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

[Tony L]

1 hour ago, Iceverge said:

 

From my understanding the applied fools don't last as long. Ligter colours suffer less thermal exoasuand contraction too. 

 

This must be why Veka's T&Cs say, "In the absence of any express warranty to be specified by the Seller in respect of Goods which are not “white PVC -U profile” no warranty shall be given in respect of the Goods.".

 

If you want a warranty for a colour, you have to ask them for one, & I don't suppose it's going to be 10 years, which is what they're offering for the white PVC-U.

[Crofter]

On 16/02/2026 at 17:18, fatgus said:

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. 

We've just taken delivery of a Neo sliding door. It was absolutely unbeatable for u value (0.74 uW) and was considerably cheaper than any aluclad iron. 

Haven't installed it yet but the quality looks good. Mine are dark grey on the outside, and as far as I can tell this is a solid colour rather than a foil wrap. 

I also took the option of having the glazing units delivered separately, which has made handling much easier. It will also allow me to install the frames by screwing straight through the frame. The glazed units were delivered in robust transport frames, unlike the cheaper units I bought from Modern UPVC Glazing.

 

All in all, I'm quite impressed so far.

[fatgus]

9 hours ago, Crofter said:

We've just taken delivery of a Neo sliding door. It was absolutely unbeatable for u value (0.74 uW) and was considerably cheaper than any aluclad iron. 

Haven't installed it yet but the quality looks good. Mine are dark grey on the outside, and as far as I can tell this is a solid colour rather than a foil wrap. 

I also took the option of having the glazing units delivered separately, which has made handling much easier. It will also allow me to install the frames by screwing straight through the frame. The glazed units were delivered in robust transport frames, unlike the cheaper units I bought from Modern UPVC Glazing.

 

All in all, I'm quite impressed so far.

 

Thanks @Crofter

 

That sounds very promising... I'm going to see whether I can visit their showroom this week 👍👍

[TerryE]

@fatgus I agree with your overall assertion that your design choices should be informed by through-life cost benefit trade-offs.  You need to granularise to make doing this practice e.g. Wall U-value 2 vs 1.5 vs 1.2, etc. Also recognise that as-built might well vary from the design calcs due to mistakes in the design or construction quality issues: missing or sloppily fitted insulation, major leaks or missed thermal bridges. For example, our house works pretty much as design except that our slab losses are maybe 30-40% higher as build from a detail that caused edge bridging.   So your heating / cooling system needs to have enough margin to cope with likely variations.

 

Also remember that your heating / cooling system must be capable of adding / removing heat as needed.  For example, we have a 3 storey house with only ground-floor in-slab UFH: no wall mounted rads and pipework anywhere; no heating on the top 2 floors. That is huge initial and ongoing cost-avoidance.  See my blog for details.

Edited yesterday at 10:12 by TerryE

[fatgus]

2 hours ago, TerryE said:

@fatgus I agree with your overall assertion that your design choices should be informed by through-life cost benefit trade-offs.  You need to granularise to make doing this practice e.g. Wall U-value 2 vs 1.5 vs 1.2, etc. Also recognise that as-built might well vary from the design calcs due to mistakes in the design or construction quality issues: missing or sloppily fitted insulation, major leaks or missed thermal bridges. For example, our house works pretty much as design except that our slab losses are maybe 30-40% higher as build from a detail that caused edge bridging.   So your heating / cooling system needs to have enough margin to cope with likely variations.

At the moment, our Whatif? table goes from 0.1 to 0.2 for walls and 0.73 to 1.2 for windows. Even the worst case (0.2 wall/1.2 windows) is only an extra £3800 over 20 years compared with the best case (0.1/0.73). Your point about “as-built” vs “design” is really important. Based on the calcs, if we go for an “as-designed” 0.13-0.14 timber frame and in reality it only performs as well as a 0.2 frame, that should still be fine 🤞😄

 

2 hours ago, TerryE said:

 

Also remember that your heating / cooling system must be capable of adding / removing heat as needed.  For example, we have a 3 storey house with only ground-floor in-slab UFH: no wall mounted rads and pipework anywhere; no heating on the top 2 floors. That is huge initial and ongoing cost-avoidance.  See my blog for details.

