CONSTRUCTION OBSERVATIONS   

Observations from construction practice have identified a number of areas where thermal bridges commonly occur as a result of issues associated with design, buildability and construction practice. These areas include:

• Around windows, doors and rooflights
• Timber studwork in timber frame construction and in pre-fabricated panels
• Stainless steel cavity wall ties
• Bridged cavity masonry construction
• Discontinuities in thermal insulation
• Complex detailing
• Around steel I-beams

All of these thermal bridges can be attributed to issues associated with the design of the building and the thermal envelope and /or issues associated with the way in which the dwellings have been constructed.

Around windows, doors & rooflights

Experience suggests that thermal bridges are common around windows, doors and rooflights. The areas where the thermal bridging occurs relate to:

• Lintels
• Jambs
• Sills
• Door thresholds
• Position of the window or door frame

Lintels
Thermal bridges commonly occur though box and top hat lintels. In these types of lintels heat flows through the path of least resistance, the perforated steel base plate, circumventing the insulation contained within the box or top hat (see figure and animation below).

Box lintel showing heat flowing through steel and bypassing the insulation within the box lintel

Thermal bridging through lintels

Even if the base plate is removed, as is the case with some insulated top hat lintels, heat will flow up and around the insulation contained within the top hat of the lintel.

Thermal image illustrating heat loss through top hat lintel

Thermal bridging can also occur if the lintels are used inappropriately. At Stamford Brook, two independent lintels were used; an L-shaped lintel for the outer leaf of masonry and a box lintel with toe for the inner leaf of masonry. The box lintel chosen was originally designed to be used in a two brick thick solid wall construction, with the inner leaf of brick being supported on by the box and the outer leaf of brick being supported by the toe. However, at Stamford Brook, the toe of the box lintel, which projected across much of the cavity leaving a 42mm gap between the lintels, was non-load bearing and was used for buildability reasons. The toe would provide a solid surface for the fixing of the plasterboard dry-lining. Modelliing work undertaken on the detail indicated that the toe provided a thermal bridge past the frame and the closer (Roberts, 2004). This is illustrated in the figure below where close isoline spacing was found directly above the window frame (the closer the isoline spacing, the greater the heat loss).

Two independent lintels, one with toe projecting across cavity

The modelling work also highlighted that if the toe was removed, the isoline spacing increased (see figure below), resulting in a decrease in heat loss through this detail. Calculations suggested that the addition of the toe increased the whole wall U-value by 2%.

Two independent lintels, no toe projecting across cavity

Observations of construction on site revealed that the size of the gap between the two lintels was often lower than the designed 42mm. In some cases, the gap was as low as 20mm (see photograph below), increasing thermal bridging at this point.

Observed gap between the two lintels

The window detail was remodelled with the observed 20mm gap which resulted in a linear thermal bridging (ψ value) of 0.203 W/m²K. This compared to a value of 0.068 W/m2K if the window had been constructed as designed with the 42mm gap.

In the worst case scenario, reducing the gap between the lintels to 20mm resulted in a 200% increase in linear thermal bridging at this detail, with the resulting ψ value for the window (0.203 W/m²K) approaching that given in Appendix K of SAP for a typical combined steel lintel (0.3 W/m²K).

In timber frame construction, it is also common to find excessive amounts of timber at the top of the window opening forming the lintel (see photograph below). In addition, where gaps exist between the timbers, it is not uncommon for insulation to be omitted from these spaces.

Excessive amounts of timber forming lintel

Jambs
Experience suggests that thermal bridging is also common at jambs. Traditionally, blocks were used to close the cavity at the jamb, resulting in a solid masonry reveal (see photograph below).

Solid masonry reveal

Nowadays, the cavity is commonly closed using a proprietary insulated cavity closer or by inserting a thin strip of insulation (a thermal break) between the masonry leafs (see photographs below).

The use of a thin strip of insulation at the reveal

Bridging can occur at the reveal if the thin strip of insulation is omitted and masonry is used to close the reveal [1], if the proprietary cavity closer that is installed is not insulated [2] or if the proprietary cavity closer that is installed is the incorrect size and the resulting gap is not closed using a suitable material [3].

Sills
Thermal bridges can also occur at the window sill if the insulation in the cavity is installed in such a way that it does not tightly abut the cavity closer and no corrective measures are taken to fill the gap in the insulation. Examples of such thermal bridges are illustrated in the photographs below. The first photograph shows the partial fill insulation installed within the cavity. However, it does not abut the cavity closer, resulting in a small gap in the thermal insulation layer just below the window sill. The second photograph shows a cavity that has been retro-filled with blown mineral fibre. The retro-filling has taken place prior to the completion of the inner leaf around the window. This has resulted some of the mineral fibre being expelled from the cavity and it can be seen lying on the floor.

