On Fri, 23 Jan 2004 12:05:18 -0000, "Neil Jones"
wrote:


"IMM" wrote in message
...

"Neil Jones" wrote in message
...

"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?



Supawrap from BuilderCenter is £12.66 exc. for 4sqm and 150mm thick.

So for 70sqm coverage you would need 35 rolls to achieve a thickness
of 300mm at a total cost of £520. This appears to be the cheapest
they have on a volume basis. You could probably get 20% discount,
so say £400.

On your assumed U value of 0.5, which is worse than it probably is,
you are saving 588W worst case.

If the temperatures were worst case all year all day then at 1.4p a
unit for gas you would save £72 per annum.

If you work on the 10 degree average which is closer to reality, then
the loss is going to be 8/21 x the figure - i.e. 220W and £27 per
annum.

In reality, the insulation you have is probably better than 0.5 and
the saving perhaps £20 per year.

A 20 year payback or 10 years if energy costs double today....

Doesn't seem interesting to me.

I'd think about using even a thin Celotex layer on a few outside walls
during redecoration first, if that's practicable. This is about
£5/sqm for 50mm, 25mm is about £3.50.





..andy

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