Target U-values… Cost/benefit sense check… What am I missing? 🤷🏻‍♂️

[fatgus]

We’re in the process of getting quotes for our build. The u-values for the timber frame range from <0.1 to 0.15 and for the windows the average ranges from 0.73 to 1.1.

 

I’ve been playing around with Jeremy Harris’s heat loss spreadsheet and have created a whatif? table looking at the 20 year impact of wall & window u-values on likely heating cost (assuming the mean minimum OAT, so hopefully a worst case).

 

Of course, I’ve made some other assumptions (floor/roof u-values, MVHR efficiency, airtightness etc) and my calculations could be wrong(!) but it looks as though the difference between the ‘best’ option (0.1 walls, 0.73 windows) and the ‘worst’ (0.15 walls, 1.1 windows) is about £8k in total over 20 years. The cost difference between the two options is many times that, so the payback is probably >100 years 🤷🏻‍♂️

 

So I’m thinking we should forget about 0.1 for the walls, maybe aim for 0.13-0.14, and save another significant chunk by going for middle of the road windows (0.8ish).
 

I guess there are other considerations, but I’m not sure I see the point of aiming for super-low numbers… What am I missing?

 

[Oz07]

I realised the same the other week when I put 150 vs 200mm cavity insulation into AI. The payback periods are insane. 

[Andeh]

There is a massive law of diminishing returns! 

 

I figure U values of high teens is the point at which it becomes futile to pour more money into it, as ROI becomes many many decades.

 

The irony we find, the house is so well Insulated the floor feels cold during mild periods when the UFH doesn't come on as house hasn't dropped below 21 degrees.... So we end up artificially boosting the heating to cancel our the cool floor feeling. 

 

Cool floor and warm ambient = feel chilly.

 

We also have a huge amount of average glazing, so that probably doesn't help. 

 

All that, and we still spent several £grands increasing all the insulation to mid teen U values. 

[MikeSharp01]

True payback can be long although it seems that payback is not always a good reason not to do it because the 'value' dimension gets lost. So while you can know both most people know the cost of things but cannot articulate the value. I suspect that if you stand back far enough on the payback side only in £ terms you will be hard pressed to make self building pay back.

[JohnMo]

24 minutes ago, Andeh said:

Cool floor and warm ambient = feel chilly.

UFH in cooling mode (during summer) summed up in 7 words.

[JohnMo]

54 minutes ago, fatgus said:

windows the average ranges from 0.73 to 1.1.

What this says if you are comparing 3G windows with 3G is the frames on 1.1 are truly rubbish thermally. You may as well install good double glazed with Krypton gas and get same performance.

 

So you need to look a bit deeper it's not just about cost and just back its also about quality of materials being offered as well.

 

It also depends what you want out of the house. Actually building a house is utter nonsense based on payback period, just get a tent! You heat a tent for several life times compared to just the basic windows for a house let alone the whole house.

 

I improved stuff based on not having to fork out much for utility bills when I retire. I can afford stuff now, on a pension I may not. Lower heat loss also drives other costs down, simpler heating system, smaller heat pump, done correctly less noise, better house feel, air quality etc 

 

 

[fatgus]

20 minutes ago, MikeSharp01 said:

I suspect that if you stand back far enough on the payback side only in £ terms you will be hard pressed to make self building pay back.

Indeed. I’m fairly sure ours will cost a little more than its value, but it’s the only way for us to get the house we want in the area we want to live. You never know what’s around the corner, but we intend it to be our last home so what it gives us is more important that how much it’s worth. That being said, once we’re gone I imagine it will be sold by our children (quite rightly) and I have limited desire to invest my hard-earned for the benefit of the next inhabitants

 

32 minutes ago, MikeSharp01 said:

True payback can be long although it seems that payback is not always a good reason not to do it because the 'value' dimension gets lost. So while you can know both most people know the cost of things but cannot articulate the value.

That’s a good point… what are the things that you would strongly advocate, even if the strict cost analysis doesn’t perhaps appear to make sense?

 

 

 

 

 

[SteamyTea]

1 hour ago, fatgus said:

it looks as though the difference between the ‘best’ option (0.1 walls, 0.73 windows) and the ‘worst’ (0.15 walls, 1.1 windows) is about £8k in total over 20 years

Energy prices may vary, they have over the last 4 years, so you need to make some assumption there.

Also, are you comparing the marginal price increase, per MWh saved.  Then there is general inflation.  When I bought my first place in 1981, I was told that it would bankrupt me and it was a silly amount of money (£17k), millstone around my neck.  Not.