Door thresholds
Experience suggests that door thresholds are also a common area for thermal bridging. It is not uncommon to find instances where the ground floor slab extends across the external cavity, rather than terminating at the inner edge of the cavity. Consequently, the cavity at this point ends up being bridged by the concrete slab, resulting in a significant thermal bridge at the ground floor/wall junction (see photographs below).

Thermal bridge at threshold where concrete slab extends to the inner face of the outer leaf of brickwork

Even if the slab does not extend beyond inner edge of the cavity, thermal bridging can also occur if construction debris is allowed to accumulate in the empty cavity between the edge of the ground floor slab and the external leaf of the external wall. If this debris is not removed prior to the insulation being installed within the cavity, discontinuities in the thermal envelope will occur.

Thermal bridge at threshold where construction debris bridges the gap between the floor slab and the outer leaf of the external wall

Position of the window or door frame
The position of the window or door frame in the opening is also known to have an impact on thermal bridging. Work undertaken by Roberts (2004) has investigated the impact that the position of the window head in the wall can have on thermal bridging. Roberts (2004) compared the psi values achieved for a number of different window head offsets from the outer face of the external wall (0mm, 35mm, 72mm, 107mm, 127mm, 145mm, 200mm and 250mm). The results of this comparison are illustrated in the animation and figures below.

Effect of window position on psi value

Isoline spacing for different window head offsets

The results indicate that the lowest psi values are achieved when the window head is positioned in line with the external wall cavity (in this case a frame offset of 107mm, 127mm or 145mm). Positioning the window head within the inner or outer leaf of the cavity wall results in a significant increase in psi value for this junction and thermal bridging.

Experience suggests that it is not uncommon for window or door frames to be installed within the outer leaf of a masonry cavity, timber-frame or steel frame external wall (see photographs below). This is particularly the case with some bay window designs. As the earlier analysis indicates, this can have a significant effect on thermal bridging.

Bay windows installed within external leaf in masonry cavity construction

Window installed within external leaf in timber-frame and steel frame construction

Door installed within external leaf in timber-frame construction

TIMBER STUDWORK IN TIMBER FRAME CONSTRUCTION AND PRE-FABRICATED PANELS

Repeating thermal bridges also exist through the solid timber studs in timber frame construction. A default value of 15% solid timber is used in the UK to account for repeating thermal bridges. However, recent work undertaken in the UK suggests that the default timber fraction of 15% may be too low. Observational work has recorded timber frame fractions of 30 to 40% for standard timber frame construction using solid framing members (see Overend, 2001) and 24% for a closed panel timber frame MMC system (see Ward, 2008), both considerably higher than the default value of 15%. The timber fractions that are actually observed in dwellings are often greater than the default design value as additional timber is used:

• To form sole plates, head plates and panel ends
• Around openings
• To provide structural strength and stability

A panoramic movie illustrating some of the typical panels observed by Ward (2008) can be seen below.

Panoramic movie illustrating observed timber fraction

Increases in the timber fraction can have a significant impact on thermal bridging and the overall U-value of a wall. Work undertaken by Ward suggests that the overall wall U-value can be as much as 50% higher than the notional U-value. This is consistent with earlier work undertaken by Overend (2001).

In addition to larger than designed timber fractions and wall U-values, it is not uncommon within timber frame construction to observe areas where large amounts of timber are concentrated together (see photograph below). This commonly occurs around openings and where additional structural support is required in the panels to support loads above.

Panels with large amounts of timber concentrated together

In one study, instances of 6 to 11 adjacent solid timber beams forming a single thermal bridge were found (Bell, Smith & Miles-Shenton, 2004). In another up to 7 adjacent beams (see photograph below) were observed (Ward, 2008). This can have a significant effect in thermal bridging at this point in the construction.

Timber panel with 7 adjacent timber beams

Another problem commonly experienced in timber frame construction is areas where it is either very difficult or impossible to insert the full thickness of the insulation material into the panel (see photographs below). This occurs where:

• Very small gaps exist between the studs.
• Gaps exist but they are not accessible. This commonly occurs at the corner of external walls or at the junction between the partition walls and the external/party wall.
• Services have been routed inside the depth of the panel.

