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Part L Energy Conservation

Appendix B - Fabric insulation

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Additional guidance for common constructions

General

B.1 This Appendix provides some basic guidance in relation to typical roof, wall and floor constructions. Guidance is not exhaustive and designers and contractors should also have regard to other sources of relevant guidance, e.g. “BR 262: 2001 Thermal Insulation; avoiding risks”, relevant standards and good building practice.

In particular, diagrams in this Appendix are intended to be illustrative of the construction to which they refer. They do not purport to provide detailed guidance on the avoidance of thermal bridging. See sections 1.3.3, 2.1.3 and Limiting Thermal Bridging and Air Infiltration - Acceptable Construction Details (ACDs) for guidance on reasonable provision in this regard.

B.2 For many typical roof, wall and floor constructions, the thickness of insulation required to achieve a particular U-value can vary depending on the thermal performance of the insulation material. Higher performing insulating materials, i.e. those with lower thermal conductivities, can achieve any given U-value with a lower thickness of insulating material.

B.3 Condensation in buildings occurs whenever warm moist air meets surfaces that are at or below the dew point of that air. There are two main types: surface condensation and interstitial condensation.

Surface condensation occurs on the surface of walls, windows, ceilings and floors and may result in mould and mildew. Interstitial condensation occurs within the construction of the building and can damage structural materials or make insulating materials less effective.

Full checks should be performed on the likelihood of surface and interstitial condensation of a construction detail in accordance with I.S. EN ISO 13788:2012. This standard contains recommended procedures for the assessment of the risk of:

  • surface condensation and mould growth;

  • interstitial condensation. The guidance in BRE document BR 497

Conventions for calculating linear thermal transmittance and temperature factors (2nd Edition) may be used to perform the checks in I.S. EN ISO 13788:2012.

I.S. EN 15026:2007 can also be used to assess the risk of surface and interstitial condensation and mould growth. The transient models covered in this standard take account of heat and moisture storage, latent heat effects, and liquid and convective transport under realistic boundary and initial conditions.

B.4 A vapour control layer (VCL) substantially reduces the water vapour transfer through any building component in which it is incorporated by limiting both vapour diffusion and air movement. The measures required to achieve a functional VCL must be carefully considered at the design stage.

A VCL should extend over the whole of the element into which it is incorporated and must be integrated with and sealed to adjoining elements, such as masonry, upstands and glazing systems and to any VCL in those elements. VCLs may be formed with a membrane within the structure or with a lining board with an integral membrane. A VCL should be of appropriate vapour resistance and should be situated on the warmside of the insulation.

The performance of a VCL depends upon the vapour resistance of the material selected, the practicability of the design and the standard of workmanship involved in its installation. The integrity of the VCL should be ensured by effective sealing of all service penetrations, e.g. electrical wiring. Methods of avoiding such penetrations should be considered in the design stage.

Side and end joints in a flexible sheet VCL should be kept to a minimum. Joints should be made over a solid backing member or substrate, lapped not less than 50 mm and 52 sealed with an appropriate sealant. Similarly, tears and splits should be repaired using the same material, jointed as above. If polyethylene sheeting is used, it should be protected from heat and sunlight to reduce the risk of degradation occurring.

Where a VCL is incorporated in a rigid board, joints between adjacent boards should be sealed to avoid mass transfer of water vapour due to air leakage. These seals should be designed to accommodate thermal or other movement which may occur during the design life of the building.

A VCL can also act as an air leakage barrier, which by reducing air movement has the added benefit of reducing the heat lost by convection.

Construction elements should be allowed sufficient time to dry in order to avoid trapped moisture prior to the installation of a VCL or internal insulation layer.

B.5 ROOF CONSTRUCTIONS

B.5.1 Construction R1: Tiled or slated pitched roof, ventilated roof space, insulation at ceiling level

B.5.1.1 - R1(a) Insulation between and over joists

Diagram HLB8 - Insulation between and over joists - Extract from TGD L
Diagram HLB8 - Insulation between and over joists - Extract from TGD L

Installation guidelines and precautions

Care is required in design and construction, particularly in regard to the following:

Provision of adequate roofspace ventilation

Adequate ventilation is particularly important to ensure the prevention of excessive condensation in cold attic areas. See relevant guidance in Technical Guidance Document F.

