sump system fitted to the cellar is not providing adequate radon reduction to the rest of the house, it may be necessary to fit an additional sump beneath the adjacent solid ground floor. In some cases a second sump can be constructed behind a wall of the cellar and under an adjacent solid floor, manifolded to the existing sump system so only requiring the expense of running one fan. This is termed a multiple sump system.


Fire and means of escape

The Building Regulations include special requirements for the structural fire protection of basements that are more onerous than for superstructures, and are set out in Approved Document. The Regulations would generally have to be complied with if conversion or material alterations are contemplated.

The Regulations also make provision for venting of heat and smoke from all basements except the very smallest and shallowest. Smoke outlets or vents ideally need to be provided for each separate space.

In many domestic cellars access is through a door inside the house, and it’s quite common to find the cellar staircase located immediately beneath the staircase giving access from the ground floor to the first floor. Walling concealing the staircase is often constructed of timber framing clad in timber boarding. Therefore fire and smoke can move directly from the cellar to the ground and first floors.

The Building Regulations make provision for firefighting shafts approached through firefighting lobbies to be provided in buildings with basements 10m or more below ground or access level, although not all firefighting shafts need to be provided with firefighting lifts. The fire resistance of walls and floors of firefighting shafts in basements need to comply with the requirements for these shafts in the remainder of the building; E.g., all construction separating the shaft from the remainder of the building needs two hours, and from the firefighting lobby, one hour.

A floor over a basement of a dwelling in single occupancy, with a basement floor area less than 50 m^2 , is required to have a full half hour fire resistance rather than the modified half hour criterion applicable to the first floor construction of a two-storey house. More exacting requirements apply to larger basement areas and basements offering multiple dwelling accommodation. In the case of a compartment wall between basement flats, non-combustible construction is required. Only limited fire protection can be attributed to existing lath and plaster.

Overlaying ceilings with fire resisting boards will increase the structural load on the floor.

Means of escape from basements in the event of fire is a major consideration. The provision of an alternative means of escape from basement bedrooms in houses is recommended. Where a flat is situated in a basement, and the flat is not provided with its own entrance, an alternative means of escape must be provided.

Timber floors over basements may be found with inadequate or complete absence of fire protection. Where a timber stairway to a basement forms part of the fire separation between flats it should have fire protection. Gas services may be found passing through non-ventilated voids in the basement construction - escapes of gas can build up to dangerous levels in these voids.

Windows into light wells may not be large enough to act as an alternative means of personal escape; the minimum dimensions needed are 850 × 500 mm, with sill height at 600-1100mm.

A careful evaluation of the suitability of the existing layout and construction should be made to try and ensure that the level of fire protection would meet current standards. This may require exposing the existing construction.

Federal Code, although intended for new-build, provides guidance applicable to rehabilitation.

Soundly constructed masonry walls are usually able to provide fire resistance for periods longer than required by building regulations. This inherent resistance to fire is however easily compromised if any unsealed holes or gaps are left through a wall.


Adequate natural light is generally more difficult to achieve in basements than in rooms above ground level, and needs careful consideration when assessing the rehabilitation potential of basements. This applies particularly in schemes involving the conversion of older properties into flats. See also Walls, windows and doors.

In accordance with the Federal Housing laws, local authorities could issue their own regulations controlling the ventilation, lighting and protection against dampness of basement rooms used for human habitation. The Federal Code specified an average minimum floor to ceiling height of 7 ft 0 in. In the absence of local requirements made under this Code, there was no mandatory prohibition on windowless habitable basement rooms, provided they had adequate mechanical ventilation and artificial light. Building Regulations no longer control the heights of rooms.

Where windows are provided in light wells extending below ground level, the wells need to be provided with guarding.


Durability of the materials from which basements and cellars are constructed have been dealt with in detail in other volumes of this series. However, basements in areas having high water tables inevitably will be at risk from salts being transported from adjacent ground. In particular the risk of sulfate damage to mortar should be investigated before conversion to habitable accommodation is attempted.

--- Provisions for the guarding of light wells: 1200 mm high guard required if vertical drop is 370 mm or more; No guard required if slope equal to or less than 26.4°

Work on site


Existing lighting levels can be evaluated subjectively or measured with a light meter.

Federal Code offers a technique for estimating how future changes in window area will affect natural lighting level.

The problems to look for are:

  • blocked air bricks
  • condensation and mould growth
  • penetrating damp
  • sulfate attack
  • means of escape inadequate
  • no guard rails round light wells
  • radon in particular areas


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Updated: Tuesday, October 16, 2012 20:03