This British Standard gives recommendations for the management of moisture in buildings using an integrated and pragmatic approach. This includes the management of moisture risk from interstitial and surface condensation, from too high or too low internal relative humidity, and from rain penetration or high levels of ground water. It covers all states of water as gas, liquid and solid, and the interactions between these states. It describes the principal sources of moisture in buildings, its transportation and deposition and provides recommendations and guidance on how to manage those risks during the assessment, design, construction and operation of buildings.

This British Standard does not cover measures specifically dealing with flooding and escape of water. These are dealt with in BS 85500.

This British Standard is intended for use by designers, builders and users of any class of building. This standard gives guidance on risks of all sorts from high moisture levels, whether in the building fabric or in indoor air, which can endanger the health and well‑being of building occupants and the integrity of the building fabric.

This British Standard is relevant to buildings of all types, whatever their form, construction and occupancy, both new and existing, and both domestic and non-domestic, except for buildings used for storage at sub-zero temperatures.

PDF Catalog

PDF Pages	PDF Title
7	Foreword
9	Introduction
12	Section 1: General 1 Scope 2 Normative references
13	3 Terms, definitions and abbreviated terms 3.1 Terms and definitions
16	3.2 Abbreviated terms
17	Section 2: Design and guidance to avoid moisture related problems 4 Design to avoid moisture related problems 4.1 Assessing the likelihood of condensation
18	4.2 Methods of assessment
19	4.3 Internal climate
20	4.4 External climate (condensation risk)
21	4.5 The external envelope
24	4.6 Alterations and extensions to buildings 5 Guidance for builders and owners
25	6 Remedial works 6.1 Actions to manage moisture risk 6.2 Heating 6.3 Ventilation
26	Section 3: Design principles – Building services 7 Application of design principles – Heating 7.1 General 7.2 Warm air heating
27	7.3 Electric storage heaters 7.4 Unflued oil and gas heaters 7.5 Heating controls
28	8 Application of design principles – Occupied space ventilation 8.1 General
29	8.2 Natural ventilation
30	8.3 Continuous mechanical ventilation
31	8.4 Cooker hood (range hood)
32	8.5 Purge ventilation 8.6 Drying rooms/cupboards 8.7 Dehumidifiers
33	Section 4: Design principles – fabric details 9 Application of design principles – junctions 9.1 Heat loss and surface temperatures
34	9.2 Principles for reducing the effects of thermal bridging 10 Application of design principles – Floors 10.1 General
35	Figure 1 — Key to the figures 10.2 Categories of floors
36	Table 1 — Summary of floor constructions 10.3 Groundbearing floors with DPM
37	Figure 2 — Connective/systemic effects for groundbearing floors
39	Figure 3 — Insulation above concrete slab Figure 4 — Insulation below concrete slab 10.4 Groundbearing floor without DPM, with or without insulation either above or below the floor slab/tiles
42	Figure 5 — Existing groundbearing floors either without DPM or without insulation above or below the floor 10.5 Suspended floors
44	Figure 6 — Connective/systemic effects for suspended floors
46	Figure 7 — Suspended concrete floor with insulation above the floor slab Figure 8 — Suspended concrete floor with insulation below the floor slab
47	Figure 9 — Suspended timber floor with insulation above the joists Figure 10 — Suspended timber floor with insulation between the joists
48	Figure 11 — Timber suspended floor with a soffit of high vapour resistance 10.6 Basements
51	11 Application of design principles – Walls 11.1 General
52	Table 2 — Checklist for design principles – Walls
58	Figure 12 — Key to the figures 11.2 Categories of walls Table 3 — Index of wall types
59	11.3 Masonry walls
64	Figure 13 — Solid traditional masonry wall – areas to inspect
66	Figure 14 — Existing solid masonry wall with external wall insulation – window details Figure 15 — Existing solid masonry wall with external wall insulation – wall-roof
67	Figure 16 — Existing solid masonry wall with external wall insulation – parapet detail Figure 17 — Existing solid masonry wall – external insulation [Example of below damp‑proofing course (DPC) or joist ends]
68	Figure 18 — Solid masonry wall – external insulation Figure 19 — External wall insulation to roof – detail
69	Figure 20 — External wall insulation at window – detail Figure 21 — External wall insulation to ground level – detail
70	Figure 22 — Recessed window head – detail
71	Figure 23 — Existing solid masonry wall with internal wall insulations – key considerations before and during installation
73	Figure 24 — Solid masonry wall – internal insulation at window – reducing thermal bridging Figure 25 — Solid masonry wall – internal insulation at joists – reducing thermal bridging
77	Figure 26 — Cavity masonry wall – insulation at junction with window frames Figure 27 — Cavity masonry wall – insulation at junction with roof (Example of eaves and continuous insulation)
78	11.4 Framed walls
81	Figure 28 — Framed wall – external insulation
82	Figure 29 — Framed wall – lintel and sill
83	Figure 30 — Framed wall – junction with bearing floor