Minimising transfer of water vapour from occupied dwelling area to cold attic space

In addition to ensuring adequate ventilation, measures should be taken to limit transfer of water vapour to the cold attic. Care should be taken to seal around all penetrations of pipes, ducts, wiring, etc. through the ceiling, including provision of an effective seal to the attic access hatch. Use of a VCL at ceiling level, on the warm side of the insulation, will assist in limiting vapour transfer and should therefore be used, but cannot be relied on as an alternative to ventilation. Where the roof pitch is less than 15o , or where the shape of the roof is such that there is difficulty in ensuring adequate ventilation, e.g. room-inthe-roof construction, the VCL is essential.

Minimising the extent of cold bridging

Particular areas of potential cold bridging include junctions with external walls at eaves and gables, and junctions with solid party walls. Gaps in the insulation should be avoided and the insulation should fit tightly against joists, noggings, bracing etc. Insulation joints should be closely butted and joints in upper and lower layers of insulation should be staggered (see Acceptable Construction Details, details G01 - G04).

Protecting water tanks and pipework against the risk of freezing

All pipework on the cold side of the insulation should be adequately insulated. Where the cold water cistern is located in the attic, as is normally the case, the top and sides of the cistern should be insulated. The area underneath the cistern should be left uninsulated and continuity of tank and ceiling insulation should be ensured, e.g. by overlapping the tank and ceiling insulation. Provision should be made to ensure ventilation of the tank. Where raised tanks are used, (to aid head pressure), the ceiling should be insulated normally and the tank insulated separately.

Ensuring that there is no danger from overheating of electric cables or fittings

Cables should be installed above the insulation. Cables which pass through or are enclosed in insulation should be adequately rated to ensure that they do not overheat. Recessed fittings and transformers should have adequate ventilation or other means to prevent overheating.

Providing for access to tanks, services and fittings in the roofspace

Because the depth of insulation will obscure the location of ceiling joists, provision should be made for access from the access hatch to the cold water tank and to other fittings to which access for occasional maintenance and servicing may be required. This can be done by provision of walkways without compressing the installed insulation or by the use of high density insulation under the walkway or by installing composite attic boards.

B.5.1.2R1(b) Insulation between and below joists

Insulation is laid in one layer between the joists, protruding above them where its depth is greater, and leaving air gaps above the joists.A composite board of plasterboard with insulation backing is used for the ceiling.

Diagram HLB9 - Insulation between and below joists - Extract from TGD L
Diagram HLB9 - Insulation between and below joists - Extract from TGD L

Installation guidelines and precautions

Similar guidelines and precautions apply as for R1(a) above. Where the thermal conductivity of insulation between and below the joists is different, the material on the warm side (i.e. below the joists) should have a vapour resistance no lower than that on the cold side (i.e. between the joists). It is preferable if the insulation on the cold side is more permeable than that on the warm side (e.g. mineral wool outside with expanded polystyrene inside).

B.5.2 Construction R2: Tiled or slated pitched roof, occupied or unventilated roof space, insulation on roof slope

B.5.2.1R2(a) Insulation between and below rafters, 50 mm ventilated cavity between insulation and breathable membrane

Diagram HLB10 - Insulation between and below rafters - Extract from TGD L
Diagram HLB10 - Insulation between and below rafters - Extract from TGD L

Installation guidelines and precautions

The insulation is installed in two layers, one between the rafters (and battens) and the second below and across them. To limit water vapour transfer and minimise condensation risks, a VCL is required on the warm side of the insulation. No material of high vapour resistance, e.g. facing layer attached to insulation to facilitate fixing, should be included within the overall thickness of insulation. Care must be taken to prevent roof timbers and access problems interfering with the continuity of insulation and VCL.

Provision should be made for ventilation top and bottom of the 50 mm ventilation gap on the cold side of the insulation.

An alternative construction using a breathable membrane may be used. In this case the membrane should be certified in accordance with Part D of the Building Regulations and installed in accordance with the guidance on the certificate.

Care should be taken to avoid thermal bridging at roof/wall junctions at eaves, gable walls and party walls (see Acceptable Construction Details, details 1.13 and 1.16).

