7.1 Flat roofs, terraces and balconies

Also see:

BRE Report ‘Thermal insulation: avoiding risks’

BS 5250

7.1.10 Thermal insulation and vapour control

Thermal insulation, vapour control and ventilation shall ensure satisfactory performance, and prevent the formation of condensation which could adversely affect the construction. Issues to be considered include:

thermal characteristics of roof types
types of insulation
control of water vapour.

Thermal characteristics of roof types#

Uninsulated roof – The temperature of the deck is at or close to that of the interior or exterior of the building whichever is the lesser.

Cold roof – The temperature of the deck is at or close to that of the external climate. Cold roof designs should be limited to roofs where it can be shown that effective cross ventilation can be provided to reduce the risk of interstitial condensation.

Warm roof – The temperature of the deck is at or close to that of the building interior.

Inverted warm roof – The temperature of the deck is at or close to that of the building interior. The WFRL should be designed
and installed to collect and drain most of the rainwater to reduce water entering and cooling the insulation layer.

Condensation analysis should be calculated using an external temperature of -5C in accordance with BS 6229.

Types of insulations#

Types of insulations and their use should be in accordance with Table 8.

Table 8:

Insulation material	Abbreviation	BS EN Standard	Applications Warm roof	Applications Inverted warm roof ¹
Expanded polystyrene board	EPS	13163	yes ²	no ³
Extruded polystyrene board	XPS	13164	yes ²	yes
Rigid polyurethane foam board	PUR, or PU,	13165	yes	no
Phenolic foam board	PF	13166	yes	no
Polyisocyanurate board	PIR	13165	yes	no
Cellular glass board	CG	13167	yes	no ³
Expanded perlite board	EPB	13169	yes	no
Cork board	ICB	13170	yes	no
Fibreboard, bitumen-impregnated (used in conjunction with EPS with hot applied waterproofing)			yes	no
Composite boards or decks (eg Plywood + PIR)			yes	no
Resin bonded mineral wool rigid board	MW	13162	yes	no
Mineral wool quilts ⁴	MW	13162	no	no
Vacuum insulation panels ⁵	VIP		yes	yes

Notes

1. Thermal insulation used in inverted roofs should be designed in accordance with ETAG 031-1.

2. Suitably protected when using hot applied waterproofing.

3. Unless the product has an independent third-party assessment for use in inverted warm roofs.

4. Not suitable for warm or inverted warm roofs and only used in cold roofs.

5. Suitably assessed in accordance with Technical Requirement R3.

Insulation for warm roofs should be:

either: a) bonded or mechanically fixed to resist wind uplift in accordance with the manufacturer’s recommendations; where mechanically fixed, it should be installed using fixings of sufficient length to ensure adequate penetration into the supporting structure, or b) part of a loose laid roof system which is ballasted to resist wind uplift
kept dry and installed in quantities which can be quickly covered if it rains (to aid bonding and to avoid trapping moisture)
dimensionally stable at working temperatures (eg 100°C under metal roofs)
lightly butted to avoid gaps.

Insulation for inverted roofs should be:

extruded polystyrene (XPS), or expanded polystyrene (EPS) suitably assessed in accordance with Technical Requirement R3
suitable for external use
ballasted to avoid floatation and wind uplift
suitable for the weight of the ballast and able to withstand anticipated traffic and design loads
protected by a breathable water flow reducing layer (WFRL) to reduce the cooling effect of cold water flowing into and under the thermal insulation and across the waterproofing layer
calculated to take account of the cooling effect of rainwater seeping through the insulation board joints in accordance with BS 6229 and ETAG 031-1 and the dry calculated thickness increased to allow for this cooling effect.

Notes

A WFRL should be designed and installed to drain most of the water from the roof. It should be installed at right angles to the designed slope of the roof, starting at the bottom of the slope. All side and end laps should be a minimum of 300mm. The WFRL should be star cut around pipes and wrapped with additional piece of membrane, turned up at upstands and penetrations, and turned down at drainage locations/outlets.

Composite insulated timber decks should:

have two beads of sealant along each board joint at the foil under face (to maintain the integrity of the vapour control layer) in accordance with the board manufacturer’s recommendations.

Control of water vapour#

The movement of water vapour within the roof construction must be controlled to avoid the risk of interstitial condensation. This can be achieved by providing an air and vapour control layer (AVCL) on the warm side of the insulation.

In a cold roof the addition of ventilation to the void(s) between the insulation and decking can also assist by removing water vapour from within the construction.

Warm roofs do not incorporate ventilation and rely entirely on the AVCL to avoid interstitial condensation. It is therefore essential that a good quality AVCL is used and that all laps, joins or penetrations are fully sealed. All AVCL’s should be fully checked for any damage just prior to being covered over. Any damage should be repaired using a full width section of membrane. Penetrations should be kept to a minimum and fully sealed to the AVCL.

In an inverted warm roof, the waterproofing layer also acts as the AVCL. Control of interstitial condensation also relies on the thermal insulation above the waterproofing layer being kept free from running or ponding water between the insulation and waterproofing layer and within joints between the insulation boards. This should be controlled by the correct installation of the WFRL laid over the insulation.

AVCL’s can be formed with one of the materials listed in Table 9.

Table 9: Materials for AVCL

Material	Specification
Reinforced bitumen membranes	BS 8747 BS EN 13970
Self-adhesive Polythene/ metal foil/ bitumen membranes for cold self-adhesion	BS EN 13970 BS EN 13984
High density Polyethylene sheet with metal foil core	BS EN 13984
High density Polyethylene sheet	BS EN 13984
Plastic and rubber sheets	BS EN 13984
12mm one coat mastic asphalt on glass fibre tissue	BS 6925

Notes

Metal-cored flexible reinforced bitumen membranes are also suitable to form an AVCL.

AVCL’s to warm roofs should:

include at least one layer of reinforced bitumen roofing membrane (S2P3) below the insulation, fully or partially bonded to the structural deck as appropriate, or a suitable self-adhesive or torch-on membrane
be self-sealing (eg self-adhesive aluminium foil-backed modified bitumen membrane) if penetrated by mechanical fixings, (eg those securing the insulation boards to the deck)
be sealed and lapped to the waterproofing layer (where compatible) at the perimeters and at each penetration, eg at outlets and pipes, rooflights etc or extended up a minimum 50mm above the insulation and sealed to the wall, rooflights etc
be fixed to resist the calculated wind uplift forces

AVCL’s for warm roof constructions supporting traditional hard metal roofs should have a minimum vapour resistance of 4000MNs/g and be fully supported.

Any fixings that penetrate the AVCL eg those securing insulation boards to the deck, should be carefully installed to avoid creating open perforations that would allow water vapour to pass through.

Cold flat roofs are difficult to detail correctly but, where used, they should be in accordance with BS 6229 and BS 5250 and have:

an effective air and vapour control layer at ceiling level
an unobstructed minimum 50mm ventilation space above the insulation
adequate cross ventilation (openings at both ends of each joist void equivalent to a minimum 25mm gap)
a maximum clear distance of 5m between ventilators on opposite sides of the roof.

Notes

All ventilation gaps should have protective mesh or grilles (nominal 4mm openings) to avoid entry of large insects and birds.

Last updated: 2nd January 2024