I’ll take a look at your blog for sure  Our intention is for UFH on ground floor only, with electric towel rails and possibly electric under-tile heating in the 1st floor bath/shower rooms. We’ll possibly install wiring to locations for bedroom electric rads “just in case”, only because it’ll be easy and cheap to do now.

 

 

[Mike]

On 15/02/2026 at 11:29, fatgus said:

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?

You may have already discounted it, but if not then you may want to consider a PassifHaus design (there are several threads on the topic).

 

The underlying concept is that if you improve the thermal efficiency to a high level, then you no longer need a conventional heating system - and that former is paid for by the saving on the latter. That trade-off may not fully work in the UK, where the prices of certain components is higher (than in Germany & Europe more generally), but factoring in very low future heating bills (negative if you add PV) may compensate for that in your evaluation.

[Bramco]

14 minutes ago, fatgus said:

with electric towel rails and possibly electric under-tile heating in the 1st floor bath/shower rooms. We’ll possibly install wiring to locations for bedroom electric rads “just in case”, only because it’ll be easy and cheap to do now.

Make sure to do the electric UF in the 1st floor bath/shower rooms - in the grand scheme of things it's a minor cost when building but a real headache if you decide you need it later on. 

We also made sure there were sockets just in case on the 1st floor bedrooms - haven't needed them so far - and the way things are going with global warming, we won't in the future. Also, heated towel rails haven't been used since we moved in a few years ago.

[jack]

12 minutes ago, Mike said:

You may have already discounted it, but if not then you may want to consider a PassifHaus design (there are several threads on the topic).

 

The underlying concept is that if you improve the thermal efficiency to a high level, then you no longer need a conventional heating system - and that former is paid for by the saving on the latter. That trade-off may not fully work in the UK, where the prices of certain components is higher (than in Germany & Europe more generally), but factoring in very low future heating bills (negative if you add PV) may compensate for that in your evaluation.

 

In thermal efficiency terms, the PassivHaus standard is more about reducing power consumpton to a particular level than to removing the need for a heating source. Very few PassivHaus-class builds in the UK would be comfortable to live in year round without any form of heating.

[fatgus]

15 hours ago, Crofter said:

We've just taken delivery of a Neo sliding door. It was absolutely unbeatable for u value (0.74 uW) and was considerably cheaper than any aluclad iron. 

Haven't installed it yet but the quality looks good. Mine are dark grey on the outside, and as far as I can tell this is a solid colour rather than a foil wrap. 

I also took the option of having the glazing units delivered separately, which has made handling much easier. It will also allow me to install the frames by screwing straight through the frame. The glazed units were delivered in robust transport frames, unlike the cheaper units I bought from Modern UPVC Glazing.

 

All in all, I'm quite impressed so far.

Hi @Crofter Can I ask where you sourced the Neo slider? I’ve just spoken with Aluplast and they said there are currently no UK fabricators of the Neo range. I’d be happy to import if it’s the right product, so wondering where you got yours?

[fatgus]

1 hour ago, Bramco said:

Make sure to do the electric UF in the 1st floor bath/shower rooms - in the grand scheme of things it's a minor cost when building but a real headache if you decide you need it later on. 

We also made sure there were sockets just in case on the 1st floor bedrooms - haven't needed them so far - and the way things are going with global warming, we won't in the future. Also, heated towel rails haven't been used since we moved in a few years ago.

 

Can never have too many sockets! Have to say, I’m looking forward to warm towels all year round. Our current bathroom rads only get hot when the rest of the heating is on, so in the milder months our towels are never warm. 1st world problems 😁

[Mike]

1 hour ago, jack said:

the PassivHaus standard is more about reducing power consumpton to a particular level than to removing the need for a heating source.

