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CPD PROGRAMME


PANEL 1 Considering the wall in Figure 2, it has already been determined that the only non- homogenous layer that will be taken into account is the blockwork. The proportions (as viewed from the face of the wall) of the mortar to blockwork is, in this case, 0.067 (ie 6.7% mortar). This can be readily determined – a simple calculation is given in BRE443 (freely downloadable from the web). So the thermal resistances of the layers can be determined. Rse and Rsi are taken from CIBSE Guide A3.


Layer


Figure 4: Simulated temperature profile through lower section of double glazed window (source http://windows.lbl.gov/software/therm/ images/therm2-3.gif)


various UK Building Regulations recommend using accredited construction details[5] [6] (ACDs) to ensure that the edge losses are minimised. The term ‘y-value’ is used to describe


the sum of (length × Ψ) for all junctions in a building envelope divided by the total area of external elements, and provides a performance metric for thermal bridging, (not to be confused with the ‘Y value’ that is the abbreviation for thermal admittance and completely different!) As an example, the application of linear


thermal transmittance as part of the U value calculation of a simple window is given in panel 2. Each of the heat flow paths in a building requires appropriate analysis to ensure that the integrity of the predicted heat flows, and hence forecast building energy use, is maintained. Aside from the important issues identified in this article, others include the anomalies when considering heat flow through ground floors, air spaces and basements that will need careful consideration. CIBSE Guide A3 and ASHRAE Fundamentals Handbook chapters 17 & 18 are great places to find out more about this essential area of knowledge and application. © Tim Dwyer


Correction


In the May edition of the CIBSE Journal CPD, the following comment in the sixth paragraph was incorrect: ‘... [in] Building Regulations Approved Document Part L1A for houses, there are maximum acceptable “back stop” values for the U value of 0.2 W/m2K for external walls, 0.3 W/m2K for a roof and 2.0 W/m2K for windows.’ The sentence should have read: ‘...[in] Building Regulations Approved Document Part L1A for houses, there are maximum acceptable “back stop” values for the U value of 0.3 W/m2K for external walls, 0.2 W/m2K for a roof and 2.0 W/m2K for windows.’ This change may have affected your answer to question one. We apologise for the error and hope it has not caused our readers too much inconvenience.


www.cibsejournal.com 1


2 3


4a 4b 5


6 Description Rse Brickwork Mortar


Proportion, P


100% 100%


EPS insulation 100% Block


Light Plaster 100% Rsi


93.3% 6.7%


100%


Thickness, d (m)


-


0.105 0.050 0.100 0.100 0.013 -


Thermal


conductivity, λ (W/mK)


- 0.77


0.040 0.20 0.88 0.18 -


There are just two resistance paths to consider – through the block: Path A = Rse + Rbrick + Rinsulation + Rblock + Rplaster + Rsi = 2.128 m2


Path B = Rse + Rbrick + Rinsulation + Rmortar + Rplaster + Rsi = 1.742 m2 1


1


And Ru may be determined by


PPathA + PPathB RPathA RPathB


=


0.933 + 0.067 2.128 1.742


1


And RL may be determined by Rse + Rbrick + Rinsulation +


1


= 1.628 + 1.866 + 0.588 = 1.628 + 0.408 = 2.036 m2 Hence Rb = (2.101 + 2.036)/2 = 2.069 m2


So RL = 0.04 + 0.136 + 1.250 + 1


PANEL 2 1.5 metres


Uncoated double glazed glass with 12mm air space


Frame made of 75mm x 75mm softwood


Window in ‘normal’ exposure And so for the window


UW = ∑(AgUg) + ∑(AfUf) + ∑(pfΨs) ∑Ag + ∑Af


= (1.147 × 2.85) + (0.353 × 2.02) + (4.4 × 0.06) 1.147 + 0.353


References


1. BS ISO 10456:2007 Building materials and products - Hygrothermal properties - Tabulated design values and procedures for determining declared and design thermal values


2. Conventions for U-value calculations BRE Report BR 443: BRE 2006, www.bre.co.uk/filelibrary/pdf/rpts/BR_443_(2006_Edition).pdf


3. CIBSE Guide A 2006, Section 3.3.11


4. Doran SM, and Kosmina L, ‘Examples of U-value calculations using BS EN ISO 6946:1997’ December 1999 www.communities.gov.uk/documents/planningandbuilding/pdf/133394.pdf


5. Accredited Construction Details , DCLG, June 2007 www.planningportal.gov.uk/uploads/br/accredconbk.pdf


6. Accredited Construction Details (Scotland) www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/profinfo/techguide/acdscot


June 2011 CIBSE Journal


= 3.269 + 0.173 + 0.264 1.5


= 2.831 W/m2 K


From tables in CIBSE Guide A and from sketch left: Frame - Uf = 2.02 W/m2


= 0.353m2 Glass - Ug = 2.85 W/m2


K (Table 3.25) and calculated area, Af K (Table 3.23) and calculated area, Ag


= 1.147m2 Linear thermal transmittance (in the CIBSE table related to


spacer s between the two panes of glass), ΨS = 0.06 W/mK (Table 3.26), perimeter length, pf=4.4m


0.933 + 0.067 0.500 0.114


+ 0.072 + 0.13 K/W K/W and so U = 1/2.069 = 0.483W/m2 K


PPathA + PPathB RBlock RMortar


+ Rplaster + Rsi = 0.438 + 0.038 = 2.101 m2 1


resistance, R (m2


Thermal K/W)


0.04 0.136


1.250 0.500 0.114 0.072 0.13


K/W and through the mortar, K/W


K/W


59


1.0 metres


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