Of course I'm looking at the whole house. I pay the whole gas bill,
which covers the heating for the whole house. If I only paid for my
study, and my wife paid for the rest, then I'd turn the radiator off in
that room and buy a few jumpers.
That's right - the extra 300mm required to take it up to the 400mm you
requested. Well spotted.
You have to
I did, actually, because the costs I have been quoted for the Kingspan
alone are less than I mentioned, but I allowed extra for battening etc.
As I have shown, the loft insulation is the same cost and has a much
smaller benefit, so it is lower priority.
Also do downstairs too with
Do you think I am made of money? If my budget is (lets say) £400, it's
either/or, not both. Insulating the downstairs wins hands down because
it has a much bigger effect for the same outlay.
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
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.
To email, substitute .nospam with .gl
The U value for 100mm of mineral fibre in a pitched roof construction
is 0.36 according to BS 5449.
So in fact Neil's heat loss via this route is less than suggested and
70 x 0.36 x 21 = 529W.
This is about 4% of his total 12.3kW
Increasing the insulation to your suggested level might achieve a U
value of 0.15 taking the heatloss down to 220W.
This represents a difference of 2.4%
Not very interesting in the context of 20% going out through the solid
The only effect that can happen there is to assume that the hallway
downstairs is at downstairs temperature and that the landing above
will achieve the same temperature via convection.
In most houses the upstairs landing might be 15-20% of the total
upstairs area, so in essence the landing becomes 3 degrees warmer than
the bedrooms. the impact is demonstrably marginal. Another corner
They look remarkably similar, even accounting for convection from
If you think that that is a win, win then you are missing out on much
Somebody once related the following tale to me, which seems apropos.
There were too bulls standing on the top of a hill and below them a
field of cows.
One was always enthusiastic about chasing the latest idea, so he said
"Cor. Look at that lot. Let's run down and f*ck one of them"
The other one said
"Let's walk down and f*ck all of them"
In fact I deliberately increased the U-value of the loft because I don't
know exactly how much insulation there is throughout the whole thing -
part of it is inaccessible at the moment - and some of it appears to be
somewhat low quality.
These are quite typical numbers, Neil.
I guess that this is an older house with a new extension?
It's interesting to note that the losses are close to being the same
upstairs and downstairs.
To email, substitute .nospam with .gl
Just to add my 2p. I insulated my loft to 150mm. I had lots of
insulation over so doubled up over the main bedroom where I sleep.
This bedroom is warmer in winter, and thankfully last summer a lot
cooler. I could always sleep while neighbours complained they were too
hot to sleep in similar houses to mine. When I have time I will do
the whole loft to at least 300mm. If I can get a decent deal maybe
thicker. To me it will be worth it. I don't know about economics as I
haven't kept a watch on the gas bills. To me that is not the real
issue during a hot summer.
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