Escaping heat follows the path of least resistance, flowing from warm areas to cold ones. Reducing the impact of potential thermal bridges in a building will help keep the warmth inside.
IN MOUNTAINOUS terrain, Italian auto-strada designers employ a tunnel and bridge approach. To relate this back to building, let’s think of the tunnel as a thermal bridge that creates an easy path for the cars – often at speed – to traverse the mountain.
Timber and particularly steel wall framing is like the tunnel. It provides an easier path than through the insulation material for heat to escape through the wall of the building.
Materials that act as thermal bridges
Thermal bridges are materials or elements that are better at conducting heat. To put it more simply, heat flows more easily through them than other materials from the warmer interior to the colder exterior.
In addition to timber and steel, heat flow or loss will be higher through:
- glass – double glazing reduces the amount
- aluminium, such as window frames
- compressed insulation material
- air when it leaks around insulation material and adjacent framing
- concrete, such as floor slabs.
The impact of a thermal bridge is to reduce the overall thermal performance of a building element, such as a wall. How much performance is reduced depends on the area of the thermal bridge with respect to the total wall area.
The impact will also be greater the more thermally conductive the bridge is. As an example, heat loss through steel studs installed without the required thermal break is greater than for timber given the same framing layout.
The BRANZ House Insulation Guide shows the effect of thermal bridging from the framing in the tables (Figure 1). In almost all cases, the R-value of an area of framed wall will be less than the R-value of the well installed insulation material. The more framing in the wall, the bigger the drop in R-value.
How can we see the effect?
Effects of thermal bridging can often be seen on the outside of a building. After a cold night, the position of the wall framing might be quite visible on the face of the cladding.
The face of the cladding will be slightly warmer because of the higher heat loss where the framing is and less likely to have visible condensation at those points.
It can also be evident on the inside where the wall linings adjacent to the stud are colder and may have visible condensation droplets and ultimately lines of mould (Figure 2). Thermal bridges can also create cold spots where condensation or mould can form.
Reducing the impact of framing
For framing, the effect of thermal bridging can be reduced:
- Keep the percentage of framing in the wall as low as practical (don’t overspecify) while still complying with NZS 3604:2011 Timber-framed buildings for timber framing or NASH N11 House Insulation Guide for steel framing.
- Ensure there is a thermal break (strips of sheet material) on steel framing installed on the outside face of the framing (Figure 3). The thermal break must have an R-value of at least R0.25 – specifying a higher R-value will further reduce heat loss at framing locations. NASH recommends R0.35 as a better solution in climate zones 1 and 2 and R0.45 in climate zone 3 or, for the best option, R0.4 in climate zones 1 and 2 and R0.5 in climate zone 3.
- Ensure boxed sections of steel framing have insulation fitted into the concealed spaces.
- Install insulation into voids at framing junctions (external corners and where internal walls abut external walls) before cladding or linings are installed.
- Install insulation for timber framing with the maximum R-value for the framing thickness — as an example, this is currently R2.8 for 90 mm framing — or increasing framing depth to 140 mm. It is usually easier and cheaper to use a higher R-value insulation product than to add thermal breaks to the frame.
- Increase utilisation of structural insulated panel (SIP) construction.
Other potential thermal bridges in wall construction are avoided by good-quality insulation installation and ensuring it:
- fits firmly within the framing
- has no creases or folds
- is not compressed – installing 100 mm thick insulation into 90 mm framing will compress it and reduce its effectiveness.
Other improvements to reduce other areas of thermal bridging are:
- specifying thermally broken aluminium joinery, uPVC or timber joinery
- up-specifying glazing – insulating glass units (IGUs) with argon fill rather than air-filled IGUs
- reducing the area of glazing, particularly on south-facing elevations.
What about roofs, ceilings and more?
Thermal bridging also occurs with roof and ceiling insulation as, like walls, insulation is typically fitted between the framing members. Options to minimise the thermal bridging impact are to:
- lay a second layer of insulation at right angles to the framing across the first layer
- install insulation that is around 50 mm thicker than the ceiling joist or truss bottom chord – once installed, the insulation tends to expand over the top of the framing, limiting the bridging effect
- ensure that ceiling insulation is carried halfway across the top plate of external walls
- insulate the top of ceiling access hatches
- install thermal breaks for steel-framed roofs as shown in NASH N11 — for example, across the top plate, rafters and top chords of trusses
- utilise warm roof construction for membrane roofs
- insert a 10 mm XPS sliver between a foundation wall and the slab edge for a heated concrete slab on ground (Figure 4).
Articles are correct at the time of publication but may have since become outdated.