 

As many have found out on here, it is possible to do a good job, for a similar price, just don't accept the contractor's initial price.

[Crofter]

I think as you approach very low u values it's better to switch your attention to better airtightness detailing, because that's going to be where the bulk of your heat loss occurs. 

 

But it's not just about saving money. You're creating a comfortable house without damp, drafts, condensation, and mould. And it's extremely hard to improve insulation and airtightness later.

 

[fatgus]

36 minutes ago, JohnMo said:

What this says if you are comparing 3G windows with 3G is the frames on 1.1 are truly rubbish thermally. You may as well install good double glazed with Krypton gas and get same performance.

 

So you need to look a bit deeper it's not just about cost and just back its also about quality of materials being offered as well.

 

 

At the low end, it’s 3G aluclad timber (e.g. Internorm) and at the higher end it’s aluminium, which we’re most unlikely to go choose. I’d say the bang/buck sweet spot would be the likes of Zyle Fenster, Rawington or Nordan, all of which are a lot cheaper than Internorm but are still around 0.8.

 

 

36 minutes ago, JohnMo said:

I improved stuff based on not having to fork out much for utility bills when I retire. I can afford stuff now, on a pension I may not. Lower heat loss also drives other costs down, simpler heating system, smaller heat pump, done correctly less noise, better house feel, air quality etc 

 

That’s similar to our rationale… minimising ongoing running costs is important. This house gives us everything we want so we need to be confident that we’ll be able to afford to stay in it, come what may 😬

 

[fatgus]

17 minutes ago, SteamyTea said:

Energy prices may vary, they have over the last 4 years, so you need to make some assumption there.

Also, are you comparing the marginal price increase, per MWh saved.  Then there is general inflation.  When I bought my first place in 1981, I was told that it would bankrupt me and it was a silly amount of money (£17k), millstone around my neck.  Not.

Fair point. In our calcs, I’ve used our current standard rate for the electricity, but have ignored the fact that we’ll also have PV & batteries, so provided there continue to be off-peak rates for charging the batteries in the months with low generation, we should be paying very significantly less on average. Our current average cost per KWh is around half the rate that I’ve used in the calculations (I was aiming for ‘worst case’) 🤞

 

 

[fatgus]

23 minutes ago, Crofter said:

I think as you approach very low u values it's better to switch your attention to better airtightness detailing, because that's going to be where the bulk of your heat loss occurs. 

 

But it's not just about saving money. You're creating a comfortable house without damp, drafts, condensation, and mould. And it's extremely hard to improve insulation and airtightness later.

Absolutely 👍

[Russell griffiths]

I couldn’t give a monkeys what these companies say on paper I would want to see the build quality and airtightness results. 
the figures for my walls would not pass current building regs, but I know every square mm is insulated exactly the same with zero gaps and no cold bridging, then with an airtight score below passive standard, I’m sitting in a house that is so comfortable, the wife put a chicken in the oven this afternoon and I went and opened the patio door for 15mins. 

[Iceverge]

Good on you for doing the sums. 

They're the only place which hides the truth, forget payback times like @JohnMo says. Think thermal comfort. 

 

Once you get anywhere near Bregs airtightness and thermal bridging become massively more important than U values. Detailed excellently a 0.3 U value house would be very nice to live in. 

 

As for the windows why not PVC? 

 

https://sbwgroup.co.uk/products/veka-softline-82-md/

 

These are the same as ours (Different installer).They've been excellent. €16k inc VAT for 32m² of glazing in 2020.

 

Gealan, kommerling, rehau and Internorm all do high quality units.im sure there's more, just avoid the cheap crap. 

 

 

[Crofter]

Due to damage I've had to replace a couple of aluclad units, and have decided to go uPVC this time. I was able to improve from 0.98 to 0.74.

[fatgus]

17 hours ago, Iceverge said:

Good on you for doing the sums. 

They're the only place which hides the truth, forget payback times like @JohnMo says. Think thermal comfort. 

 

Once you get anywhere near Bregs airtightness and thermal bridging become massively more important than U values. Detailed excellently a 0.3 U value house would be very nice to live in. 

 

👍👍

 

 

17 hours ago, Iceverge said:

As for the windows why not PVC? 

 

https://sbwgroup.co.uk/products/veka-softline-82-md/

 

These are the same as ours (Different installer).They've been excellent. €16k inc VAT for 32m² of glazing in 2020.

