"Neil Jones" wrote in message
...

"IMM" wrote in message
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"Neil Jones" wrote in message
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"IMM" wrote in message
...

"Neil Jones" wrote in message
[snip]

Total losses for house = 12.3kW

Downstairs losses are 51.9% of total losses from house.

Losses through:-

Ground floor 4%
Downstairs solid wall 19%
Downstairs cavity wall 1%
Downstairs windows 6%
Ceiling to upstairs 3%

Roof 6%
Upstairs solid wall 18%
Upstairs cavity wall 2%
Upstairs windows 6%

Increase the loft insulation to 350-400mm and do the calcs
again.

OK.
Thermal conductivity of mineral wool = 0.038

1/0.5 + (0.3/0.038) = 2 + 7.9 = 9.9.

U val = 1/9.9 ~ 0.10

Losses through ceiling reduced to 70 * 0.10 * 21 = 147W

Saving 588W, 5% of the total loss previously. How much does it
cost
to
buy 70 m2 loft insulation, at a depth of 300mm?


What was the answer to this?



I'll answer my own question. About £300, as far as I can tell.

For the same price I can get enough boards of 25mm Kingspan to dryline
the solid walls downstairs, which would reduce the U-value to 0.6.

So the heat loss here would become

Walls = (27*2.1 - 5.9 - 3) * 0.6 * 24 = 688 W
+( 8*2.1 - 2.2 - 3) * 0.6 * 24 = 167 W
Walls Total = 855 W
Previous Walls Total = 2462 W

Saving = 1607 W

So I can either spend £300 to save 588 W, (£1.96/W) or I can spend the
same and save 1607 W. (£5.35/W)

If I go for 50mm Kingspan, the price goes up to £400, but the saving
goes to 1870 W. (£2.63/W for the extra £100, which still gives a better
return than more loft insulation).

Alternatively, I could spend another £300 on 25mm Kingspan for the
upstairs and get another 1573 W saving, at £5.24/W.

[If Andy is right, and I'm only saving 392 W with the extra loft
insulation, it's £1.31/W.]

Extra loft insulation comes well down the priority list for my house,
because the cost/Watt saved is easily beaten elsewhere.