I evidently should have put this bit in bold:

 

1 hour ago, Mike said:

if you improve the thermal efficiency to a high level, then you no longer need a conventional heating system

That is, when properly designed and built, a post-heater(s) on the MVHR system is sufficient to sustain an acceptable internal temperature.

 

Wolfgang Feist, co-creater of the PassivHaus, has said that this is the fundamental definition of PassivHaus in many interviews over the years, including in this one from my archive (Energy Design Update, Vol. 28, No. I, January 2008):

 

 

That's typically topped up on very cold days with a panel heater(s) or the like. In the UK at least, UFCH pipes are sometimes added for reassurance and / or for future summer cooing.

 

[Iceverge]

2 hours ago, fatgus said:

Based on the calcs, if we go for an “as-designed” 0.13-0.14 timber frame and in reality it only performs as well as a 0.2 frame, that should still be fine 🤞😄

 

I would go for an as designed and as built U value of 0.2W/m2K. Otherwise you'll be charged for 0.13 and not see the benefit. 

 

I prattle on about U values as much as anyone else here but there's other fish to fry. Noise and Fire etc. I would disregard anything with PIR between the studs in these grounds alone. Likewise triple gasketed 3g glazing is important for noise and thermal comfort. 

 

Have you priced stick build? Pump it with cellulose and you get the best of a lot of worlds. 

 

 

 

[fatgus]

1 hour ago, Iceverge said:

 

I would go for an as designed and as built U value of 0.2W/m2K. Otherwise you'll be charged for 0.13 and not see the benefit. 

 

I prattle on about U values as much as anyone else here but there's other fish to fry. Noise and Fire etc. I would disregard anything with PIR between the studs in these grounds alone. Likewise triple gasketed 3g glazing is important for noise and thermal comfort. 

 

Have you priced stick build? Pump it with cellulose and you get the best of a lot of worlds. 

 

 

One of the tempting routes is an i-beam stick build, designed by Cullen Timber Design, manufactured from their cutting list by a company not too far from us (Gloucester, I believe) and then put together by our builders. We'd use blown cellulose. Certainly no PIR.

 

Fire is one of our main concerns as the external walls will be almost 80% timber frame and 100% clad in timber (it was originally 50% metal, 50% timber but that was scuppered by the planners). We want to mitigate risk, both for our peace of mind and to reduce insurance, but it's hard to find breathable FR boards. We think we have an option from Magply, but the fire risk is one reason we even considered reverting to block build.

 

 

Edited yesterday at 16:46 by fatgus

[jack]

Quote

The definition of a PassivHaus doesn't need any number.

 

The PassivHaus Institute thinks otherwise.

 

Sure, their heat demand/load numbers are based on the theoretical ability of an MVHR system to deliver the required heat based on several other assumptions, but imo that context is of little practical relevance given they don't actually require heat to be distributed in that fashion.

 

1 hour ago, Mike said:

In the UK at least, UFCH pipes are sometimes added for reassurance and / or for future summer cooing.

 

But UFH isn't "sometimes" installed for "reassurance" in the UK. UFH is widely installed as the primary heating distribution system in PassivHaus-class buildings in the UK, presumably because people prefer to heat their homes that way than via hot air through a ventilation system.  

[Oz07]

5 hours ago, Bramco said:

Make sure to do the electric UF in the 1st floor bath/shower rooms - in the grand scheme of things it's a minor cost when building but a real headache if you decide you need it later on. 

We also made sure there were sockets just in case on the 1st floor bedrooms - haven't needed them so far - and the way things are going with global warming, we won't in the future. Also, heated towel rails haven't been used since we moved in a few years ago.

I found with lvt never felt cold feet in bathrooms no need for ufh. Electric towel rad was great on other hand. Drying towels for a few hours after shower when the heating wasn't coming on. 

