Surface resistance

legs_akimbo

Surface resistance ?
Something that I am not familiar with

How is this accounted for in the calculation for a wall element ?

It is part of this particular detail :




Thermal Transmittance (U-value) Calculation Report
Calculation performed by Uvaluate, version 2.0.9 UK
Xtratherm UK ltd
Produced on 20-Jul-09
Reference U value calculations
Fax Unknown
Site West of Bedford
Assessed by Paschal Gallagher, Xtratherm ltd
Comment All measurements must be verified on site
If corrections for air gaps / mechanical fasteners are less than 3% of U (EN 6946), they are: Applied
Element details
PF Cavity Wall - XT/CW Plus L'weight Block 0.19 Dot & Dab
Element type Exposed element
Area [m2] 1
U-value [W/m2 K] 0.22
Layers Thickness Conduct'y Resistance
[mm] [W/m K] [m2 K/W]
Internal surface resistance 0.130
Plasterboard 13 0.180 0.069
Default air layer 15 0.096
Concrete block Celcon Hi Seven 100 0.190 0.408
Xtratherm XT Cavity Plus 60 0.021 2.857
Cavity unventilated Low E 50 0.650
Clay brickwork (outer leaf) 102 0.770 0.132
External surface resistance 0.040
Upper/lower resistance 4.512 / 4.383 W/m2 K
Correction for air gaps / mechanical fasteners 0.000 W/m2 K
Details of layers with thermal bridging or corrections for air gaps or mechanical fasteners are given
below.
Default air layer
Is there repeating thermal bridging? Yes
Bridging conductivity [W/m K] 0.43
Bridging material Plaster adhesive
Bridging thickness [mm] 15
Fractional area of bridging [-] 0.2
Correction for air gaps Not applicable
Correction for mechanical fasteners Not applicable
Concrete block Celcon Hi Seven
Is there repeating thermal bridging? Yes
Bridging conductivity [W/m K] 0.88
Bridging material Mortar - protected
Bridging thickness [mm] 100
Fractional area of bridging [-] 0.08
Correction for air gaps Not applicable
Correction for mechanical fasteners Not applicable


Also in the above would I be coorect in noticing errors for example in the underlined materials as in calculating the thermal resistance
eg : Concrete block Celcon Hi Seven 100 0.190 0.408
expressed as 0.100 / 0.19 = 0.408 (should be 0.526)

Also this

Plasterboard 13 0.180 0.069

Whereas these are correct


Clay brickwork (outer leaf) 102 0.770 0.132
0.102 / 0.77 = 1.32

Xtratherm XT Cavity Plus 60 0.021 2.857
0.060 / 0.021 = 2.857

This wall has according to those calcs a u value of 0.22
Would you agree ?
Where does the surface resistance come into play.

sinnerboy

legs_akimbo wrote: »

Surface resistance ?
Something that I am not familiar with

How is this accounted for in the calculation for a wall element ?

heat flow is slowed at the internal and external surfaces of an enclosing element

the "rules" - as far as b regs are concerned are set out in BR 443 2006

see page 7 for mandated figs for surface resistances

http://www.bre.co.uk/filelibrary/rpts/uvalue/BR_443_(2006_Edition).pdf

sydthebeat

legs.... i think the concrete block resistance figure takes into account the mortar bridging factor as well.. thus the lower resistance....

but on a point... is that an AAC block you are proposing to use??

legs_akimbo

sydthebeat wrote: »

legs.... i think the concrete block resistance figure takes into account the mortar bridging factor as well.. thus the lower resistance....

but on a point... is that an AAC block you are proposing to use??

if by AAC you mean an aereated concrete block (Quinnlite, Celcon, Thermalite) then yes Syd.
This is not something I am working on but just something that I came across and caught my attention.