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I have recently moved into a property that needs refurbishment and insulation. It has late 60s - early 70s cavity walls.

I wanted to work out what the current U value is across the property and what EPS insulation in the cavity would do to improve it based on published thermal conductivity values. I have also added calculations for the loft space, using a base layer and top-up layer of insulation.

Walls           
Outer leaf value        Brick   0.90    W/m.K
Outer leaf thickness            102.50  mm
Outer leaf R value              0.11    m²K/W
Insulation value        EPS     0.03    W/m.K
Insulation thickness            100.00  mm
Insulation R value              3.23    m²K/W
Inner leaf value        Block   0.20    W/m.K
Inner leaf thickness            100.00  mm
Inner leaf R value              0.50    m²K/W
Total R value                   3.84    m²K/W
Total U value                   0.26    W/m².K
            
Loft            
Tiles value             Tile    0.56    W/m.K
Tiles thickness                 20.00   mm
Tiles R value                   0.04    m²K/W
Insulation value        Bottom  0.04    W/m.K
Insulation thickness            100.00  mm
Insulation R value              2.27    m²K/W
Insulation 2 value      Top up  0.04    W/m.K
Insulation 2 thickness          170.00  mm
Insulation 2 R value            3.86    m²K/W
Total R value                   6.17    m²K/W
Total U value                   0.16    W/m².K
            
Windows (stated in brochure)            
Total U value                   1.40    W/m².K
            
Sliding doors (stated in brochure)          
Total U value                   1.60    W/m².K

Am I roughly along the right lines for my calculations? There are some pretty big tolerances based on the type of brick and some confusion around how to get a total U value for a combination of layers. I'd love to nail these values so I can show measurable improvement in the property's heat retention credentials.

  • Having trouble picturing this... does your wall look like this? How will you replace the cavity insulation without having to completely rebuild one leaf? – LShaver Oct 29 '20 at 14:38
  • Hi - It does but without block insulation as pictured. We plan on blowing in EPS blown bead insulation which is done by drilling holes in the external leaf and using an air blower and 'ball injector' to fill the cavities – MrNorm Oct 29 '20 at 14:45
  • Got it! So there's no insulation in the cavity currently? – LShaver Oct 29 '20 at 14:51
  • Correct! There's nothing in place which according to my sheet would be 1.63W/m².K - I'm just not sure if that's 'pie in the sky' or if it looks correct – MrNorm Oct 29 '20 at 15:14
  • You're right, and also correct to notice that there are very wide tolerances. The only way around those is measurement, which can be as expensive as you like. The common pragmatic solution is to measure the house as a whole since that's what you care about - PassivHaus do that, for example. That can be as simple as waiting for a cool night and leaving a heater on inside while using a non-contact thermometer to measure the temperature difference. Easier to use an IR camera, which you can hire, but the approach is "thermodynamics for dummies"... insulation = heat transfer/temperature difference – Móż Oct 29 '20 at 22:38

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