Should I ask for more insulation?

[JP_]

The roof is a cold roof. There is ventilation above the insulation, with air bricks - ignore what I originally wrote about the type of flat roof.

 

[JP_]

The better question for you is, realistically how much will it save? What are you thinking of putting in there, what will it's insulative value be, how does it affect the U value of the roof and hence what overall effect wil it have. You might find that it will cost you an extra 300 quid, and save you £20 a year on your heating bill. That's a 15 year payback

I have been trying to find out if there is any rule / calculation for how U values affect bills. So far, not discovered anything.

Is there a way of calculating the difference between 0.25 and 0.17 in terms of heating costs for a room that has walls with 0.25, smallish window (about 1m square), with about 15m square floor area?

 

[cjard]

FWIW My daughter has had a very large dormer conversion done on her bungalow complete with £900 worth of Celotex installed (150mm?). It has led to the upstairs bedrooms getting too hot, because they have gone open plan including the stair case, so all the downstairs heat ends up in the upper bedrooms.
Frank

Sounds ideal for one of these http://www.esavep.com/online-store/ecocent-domestic-hot-water?route=product/category&path=41

 

[cjard]

I have been trying to find out if there is any rule / calculation for how U values affect bills. So far, not discovered anything.

Is there a way of calculating the difference between 0.25 and 0.17 in terms of heating costs for a room that has walls with 0.25, smallish window (about 1m square), with about 15m square floor area?

If the walls are 0.25 then they lose a quarter watt of energy for every square metre of wall, for every degree temp difference between the wall faces. Say outside its 1 and inside is 21, diff is 20.. 15sqm assume your room is 3x5 and 2m high to make the maths easy, the walls are two at 2x5 and two at 2x3 and the roof is 15 = 47sqm less the window, so we have 0.25 x 46 x 20 = 230 watts for the walls. Let's say the window is 2.0, loss is 2.0 x 1 x 20 so you're knocking on for 270watts

In other words a 270 watt electic fire running constantly will keep your room at 21 when its 5 outside. Cost of running a 0.27kilowatt fire per hour when a kilowatthour is 10pence is 2.7pence an hour. Doing it over for 0.17 is approx 196watts loss, so you're looking at ~2pence an hour

So in other words, going from .25 to .17 will save you a little over half a pence per hour per 20 degree difference. If it's -15 outside you save nearly 1.5p an hour. If your heater cost is different to a simple electric heater then you must factor that in too

 

[JP_]

... so not a lot then!

Looked at the 2006 building regs - that was 0.25 u value for a roof. So some the roof might only meet 2006 standards, but some is up to current standards - assuming I am not missing something!

It is a mild evening, but it is not cold up there at the moment (with a jumper on), and still no insulation in the dormer wall. Let's see what tomorrow brings.

 

[theprinceofdarkness]

There are about 8,000 Hrs/years, for 6 months you do not need heating, so that gives 4,000 Hrs where you do, 1 month at 0 degs ? = 666 Hrs at 20 degrees of heating and the remainder at 10 degrees of heating. This would give 666 X 20 + 3333 X 10 = 13200 + 33330 = 46530 degree/Hrs of heating. From Cjards figures .5p for 20 degrees would give a cost of 46530/20 X .5 p = 46530 /10 p or 4653p or 46.53£ per year.
Does this make it worth while?
Frank

 

[JP_]

£46.53 a year is a lot really! £465 in only 10 years.

 

[mfarrow]

46.53 per year assumes the alternative is a U=0, which it will not. At 0.17 you'll spend (.17/.25)*46.53 = £31.64.

 

[theprinceofdarkness]

So in other words, going from .25 to .17 will save you a little over half a pence per hour per 20 degree difference. If it's -15 outside you save nearly 1.5p an hour. If your heater cost is different to a simple electric heater then you must factor that in too

My figures are for the improvement from .25 to .17
Frank

 

[AronSearle]

Where is your ventillation, is there foil faced insulation above the OSB layer, in which case how do you think your ventillation layer is going to work?

And how do brick purpends ventilate the area above the OSB, does it have a parapit wall detail?

Is there insulation at the end above the wall plate, (between the joists like noggins), otherwise there is an oppertunity for cold bridging and possible condensation between the insulation layers.

It's all a bit ass backwards and odd from the details provided, might be fine, but sounds like an iffy hybrid wall (neither cold or warm).

costs of energy

You need to account for inflation as well.

Also energy prices will rise due to non inflationary causes, but the rate is not predictable.

 

[JP_]

Where is your ventillation, is there foil faced insulation above the OSB layer, in which case how do you think your ventillation layer is going to work?
And how do brick purpends ventilate the area above the OSB, does it have a parapit wall detail?
..................

I am not the builder, so honestly have no idea!

Have sent request for more insulation all round. Will have to pay for some materials and labour, hopefully other parts were already in the plans etc. Either way, getting more insulation.

Still not very wise about it all, but have learned a lot about U-values and building regs. By the sounds of things, it will all be ramped up soonish, what with this carbon neutral thing coming. 125mm might be half of what roofs end up with!

 

[AronSearle]

I don't like to be too alarmist, but tread carefully.

The information provided so far seems to suggest it is neither a cold or warm roof, so done inproperly there is a risk of condensation within the structure.

Time to ask questions.