What U value in walls is it worth paying for and having? My calcs must be wrong! Please help.

[Norbert]

I have a number of quotes from TF manufacturers. Taking just 2 of these I can have a U 1.4 wall build-up, or for an additional £6k U 1.1. I am trying to work out if it is worth paying the extra?

Please check my assumptions and calcs. My answer must be wrong

 

Ignoring all other sources of heat loss, floor, roof, windows and doors, MVHR ventilation, air tightness etc. and homing in on heat loss through the walls my calculation is based on the following.

 

Average temp difference internal to external over a year = 10C

Area of walls = 220 sq/mtrs

 

Energy loss = wall area x temp difference x U, so :

 

U 0.11 = 242 W

U 0.14 = 308 W

 

Over a year it would be the above x 24 x 365 to give annual heat loss through the walls in kWh

 

U 0.11 = 2120 kWh

U 0.14 = 2698 kWh

 

If my heat pump had a SCOP of say 3.5, then electricity used to generate this heat would be :

 

U 0.11 = 606 kWh

U 0.14 = 771 kWh

 

If the cost of electricity was £0.30 per kWh then annual cost of energy lost through the walls would be :

 

U 0.11 = £182

U 0.14 = £231

 

Annual difference £50. Pay back period 6000/50 = 120 years.

 

As I say, that must be wrong. Where is my error?

[JohnMo]

10 minutes ago, Norbert said:

Average temp difference internal to external over a year = 10C

You need to use the heating season average not the whole year. So will be nearer 13 to 14 I would think.

 

12 minutes ago, Norbert said:

Over a year it would be the above x 24 x 365

That again would be just for the heating season say 180 days

 

13 minutes ago, Norbert said:

cost of electricity was £0.30 per kWh

If a heat pump, your CoP of 3 to 4, so nearer 10p.

 

I assume U values are  0.14 or 0.11?

 

About £40 per year difference based on a heat pump.

[ProDave]

0.14 is quite respectable, it's what our house is.

 

Another decision is what insulation material are they using to achieve that.  Something like PIR or PUR (solid foam) has a short decrement delay.  Things like glass wool and celulose and even wood fibre have a much longer decrement delay.  So you can have 2 different walls with the same U value, but one will  cool down quicker than the other.

 

So a more important decision is select the TF manufacturer on what type of insulation they are proposing.

[PNAmble]

We did some thermal modelling on three  options. 
 

 

And 

 

 

Once you hit a level of u values airtightness is more important.  
 

 

 

[SteamyTea]

What is the fraction of wall area to over all area.

Would it cost more or less to improve windows and doors, extra ground insulation.

Adding PV could be a cheap option to reduce ongoing cash expenditure.

 

Get the airtightness better, which isn't cheap to do, but saves loads.

[Alan Ambrose]

>>> Ignoring all other sources of heat loss, floor, roof, windows and doors, MVHR ventilation, air tightness etc.

 

Is that the problem? Heat loss through the walls is probably 20% of the total for a well insulated house. Windows are critical.

 

You could use Mr Harris’s spreadsheet for a more accurate feel. And ubakus for to check the supplier’s U-value calcs.

[Norbert]

So it seems like that while I had some gross assumptions as a basis for my calculations, they were correct. This begs the question why would anyone go for an additional expenditure that had a 120 year payback? I was thinking that I must be an order of magnitude out somehow. 12 year payback fine, 120 year pay back, well that seems barking mad. I intend to have a largish PV area on the roof, plus battery storage, so some of my heat pump input will be 'free', which will further push out the payback period.

 

I have already completed Mr Harris’s spreadsheet for my design and used it to try various U values. This showed me that window and door loss, even with triple glazing, was similar to wall loss, both at ca. 30% of the total. Changing the U value from 1.4 to 1.1 did not make a huge difference. That is why I homed in on wall loss calcs to try to understand things better.

 

The spreadsheet does not give the option to put in air tightness values, but based on comments above I understand that this becomes more important as other factors are improved.

 

At the moment I am thinking that the sweet spot for me is U 1.4/1.5 for walls and roof, triple glazing U 0.79, good airtightness, PV and battery, quality MVHR, smallish ASHP to UFH. Seem reasonable?

