# Loft Insulation

Page 4 of 10
• posted on January 22, 2004, 6:11 pm

I didn't say it wasn't unless the space lost matters. That can be worth a lot more than £10 a year.
My point was that there are much larger losses of heat.
Take the same house of 7m square and 5m high. Take out 10% for windows.
The total wall area is 125 sq m.
Under the worst case conditions of temperature outside of -3 and inside of 21 degrees then the numbers work out as follows:-
Solid walls U=2.0 ============== Heat loss = 125 x 24 x 2 = 6kW
Cavity walls with no insulation U=1.5 ============================= Heat loss = 125 x 24 x 1.5 = 4.5kW
Cavity walls with insulation U=0.55 ============================ Heat loss = 125 x 24 x 0.55 = 1.65kW
Under the typical 10 degree average outside temperature the figures become 3.25kW, 2.44kW and 894W respectively
So looking at cavity insulation or not in the cavity wall, there is an energy saving of 1546W.
At a gas unit price of 1.4p and assuming 24*365 heating at 10 degrees outside (which was the basis of the £10-12 figure) then this comes to a saving of £190 per annum.
Typical costs for cavity insulating a house are about £1000 so the payback is in 5 years rather than the the 10-20 years of incremental loft insulation.
The difference is obvious and significant.
.andy
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• posted on January 22, 2004, 10:12 pm

NO! Take a house of 7m square and 5m high and split it in two and calulate it separtely. Then adding a lot of insulation in the loft makes one half of the house, the upper floors, very cheap to run and very comfirtable in winter and summer.
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• posted on January 22, 2004, 10:27 pm

I did this assuming different upstairs temperatures. It doesn't alter the point in any significant way.
In fact if the upstairs is cooler as is typically recommended, then there is even less point in your argument because the heat loss is less anyway.

.andy
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• posted on January 23, 2004, 12:02 am

done.
calulate
of
It does!!!!!!!!!! The upstairs then is much more well insulated, warmer and consumes les fuel to heat. The upstairs room benefit greatly by heavy insulation in the loft.

You have strange logic.
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• posted on January 23, 2004, 12:33 am

Do I need to do the sums again for you to demonstrate the point?

I suppose logic must seem strange to you.......

.andy
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• posted on January 23, 2004, 12:44 am

and
Well do them by having a before and after of the ground floor and the 1st floor. Then see how much fuel is saved in each. I bet the 1st floor saves more fuel. Then there is superior cooling of the 1st floor and higher comfort conditions.
You have strange logic.
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• posted on January 23, 2004, 12:53 am

It doesn't use it in the first place if the temperature is lower.
For the ground floor, the heat calculation is generally what passes to the first floor and may be taken off of the local heating requirement of the first floor.
.andy
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• posted on January 23, 2004, 11:00 am

warmer
heavy
1st
Here's my house as an exercise:-
Foot print ~ 70m2 Downstairs wall height ~ 2.1m Upstairs wall height ~ 2.2m Wall construction:- downstairs 27 m solid wall construction U-val ~ 2.0 + 8 m timber clad cavity wall construction U-val ~ 0.6 upstairs 27 m tile hung lath/plaster construction U-val ~ 2.0 + 8 m timber clad cavity wall construction U-val ~ 0.6 Ground floor :- thick concrete + screed + covering U-val ~ 0.3 Ceiling Floorbaord/Joist/Plaster U-val ~ 1.6 Loft currently insulated with ~ 100mm mineral wool U-val ~ 0.5
Windows:- Downstairs 2.2 m2 single glazed timber frames U-val ~ 4.8 5.9 m2 sealed double glazed units in plastic and metal U-val ~ 3.8 Upstairs 2.2 m2 single glazed timber frames U-val ~ 4.8 5.9 m2 sealed double glazed units in plastic and metal U-val ~ 3.8
Doors:- 6 m2 mixed construction U-val ~ 3.0
Air changes :- 1.5 per hour
Design parameters ================Outside temp -3 C
Downstairs temp 21 C (delta-T = 24) Upstairs temp 18 C (delta-T = 21)
Heat loss - downstairs =====================Floor = 70 x 0.3 x 24 = 504 W Walls = (27*2.1 - 5.9 - 3) * 2.0 * 24 ~ 2295 W +( 8*2.1 - 2.2 - 3) * 0.6 * 24 = 167 W Walls Total = 2462 W
Windows = 2.2 * 4.8 * 24 = 254 W +5.9 * 3.8 * 24 = 538 W Windows Total = 792 W
Doors = 6 * 3 * 24 = 432 W Ceiling = 70 * 1.6 * 3 = 336 W Air changes = 70 * 2.1 * 0.36 * 1.5 * 24 = 1896 W
Downstairs losses = 6422 W
Heat loss - upstairs ===================Floor = 70 x 1.6 x -3 = 336 W Walls = (27*2.2 - 5.9) * 2.0 * 21 ~ 2247 W +( 8*2.2 - 2.2) * 0.6 * 21 = 194 W Walls Total = 2441 W
Windows = 2.2 * 4.8 * 21 = 222 W +5.9 * 3.8 * 21 = 471 W Windows Total = 693 W
Ceiling = 70 * 0.5 * 21 = 735 W Air changes = 70 * 2.2 * 0.36 * 1.5 * 21 = 1746 W
Upstairs losses = 5951 W
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%
Then see how much fuel is saved in each. I bet the 1st floor saves

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• posted on January 23, 2004, 11:15 am

Increase the loft insulation to 350-400mm and do the calcs again. Also what about the hallway which is also upstairs and downstairs. Look specifically at the upper rooms and see the difference. Also there is the matters of keeping the upper rooms cool in summer too. 400mm does that wonderfully. It is a win, win situation.
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• posted on January 23, 2004, 11:41 am
wrote in message [snip]

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?
Also what

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• posted on January 23, 2004, 11:56 am
On Fri, 23 Jan 2004 11:41:24 -0000, "Neil Jones"

U value of mineral wool of 100mm is 0.36 according to the British Standard, Neil, so I think you are probably better off than you think with respect to the roof, and the difference is not as substantial as this suggests. I reckoned that this would save about 2.5%.

.andy
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• posted on January 23, 2004, 12:16 pm
wrote in message

So worth it.
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• posted on January 23, 2004, 12:55 pm

ROTFL.
.andy
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• posted on January 23, 2004, 2:00 pm
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You did? Do you laugh at the Vicar of Dibley too?
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• posted on January 23, 2004, 6:35 pm

No, just at your notion that 2.5% is meaningful when compared to a context of over 30% for the solid walls.

.andy
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• posted on January 23, 2004, 11:57 am
wrote in message

Must be regarded as a ground floor room.
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• posted on January 23, 2004, 12:05 pm
wrote in message

to
What was the answer to this?

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• posted on January 23, 2004, 12:47 pm
wrote in message

again.
cost
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.

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• posted on January 23, 2004, 12:52 pm

better
Whoops! These are all in Watts saved / £ spent, not the other way around.
Neil

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• posted on January 23, 2004, 1:05 pm
wrote in message

You are looking at the house as a whole again. Duh! The extra insulation benefits greatly the upper rooms.
The loft insulation is the depth over and above the 100mm. You have to take into account the fixing of the downstairs wall Kingspan. Loft insulation is cheap and easy, so do it. Also do downstairs too with Kingspan, but the installation cost is greater.
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