Cavity blocks

SilverBER

I have an assessment at the moment and I need to calculate the U Value for a wall in an externsion built last year(2008). I have all the information I need from the homeowner but they used 225mm cavity blocks for the wall construction with added internal insulation. Anyone got any idea what the TR of a cavity block is?

sinnerboy

215 mm hollow concrete block (thermal resistance = 0.21 W/m2K),

from table B17 , page 56 of

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,19069,en.pdf

recedite

Unfortunately that Table B 17 is a complete ***k-up.
The figure of 0.21 is given in units of U-value.
The thermal conductivity of external render is given as 1 instead of 0.57
Here is what DEAP says;
Thermal conductivity values for common building materials can be obtained from Building Regulations 2008 TGD L (Table A1), IS EN 12524 or the CIBSE Guide, Section A3. For specific insulation products, data should be obtained from accredited test data. Note that Building Regulations 2008 TGD L, Table A2 may be used for
insulation products' conductivity for design purposes (such as provisional ratings) but not for final BER assessments
Note it does not mention table B17 of the regs
I reckon the only DEAP compliant figure for thermal resistance of a hollow block wall is 0.417 W/m/k but this includes the render & hardwall.
I extrapolate this from DEAP table S3 default U-value of a hollow block wall is 2.4, ie. 0.225 divided by the inverse of 2.4 is 0.417
Then you just add on the extra resistance of your insulation.Clever or what? They can't argue with that.

SilverBER

Thanks Recedite, I had a look at table S3 before I asked the question and I was using the J age band for reference (as its a new build) but what is confusing me is the fact that in this age band the U Value for a cavity block is the same as every other type of material including Timber Frame.

recedite

The assumption is that whatever form of construction you use, the overall structure including insulation will have to come up to the same U-Value standard to comply with the building regs

SilverBER wrote: »

in this age band the U Value for a cavity block is the same as every other type of material including Timber Frame.

sinnerboy

The unit is wrong in Table B17 - but the numeric value is repeated in these software packages

here

http://www.xtratherm.com/software/software.php

and

http://www.builddesk.co.uk/our+software/builddesk+u

here

i.e - 0.21 m2/k/w

sinnerboy

http://www.josephlittlearchitects.com/documents/hollow%20block%20wall%20assessment_BLC_260609.pdf

0.238 m2/k/w