 

Gealan, kommerling, rehau and Internorm all do high quality units.im sure there's more, just avoid the cheap crap. 

 

 

 

8 hours ago, Crofter said:

Due to damage I've had to replace a couple of aluclad units, and have decided to go uPVC this time. I was able to improve from 0.98 to 0.74.

 

We've looked at the Internorm aluclad PVC for the wet rooms. It's a saving for sure and the Uw values are still great.

 

Will take a look at that Veka option... thanks

 

A while ago we looked at the Aluplast's Neo range, but at that time it wasn't commercially available. Might be worth another look. Also like the Rehau Artevo.

 

The only possible hurdle would be planning... we're in a national park and the approved drawings state 'aluminium'. I think the Neo has an aluminium clad option... not sure about Rehau or Veka. More fun investigation!

 

 

 

 

 

 

[fatgus]

19 hours ago, Russell griffiths said:

I couldn’t give a monkeys what these companies say on paper I would want to see the build quality and airtightness results. 
the figures for my walls would not pass current building regs, but I know every square mm is insulated exactly the same with zero gaps and no cold bridging, then with an airtight score below passive standard, I’m sitting in a house that is so comfortable, the wife put a chicken in the oven this afternoon and I went and opened the patio door for 15mins. 

 

I recently visited a lovely home being built by one of the TF companies we've spoken with... the homeowner was very complimentary about the TF firm and to be fair the quality of the work looked great, but he was bemused by my questions about airtightness. I'm not sure it had even been considered. As we dig into this more, I'm thinking that a wall of 1.3-1.4 will be more than good enough provided we nail the airtightness.

 

 

[Tony L]

I had a look at the Veka website, earlier today, following @Iceverge's tip, above.  They offer some colours in what's called "Feinstruktur" finish, which they say looks like aluminium.

 

19 minutes ago, fatgus said:

approved drawings state 'aluminium'. I think the Neo has an aluminium clad option... not sure about Rehau or Veka.

[fatgus]

1 hour ago, Tony L said:

I had a look at the Veka website, earlier today, following @Iceverge's tip, above.  They offer some colours in what's called "Feinstruktur" finish, which they say looks like aluminium.

 

Excellent… thanks @Tony L 👍👍

 

 

[Iceverge]

4 hours ago, fatgus said:

. we're in a national park and the approved drawings state 'aluminium'

 

What they really mean is we don't want to have a house that looks like this,(because there may be poor people inside). What a bunch of snobs they are. 

 

 

White uPvc looks just fine if appropriate care is taken of the proportions of the windows. 

 

 

It's amazingly durable if you get good stuff. I don't know what the aluminum adds really. 

 

That's ours below. White T+T Veka Softline 82. 

 

 

 

White uPvc fascia and soffit and black guttering and down pipes whilst we're at it. 

 

 

[Gus Potter]

On 15/02/2026 at 10:29, fatgus said:

We’re in the process of getting quotes for our build. The u-values for the timber frame range from <0.1 to 0.15 and for the windows the average ranges from 0.73 to 1.1.

Ok lets look at some hard economics first, I'll assume this is your forever home. 

 

Ask your glazing manufacturer what their guarantee period is. Now in the past the adverts on tv said 10 years.. but now often in the fine print it's 10 for the frames and 7 - 8 for the glass. So you truck on with all your calcs.. but it could be that in 20 years time or a lot less your glazing is kaputt! So maybe it best to take the view that the least risk / long term value lies in insulating the things that are less likely to go wrong.. like the walls and roof. But what if you high end glass fails in year 9 .. it will be heavy.. lots of thermal stress and so on? 

 

You mention that you have some glazing at 0.73 W/m^2K. That is a very big ask.. is this a value for the glass or the window . doors as a whole? If so they are going to be very expensive. But you will be wasting money if you don't insulate the reveals properly. Someone is going to have to detail that and a builder is going to have to follow what is likely to be a detailed drawing. In doing stuff like this you'll also encounter what I call the half day problem. 

 

This is where you have to sequence the works to comply with a demanding detail. The operatives may only be required on site for a couple of hours and then have to get back in the van and go to another job.. all that needs to be paid for by you.

On 15/02/2026 at 10:29, fatgus said:

significant chunk by going for middle of the road windows (0.8ish)

Start with conceptually, windows an doors with an overall u value of 1.4 if in England and 1.2 if in Scotland. This will then allow a wider variety of suppliers. You are trying to refine the design too much at this stage.. you are leaving yourself no redundancy and this will come back to bite you in the arse cost wise.