[Iceverge]

2 hours ago, fatgus said:

 

One of the tempting routes is an i-beam stick build, designed by Cullen Timber Design, manufactured from their cutting list by a company not too far from us (Gloucester, I believe) and then put together by our builders. We'd use blown cellulose. Certainly no PIR.

 

Fire is one of our main concerns as the external walls will be almost 80% timber frame and 100% clad in timber (it was originally 50% metal, 50% timber but that was scuppered by the planners). We want to mitigate risk, both for our peace of mind and to reduce insurance, but it's hard to find breathable FR boards. We think we have an option from Magply, but the fire risk is one reason we even considered reverting to block build.

 

 

 

 

They do an option for Softwood stick builds too. 

 

Something like an 8*2 won't actually carry much more heat than an I joist. Espically if you add a cross battened insulated internal service cavity. Run it through uBakus to compare.

 

Agreed fire is a concern. Would something like cereal fiber cement be an option? An layer of Rockwool Frontrock outside the OSB sheathing would protect  from a fire in the cavity. 

 

 

 

 

 

[Gus Potter]

On 17/02/2026 at 12:46, fatgus said:

I think we're actually approaching this from a very similar perspective.

Ah, tongue in cheek. You can ask ten designers like me and get ten different answers. So long as each answer is reasoned out and evidence based then that lets you make an informed descision. Sometimes these descisions are based on you just saying.. we like and want that, yes we recognise that it may not be the best technically and SE wise, but we want it! So long as it does not fall down, leak water etc this is our choice and just get on with the design.

 

This happens a lot.  

 

On 17/02/2026 at 12:46, fatgus said:

I love a good spreadsheet and find it the best way, for me, to work towards a cost/benefit decision.

Ok, that is good. 

 

If you like a spread sheet then set up another tab for labour cost. What I'm leading you towards is this. 

 

When you design things you are aware that the Client has a budget. When I'm doing this I always have at the back of my mind that if I introduce complexity, design some steelwork say, that very quickly the labour cost can escalate. Insulation/ air tightness details can be very labour intensive.. and to get them right you need a level of site supervision. Builders will often get apprentices to stick on the tapes that you are basing a lot of your calculations on.. mistakes happen!

 

Complexity also adds additional risk to the Contractor who has to make a profit. You, at the stage you are at, don't have the experience yet, but you can experiment, just using your common sense with a bit more research. 

 

Set up a tab that looks a bit like this. I made this cost sheet for a Client like you, to play with.

 

 

And another part of it..

 

 

I hope you can read it as have screenshotted it. The labour rates are maybe a bit low now. The main thing to take away from this is to say for two site operatives you are looking at well over 2.0k a week, before profit etc, down south or in say Edinburgh you are looking at a lot more. My inclination is to look for where we can easily insulate, do the structure in a buildable way. If you take this approach it frees up cash to spend on the things you see and enjoy. 

 

What this does is to let you see how quickly the labour cost can get out of hand, but also make savings. If you look at the top bit you can see I have contingency, contractor profit and so on.

 

If you just set up a tab like this for now you can add to it as you go. This is a bit of philistine stuff but you can have a look at commercial websites like check a trade to get a feel for labour and cost rates. You can ask your local builders, here on BH.. feed all of that into your spread sheet to get a reasonable rate for your area. 

On 17/02/2026 at 12:46, fatgus said:

Norrsken and Zyle Fenster is around £30k for a relatively small improvement in average Uw. Norrsken are lovely windows, but make zero economic sense for us.

 

In the round you need to strick a balance.. but with a bit of thought you can often get to install the things you love..call these a luxury item, if you can make pragmatic savings elsewhere. 

 

 

 

 

[Ed_]

Having similar debates myself.

 

My main concern is that door/window u values are, at best, much worse than walls. Given a heat source like UFH, which is uniformly distributed, it is inevitable that close to the doors/windows will be colder than the rest of the room, more so with large expanses of glass.

 

So it seems to me that saving money on the u value of doors/windows might be economic in cost terms but possibly has a significant effect on comfort.

