Thermal Bridging at Junctions and Openings

With improving U-values for walls and other elements, it has become necessary to ensure that no cold bridges exist at junctions and openings. In England and Wales Accredited Details have been issued for housing, and these can be used to demonstrate Building Regulations compliance in non-residential buildings of similar construction. If these details are not suitable, the designer may use other approved details or calculate heat loss through the cold bridges.

The opposite page demonstrates suitable ways to avoid cold bridging at junctions and openings. Polyfoam System Closer exceeds the performance required for closing reveals and is suitable for cavities up to 100mm wide.

Wall Ties

Apart from structural considerations, which are obviously pre-eminent, the correct specification of wall ties is crucial in two additional respects. Firstly, it is necessary that the tie does not compromise performance of the cavity wall insulation with regard to liquid water penetration.

Secondly, that the U-value calculation method takes into consideration the number of wall ties per square metre as well as the cross sectional area and thermal performance of the tie. Where cavities are 100mm or less, stainless steel wire ties are recommended as they have negligible effect on the wall U-value. Where the cavity is over 100mm wide, a more robust tie is required, which can have a significant effect on the wall U-value.

Masonry cavity walls and fire performance

Open cavities must be stopped to prevent the passage of fire. This is required at specific intervals and the cavity stop has to provide at least 30 minutes fire resistance. If the cavity is fully or partially filled and is built in accordance with Diagram 34 of Approved Document B, fire barriers are not required.

Prevention of liquid water penetration

Prevention of liquid water penetration from the outer to inner leaf is one of the major considerations when designing cavity walls. The selection of appropriate materials and pointing methods for the outer leaf are crucial.

Cavity trays

Cavity trays should be provided:

 

 

Cavity trays should rise at least 140mm within the cavity, be self-supporting or fully supported with joints lapped and sealed. Stop ends should be provided to the ends of all cavity trays.

Weepholes should be provided at not more than 900mm centres to drain each cavity tray, with at least two weepholes per cavity tray.

Condensation risk

In cavity wall construction it is necessary to ensure that there is no danger of condensation forming within the inner leaf. Condensation may have a detrimental effect on the thermal performance of a structure or cause damp on the inside. Unfaced mineral wool products, being ‘vapour open’ offer virtually no resistance to the passage of water vapour and consequently will not suffer from interstitial condensation.

The Knauf Insulation Academy Technical Advisory Centre are able to carry out condensation risk calculations if further reassurance is required.

 

1) Partial fill: Crown DriTherm Cavity Slab

Crown DriTherm Cavity Slab ......mm thick to be located in the cavity of all external masonry cavity walls.

The residual cavity width to be 25mm/ 50mm. Insulation to be retained against the cavity face of the inner leaf of masonry by means of insulation retaining wall ties as manufactured by ..... . The wall ties to have one of their drip centres half-way across the residual cavity.

The first run of wall ties to be located at 600mm centres horizontally (at a level to be decided by the specifier). Ties thereafter to be at 450mm centres vertically and a maximum of 600mm horizontally.

All work under construction must be protected overnight and during adverse weather conditions in accordance with BS 5628: Part 3: 1985.

Alternatively, refer to NBS clause: F30/12,151

2) Partial fill: Polyfoam Cavityboard

Polyfoam Cavityboard, ......mm thick, to be located in the cavity of all external masonry cavity walls; residual cavity width to be ....mm. Insulation to be retained against the cavity face of the inner leaf of masonry by means of insulation retaining wall ties as manufactured by ....... .

The wall ties to have one of their drip centres half-way across the residual cavity.

The residual cavity width to be 25mm/50mm.

The first run of wall ties to be located at the bottom level of the insulation (to be decided by the specifier). Ties to be at 450mm centres vertically and a maximum of 900mm horizontally.

All work under construction must be protected overnight and during adverse weather conditions in accordance with BS 5628: Part 3: 1985.

All work to be in strict accordance with this specification and the recommendations contained in Knauf Insulation’s literature.

Alternatively, refer to NBS clause: F30/12,151

3) Crown Supafil

Crown Supafil Cavity Wall Insulation glass mineral wool to be injected into the cavity by an Knauf Insulation/BBA Approved Installer.

New cavity walls to receive Crown Supafil Cavity Wall Insulation by injecting insulation into the cavity should be constructed so that insulation cannot penetrate ventilation ducts or pass through to cavities in adjoining buildings or compartments which are not to be insulated. Close fitting ducts or sleeves should be installed across the cavity to serve air bricks and other ventilation openings. Cavity barriers should be installed at junctions with other properties and compartments, and as required by the Building Regulations.

Existing cavity walls to be inspected by the Approved Installer and all necessary builder’s work carried out prior to Crown Supafil Cavity Wall Insulation being injected.

All work to be in strict accordance with the procedures laid out in the relevant BBA Certificate and the Knauf Insulation “Operators” and “Survey & Assessment” manuals.

Alternatively, refer to NBS clause: P11/40,220

4) Full fill: Crown DriTherm Cavity Slab

All external walls to be insulated during construction by completely filling the cavities with Crown DriTherm Cavity Slab

The first run of wall ties to be located at 600mm centres horizontally (at a level to be decided by the specifier). Subsequent runs of wall ties to be at not more than 900mm centres horizontally, or as otherwise required by the structure, and at 450mm vertically.

All work under construction must be protected overnight and during adverse weather conditions in accordance with BS 5628: Part 3: 1985.

Alternatively, refer to NBS clause: F30/10,150

5) Polyfoam System Closer

Install Polyfoam System Closers, 25mm thick ….mm wide (to equal cavity width) at jambs and sills of all openings in masonry cavity walls. Secure with the proprietary ties at maximum 500mm centres. Ensure that closers fit tightly to each other and to the lintel. The closer at the jamb must completely lap the closer at the sill.

Alternatively, refer to NBS clause: F30/18,180

6) Polyfoam Combi Closer

Install Polyfoam Combi Closers, 25mm thick ….mm wide (to be cut, where necessary, to equal cavity width) at jambs and sills of all openings in masonry cavity walls. Secure with the proprietary ties at maximum 500mm centres. Ensure that closers fit tightly to each other and to the lintel. The closer at the jamb must completely lap the closer at the sill.

Alternatively, refer to NBS clause: F30/18,180

7) Polyfoam Linerboard

Polyfoam Linerboard of insulation thickness .....mm with integral 9.5mm plasterboard to be fixed directly to inner masonry leaf by adhesive*/ plaster dabs*/ mechanical fixing* (*delete as required) using the method and materials recommended in Knauf insulation literature.

When the adhesive has set, secure with two ..... (manufacturer/reference) mechanical fixings along the top edge of each board (for adhesive/plaster dabs).

Alternatively, refer to NBS clause: K10/175,185,205