Calculating U values - Am I doing it correctly?

Question

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.

Answer 1

You are right about the complexity of the calculations, and also about the very wide tolerances. Normally those are done from CAD drawings but those contain a lot of assumptions (some more complex than you might expect - they will often give you values for "perfectly installed" vs "average install" where the latter comes from manufacturers surveying their products in use).

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 between inside and outside. That's what the final result will be anyway... insulation = heat transfer/temperature difference

In terms of finding faults or problems, it's easier to use an IR camera (which you can hire) rather than trying to measure hundreds or thousands of points on your building. That will often reveal defects that aren't just insulation, because if air is getting through sometimes so is water.

Note that in practice air movement normally obviates insulation, so the first step is to decide how much you want to seal the building. But since you're familiar with Passivhaus you're likely aware of all those issues and the technologies to make them work.