Where the thermal conductivity of insulation between and below the rafters is different, the material on the warm side (i.e. below the rafters) should have a vapour resistance no lower than that on the cold side (i.e. between the rafters). It is preferable if the insulation on the cold side is more permeable than that on the warm side (e.g. mineral wool outside with expanded polystyrene inside).

Where walls are installed in attics for room in roof constructions the U-value for the wall should meet Table 1 provisions and be accounted for in the DEAP calculation.

Access hatches and doors to the ventilated attic void should be sealed and insulated.

B.5.2.2R2(b):Insulation above and between rafters, slate or tile underlay of breathable membrane type

general G6
Diagram HLB11 - Insulation above and between rafters - Extract from TGD L

Installation guidelines and precautions

The effective performance of this system is critically dependent on the prevention of air and water vapour movement between the warm and cold sides of the insulation. Only systems which are certified or shown by test and calculation as appropriate for this function, (see Technical Guidance Document D, paragraphs 1.1 (a) and (b)) should be used. The precise details of construction are dependent on the insulation and roof underlay materials to be used. Installation should be carried out precisely in accordance with the procedures described in the relevant certificate.

In general, the insulation material must be of low vapour permeability, there should be a tight fit between adjacent insulation boards, and between insulation boards and rafters. All gaps in the insulation (e.g. at eaves, ridge, gable ends, around rooflights and chimneys, etc.) should be sealed with flexible sealant or expanding foam.

Care should be taken to avoid thermal bridging at roof/wall junctions at eaves, gable walls and party walls (see Acceptable Construction Details, details 1.14 and 1.18).
Where walls are installed in attics for room in roof constructions, the U-value for the wall should meet Table 1 provisions and be accounted for in the DEAP calculation.

Access hatches and doors to the ventilated attic void should be sealed and insulated.

B.5.3 Construction R3: Flat roof, timber joists, insulation below deck (Cold Deck)

B.5.3 R3: Insulation between and below joists, 50 mm air gap between insulation and roof decking

Diagram HLB12  - Timber flat roof, insulation between joists and below joists- Extract from TGD L
Diagram HLB12 - Timber flat roof, insulation between joists and below joists- Extract from TGD L

Installation guidelines and precautions

The insulation is installed in two layers, one between the joists, and the second below the joists. A ventilated air space as specified in Technical Guidance Document F should be provided above the insulation. Cross ventilation should be provided to each and every void. When installing the insulation, care is needed to ensure that it does not block the ventilation flow paths. The roof insulation should connect with the wall insulation so as to avoid a cold bridge at this point.

The lower layer may be in the form of composite boards of plasterboard backed with insulation, with integral vapour barrier, fixed to the joists. The edges of boards should be sealed with vapour-resistant tape. Where the thermal conductivity of insulation between and below the joists is different, the material on the warm side (i.e. below the joists) should have a vapour resistance no lower than that on the cold side (i.e. between the joists). It is preferable if the insulation on the cold side is more permeable than that on the warm side (e.g. mineral wool outside with expanded polystyrene inside).

B.5.4 Construction R4: Sandwich warm deck flat roof

The insulation is installed above the roof deck but below the weatherproof membrane. The structural deck may be of timber, concrete or metal.

Diagram HLB13  - Sandwich warm deck flat roof above a concrete structure- Extract from TGD L
Diagram HLB13 - Sandwich warm deck flat roof above a concrete structure- Extract from TGD L

Installation guidelines and precautions

The insulation boards are laid over and normally fully bonded to a high performance vapour barrier complying with I.S. EN 13707: 2013 which is bonded to the roof deck. The insulation is overlaid with a waterproof membrane, which may consist of a single layer membrane, a fully-bonded built-up bitumen roofing system, or mastic asphalt on an isolating layer. At the perimeter, the vapour barrier is turned up and back over the insulation and bonded to it and the weatherproof membrane. Extreme care is required to ensure that moisture can not penetrate the vapour barrier. The insulation should not be allowed to get wet during installation. There should be no insulation below the deck nor should this area be ventilated as this could give rise to a risk of condensation on the underside of the vapour barrier. Thermal bridging at roof/wall junctions should be avoided (see Acceptable Construction Details, details 1.19 and 1.20).

