Thermal Assessment of a Facade

Why do we need to assess the thermal transmittance in a facade? Why do we need to identify whether there is a risk that condensation might occur?  What types of condensation do we need to analyse and how do we quantify it, if it occurs while demonstrating compliance with the regulations? Which regulations do we need to follow? Presently we face these challenges when we start the design of a facade project to provide our clients with the best levels of energy performance on their buildings.

Condensation affects many buildings in the UK and can bring drastic consequences such as dampness, mould growth, wallpaper peeling and material corrosion which can result in long-term deterioration of the building fabric and even structural failure. The parameters used to analyse condensation within the façade are described in BRE BR 497, BRE IP 1/06 and BS EN ISO 13788 plus BS EN ISO 10211 with additional guidance in  CWCT Guidelines and Technical Notes which are “good practice” documents. This analysis should be carried out for all systems, interfaces and assemblies confirming the location of the vapour control layer, dew point (temperature at which naturally occurring moisture vapour in air condenses and becomes liquid) and u-value.

Analysing the thermal effect of brackets in a façade can be quite challenging as these are often the position where thermal bridges can occur making the likelihood of condensation occurring in undesirable places. In general brackets and other fixings are relatively small in comparison to the entire façade and are often excluded from the analysis unless considered to be significant.  Good practice would dictate that some form of isolation is introduced wherever there is the possibility of uninterrupted metal passing from outside to inside the building. Thermal bridges can also be minimized by changing the type or thickness of insulation. The thermal model in the pictures shows an example of condensation risk identified by the position of the dewpoint (red) line on a bracket detail without a thermal pad and thinner insulation (Figure 1) and no condensation with a thermal pad and thicker insulation (Figure 2):

Figure 1 & 2 – 2D Condensation Analysis – Dew Point Line Temperature

If a condensation risk is identified on a detail and nothing can be done to eliminate it, it is possible to estimate the quantity and frequency of surface liquid appearing and thereby assess the risk and potentially a management strategy.  This can be done by referring to and using BS 5250:2011 (Code of practice for control of condensation in buildings + A1:2016).  This evaluates the effect of condensation on impermeable surfaces given specific volumes of condensation parameters in g/m2.  This can happen for example on a sliding door track, if condensation occurs in the tracking area and the mechanism is adequately drained there will be no risk to the materials or surfaces and therefore will not compromise the detail or the system performance. To have this assessed and clearly identified in the design stage is essential for our client’s confidence, that the details and systems work without compromising the efficiency of the overall building.

3D models are usually assessed on building beams penetrating a facade to support canopies or other external features supported either directly by the structure or indirectly via the façade.  These can be continuous or thermally broken brackets supporting external structures such as large brise soleil considered to be significant enough for condensation to be a potential factor. The model (Figure 3) shows an example of a 3D analysis of a steel beam penetrating a façade:

Figure 3 – 3D Condensation Analysis – Dew Point Line Temperature

The thermal transmittance is assessed for each facade type area-weighted average of 1D centre-pane U-values and 2D sections around it and carried out in accordance with BS EN ISO 6946 (Building components and building elements. Thermal resistance and thermal transmittance. Calculation methods) and BS EN ISO 10077-2 (Thermal performance of windows, doors and shutters. Calculation of thermal transmittance. Numerical method depending on what we are analysing: windows/doors or curtain walls – for curtain walls we use the BS EN ISO 12631. The calculated overall U-value is then compared with the target U-value identified in the M&E client’s documentation façade performance specification. (Figure 4&5) This shows an example of this assessment with overall U-value.

Figure 4 – Facade Geometry and Details Description

Figure 5 – Overall U-Value Calculations

If no condensation occurs in those details and the overall U-value is equal or better than the project requirements, it can be concluded the proposed design suits the design intent and is compliant with the relevant standards and regulations.

Provision for the prevention of condensation in buildings in UK can be traced back at least as far as 1962 and conservation of fuel became a serious issue in 1974 when the oil crisis made energy conservation a political imperative; however the consideration of all the above was pretty much left to “best practice”:  Good designers knew that thermal breaks should be used to prevent cold bridging and condensation issues and that general insulation was a clever idea but there was very little in the way of regulatory requirement.  There was provision made in the 1984 Building Act which included Part L in 1985 and required that reasonable provision was made for the conservation of fuel and power.  This all changed following the Kyoto Protocol in 1992 which resulted in regulations having an increasing part to play in the thermal performance of buildings with Approved Document L: Conservation of Fuel and Power, being introduced in 1995 then revised and tightened in the years following and introduced into the Building Regulations as Part L in 2010.

Many people now comment that the regulation has overtaken good practice and that our new buildings are not functioning as we would want them to with issues such as overheating due to highly insulated envelopes plus high solar gains (poor design and glass selection), lack of sufficient ventilation and poor operation.  Additionally, there are reports of “sick buildings” where issues of poor health due to air quality, mainly created by air tightness, small opening areas and again poor operation of the building; possibly because of poor understanding of how such a complex “machine” needs to work.

CIBSE recently published CIBSE TM59 which sets out a standardised methodology for improved design consideration of overheating in residential homes covering such aspects as occupancy, equipment and lighting etc. which would indicate that this is no longer an anecdotal issue used to complain about regulation but is being taken seriously and should be addressed as early in the process as possible.

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