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 👍