84	Figure 31 — Framed wall – junction with roof – continuous insulation
85	Figure 32 — Framed wall – internal insulation
86	Figure 33 — Framed wall – internal sheathing – thermal insulation between and to the inside of the structural framing Figure 34 — Framed wall – external sheathing – thermal insulation between and to the inside of the structural framing
87	Figure 35 — Framed wall – external sheathing – thermal insulation between the framing and on the outside of the sheathing
90	11.5 Structural insulated panel systems (SIPS): walls
92	11.6 Cladding systems
94	Figure 36 — Cladding systems – Integral thermal insulation – Prefabricated
95	Figure 37 — Cladding systems – Integral thermal insulation – Built in situ
97	Figure 38 — Closed joint rainscreen cladding
98	Figure 39 — Open joint rainscreen cladding 11.7 Cavity trays and weep holes
100	11.8 Openings in walls 12 Application of design principles – Roofs 12.1 General
101	Figure 40 — Key to the figures 12.2 Categories of roofs
103	Figure 41 — Roofs categorized by position of the insulation 12.3 Methods for assessing moisture risks in roofs
104	Table 4 — Calculation methods for different roof types
105	12.4 Design considerations
108	12.5 Cold pitched roofs
112	Figure 42 — Cold pitched roof – LR underlay – Air permeable outer weatherproof covering Figure 43 — Cold pitched roof – LR underlay – Air impermeable outer weatherproof covering
113	Table 5 — Minimum free area of openings for loft-space ventilation
114	12.6 Warm pitched roofs
116	Figure 44 — Warm pitched roof with HR underlay – Any roof covering
117	Figure 45 — Warm pitched roof with LR underlay and air permeable outer weatherproof covering Figure 46 — Warm pitched roof with LR underlay and air impermeable outer weatherproof covering
118	12.7 Hybrid pitched roofs Figure 47 — Routing insulation and AVCL in a hybrid pitched roof to achieve continuity – insulation follows slope of roof and horizontal ceiling
119	Figure 48 — Routing insulation and AVCL in a hybrid pitched roof to achieve continuity – insulation follows horizontal ceiling, vertical wall and slope of roof 12.8 Flat roofs
121	Figure 49 — Cold flat roof – roof deck
122	Figure 50 — Warm flat roof – roof deck Figure 51 — Warm flat roof – roof slab
123	Figure 52 — Inverted flat roof – roof deck Figure 53 — Inverted flat roof – roof slab
124	Figure 54 — Cold pitched roof with flat roof apex 12.9 Self-supporting sheeted metal
127	Figure 55 — Site-assembled metal roof
128	Figure 56 — Composite metal roof 12.10 Structural insulated panel systems (SIPS): pitched roofs and flat roofs
131	12.11 Openings in roofs 12.12 Refurbishment
133	Section 5: Condensation risk 13 Calculating condensation risk 13.1 Mould growth and surface condensation
134	13.2 Assessment methods
136	Table 6 — Monthly mean temperature and relative humidity for interstitial condensation calculations Table 7 — Corrections to monthly mean temperatures and relative humidities to create condensation risk years with various return periods
138	Table 8 — Example of the calculation of estimated ground temperatures
139	Table 9 — Moisture production rates in housing Table 10 — Typical moisture production rates from fuels
140	Table 11 — Typical moisture generation rates for household activities
141	Figure 57 — Variation of internal humidity classes with external temperature Table 12 — Internal humidity classes: building types and limiting relative humidities at T3 = 0 °C
142	13.3 Calculation of condensation risk in a cold pitched roof with thermal insulation applied on a horizontal ceiling
143	Annex A (informative) Guidance for designers and builders: a whole‑building approach
148	Annex B (informative) Properties of materials
149	Table B.1 — Thermal conductivity and vapour resistivity of building materials
152	Table B.2 — Vapour resistances of thin membranes and foils
153	Table B.3 — Thermal resistance, in m2K/W, unventilated cavities 5 mm and 25 mm wide with high emissivity surfaces
154	Table B.4 — Thermal resistance of roof spaces Table B.5 — Thermal resistances of surfaces
155	Table B.6 — Factors for converting permeance units to µg/N·s Annex C (informative) Diagnosis of dampness problems
162	Annex D (informative) Moisture in buildings
165	Table D.1 — Effect of condensate on impermeable surfaces Annex E (informative) Guidance for builders
167	Annex F (informative) Guidance for occupiers on how to avoid damaging condensation
169	Annex G (informative) The temperature and moisture content of air
170	Figure G.1 — Psychrometric chart showing the derivation of relative humidity from temperature and vapour pressure
173	Bibliography

Published By	Publication Date	Number of Pages
BSI	2021	178


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Status	Definitive
Pages	178
Publication Date	2021-10-05
ISBN	978 0 539 18975 9
Standard Number	BS 5250:2021
Title	Management of moisture in buildings. Code of practice
Corrects	BS 5250:2021
Replaces	BS 5250:2011+A1:2016
Descriptors	Condensation, Heating equipment, Thermal design of buildings, Design, Surfaces, Damage prevention, Ventilation, Buildings, Structural design, Mathematical calculations
Publisher	BSI
Committee	B/540/2
ICS Codes	91.120.99 - Other standards related to protection of and in buildings

BS 5250:2021

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