B.5.5 Construction R5: Inverted warm deck flat roof: insulation to falls above both roof deck and weatherproof membrane

Insulation materials should have low water absorption, be frost resistant and should maintain performance in damp conditions over the long term. To balance loss of performance due to the damp conditions and the intermittent cooling effect of water passing through and draining off from the warm side of the insulation, the extra heat loss should be calculated in accordance with Clause F.4 of Annex D in I.S. EN ISO 6946:2017.

Diagram HLB14  - Inverted warm deck roof with concrete structure - Extract from TGD L
Diagram HLB14 - Inverted warm deck roof with concrete structure - Extract from TGD L

Installation guidelines and precautions

The insulation is laid on the waterproof membrane. A filtration layer is used to keep out grit, which could eventually damage the weatherproof membrane. The insulation must be restrained to prevent wind uplift and protected against ultraviolet degradation. This is usually achieved by use of gravel ballast, paving stones or equivalent restraint and protection. The insulation should have sufficient compressive strength to withstand the weight of the ballast and any other loads.

Rainwater will penetrate the insulation as far as the waterproof membrane. Drainage should be provided to remove this rainwater both at the upper surface and at the membrane level where practicable. To minimise the effect of rain on performance, insulation boards should be tightly jointed (rebated or tongued-and-grooved edges are preferred), and trimmed to give a close fit around upstands and service penetrations.

To avoid condensation problems, the thermal resistance of the construction between the weatherproof membrane and the heated space should be at least 0.15 m2K/W. However, this thermal resistance should not exceed 25 % of the thermal resistance of the whole construction.

Thermal bridging at roof/wall junctions should be avoided.

B.6 WALL CONSTRUCTIONS

B.6.1 W1: Cavity walls, insulation in cavity, cavity retained (partial fill)

B.6.1.1W1(a): Brick or rendered dense concrete block external leaf, partial fill insulation, dense concrete block inner leaf, plaster or plasterboard internal finish.

Masonry walls should comply with the relevant requirements of Eurocode 6 - Design of Masonry Structures (series) and the additional guidance given in S.R. 325:2013 Recommendations for the design of masonry structures in Ireland to Eurocode 6 (+A1:2014/ A2:2018). It should be noted that the details below refer to cavity walls with a maximum overall cavity width of 150 mm. Where cavity widths are greater than 150
mm, specialist design of wall ties is required and manufacturer’s guidance should be sought.

The insulation thickness required to achieve a given U-value may be reduced by using lightweight concrete insulating blocks for the inner leaf. When calculating the U-value in accordance with Appendix A, the inner leaf is considered a bridged layer due to the mortar with a 7 % fractional area.

Due to the sound attenuation properties of lightweight blocks their suitability for use in the inner leaves adjoining party walls may be limited when flanking sound transmission is considered.

Diagram HLB15 - Cavity wall with partial-fill insulation - Extract from TGD L
Diagram HLB15 - Cavity wall with partial-fill insulation - Extract from TGD L

Installation guidelines and precautions

Insulation should be tight against the inner leaf. Any excess mortar should be cleaned off before fixing insulation. The insulation layer should be continuous and without gaps. Insulation batts should butt tightly against each other. Mortar droppings on batts should be avoided. Batts should be cut and trimmed to fit tightly around openings, cavity trays, lintels, sleeved vents and other components bridging the cavity, and should be adequately supported in position. BRE “Good Building Guide 68 Part 2 Installing thermal insulation: Good site practice” provides further guidance on installing insulation in cavity walls.

Critical locations where care should be taken to limit thermal bridging include lintels, jambs, sills, roof/wall junctions and wall/floor junctions. The method of cavity closure used should not cause thermal bridging at the roof/wall junction (see Acceptable Construction Details, section 1 details).

B.6.1.2W1(b): As W1(a) except with insulation partly in cavity and partly as internal lining

If composite boards of plasterboard backed with insulation (of similar conductivity to that used in the cavity) are used internally, then the total insulation thickness (cavity plus internal) may be used to calculate the Uvalue. If internal insulation is placed between timber studs/battens, then the insulation must be treated as two separate layers with the bridging effect allowed for in the internal layer (similar to Example A2 in Appendix A).

Where the thermal conductivity of the insulation is different, the material on the warm side (i.e. internally) should have a vapour resistance no lower than the insulation on the cold side (i.e. in the cavity).

