Building Standards technical handbook 2017: domestic buildings

Condensation can occur in heated buildings when water vapour, usually produced by the occupants and their activities, condenses on exposed building surfaces (surface condensation) where it supports mould growth, or within building elements (interstitial condensation).

The occurrence of condensation is governed by complex interrelationships between heating, ventilation, moisture production, building layout and properties of materials. Condensation need not always be a problem, for example it regularly occurs on the inner surface of the outer leaf of a cavity wall which receives very much more water from driving rain. However excess condensation can damage the building fabric and contents and the dampness associated with mould growth can be a major cause of respiratory allergies.

Condensation can also affect thermal insulation materials as the measured thermal performance reduces with increased moisture content. For all of the above reasons the control of condensation is an important consideration in building design and construction.

There are buildings designed and constructed for specialist activities, controlled environments or factory processes that normally involve high humidity levels. The guidance to this standard may not be fully appropriate for such buildings as their design is generally by specialists and often involves distinctive construction methods and materials required to produce buildings that are fit for purpose under the known conditions.

The effects of climate change may exacerbate problems of condensation in buildings due to higher relative humidity. Higher winter temperatures combined with increased vapour pressures could result in more severe problems, particularly in roof spaces. Very careful consideration of the issues is essential and the correct detailing will therefore be critical.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted must be improved to as close to the requirement of that standard as is reasonably practicable, and in no case be worse than before the conversions (regulation 12, schedule 6).

A dwelling should be constructed to reduce the risk of both interstitial and surface condensation in order to prevent damage to the fabric and harmful effects on the health of people using the dwelling. The guidance given in BS 5250: 2002 ‘Code of Practice for the control of condensation in buildings' is helpful in preventing both interstitial and surface condensation.

If the average relative humidity within a room stays at or above 70% for a long period of time, the localised relative humidity at the external wall will be higher and is likely to support the germination and growth of moulds.

The fundamental principle of designing to control humidity is to maintain a balance between, the thermal and vapour properties of the structure, heat input and ventilation rate. The thermal and vapour properties of the structure are covered in Standard 3.15 and heat input in Section 6, Energy.

The kitchen, bathroom and utility room are the 3 areas in a dwelling where most moisture is generated. Control of this moisture can be by active or passive means. Guidance to Standard 3.14 provides various methods of controlling humidity in high humidity areas.

Section 8.4 of BS 5250: 2002 provides guidance on the control of condensation in the principal forms of roof construction. Clause 8.4.1 of BS 5250 lists various issues that should be considered in the design of roofs to reduce the possibility of excess condensation forming that might damage the building and endanger the health of the occupants. However cold, level-deck roofs, should be avoided because interstitial condensation is likely and its effect on the structure and insulation can be severe and many instances of failure in such systems have been recorded. It is considered that more reliable forms of construction are available. Both the warm deck and warm deck inverted roof constructions, where the insulation is placed above the roof deck, are considered preferable. However fully supported metal roof finishes including aluminium, copper, lead, stainless steel and zinc are regularly used in conversion work, and they should have a ventilated air space on the cold side of the insulation in addition to a high performance vapour control layer near the inner surface. Further information may be obtained from the relevant metal associations.

Thermal bridging occurs when the continuity of the building fabric is broken by the penetration of an element allowing a significantly higher heat loss than its surroundings. These ‘bridges’ commonly occur around openings such as lintels, jambs and sills and at wall/roof junctions, wall/floor junctions and where internal walls penetrate the outer fabric. Thermal bridges provide a ready passage of heat transfer to the outside air and allow a heat flow entirely disproportionate to their surface area resulting in excessive heat losses. Condensation may occur on the inner surfaces that can damage the dwelling or threaten the health of the occupants.

To minimise the risk of condensation on any inner surface, cold bridging at a floor, wall, roof or other building element should be avoided. Detailing should be in accordance with the recommendations in Section 8 of BS 5250: 2002. Also, to maintain an adequate internal surface temperature and thus minimise the risk of surface condensation, it is recommended that the thermal transmittance (U-value) of any part and at any point of the external fabric does not exceed 1.2W/m²k.

Further guidance on acceptable thermal insulation may be obtained from BRE Report, BR 262, Thermal insulation: avoiding risks.

A floor, wall, roof or other building element should minimise the risk of interstitial condensation in any part of a dwelling that it could damage. Walls, roofs and floors should be assessed and/or constructed in accordance with Section 8 and Annex D of BS 5250: 2002.

For the control of condensation in roofs, including cold deck roofs, BS 5250: 2002 provides guidance on the principal forms of construction. There is evidence that suggests that condensation in cold deck flat roofs is a problem. They should be avoided therefore because interstitial condensation is likely and its effect on the structure and insulation can be severe. Many instances of failure in such systems have been recorded and it is considered that more reliable forms of construction are available. However fully supported metal roof finishes including aluminium, copper, lead, stainless steel and zinc are regularly used in conversion work, and they should have a ventilated air space on the cold side of the insulation in addition to a high performance vapour control layer near the inner surface. Further information may be obtained from the relevant metal associations.

Both the warm deck and warm deck inverted roof constructions, where the insulation is placed above the roof deck, are considered preferable.

Ventilation is vital for preventing excessive build-up of condensation in cold, pitched roof spaces. Where the insulation is at ceiling level the roof space should be cross ventilated. Special care should be taken with ventilation where ceilings following the roof pitch. The recommendations in BS 5250: 2002 should be followed.

Roof type A insulation on a level ceiling Pitched roof structure of timber or metal framed construction. External weatherproof covering of slates or tiles on under slating felt with or without boards or battens. Insulation laid on a level ceiling with a ventilated space between the insulation and the roof structure.

Figure 3.34. Roof type A insulation on a level ceiling

Roof type B insulation on a sloping ceiling Pitched roof structure as (A) above with a decking of low permeability insulation fitted to and between the roof framing. External weatherproof covering of slates or tiles, with tiling battens and counter battens (located over roof framing), and a breather membrane laid on the insulation decking; with a sloping ceiling.

Figure 3.35. Roof type B insulation on a sloping ceiling

Roof type C insulation on decking Pitched roof structure as (A) above with a decking of low permeability insulation fitted to and between the roof framing. External weatherproof covering of slates or tiles, with tiling battens and counter battens (located over roof framing), and a breather membrane laid on the insulation decking; with a sloping ceiling.

Figure 3.36. Roof type B insulation on a sloping ceiling