Edited January 21 by Norbert

[JohnMo]

2 minutes ago, Norbert said:

This begs the question why would anyone go for an additional

Also have to concider location and building form factor, also some people live in colder locations, so payback is different. 

 

If you are doing UFH get yourself down to 0.1, to mitigate downwards heat loss. And the roof is generally quite cheap to get down to 0.1 also.

 

All things house related are not about pay back period. If you built all houses based on payback we would all live in tents.

 

7 minutes ago, Norbert said:

sweet spot for me is U 1.4/1.5 for walls and roof

You mean 0.14/0.15

[Norbert]

6 minutes ago, JohnMo said:

If you are doing UFH get yourself down to 0.1, to mitigate downwards heat loss.

 

Yes I can see that floor insulation is more important with UFH.

 

You mean 0.14/0.15 -

 

Yes, of course, thanks

 

Edited January 21 by Norbert

[ProDave]

20 minutes ago, Norbert said:

This begs the question why would anyone go for an additional expenditure that had a 120 year payback?

If payback was the only thing, I would not have spent £200K building a really nice house, I would have stayed in the static caravan and the £200K would have paid the very high heating bills of the static caravan for the rest of my life.

[SimonD]

1 hour ago, JohnMo said:

If you built all houses based on payback we would all live in tents.

 

57 minutes ago, ProDave said:

If payback was the only thing, I would not have spent £200K building a really nice house, I would have stayed in the static caravan and the £200K would have paid the very high heating bills of the static caravan for the rest of my life.

 

It's interesting that payback period is one of those things so often raised as a problem when it's down to spending on the fabric of the building or the heating system, but never gets a look in when it comes to a bathroom, kitchen or some other internal home improvement.

 

Yet from a daily living perspective it's worth so much more. Everyone who visits our house for the first time, especially in the winter remarks on how comfortable and warm it is - which I explain is because it is well insulated and doesn't have any draughts. Is that worth the just the extra £10k I decided to spend on insulation? Absolutely.

[Norbert]

Let me explain why it seems right to be focusing on payback period right now wrt wall U values. I have a choice to make .14 or for £6k more .11. Some of the remarks above, while obviously correct, do not help me decide.

Let's just say I decide to go ahead with U .11, while my identical brother decides to build an identical house next door, but with just one exception, he uses .14.

 

Some time later both houses are finished. ASHP, UFH, MVHR, air tightness, windows, all other insulation, .... all identical. We are both very pleased with our comfortable houses.

 

How can we, or anyone else tell the difference?

 

It seems to me that the only difference is that I am spending ca. £50 per annum less on electricity, and he paid £6K less on his build. Thus it seems perfectly reasonable to focus on payback in this instance as there is no other difference. Or am I missing something?

[ProDave]

You are probably at the point of diminishing returns.  As I have said, 0.14 is quite respectable.  I would still be wanting to know more about what type of insulation they were proposing for each option?  What details have they given about the wall make up?

[SteamyTea]

19 minutes ago, Norbert said:

£50 per annum less on electricity

What unit rate is the electricity at?

 

6 minutes ago, ProDave said:

You are probably at the point of diminishing returns

Insulation, for any given k value, is by the nature of arithmetic, a diminishing returns calculation.

 

Why not just add some extra insulation to the north facing walls. They will always have a lower mean temperature, so will benefit the most.

Edited January 21 by SteamyTea

[Kelvin]

What are all the other elements of the house specification so air tightness target, type of windows, etc. When you get to this level of insulation air tightness becomes the thing to focus on if you are going for a very airtight house (and why wouldn’t you be) then ventilation becomes a crucial aspect. 

Edited January 21 by Kelvin

[Norbert]

4 minutes ago, ProDave said:

You are probably at the point of diminishing returns.  As I have said, 0.14 is quite respectable.  I would still be wanting to know more about what type of insulation they were proposing for each option?  What details have they given about the wall make up?

Various, blown cellulose in the higher 'Passivhouse' specs, PIR and mineral wool in the others. The difference between the .11 and the .14 in my example is 140mm of mineral wool vs PIR.