Lower U-values, or reduced insulation thickness, can be achieved by using insulating concrete blockwork (rather than dense concrete) between the cavity and internal insulation.

Insulation partly in cavity and partly as internal lining helps minimise thermal bridging. Internal insulation limits thermal bridging at floor and roof junctions, whereas cavity insulation minimises thermal bridging at separating walls and internal fixtures.

Installation guidelines and precautions

Installation of insulation in the cavity should follow the guidelines given above for construction W1(a) (partial-fill cavity insulation), and installation of the internal lining should follow the guidelines given below for construction W4 (hollow-block). BRE “Good Building Guide 68 Part 2 Installing thermal insulation: Good site practice” provides further guidance on installing insulation in cavity walls.

B.6.2 Construction W2: Cavity walls, insulation in cavity, no residual cavity (full-fill)

The insulation fully fills the cavity. Insulation may be in the form of semi-rigid or rigid batts installed as wall construction proceeds, or loose-fill material blown into the cavity after the wall is constructed; the former is considered here. Insulation material suitable for cavity fill should not absorb water by capillary action and should not transmit water from outer to inner leaf. Such insulation may extend below damp proof coursing (dpc) level.

Diagram HLB16 - Cavity wall with full-fill insulation - Extract from TGD L
Diagram HLB16 - Cavity wall with full-fill insulation - Extract from TGD L

The insulation thickness required to achieve a given U-value may be reduced by using insulating concrete blocks for the inner leaf, as advised in W1(a) above.

Installation guidelines and precautions

Only certified insulation products should be used, and the installation and other requirements specified in such certificates should be fully complied with. In particular, regard should be had to the exposure conditions under which use is certified and any limitations on external finish associated therewith.

Guidance on minimising air gaps and infiltration in partial-fill cavity insulation applies also to full-fill insulation. Similar issues regarding avoidance of thermal bridging as for partial-fill construction apply.

BRE “Good Building Guide 68 Part 2 Installing thermal insulation: Good site practice” provides further guidance on installing insulation in cavity walls.

B.6.3 Construction W3: Timber frame wall, brick or rendered concrete block external leaf

B.6.3.1W3(a): Insulation between studs

The insulation is installed between studs, whose depth equals or exceeds the thickness of insulation specified.

In calculating U-values, the fractional area of timber bridging the insulation should be checked. Account should be taken of all repeating timber elements which fully bridge the insulation. In Table A2, a fractional area of 15 % is given as the default percentage and is used in Example A2 to calculate the upper and lower thermal resistance of the bridged construction.

Diagram HLB 17 - Timber frame wall, insulation between framing timbers - Extract from TGD L
Diagram HLB 17 - Timber frame wall, insulation between framing timbers - Extract from TGD L

Installation guidelines and precautions

Air gaps in the insulation layer, and between it and the vapour barrier, should be avoided. Insulation batts should be friction fitted between studs to minimise gaps between insulation and joists. Adjacent insulation pieces should butt tightly together. Particular care is needed to fill gaps between closely spaced studs at wall/wall and wall/floor junctions, and at corners of external walls.

A VCL should be installed on the warm side of the insulation. There should be no layers of high vapour resistance on the cold side of the insulation.

Care is required to minimise thermal bridging of the insulation by timber noggings and other inserts (see Acceptable Construction Details, section 4 details).

B.6.3.2W3(b): Insulation between and across studs

Where the chosen stud depth is not sufficient to accommodate the required thickness of insulation, insulation can be installed to the full depth between the studs with additional insulation being provided as an internal lining. This additional insulation may be either in the form of plasterboard/insulation composite board or insulation between timber battens, to which the plasterboard is fixed.

The VCL should be on the warm side of the insulation. If different types of insulation are used between and inside the studs, the vapour resistance of the material between the studs should not exceed that of the material across them (see B.4 regarding VCLs).

B.6.4 Construction W4: Hollow concrete block wall, rendered externally, internal insulation lining with plasterboard finish

Diagram HLB18 - Hollow-block wall, internal insulation lining - Extract from TGD L
Diagram HLB18 - Hollow-block wall, internal insulation lining - Extract from TGD L

The insulation is installed on the inner face of the masonry walls. It may be installed between preservative-treated timber studs fixed to the wall, or in the form of composite boards of plaster backed with insulation, or as a combination of these.

