Holdfast Training Services Trial Blog

Although it has been a relatively sedate pace at BP it has certainly not been slow at home. Having moved into the new house 3 weeks ago I have already stripped 3 rooms back to plaster and floor boards ready for windows, plastering, electrics and plumbing.

Being a 1930s semi built with solid walls and suspended flooring, it doesn’t lend its self to being thermally efficient, so with my new found knowledge (thanks Mark) I aim to change that.

Approximate heat losses through building fabric

A quick lesson

For those not in the know, the U value, or thermal transmittance, represents the amount of heat lost (in watts) through a square metre of the considered fabric per degree of temperature difference between the inside and outside temperatures. So if the temperature difference is 20^OC, U value is 1.4 and the surface area covers 2m² then heat loss would be U x A x (Ti-To) = 1.4 x 2 x 20 = 56 watts.

When there is a combination of materials through which heat is lost, i.e. insulation, air gap and brick, then the overall U value can be determined by calculating the reciprocal of the sum of the thermal resistance, R values. In the case of 25mm insulation, 10mm air gap and 225mm brick the relative R values are 1.19, 0.15 and 0.3 m²K/W respectively and additional thermal resistance of 0.04 and 0.13m²K/W are also considered for the outside and inside surface resistances. Therefore giving a total U value of 1/(1.19+0.15+0.3+0.04+0.13) = 0.55W/m²K. Simples!

Windows

Current windows are approx. 15 years old and in some places, much older. Solution: Upgrade with new uPVC double glazing filled with argon with a U Value of 1.4 w/m²K. The value of 1.4 is below building regs part L1a value of 2.0 for new dwellings.

General window U values

Walls

Current walls are 225mm solid brick with a U value typically of between 2 and 3 W/m²K. Ouch. Solution: apply 25mm rigid polyisocyanurate (PIR) insulation to the inside of external walls to reduce overall U value to 0.55. This does not reach the part L1a value of 0.3W/m²K but in order to reach that value then an insulation thickness of approx. 60mm would be required and a compromise had to be made.

General wall U Values

Ground floor

Currently the flooring consists of 18mm floor boards and carpet giving a U value of approx. 2W/m²K. Solution: addition of 50mm PIR insulation will reduce the U value to 0.35W/m2K which is higher than 0.25 required in new buildings but a compromise had to be made with the increased cost and depth of floor joists.

Progress as at 6 Jun 15

(current state above. You will notice a whole host of cat6, coax and HDMI cables running under the floor)

So what?

Reducing the U value of windows from 2.8 to 1.4, reducing the walls from 2 to 0.6 and reducing the floor u value from 2 to 0.37 will ultimately reduce the heat loss by approx. 75% and therefore reduce gas bills. I forecast the total cost of these upgrades across the house will be approx. 16k with windows taking up 12k of that. Average current gas bill, say £100/month, therefore savings £900 a year, therefore a payback period of nearly 18 years!! But at least I have that warm (excuse the pun) fuzzy feeling of doing my bit for the environment.

Next step

Size emitters (radiators) to each room given the surface areas, U values above and internal and external temperatures of 21 and -1^OC respectively and size boiler given the emitter sizes and hot water demand calculated.

Final thought.

During a conversation with a sales man from Everest Windows (other window suppliers are available), he said that it is not worth getting triple glazing as the additional money you would save on the gas bill due to the higher U value would be countered by the need for additional lighting due to a reduced level of light because of the additional pane of glass.