Condensation and thermal bridges

By - , Build 87

Although steel framing in domestic construction has only a small market share, there are signs it may be on the increase. Since steel is a good conductor, builders need to be aware of potential pitfalls, such as thermal bridges and condensation.

Figure 2: The effectiveness of thermal breaks can be seen in the R-values of a steel-frame wall with a  range of claddings, (The wall has galvanised steel studs at 600 mm centres filled with R1.8 insulation.)
Figure 3: Typical wall construction with thermal break.

You only need to think of steamy  windows on a cold day to realise  how easy it is for condensation  to form, on a cold surface. The warm  moist air on. the inside hits the cold  glass, releasing its moisture.

A thermal bridge exists wherever  there is a high heat conductance path.  in buildings, a steel framing member  can act as a bridge allowing heat to  move from a warm interior, through,  the wall lining to the external  cladding and outside, effectively  bypassing the insulation.

Thermal bridges cause  condensation

This means the temperature of the  indoor side of the bridge will he closer  to the outside temperature. On cold  days this will cool the wall lining  along the bridge. If this temperature Is  well below the dewpoint of the indoor  air, moisture will condense on the  lining, as shown in. Figure 1.

If the steel framing toadies the  external cladding but not the internal  lining, i.e. it is buffered from the  inside, condensation can form instead  or the steel in the wall.

Walls or roofs that have thermal  bridges where condensation may occur  on the internal linings will not comply  with clause E3 Internal moisture of the  New Zealand Building Code. The  Acceptable Solution E3/AS1 requires  thermal breaks when steel studs are  used in wall construction.

Thermal bridges degrade  R-values

Condensation is not the only potential  problem with thermal bridges – the  R-values can be seriously degraded.

Figure 2 shows the improvement to  R-value if thermal breaks are used.  With no thermal break (shown as  ‘none’ in the graph), there is a possible  30% loss in R-value. Since R1.5  is the minimum required for walls  in Climate Zones 1 and 2, this may  mean a building fails to meet: the  code requirements.

What are thermal breaks?

Thermal breaks are insulating  materials placed on the cold side of a  thermal bridge (see Figure 3) so that heat passing across the bridge meets a  high thermal resistance barrier. Such  barriers can either be strips of material  across the width of the members where  they meet the cladding, or sheathing  behind the cladding. The aim is to  keep as much of the structure as warm  as possible, thus avoiding excessive  heat loss and condensation.

Figure 2: The effectiveness of thermal breaks can be seen in the R-values of a steel-frame wall with a  range of claddings, (The wall has galvanised steel studs at 600 mm centres filled with R1.8 insulation.)
Figure 3: Typical wall construction with thermal break.

The breaks should be installed on  the outside of all steel exterior wall  framing, including all dwangs and  girts that have cladding fixed to them.  If the floor is steel framed, breaks  should also be used wherever the  cladding is fixed to the floor members.

Insulation in a ceiling space with  steel framing should be carried over  ceiling joists, not just placed between  them. Otherwise, the framing acts as a  thermal bridge between the indoor  space and the ceiling space.

What materials can be used?

The material and its thickness  must have an R-value greater than  0.2 m2°C/W. It must support the  exterior cladding, so needs to be  non-compressible. Materials include  12 mm low-density wood fibreboard  (softboard), 10 mm or extra high- density expanded polystyrene  (EPS), or 10.5 mm bitumen-coated  wood fibreboard.

What is the dew point?

The key to managing condensation  within a building is understanding  dew point.

When warm, moist air passes  over a cooler surface, condensation  can form. Condensation is the  collection of moisture on a surface  at or below the dew point. The dew  point is the temperature air must  fall to for saturation to occur. If a  building material/component has a  temperature belowthe dewpoint,  condensation will accumulate on  that material.

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Figure 2: The effectiveness of thermal breaks can be seen in the R-values of a steel-frame wall with a  range of claddings, (The wall has galvanised steel studs at 600 mm centres filled with R1.8 insulation.)
Figure 3: Typical wall construction with thermal break.

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