Installation guidelines and precautions

Exposure

Table 11 of S.R.325:2013 (+A1:2014) provides guidance on exposure categories for single leaf masonry walls for the assessment of resistance to rain penetration.

Air movement

Air gaps in the insulation layer should be kept to a minimum. If using insulation between timber studs, there should be no gaps between insulation and studs, between insulation and the VCL, between butt joints in the insulation, around service penetrations, etc. If using composite boards, they should be tightly butted at edges, and should provide complete and continuous coverage of the external wall.

When mounting composite boards on plaster dabs or timber battens, there is a danger that air will be able to circulate behind the insulation, reducing its effectiveness. To minimise such air movement, the air gap behind the boards should be sealed along top and bottom, at corners and around window and door openings, e.g. with continuous ribbon of plaster or timber studs. This also serves as a fire barrier.

Condensation

A suitable VCL should be installed on the warm side of the insulation to minimise the risk of interstitial condensation on the cold masonry behind the insulation. Care should be taken to avoid gaps in the VCL at all joints, edges and service penetrations. The location of service runs in the air gap on the cold side of the insulation should be avoided. Refer to paragraph B.4 for further guidance.

Thermal bridging

Care should be taken to minimise the impact of thermal bridging. Critical locations where care should be taken to limit thermal bridging include lintels, jambs, sills, roof/wall junctions and wall/floor junctions (see Acceptable Construction Details, section 6 details). Other areas where there is a risk of significant thermal bridging include:

Junctions with solid party walls and partitions

Internal partition or party walls of solid dense concrete blockwork can create significant thermal bridge effects at junctions with single leaf masonry external walls (see Acceptable Construction Details, details 6.05 and 6.06).

Junctions with intermediate floors

The external walls in the floor space of intermediate floors should be insulated and protected against vapour movement. Along the wall running parallel to the joists, insulation can be placed between the last joist and the wall. Where the joists are perpendicular to the wall, the insulation and VCL should be continuous through the intermediate floor space and should be carefully cut to fit around the joist ends (see Acceptable Construction Details, detail 6.04).

Stairs, cupboards and other fittings supported on or abutting the external wall

Insulation should be carried through behind such fittings.

Ducts, e.g. soil and vent pipe ducts, against external walls

Insulation should be continuous at all such ducts, i.e. the insulation should be carried through on either the external or internal side of such ducts. Where the insulation is on the external side, particular care should be taken to prevent ingress of cold external air where ducts etc. penetrate the insulation.

B.7 FLOOR CONSTRUCTIONS

B.7.1 Construction F1: Ground floor: concrete slab-on-ground. Insulation under slab or under screed

For continuous and uniform insulation under the full ground floor area, the insulation thickness required to achieve prescribed U-values for slab-on-ground floors varies depending on the floor area to exposed perimeter ratio as shown in Example A4 in Appendix A. When calculating the U-value, the ground conductivity should be taken as 2.0 W/mK and the full wall thickness as per design.

Diagram HLB19 - Concrete slab-on-ground floor, insulation under slab - Extract from TGD L
Diagram HLB19 - Concrete slab-on-ground floor, insulation under slab - Extract from TGD L

Diagram HLB20 - Concrete slab on ground floor, insulation under screed - Extract from TGD L
Diagram HLB20 - Concrete slab on ground floor, insulation under screed - Extract from TGD L

Installation guidelines and precautions

The insulation may be placed above or below the damp proof membrane (dpm)/radon barrier. The insulation should not absorb moisture and, where placed below the dpm/radon barrier, should perform well under prolonged damp conditions and should not be degraded by any waterborne contaminants in the soil or fill.

The insulation should have sufficient load-bearing capacity to support the floor and its loading.

The insulation is laid horizontally over the whole area of the floor. Insulation boards should be tightly butted, and cut to fit tightly at edges and around service penetrations.

Care should be taken to prevent damage or dislodgement of insulation during floor laying. If the dpm is placed below the insulation, the joints between insulation boards should be taped to prevent wet screed from entering when being poured. If the slab/screed is power-floated, the exposed edges of perimeter insulation should be protected during power-floating, e.g. by boards, or the areas close to the edge of the floor should be hand trowelled.