Very small gaps between studs in panel

Services incorporated within the panel

Gaps behind studs

Even if it is possible to insert insulation into the gaps in the panels, care needs to be taken to ensure that the insulation is not over compressed. Over compressing the insulation will reduce its effectiveness.

STAINLESS STEEL CAVITY WALL TIES

Wall ties are commonly used in masonry cavity wall construction in the UK for structural stability and to resist the wind loads applied to the non-load bearing external leaf. If stainless steel wall ties are used, they act as a thermal bridge through the construction. When levels of insulation within the cavity were low, the additional heat loss attributable to the stainless steel wall ties was relatively small. However, as insulation levels and cavity widths increase (see animation below), the wall ties that are required within the cavity become larger (they have a greater cross-sectional area) and have to be installed at more frequent intervals (greater density). This, coupled with the fact that the difference in thermal conductivity between the bridged and non-bridged construction is also larger, means that stainless steel wall ties can represent a significant source of heat loss in large well insulated cavities.

Comparison of the effect of wall ties on external wall U-values

Generally speaking:

BRIDGED CAVITY MASONRY CONSTRUCTION

Experience suggests that it is non uncommon to find that the cavity in masonry cavity construction is bridged either with construction debris and/or mortar snots and droppings, resulting in thermal bridging. The photographs below illustrate examples of excessive mortar snots and mortar droppings within the cavity of masonry cavity walls caused by poor construction practice.

Mortar snots within the cavity

Mortar droppings at the base of the cavity

Accumulations of mortar droppings are commonly observed at the bottom of the cavity in masonry cavity walls, on top of cavity trays, on top of cavity socks and on the top of wall ties.

The method used to insulate masonry cavity external walls can be important. With partial fill or retro fill insulation, all or part of the cavity remains clear during the construction of the wall. Consequently, opportunities exist for construction debris or mortar to fall down and accumulate within the cavity unless measures are taken to prevent material entering the cavity, for instance the use of cavity boards (see photograph below). With built-in full-fill insulation, the cavity is generally always filled with insulation as the wall is being constructed (see photograph below). Although this can prevent debris and mortar entering the cavity, this material can accumulate on the top of the cavity batts. This material should be removed before the installation of further batts.

Use of cavity boards with partial fill insulation

Full-fill cavity insulation

DISCONTINUITIES IN THERMAL INSULATION

Observations suggest that it is also not uncommon to find areas where there are discontinuities in the thermal insulation. Discontinuities that have been observed include:

• Areas of missing insulation. With retro filled walls, sometimes the filling process results in areas of wall that are not filled [1]. It is also very difficult to properly insulate above and between windows in retro filled walls due to the use of cavity trays.
• Poor fitting of mineral wool insulation batts at lintels [2 & 3].
• Gaps in partial fill insulation [4, 5 & 6].
• Missing insulation in dormer wall [7].
• Debris within the cavity which results in discontinuities in the insulation layer [8].
• Discontinuities in the thickness of the insulation and gaps in the insulation at the eaves [9].

Missing loose fill mineral wool insulation at window sill

Poor fitting of insulation between lintels at window and door heads

Gaps in partial fill insulation

Thermal image illustrating area of missing insulation on dormer wall

Construction debris within the cavity that will cause the partial fill insulation layer to be discontinuous

Thermal image illustrating area of missing insulation at eaves

An example of the sorts of discontinuities in thermal insulation that can occur at the wall/roof junction in cavity walled dwellings that have been constructed with a cold pitched roof was observed at Stamford Brook. In the dwellings constructed by one of the developers, the roof truss had been designed to overhang the wall plate by 50mm (see [1] below). However, observations of actual construction indicated that actual overhang that was being achieved on-site was closer to 10mm (see [2] below). The reduction in overhang meant that the full thickness of loft insulation could not be fully maintained at this junction, leading to an increase in thermal bridging at this point. In addition, the cellulose insulation that was used in the loft was not installed correctly, resulting in gaps in the insulation at the eaves (see [3] below).

Roof truss overhang as designed [1] and as built [2]

Gaps in the insulation at the eaves

COMPLEX DETAILING

Thermal bridging is also more common around areas of complex detailing, as these areas often contain inherent design difficulties that make it more difficult to maintain continuity of thermal insulation layer. Such areas typically include recessed front doors, balconies, covered alleyways and bay windows.

An example of the potential thermal bridging problems that can be encountered with recessed front doors was observed at Stamford Brook, where a recessed front door porch detail was incorporated within some of the dwelling types. Observations of the door at various stages of construction revealed that in order to accommodate the recess, 4 lintels were used (see photographs below). 3 single leaf lintels were used to support the recessed inner blockwork (2 lintels) and the inner leaf of the main external wall, whilst a single leaf arched lintel was used to support the external leaf of brickwork for the outer wall.