I agree about the diminishing returns.

[Kelvin]

4 minutes ago, SteamyTea said:

 

Why not just add some extra insulation to the north facing walls. They will always have a lower mean temperature, so will benefit the most.

Given that windows are typically something like 5 times worse than walls, thermal bridges, hard to make air tight etc we removed two windows from our North facing wall and made the others smaller. We only have three windows and a door on this elevation. 

[Kelvin]

5 minutes ago, Norbert said:

Various, blown cellulose in the higher 'Passivhouse' specs, PIR and mineral wool in the others. The difference between the .11 and the .14 in my example is 140mm of mineral wool vs PIR.

I agree about the diminishing returns.

You also need to consider decrement delay as already mentioned. Therefore it’s not just about U value and payback it’s about comfort both in the winter and summer. Building physics isn’t a one dimensional thing with everything acting in isolation.  Therefore using your example twin brother if he builds using blown cellulose and you build using PIR his house will be more comfortable to live all year round. 

Edited January 21 by Kelvin

[Alan Ambrose]

My view is that these builds are a huge amount of time and energy and that we might just as well do as good a job as we can of them. Hopefully they’ll be around in 100 years time.

[Kelvin]

21 minutes ago, Alan Ambrose said:

My view is that these builds are a huge amount of time and energy and that we might just as well do as good a job as we can of them. Hopefully they’ll be around in 100 years time.

Sure but budgets aren’t unlimited and there is definitely a line to drawn on the diminishing returns graph. However I do think the op is looking at the problem too one dimensionally. There’s a lot going on in a wall/ceiling/floor  build up beyond U value. 

[BadgerBadger]

There are dozens of things you could do to improve the efficiency of your build and, as you're finding, at some point you reach the point where you have to say "I'm just not going to do that".  The key is to make sure you're doing the things that are most effective and being consistent with yourself, there's no point spending thousands extra on one aspect early on in the build and opting out of something else later on that was actually much more cost effective.

 

I ended calculating a "pounds per W/K improvement" measure that I used to decide to make sure I was being consistent with myself across the project.  If I was debating adding a new efficiency improvement (e.g. increasing insulation, introducing marmox thermoblocks on our masonry build, thermally broken lintels etc.)  I would assess the impact of the money I was spending on the improvement to see if it was worthwhile.

 

So in your example:

A U-value of 0.14 over a 220sq m wall area, gives the heat loss through the wall as 30.8 W/K.  A U-value of 0.11, gives you 24.2 W/K.

So £6000 gives you 6.6 W/K improvement, meaning just over £900 per W/K.

 

For info, my cut-off was somewhere around £350 per W/K (based on raw materials as we DIY'd a lot, I'm aware your figure is fully finished)

[Norbert]

12 minutes ago, Kelvin said:

However I do think the op is looking at the problem too one dimensionally. There’s a lot going on in a wall/ceiling/floor  build up beyond U value. 

As the op I do not think that I am. I started of filling in the spreadsheet with all factors and trying to assess the results, which were somewhat different to what I expected. I then tried to dig deeper and got confused with trying to juggle to many balls. That is when I switched to just looking a wall U values so that I could make sure my understanding was correct. I have now gone back to considering the whole picture, but with confidence that I have a better understanding of the basics and options available.

[Kelvin]

You are based on the example you gave of two houses with differing U values using two entirely different insulation materials. PIR is very different from blown cellulose or a mineral fibre in the overall performance of the building but your only measure appears to be payback based on U value. Both buildings will perform very differently over the whole year especially in the summer. You might find the PIR building overheats more compared to the blown cellulose building and you need to spend £6000 fitting measures to deal with it. 
 

 

Edited January 21 by Kelvin

[ProDave]

4 hours ago, Norbert said:

 The difference between the .11 and the .14 in my example is 140mm of mineral wool vs PIR.

I agree about the diminishing returns.

So are you saying the 140mm mineral wool gives the 0.14U and the PIR, presumably also 140mm gives the 0.11U

 

You would expect with the same thickness, mineral wool will give the lower U value, BUT will have a longer decrement delay and would be the one I would choose.

[Kelvin]

Can you show us the exact wall build up?