[Crofter]

8 hours ago, fatgus said:

Hi @Crofter Can I ask where you sourced the Neo slider? I’ve just spoken with Aluplast and they said there are currently no UK fabricators of the Neo range. I’d be happy to import if it’s the right product, so wondering where you got yours?

They were imported from Poland. I got mine via a small outfit in Kirkcaldy, but @craigcan supply them too.

[Bramco]

10 hours ago, Ed_ said:

Given a heat source like UFH, which is uniformly distributed, it is inevitable that close to the doors/windows will be colder than the rest of the room, more so with large expanses of glass.

Yes but.....

 

We have UFH and 3 large sets of sliding patio doors on our living space. We worried that these could cause a cold down draft - however after much reading here and elsewhere we specced triple glazing and it works.  There aren't any cold down drafts and yes, if you hold the back of your hand near one of them, you can feel it is cooler but only just. And our doors aren't the most expensive on the market - aluminum frames from Express Bifolds in Leeds. Great to deal with and excellent after sales service.

[fatgus]

10 hours ago, Crofter said:

They were imported from Poland. I got mine via a small outfit in Kirkcaldy, but @craigcan supply them too.

Thanks @Crofter 👍👍

[craig]

8 minutes ago, fatgus said:

Thanks @Crofter 👍👍

 

Let me know if I can help @fatgus drop me a message if you would like to.

[fatgus]

@Gus Potter Thanks for the screen shots... really appreciate it  We've already got some labour costs in the spreadsheet, mostly because we're aiming for supply & fit for the big ticket items (for VAT reasons... 5% on supply & fit, 20% on supply only). Thankfully, labour rates here are not too outrageous if you choose wisely  Once it's watertight, we're in no huge rush and looking forward to doing as much as possible ourselves... we've done it before, albeit on a smaller scale, and while we're admittedly getting older, being hands-on is one of the drivers for the project.

 

12 hours ago, Gus Potter said:

In the round you need to strick a balance.. but with a bit of thought you can often get to install the things you love..call these a luxury item, if you can make pragmatic savings elsewhere. 

 

This is bang on... There are quite a few aspects of our plan that many would question, but we are determined to have... a few economies elsewhere make the justification a bit easier 

 

 

[Mike]

17 hours ago, jack said:

UFH is widely installed as the primary heating distribution system in PassivHaus-class buildings in the UK, presumably because people prefer to heat their homes that way than via hot air through a ventilation system. 

Sure, you can choose to heat a Passihaus however you like without restriction.

 

Perhaps because of that, many people in the UK seem to be unaware of the underlying principle behind why 15 kWh/m²/year was chosen (for European climates) - it's not a random number - and think that they need to pay for a conventional heating system in addition to all the other costs of achieving the standard, rather than offsetting the cost saving.

 

Indeed it's so fundamental that it's the reason that the word 'passive' is in the name - it doesn't require traditional 'active' heating or cooling, because it can maintain a comfortable temperature largely through passive means.

 

17 hours ago, jack said:

imo that context is of little practical relevance given they don't actually require heat to be distributed in that fashion.

It's pretty useful to know on a thread discussing costs vs benefits.

 

Worth mentioning too the innovative use of Willis Heaters as a low-cost heat source for those who do choose to add UFCH; there are multiple threads on the topic.

[JohnMo]

19 minutes ago, Mike said:

unaware of the underlying principle behind why 15 kWh/m²/year was chosen (for European climates)

So elaborate!

 

20 minutes ago, Mike said:

it doesn't require traditional 'active' heating or cooling, because it can maintain a comfortable temperature largely through passive means.

Not exactly correct - the heating demand rate if 10W per m² max demand is a good indicator that us incorrect. That number being chosen for a specific reason - a heating element in the MVHR (flowing at passivhaus flow rates) can supply that heat without getting a burning dust smell from the ventilation air.

 

A passivhaus still needs heat not much, but some, you don't need a huge gas or oil boiler though.