Completed recessed front door detail

Use of 4 lintels to construct the recessed front door porch detail

It is clear from the photographs above that this detail contains a number of thermal bridging and air barrier continuity issues, due to the multiple changes in plane that exist. With respect to thermal bridging, the lintel that supports the inner blockwork leaf of the main external wall bypasses the cavity wall insulation forming a significant thermal bridge. This is illustrated in the drawing and photographs below. In addition, a plain piece of uninsulated cement board was attached to the underside of this lintel to finish off the recess.

Thermal bridge through lintel

In addition to the thermal bridge, there is also a discontinuity in the primary air barrier between the parging layer on the upper blockwork leaf (A) and the parging layer on the lower blockwork leaf (B). This discontinuity is likely to result in air leakage occurring at this point, particularly given the fact that a number of services are known to penetrate the intermediate floor and these were not all adequately sealed. A detailed analysis of the air leakage around the recessed front porch detail can be found within Miles-Shenton, Wingfield & Bell (2007).

Complex detailing at balconies can also lead to potential thermal bridging problems. The animations below illustrate some common thermal bridges encountered at balconies.

Thermal bridging at balconies

A detailed example of the thermal bridging problems that can be encountered with balconies was observed with the intermediate floor ‘Juliet’ balconies at Stamford Brook (see photograph below). A number of ad-hoc changes were made to the balcony details on site, such that the balcony threshold detail as designed [1] differed from that that was actually built [2]. These changes resulted in the occurrence of a thermal bridge at the point where the flooring continued over the cavity to meet the external leaf of brickwork along with a direct air leakage path from the inside to the outside of the dwelling.

Completed dwellings incorporating Juliet balconies

Juliet balcony as designed [A] and as built [B]

The animation below illustrates the various stages of construction of the Juliet balconies at Stamford Brook. It is clear from the animation that the lack of thermal insulation at the threshold causes significant thermal bridging and air leakage problems.

Sequential construction of the Juliet balcony at Stamford Brook

Thermal images of the Juliet balconies illustrated areas of significant heat loss around the thresholds (see photograph and thermal image below). This was confirmed during a pressurisation test, where air leakage was detected at the threshold of the Juliet balcony (see images 1 and 2 below).

Photograph and thermal image of Juliet balcony

Air leakage at the threshold of Juliet balcony:
[A] Thermal image under depressurisation and
[B] Smoke flowing into the floor cavity at threshold under pressurisation

Covered passageways also present complex detailing problems that can lead to thermal bridging. The animation below illustrates a common detail that is used to support the floors above the passageway. With such a detail, it is common for the soffit of the passageway to be finished off with a piece of uninsulated cement board attached to the underside of the steel I-beams, resulting in a significant thermal bridge.

Thermal bridging at passageway

Another example of the sorts of thermal bridging problems that can be encountered with complex detailing was observed around the ground floor bay windows at Stamford Brook (see photograph below).

Ground floor bay windows in completed dwellings

Thermal imaging of the ground floor bay windows illustrated significant heat loss around the bay window head (see images below).

Photograph and thermal image of the ground floor bay window

An analysis of the detail drawing associated with the bay window shows that the ceiling of the bay should have been constructed with insulated plasterboard in order to reduce the thermal bridge created by the outer lintel which was used to support the external leaf of brick However, observations of construction showed that plain plasterboard was used instead, resulting in a thermal bridge through the lintel (see drawing below).

Detail drawing of bay window

Thermal images of the bay window undertaken from the inside during depressurisation not only highlighted the thermal bridge through the lintel, but they also identified an air leakage path into the plasterboard plenum at the junction between the timber roof and external wall, which had not been sealed during construction, and an air leakage path via the cavity (see below).

Photographs of bay window head illustrating thermal bridge and air leakage paths

AROUND STEEL I-BEAMS

Thermal bridging has also been observed around steel I-beams. These beams are commonly used in new build in areas that require additional support. For instance, they are commonly used in room-in-the-roofs to support the roof trusses, where the roof comprises a combination of both warm and cold roof constructions.

The animation and photograph below illustrates the steel I-beam puncturing through the insulation layer, resulting in a significant thermal bridge. In addition, the construction also results in a direct air leakage path around the I-beam from the inside to the outside of the dwelling.

Thermal bridging and air leakage around steel I-beam