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 9 hours ago 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 2 hours ago 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.