Thermal bridging at floor/wall junctions should be minimised (see Acceptable Construction Details, details 1.01a and 6.02)

B.7.2 Construction F2: Ground floor: suspended timber floor, insulation between joists

Diagram HLB21 - Suspended timber floor with quilt insulation - Extract from TGD L
Diagram HLB21 - Suspended timber floor with quilt insulation - Extract from TGD L

Diagram HLB22 - Suspended timber floor with rigid or semi-rigid board insulation - Extract from TGD L
Diagram HLB22 - Suspended timber floor with rigid or semi-rigid board insulation - Extract from TGD L

Installation guidelines and precautions

Where mineral wool quilt insulation is used, the insulation is supported on polypropylene netting or a breather membrane draped over the joists and held against their sides with staples or battens. The full thickness of insulation should extend for the full width between joists. Insulation should not be draped over joists, but cut to fit tightly between them.

Alternatively, rigid or semi-rigid insulation boards, supported on battens nailed to the sides of the joists, may be used.

When calculating the U-value for a timber floor the fractional area of timber thermal bridging can be calculated or assumed as 11 % as per Table A2.

Thermal bridging, and air circulation around the insulation from the cold vented air space below, should be minimised. The insulation should fit tightly against the joists and the flooring above. Careful placement of supporting battens (or staples) is required to achieve this. At floor/wall junctions the insulation should extend to the walls. The space between the last joist and the wall should be packed with insulation to the full depth of the joist. Where internal wall insulation is used, the floor and wall insulation should meet. Where cavity insulation is used, the floor insulation should be turned down on the internal face and overlap the cavity insulation, or insulating blocks used in the wall at this level (see Acceptable Construction Details, details 1.03 and 6.03).

Cross-ventilation should be provided to the sub-floor space to remove moisture.

Water pipes in the sub-floor space should be insulated to prevent freezing.

B.7.3 Construction F3: Ground floor: suspended concrete floor

Diagram HLB23 - Suspended reinforced concrete floor, internally insulated walls - Extract from TGD L
Diagram HLB23 - Suspended reinforced concrete floor, internally insulated walls - Extract from TGD L

Installation guidance and precautions

If the walls are internally insulated, it is recommended that the floor insulation be placed above the floor structure, since it can then connect with the wall insulation. Thermal bridging at the floor/wall junction is difficult to avoid when insulation is placed below the floor structure (see Acceptable Construction Details, details 6.01 and 6.02).

If the walls are cavity insulated, floor insulation can not connect with wall insulation, so some thermal bridging is inevitable. It can be minimised by using insulating blocks for the inner leaf between overlapping floor and wall insulation. Insulation and insulating blocks may be either above or below the floor structure, but above is recommended. This will allow the use of less dense blocks (of lower thermal conductivity), since they will not have to support the weight of the floor. Also, above the structure they will be above the dpc, where their lower moisture content will give a lower thermal conductivity than below the dpc. Heat loss from the floor can be further reduced by extending the cavity insulation down to, or below, the lower edge of the suspended floor.

Diagram HLB24 - Suspended beam and block floor - Extract from TGD L
Diagram HLB24 - Suspended beam and block floor - Extract from TGD L

B.7.4 Construction F4: Exposed floor: timber joists, insulation between joists

Diagram HLB25 - Exposed timber floor, insulation between joists - Extract from TGD L
Diagram HLB25 - Exposed timber floor, insulation between joists - Extract from TGD L

Installation guidance and precautions

The flooring on the warm side of the insulation should have a higher vapour resistance than the outer board on the cold side. If necessary, a vapour check should be laid across the warm side of the insulation. Methods of avoiding thermal bridging at junctions with internally insulated and cavity insulated walls are similar to those described for suspended timber ground floors above.

B.7.5 Construction F5: Exposed floor: solid concrete, insulation external

Diagram HLB26 - Exposed concrete floor, external insulation - Extract from TGD L
Diagram HLB26 - Exposed concrete floor, external insulation - Extract from TGD L

Installation guidance and precautions

If the walls are internally insulated, this floor construction is not recommended. Floor insulation should instead be located internally in order to connect with the wall insulation. With cavity wall insulation, thermal bridging may be minimised by supporting the external leaf independently, and continuing the external floor insulation around the edge beam to connect with the cavity insulation as shown i n Diagram B19.

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